GPS Reference Stations and Networks An introductory guide
Contents Page Page 1. Introduction: aim of this book 1 11. Power supply 15 2. What is a Continuously Operating Reference 11.1 Power supply at the receivers 15 Station (CORS)? What is a network? 2 11.2 Power supply for the computer running the reference station software 16 3. What are GPS reference stations used for? 3 3.1 Geodetic control for surveying, engineering, 12. GPS reference station software and control mapping, cadastre, etc. 3 computer (server) 17 3.2 Monitoring the Earth's crust, natural and 12.1 Reference station software and control man-made structures 4 computer (server) 19 3.3 Machine guidance 5 12.2 Reference station software: calculation of 3.4 GIS data collection 5 RTK/DGPS data 19 3.5 Endless possibilities 5 12.3 Reference station software: continuous network analysis and calculation of correction 4. What does a GPS reference station or a network parameters for enhanced RTK 20 of stations have to do? 7 12.4 Reference station software: calculation of the positions of the antennas 20 5. Selecting suitable sites 8 6. Open view of the sky 9 13. RTK and DGPS data 21 7. Setting up a reference station on a pillar 10 13.1 RTK and DGPS data: output from the receivers 21 8. Setting up a reference station on a building 11 13.2 RTK and DGPS data: computed and output at 9. Setting up a reference station on a man-made the server 21 structure 12 14. The need for reliable communication 22 10. GPS receivers, GPS antennas, cables 13 15. Communication between the server and the 10.1 GPS receivers 13 receivers 23 10.2 GPS antennas 14 15.1 Dial-up links (open when required) 23 10.3 Antenna cables 14 15.2 Permanently open links (continuously on) 24 Page Page
16. Communication: transmission of RTK and DGPS 20. Continuous network analysis and calculation of data 25 network correction parameters for 16.1 Transmission directly from the receivers 25 enhanced RTK 34 16.2 Distribution (transmission) from a control center 26 20.1 Accuracy and range of RTK: the influence of 16.3 Radio versus phone: advantages and distance dependent errors 34 disadvantages 27 20.2 Network analysis and computation of network correction parameters 35 17. Using the Internet, or other IP-based methods, 20.3 Network data in RTCM V3.0 format and network for communication between the server and the correction parameters are transmitted to the receivers 28 rover and the rover applies them. 36 20.4 Network correction parameters are applied at 18. Using the Internet to distribute and access RTK the server and the server transmits “corrected” and DGPS data 30 RTK data to the rover. 37 18.1 Equipment needed by RTK and GIS rovers 30 20.5 Comparison of the two methods 39 18.2 Distribution from a single stand-alone reference 20.6 Improved RTK performance 39 station 30 18.3 Distribution from a network control center 31 21. Using an FTP server to distribute RINEX data 40 18.4 Summary of the various ways by which RTK/DGPS data can be distributed 32 22. Processing baselines between stations to check antenna positions and monitor movements of 19. NTRIP - Networked Transport of RTCM via natural and man-made structures 41 Internet Protocol 33 22.1 Processing the baselines between the stations of a network 41 22.2 Checking the positions of the antennas 41 22.3 Monitoring movements of the Earth's crust, and of natural and man-made structures 42 Page Page 23. Determining the WGS84 coordinates of the 27. Meteorological and tilt sensors 53 reference stations 43 27.1 Meteorological sensors 53 27.2 Tilt sensors 53 24. Where should the stations be positioned? What should be the distances between the stations? 45 28. Protection against lightning 54 28.1 Lightning conductor 54 25. Charging for products such as RINEX and 28.2 Lightning surge arrestor 55 RTK/DGPS data 46 29. Points to take into account when deciding what 26. Some examples of stand-alone reference stations type of stations and/or networks are needed 56 and networks 48 26.1 Stand-alone reference station for a small area 30. Useful links 57 such as an opencast mine, construction site, or local community 48 26.2 A simple network of a few stations providing complete RTK coverage for a relatively small well developed area. 49 26.3 A more complex network providing complete RTK coverage for a relatively large highly developed area. 50 26.4 A network of a few stations providing RINEX data and limited RTK/DGPS for a very large undeveloped area 51 26.5 A combined approach: networks of stations providing RTK/DGPS coverage for highly developed areas, widely spaced stations providing RINEX data throughout undeveloped areas. 52 1. Introduction: aim of this book
Although the benefits of Continuously This book offers practical advice on Operating GPS Reference Stations - how to set up and run both individual often referred to by the acronym CORS GPS reference stations and networks - are generally well recognized today, of stations and to provide the services there is still considerable uncertainty that are required. within many organizations and amongst many would-be users as to As budgets are sometimes limited, the best way to establish reference and as reference stations and stations and provide appropriate levels networks can vary considerably in of services for the GPS user complexity and, therefore, the community. investment needed, emphasis is placed on efficient, cost-effective solutions.
For general background information on GPS, please see the book “GPS Basics” 713282 by Leica Geosystems. 1 2. What is a Continuously Operating Reference Station (CORS)? What is a network?
A continuously operating GPS at regular intervals and pass them to software and communicating by reference station - or permanent an FTP server for access by the GPS telephone, LAN, WAN or Internet can reference station as it is often user community. control all the stations in the network called - comprises a GPS receiver (PCs are not required at the receivers). and antenna set up in a stable manner One or more single reference stations at a safe location with a reliable power supplying GPS services to users in the This brief introduction illustrates that supply. The receiver operates immediate surrounding areas may be reference stations and networks can continuously, logging raw data, all that is required by some vary considerably in extent and perhaps also streaming (continuously organizations. Other authorities, complexity. Organizations that are outputting) raw data, and often however, may need to establish studying the establishment of outputting RTK and DGPS data for networks of reference stations - reference stations should consider transmission to RTK, GIS and GPS perhaps 5, 10, 20, 50, or even more carefully what the stations will be used navigation devices. The receiver is stations - to provide complete GPS for, what services they will have to usually controlled by a computer that services over entire regions and even provide, and what will be the can be located remotely if necessary. countries. A single server (computer) appropriate levels of sophistication The PC will usually download data files running a GPS reference station and cost.
123n
2 3. What are GPS reference stations used for?
The first reference stations, in the days than a traditional triangulation and Such a network can be of almost any when GPS was in its infancy, were set traverse network. The stations can be size. Whilst one or two stand-alone up along coastlines to transmit DGPS set up at convenient locations in areas reference stations may be all that is corrections to improve the accuracy of where they are needed (rather than required for a local area, town, ship navigation. on remote hilltops). Network geometry municipality, opencast mine or is not as critical as with traditional engineering site, a multi-station Today, with the widespread networks, and the accuracy is higher network will usually be needed to acceptance of high-precision GPS and more consistent. Users set up provide full GPS service coverage for a measurement techniques, GPS their field receivers in the areas in large county, region or entire reference stations are being which they are working, download country. established all over the world in ever reference station data via the Internet, increasing numbers. They are used to and compute their positions. The sta- monitor the Earth's crust, to provide tions can also transmit RTK and DGPS geodetic control, to support surveying, data for direct use by RTK and GIS engineering, GIS data collection, field rover equipment. machine control and precise positioning, as well as to monitor natural and man-made structures. GPS reference stations provide the control needed for a wide variety of applications.
3.1 Geodetic control for surveying, engineering, mapping, cadastre etc.
A network of continuously operating GPS reference stations is more efficient 3 3. What are GPS reference stations used for? (continued)
3.2 Monitoring the Earth's crust, natural and man-made structures
In regions where earthquakes are likely to occur, along major fault lines, and in areas of volcanic activity, networks of suitably positioned GPS reference stations are often used to monitor movements of the Earth's crust.
A central computer with a reference station software controls the receivers, downloads data, and computes the network to determine the positions of the antennas. Movements can be analyzed.
Similar networks, though usually on a smaller scale, are used to monitor the positions and movements of natural and man-made structures such as glaciers, landslides, dams, bridges, buildings, towers, offshore oilrigs etc.
4 3. What are GPS reference stations used for? (continued)
3.3 Machine guidance 3.4 GIS data collection 3.5 Endless possibilities
On large construction sites and in Water, electricity, gas and telephone GPS reference stations and networks opencast mines, work can be carried authorities, cadastral offices, can be used in many ways for many out faster, with higher accuracy and municipalities and other similar applications. using less material when machines are organizations often operate equipped with automatic guidance and Geographical Information Systems. Stations and networks can be set up height-control systems. Bulldozers, The GIS database shows the location and configured for just one particular graders, scrapers, drilling machines of property boundaries, infrastructure, application and one user group. Or etc. can be positioned and steered to and assets such as water pipes, they can be designed to be multi- centimeter-accuracy with RTK. The hydrants, power lines, gas pipes, functional to support a wide range of height of the blade can be controlled telephone lines etc. applications and a multitude of users. automatically. RTK and DGPS measurement A single reference station may be When fields are large, agricultural techniques are widely used for perfectly sufficient for a small locality. machines for sowing seed, distributing updating the database, A multi-station, multi-purpose network fertilizer, harvesting, spreading surveying new features, and will often be preferred for an entire insecticide etc. operate more re-surveying existing features to region. The permutations are endless. efficiently and cost effectively when improve the accuracy of the data. guided by RTK or DGPS. One or more permanent reference GPS reference stations provide the stations will be needed whenever GPS GPS reference stations provide the control and support needed by RTK surveying or monitoring has to be control and support needed for and GIS rover equipment. carried out repeatedly over the same machine guidance. area for a long period of time.
5 GPS reference stations and networks support all types of users and applications 6 4. What does a GPS reference station or a network of stations have to do?
The GPS receivers at reference stations A server running reference station CMR, CMR+) for transmission to and run continuously. The raw code and software can control a single receiver use by RTK and GIS field rover phase measurement data are usually at a stand-alone reference station or receivers. Transmission can be logged internally in files of specified an entire network comprised of many directly from the stations or via other length. Depending on the application, receivers. In case of a single stand- suitable locations. Communication for the file length can be set to any alone station, the computer will often transmission of RTK and DGPS data required value from a few minutes to be connected directly to the receiver. will usually be by radio, high-speed several hours or even to a full day. In case of a multi-station network, the wireless (GSM, GPRS, CDMA etc) or server will usually be at a control center even by the Internet. Reference station software running on and connected to the receivers by a computer - let us call it a server - telephone, LAN, WAN or Internet (PCs controls the receivers and downloads are not required at the receivers). the data files at regular intervals. If required, the raw data can also be Once set up and configured, the
streamed continuously, second by stations and network will run fully
0101001001111011010 0110101010010011110110100
00100111101101 1
0011110110 second, from the receivers to the automatically. However, system 111101
1001101010100100111101101000 server. The reference station software supervisors can log in, inspect the 1001101010100100111101101000
running on the server converts the receivers and the network and make 1001101010100100111101101000
001101010100100111101101000
data to RINEX (Receiver INdependent any changes that are necessary. 001101010100100111101101000
0011010101001001111011010001
EXchange format) and produces 11011010001
00110101010010011 compressed RINEX files. The RINEX A main requirement today is to 0011010101001001111011010001
files are pushed to an FTP server for provide the data needed by real-time 10011010101001001111011010001 FTP
100110101010010011110110100 10011010101001001111011010001
10011010101001001111011010001
101001001111011010001001 easy Web access by the GPS user survey and GIS rover equipment. The 0011010101 1010101001001111011010001001
100110101010010 community and are also archived for receivers at the reference stations can 1001101010100100111 100110101010010011
100110101010010011110 10011010101001001111
safekeeping. output data in standard RTCM formats 10011011101001001111011 1001101010100100111101
100110101010010011110110 10011010101001001111011
1001101010100100111101101 100110101010010011110110
and in other proprietary formats (Leica, 1001101010100100100101101 1001101010100100111101101 001101010100100111101101
0011010101101000100101101 11010101001001101011010 11010101001001111011010
011010101001001001101101 011010101001001001011010 0011010101001001011011010 0011010011001001011011010
7 4. (continued) 5. Selecting suitable sites
If it is required to monitor any When selecting sites for continuously operating GPS reference stations careful movements within a network, it is also consideration must be given to the following: possible to arrange for the positions of the antennas to be • The need for an open view of • How to provide reliable power and computed automatically at regular the sky communication? intervals. • No objects in the vicinity that could • How to house and protect the cause multipath equipment? The functionality and complexity of an • No transmitters in the area that • How to ensure security against individual station or a network of could cause interference vandals and passers-by? stations will depend on a number of • What the stations will be used for • Accessibility for inspection and factors, including: • How to provide a stable mount for service the antenna? • Cost • The applications for which the station or network is to be used for • The services that have to be provided • The number and type of users that have to be supported • The most suitable methods of communication • The available infrastructure • The costs of running the station or the network • The possibility to charge for services and data • The available budget
8 6. Open view of the sky
Receivers at reference stations will One way to make fully certain that a a receiver and antenna, log data for usually be set to track satellites down site is absolutely perfect for a several days, and then analyze the to 10° above the horizon (10° cut-off geodetic reference station is to set up data using the TEQC tool from angle). For some applications it may UNAVCO (see 30. Useful links). even be required to track satellites down to the horizon, i.e. to 0° elevation. No✘ Obstructions can lead to loss of satellite signals and may also cause multipath (reflected signals). Multipath can have a negative influence on the quality of the data and, therefore, accuracy.
For these reasons sites should be selected where there will be no obstructions above 10° above the ✓ horizon of the antenna. This is OK particularly important for reference stations that will form part of a high accuracy geodetic network. The best, of course, is if there will be no obstructions at all above the antenna horizon.
9 7. Setting up a reference station on a pillar
Well-constructed pillars with solid power supply and communication Unless the reference station can be foundations, ideally bedrock, will device inside the pillar. Otherwise set up within a secure area, a high usually be preferred for stations that these items will have to be located in fence may be needed to protect the are to be used for monitoring a suitable container, shed or nearby equipment from vandals and movements of the earth. building. Depending on the climate, passers-by. ventilation or even air conditioning Pillars are also sometimes used at may be necessary. Reference stations using pillars need stations that form part of a geodetic careful planning especially as the control network (set ups on buildings If the equipment can be placed in a required infrastructure can lead to are often more practical however). nearby building, the required power high costs. Examples of pillars can be and telephone connections will often found on the UNAVCO web site (see If pillars are to be used the following be available. If the equipment has to 30. Useful links). should be considered carefully: be placed in the pillar or in a • How to provide reliable power and container, special power and communication? telephone lines may have to be • Where to place the receiver, power arranged, which can increase costs supply and communication device? significantly. • Security At remote sites in sunny climates, it Pillars can be of concrete or metal. is possible to arrange a bank of Note that, in hot climates, very tall batteries with a solar panel charging pillars could be liable to slight diurnal system to power the GPS receiver and deformations caused by the heat of a communication device, such as a the Sun. satellite phone or mobile phone.
If the pillar is suitably designed, it may be possible to place the GPS receiver,
10 8. Setting up a reference station on a building
There are significant advantages in normally be very stable and suitable located in a room below the roof setting up reference stations on for reference stations. where power and telephone buildings, as power and telephone connections are available. connections are usually available and It is usually easy to arrange a stable the equipment should be relatively mount for the GPS antenna on the In most cases infrastructure costs can safe. roof of a building. There are various be kept reasonably low when setting ways in which this can be done, for up a reference station on a building. Buildings are usually suitable for instance: using a metal plate with a It should also be easy to ensure reference stations that form part of a 5/8 inch screw fixed firmly in position, that only authorized persons geodetic control network, that support building a small pillar, attaching a pipe have access to the equipment. surveying and engineering or console to a wall, etc. applications, and that transmit RTK No✘ and DGPS data to RTK and GIS rover The GPS receiver, power supply and units. communication equipment (and possibly even a PC) can often be Although it is usually advantageous to place the GPS antenna as high as OK ✓ possible, towers and thin, tall, skyscraper-type buildings are often not suitable as they may sway in strong winds. In hot climates, very tall structures may also deform (bend) slightly due to expansion caused by the Sun.
Flat-roofed buildings of medium height with a relatively large base will
11 9. Setting up a reference station on a man-made-structure
Networks of GPS reference stations The type of structure and the can be used very effectively for movements to be monitored will monitoring deformations and determine where and how the GPS movements of man-made structures antennas have to be set up. The such as dams, bridges, buildings, availability of power and telephone offshore oilrigs etc. connections has to be considered when positioning the GPS receivers, If the need is to identify slow, power supply and communication long-term movements, data can be devices. logged in the receivers, downloaded at regular intervals to a central control computer, and then processed automatically to obtain the positions of the antennas.
In order to monitor rapid, short-term movements and even vibrations, the raw data have to be streamed continuously at a high rate from the receivers to the control computer. Software running on the computer processes the baselines between the stations continuously.
Movements and deformations can be identified.
12 10. GPS receivers, GPS antennas, cables
10.1 GPS receivers The receivers used at reference Single-frequency receivers are limited stations should have sufficient ports in performance and not suitable for Modern, universal, dual-frequency for: use at the multi-purpose reference receivers, such as those of the Leica stations that are often required today. System 1200 and System 500 series, • Connecting to a control computer They can be used, however, at are by far the most suitable for running the reference station stations that simply transmit DGPS reference stations as they produce all software data to GIS rovers and navigation types of measurement data (L1, L2, • Streaming raw data to a control receivers. Although it is possible to code, phase), generate all required computer if this is a requirement use single-frequency receivers within outputs (RTK, DGPS, NMEA) and • Attaching communication devices small, short-baseline networks that support every kind of application. for transmitting RTK and DGPS data are used for monitoring movements, • Connecting primary and backup dual-frequency receivers will usually be In order to be able to provide the power supplies preferred. required services to different users at • Connecting peripheral devices such the same time in an optimum manner, as meteorological and tilt sensors it is best if the receiver can log data at high rates, continuously stream raw data, and also output RTK and DGPS data in all commonly used formats (RTCM, Leica, CMR, CMR+).
Simultaneous data logging at two different rates to two different files can also be required for some applications.
13 10. GPS receivers, GPS antennas, cables (continued)
10.2 GPS antennas Compact geodetic antennas provide Antenna cables up to 100m or more good quality data that are sufficient for are obtainable, but the longer the If reference stations form part of a most users and applications, and they cable the thicker it has to be to national, first-order, geodetic-control are significantly less expensive than minimize loss of signal and the network, IGS type choke-ring choke-ring antennas. heavier and more unwieldy it antennas fitted with Dorne & Margolin becomes. Extra-long cables also cost elements will often be mandatory. Any 10.3 Antenna cables much more than standard cables. reference station that forms part of the global International GPS Service A standard 10m cable will often be The best is to use cables that are as (IGS) network will also usually require sufficient to connect the antenna to short as possible. Cables longer than a choke-ring antenna. This type of the receiver. If the receiver has to be 30m should rarely be needed. antenna has very high phase-center located further away from the stability, suppresses multipath to antenna, a longer cable will be needed. negligible levels, and helps to ensure that measurement data will be of the highest possible quality.
For single, stand-alone reference stations and for reference stations in networks that are used mainly to support surveying and engineering and to provide data to RTK and GIS rovers, standard, compact geodetic antennas are often suitable.
Compact geodetic antenna
Choke-ring antenna 14 11. Power supply
11.1 Power supply at the receivers When studying whether a UPS is consider the level of reliability that is required, it is worth considering demanded. Users that have to pay for A GPS reference station needs a whether the reference station is part data and services are unlikely to reliable, continuous power supply. of a network of stations or whether tolerate failures; in this case UPS units An AC to DC converter connected to it is a stand-alone station. will usually be required. the mains (line) will normally be used for powering the receiver and any If one station in a network of stations If stations form part of a monitoring other ancillary equipment such as should stop operating very network and if failure could lead to communication devices. occasionally for a short period of time danger or even loss of life, UPS units due to power failure, users of field of appropriate size will probably be A UPS (Uninterruptible Power Supply) receivers, RTK and GIS rovers may be obligatory. unit will provide backup power for a able to obtain the data and services limited time should the mains (line) that they require from other nearby Note that modern GPS receivers used supply fail. UPS units are available in stations and continue to work; thus for reference stations will restart various types and sizes (capacity). the occasional short power failure at automatically when power is The larger the capacity, the longer the one station may not be too critical. restored. UPS will power the equipment. If the station is a single, stand-alone It may sometimes be necessary to A UPS of appropriate size will almost station (not part of a network) - i.e. establish a reference station in an certainly be needed at a reference the only station supplying GPS area where there is no power. At station where the mains (line) power services to the surrounding area - a remote sites in sunny climates, it is is unreliable. However, if the power power failure will result in all users of possible to arrange a bank of supply at the mains is very reliable field equipment having to stop work. batteries with a solar panel charging and unlikely to fail, it may be decided system to power the GPS receiver and that a UPS is not necessary. When evaluating whether to install a communication device such as a UPS units and what capacity would be satellite phone or mobile phone. needed, it is also important to
15 11. Power supply (continued)
11.2 Power supply for the the software should restart computer running the automatically and download the data reference station software files from the receivers. Thus power failure may lead to a delay in down- It is advisable to consider installing a loading data but may not result in a UPS (Uninterruptible Power Supply) loss of data. unit to provide backup power for the computer running the reference station software. The UPS will bridge short power cuts and will also enable the software to shut down safely before power fails completely. AC to DC Converter A UPS of appropriate size may well be DC mandatory if the computer controls an entire network of receivers. At a stand-alone reference station, a UPS (if required) should be able to provide backup power for both the receiver AC and the computer.
If power fails at the computer but not at the reference stations, the AC receivers will continue to operate A C normally, log data, and transmit RTK and DGPS data. As soon as power is restored, the computer will reboot and UPS
16 12. GPS reference station software and control computer (server)
Although it is possible to run a single, stand-alone reference station - perhaps to support a mine, construc- tion site or community - without a computer running reference station software, such a reference station will have severe disadvantages and is unlikely to be more than a temporary solution. Such a station may be sufficient if the only requirement is to transmit RTK and DGPS data to RTK and GIS rovers. However, if raw data has to be archived and made available for users, the receiver tracking will have to be stopped - perhaps every evening - and the logged data downloaded manually.
Even for a single, stand-alone reference station, it is always advantageous if the receiver can run continuously, the data can be downloaded fully automatically at regular intervals, and the entire operation can be controlled and inspected as required by a supervisor.
17 RTK / DGPS
TCP / IP Radio Dial-up modem Serial RTK / DGPS
1234 n
GPS Reference Stations 18 12. GPS reference station software and control computer (server) (continued)
12.1 Reference station software continuously to the server instead of configurations, parameters and and control computer (server) logging data or even stream raw data operating modes, upload new at the same time as they are logging firmware to the receivers etc. Reference station software running on data. a computer - let us call it a server - Once configured and started, a can control a single receiver at a Reference station software running on reference station or a network of stand-alone station or all the the server checks the raw data for stations controlled by software receivers at all stations in a network. completeness, compresses the raw running on a server will operate fully In case of a single stand-alone data, converts it to RINEX, compacts automatically. Well-designed station, the receiver will often be RINEX, archives raw data and RINEX reference stations and networks are connected directly to the computer. files, and pushes raw data and RINEX extremely powerful yet, once set up, In case of a multi-station network, the files to an FTP server for easy access they are very easy to use. server will usually be at a control by the GPS user community. The center and connected to the receivers software also monitors the operation 12.2 Reference station software: by telephone, LAN, WAN or Internet of the receivers, the quality of the calculation of RTK/DGPS data (PCs are not required at the data, the communication links, the receivers). functioning of the entire network, and Although the receivers can output RTK generates messages and reports as and DGPS data directly in standard GPS receivers at reference stations run necessary. RTCM formats and in proprietary continuously. The raw measurement formats (Leica, CMR, CMR+) for data are usually logged internally in System supervisors have full control immediate transmission from the the receivers in files of the required over the receivers at the stations and stations to RTK and GIS field rovers, length. Reference station software the entire network. They can log in to some organizations prefer to have the running on the server controls the the server, perhaps even from remote distribution of RTK and DGPS data receivers and downloads the data files locations, inspect the operation of the controlled from one central location. automatically at regular intervals. receivers and the network, start and Receivers can also stream raw data stop the various operations, change If permanently open communication links are available, raw data can be 19 12. GPS reference station software and control computer (server) (continued)
streamed continuously from the Using a network of several reference 12.4 Reference station software: receivers to the server. A software stations, it is possible to model these calculation of the positions component running on the server distance dependent errors and to of the antennas computes the required RTK/DGPS data correct for them. continuously in the required Networks of GPS reference stations format - standard RTCM format or The raw data have to be streamed are often used for monitoring proprietary (Leica, CMR, CMR+) continuously from the receivers to movements of the Earth's crust, format - and outputs it immediately the server. Network analysis software particularly along fault lines and in for distribution to RTK and GIS rovers. running on the server continuously areas of volcanic activity. Small analyzes the “state” of the network networks can be used to monitor the data and models the distance positions and movements of natural 12.3 Reference station software: dependent errors. Correction and man-made structures such as continuous network analysis parameters are computed dams, bridges, buildings, oilrigs, and calculation of correction continuously. landslides, glaciers etc. parameters for enhanced RTK If the RTK rover receivers are A software component running on the The maximum range from a reference equipped with the appropriate server can compute the baselines of station at which a standard RTK rover algorithms, these correction the network continuously or at can operate successfully (i.e. resolve parameters can be transmitted to the regular intervals to determine the ambiguities) is usually quoted as rovers and the rovers apply them. positions (coordinates) of the about 30km. This assumes favourable Alternatively, the correction antennas. atmospheric conditions and that the parameters can be applied at the rover is receiving standard RTK data server and corrected RTK data can be Depending on the applications and from the station. The limitation in transmitted to the rovers. In both the requirements, slow, long-term range is due to the effects of distant cases, improved RTK at longer ranges movements, rapid, short-term dependent errors relating to satellite can be achieved. See 20 for full movements, and even vibrations can orbits, ionospheric delays and information. be investigated. tropospheric delays. 20 13. RTK and DGPS data
13.1 RTK and DGPS data: The data can be transmitted from 13.2 RTK and DGPS data: output from the receivers one or more ports in one or more computed and output at formats. the server The receivers at reference stations can be configured to output RTK and/or Radios and phones can be used to Some organizations prefer to have the DGPS data continuously in standard transmit RTK and DGPS data directly distribution of RTK and DGPS data RTCM V2.1/2.2/2.3/3.0 formats and/or from the receivers; see 16.1. The managed from one central location, in Leica, CMR and CMR+ proprietary Internet can also be used as rather than transmitting directly from formats. explained in 18.2. the receivers. This allows better control over user access and facilitates charging for data if this is a requirement.
RTK/DGPS Using permanently open communication links, raw data can be RTK/DGPS streamed continuously from the receivers to the server. Software running on the server computes the required RTK/DGPS data continuously in standard RTCM V2.1/2.2/2.3/3.0 formats and/or in Leica, CMR and CMR+ proprietary formats. The data are output in the required format(s) and transmitted to RTK and GIS rovers.
Radios, phones and even the Internet can be used to transmit the RTK/DGPS data. For more details see 16.2 and 18.3.
21 14. The need for reliable communication
Reliable communication is vital for • What the station or network of From the above it will be obvious that the efficient operation of GPS stations is to be used for. there is no standard solution. What is reference stations and networks of • The communication technologies best for a station or network in one stations. that are available locally and for country may not be appropriate for a • The reference station software which reliable support is available. station or network in another country. running on the server has to control • The cost of the communication the receivers and download the equipment and the installation The following chapters outline the data. costs. various communication methods that • RTK and DGPS data have to be • The running costs. may be considered. The final transmitted for use by RTK and GIS • The service and support costs. decisions as to which methods are the rover receivers. most suitable should only be taken When making a decision on the most after consulting carefully with Communication technology and suitable means for transmitting RTK communication and information information technology are evolving and DGPS data to RTK and GIS rovers, companies and specialists that very quickly. The availability of the the following should also be taken understand the requirements and various techniques - or at least the into account: that are thoroughly familiar with feasibility of using them - can also the local market. vary considerably from country to • The number of RTK and/or GIS country. rovers that the station or network has to support. When selecting the most suitable • The range at which RTK rovers have method for communication between to operate. the server and the receivers, various • The communication equipment that factors should be considered will be needed by RTK and GIS carefully, including: rovers. • The cost of this equipment and the running costs.
22 15. Communication between the server and the receivers
The reference station software running Note that the costs of permanently For a small network, one telephone on the server (computer) has to open links - in particular the running modem at the server should be control the receivers and download costs - are usually higher than those sufficient. Such a network should be the internally logged data files. of dial-up links. relatively easy to set up and should not be too expensive. For some applications it may be 15.1 Dial-up links necessary to stream the raw data (open when required) Mobile phone modems (GSM, CDMA, continuously (second by second) TDMA, GPRS etc) can be used if from the receivers to the server If standard telephones are available at standard telephones are not available instead of downloading the logged the reference station sites, standard at the reference station sites. The data files. Some organizations may landlines and telephone modems can phones have to be powered and even prefer to have both streamed be used to connect the receivers to permanently switched on. Running data and downloaded files. the server. The reference station costs (call charges) will often be software running on the server will higher than with standard landline. For controlling the receivers and dial the receivers and download the downloading logged data files, the logged files automatically at preset Another solution could be to use the communication links can be either times. Internet. See 17 for more details. dial-up (i.e. opened when required) or permanently open (i.e. continuously on). Dial-up links will usually be preferred, however, as the costs are generally lower than for permanently logged dat open links. a files
If the raw data have to be streamed continuously, permanently open communication links are essential. Dial when required 23 15. Communication between the server and the receivers (continued)
15.2 Permanently open links access router at the server. Note that (continuously on) the costs of this hardware can add up!
If the raw data have to be streamed The trend today is to try to make use continuously from the receivers to the of the Internet to achieve permanently server, the communication links open links for streamed data between between the receivers and the server the receivers and the server. Running have to be permanently open. costs with the Internet should be significantly lower than with Standard landline telephone modems telephone connections. and mobile phone modems can be See 17 for more details. used, however the running costs (call charges) will often tend to be high. It is also worth investigating the possibility of using leased lines, as these will usually provide very high reliability.
Note that a port and telephone modem will be needed at the server Streamed raw data for each reference station from which data is to be streamed. Thus a 10-station network will require 10 permanently open lines, a modem at each of the 10 receivers, and 10 modems at the server. Another Permanently open link possibility is to use a telephone
24 16. Communication: transmission of RTK and DGPS data
The GPS receivers used at reference number of RTK and GIS rover If a suitable telephone access router stations can output RTK and DGPS receivers can receive the data and or a bank of telephone modems is data from one or more ports in that the users of rover receivers do attached to the reference station standard RTCM formats and/or in not incur telephone charges. receiver, several rovers (perhaps 5 or proprietary formats. See 13.1. The 10 or even more) can dial in to the data can be transmitted directly from Another common solution is to attach station at the same time. The more the receivers. a telephone modem - landline or even rovers the router or bank of modems a mobile - to the reference has to support, the more it is likely to If raw data are streamed continuously station receiver. RTK and GIS rover cost. from the receivers to the server receivers with suitable mobile through permanently open links, telephone modems dial in to obtain Transmission by both radio and tele- software running on the server can the required RTK and DGPS data. A phone is possible if a radio modem is compute and output RTK and DGPS telephone connection usually permits attached to one port of the receiver data in standard RTCM formats and/or longer RTK and DGPS range than a and a telephone modem (or bank of in proprietary formats. See 13.2. radio modem, but the rovers incur modems or router) is attached to telephone charges and only one rover another port. 16.1 Transmission directly from at a time can dial in to one telephone the receivers at a reference station. Another solution could be to use the Internet. See 18 for more details. One way to transmit RTK and DGPS RTK / DGPS data directly from the receivers is to RTK / DGPS use radio modems. If two radio modems are attached to a receiver, they have to transmit at different frequencies (use different channels). RTK and GIS rover receivers have to be equipped with compatible radio Telephone modem (or banks of phone Radio modem modems. The advantages are that any modems or telephone access router) 25 16. Communication: transmission of RTK and DGPS data (continued)
16.2 Distribution (transmission) If radios are used, the RTK/DGPS reference station for which RTK/DGPS from a control center data for different reference stations data is available. Telephone access have to be transmitted on different routers or banks of modems will be Some organizations prefer to frequencies (use different channels) in needed in order to ensure that distribute the RTK/DGPS data from order to prevent interference. As the several rovers can access the one central location rather than trans- range of radio modem transmission is RTK/DGPS data from the same mitting directly from the receivers. usually limited, radio transmission station at the same time. stations or repeaters may be needed The raw data have to be streamed to provide full coverage for a network It is possible to have a single tele- continuously from the receivers to the area. phone number for all reference server thorough permanently open stations if the rover receivers that are communication links as outlined in If phones are used, a unique phone equipped with mobile phones can 15.2. Software running on the number will be needed for each send their positions to the server. server computes the required RTK/DGPS data in standard RTCM RTK / DGPS formats and/or in proprietary formats RTK / DGPS for each reference station.
The RTK/DGPS data for a specific reference station will be output via a dedicated port, thus the number of RTK/DGPS output ports at the server should equal the number of reference stations. Radios, phones or even the Radios: different frequencies for different stations Internet can then be used to Banks of phone modems distribute the RTK/DGPS data. or telephone access router Server computes RTK / DGPS data
26 16. Communication: transmission of RTK and DGPS data (continued)
The rover phones the control center The disadvantage is that the range at Phones and sends its position coordinates in which RTK and GIS rovers can operate The advantage of using phones is that NMEA format. A software component is usually less than if phones are the range at which rovers can operate running on the server decides which used. is usually not limited by the reference station is closest to the communication means. The rover. A telephone access router • The higher the output power of the connection is also very reliable with transmits the RTK/DGPS data for this transmitting radio, the greater will no interference and little loss of station to the rover. The router has to be the range at which rovers can reception due to obstructions. A be of sufficient size to allow operate. Output power is restricted disadvantage is that the rovers simultaneous multi-user access to in many countries, however. incur call charges. the RTK/DGPS data from all stations. • The higher the radio antenna can be set up at the transmitting station, A telephone access router or a bank It is also possible to distribute the the greater will be the range at of telephone modems will be RTK/DGPS data from the control which rovers can operate. required if several rovers have to access center using the Internet. See 18 for • Use of high quality antennas at the the RTK/DGPS data from the same more information. transmitting radios and at the rover reference station at the same time. receivers will also improve the Depending on the number of users it 16.3 Radio versus phone: range. has to support, this equipment can be advantages and disadvantages • Transmission from UHF radio quite costly. modems is “semi” line-of-sight. Radio modems Obstructions, particularly at longer Radio and phone The advantages of using radio ranges, can cause loss of reception Note that it is possible to transmit modems are that any number of RTK at the rover. RTK/DGPS data directly from the and GIS rovers can benefit from the • Interference on adjacent frequencies receivers or from a control center by service and that there are no charges. may also sometimes lead to poor both radio and phone. Users of rover reception at the rover. units can then use the service that they prefer.
27 17. Using the Internet, or other IP-based methods, for communication between the server and the receivers
There is a rapidly growing interest today in using IP-based methods for communication between the receivers and the server running the reference station software, and also for distribution of RTK and DGPS data. IP-based communication can be LAN, WAN, WLAN, Internet, Intranet, and Radio IP etc.
For simplicity, this document refers to INTERNET the Internet to represent all IP-based methods.
The main attraction of using the Internet for communication between the server and the receivers is that it is usually possible to reduce running costs (charges). If raw data have to be Modem streamed continuously from the receivers to the server as outlined in IP Ports 15.2, the running costs with the Internet will usually be much lower than with telephone connections. Server
28 17. Using the Internet, or other IP-based methods, for communication between the server and the receivers (continued)
A possible disadvantage of using the In order to access the Internet, the The reference station software Internet could be that the reliability receiver at a reference station will running on the server operates in and quality of standard Internet require a modem, a Com Server or exactly the same way with Internet connections may not be quite as high Ethernet port, and a static IP address. connections as with standard as with standard telephone The modem could be a telephone, telephone connections. connections. cable, or broadband/ADSL modem.
Internet connections of very high The server will require a suitable reliability can normally be guaranteed modem and one IP port for each if dedicated (leased) lines or MPLS reference station from which data will (MultiProtocol Label Switching) are be streamed (i.e. 10 IP ports are used between the server and the needed if data is to be streamed receivers. However, the cost is likely to from 10 stations). be higher than the cost of standard Internet connections. The modem could be a telephone, cable, or broadband/ADSL modem. The costs and reliability of the various However, if the server has to receive options can usually be clarified by continuously streamed raw data discussing carefully with the local simultaneously from several Telecom Company and Internet Service reference stations - which will often Provider. be the case - the best would be a broadband/ADSL or cable modem with a suitably large bandwidth.
29 18. Using the Internet to distribute and access RTK and DGPS data
18.1 Equipment needed by RTK 18.2 Distribution from a single formats or proprietary formats; see and GIS rovers stand-alone reference station 12.2). The server needs one IP port for output of the RTK/DGPS data to RTK/DGPS data can also be At a stand-alone reference station, the the Internet. distributed using the Internet. receiver will often be connected In order to access the Internet and directly to the server (computer) RTK and GIS rovers, equipped with obtain the required data, RTK and running the reference station soft- Internet capable devices, access the IP GIS rover receivers have to be ware. Raw data has to be streamed port and obtain the required equipped with Internet capable continuously from the receiver to the RTK/DGPS data. Multiplexing software devices such as GPRS or CDMA server. running on the server allows several phone modems. rovers to access the IP port and the Software running on the server RTK/DGPS data at the same time. computes the required RTK/DGPS data in the required format (standard RTCM
RTK / DGPS
Streamed RTK / DGPS INTERNET raw data
Server with one IP port Rovers with Internet and multiplexing software capable devices
30 18. Using the Internet to distribute and access RTK and DGPS data (continued)
18.3 Distribution from a network running on the server allows several The rover accesses the server via the control center rovers to access the RTK/DGPS data IP port and sends its position from the same the IP port at the same coordinates in NMEA format. In case of a multi-station network, time. A software component running on the the server will be at a control center. server decides which reference Raw data have to be streamed It is possible to have a single IP port station is closest to the rover. The continuously from the receivers to the for all reference stations if the rover RTK/DGPS data for this station are server. The connection between the receivers that are equipped with then transmitted to the rover. server and the receivers can be via the Internet capable devices can send Multiplexing software running on the Internet as explained in 17. Software their positions to the server. server allows several rovers to access running on the server computes the the server via the same IP port at the required RTK/DGPS data in the same time. required format (standard RTCM formats or proprietary formats; see 12.2) for each reference station.
The computed RTK/DGPS data can be output via the Internet. The server at raw d a ed needs one IP port for each reference am RTK / DGPS re station for which RTK/DGPS data are St to be output (i.e. if RTK/DGPS data are INTERNET INTERNET to be output for 10 stations, 10 IP ports are needed). RTK / DGPS
RTK and GIS rovers, equipped with Internet capable devices, access the IP Server with IP ports ports and obtain the RTK/DGPS data and multiplexing software for the stations. Multiplexing software 31 18. Using the Internet to distribute and access RTK and DGPS data (continued)
18.4 Summary of the various ways by which RTK/DGPS data can be distributed
Using radio modems (see 16) Using phones (see 16) Using the Internet
Any number of rovers can access the The connections are reliable and there The running costs (call charges) for data and the rovers do not incur are few restrictions to range. However, rovers should be less than with running costs (charges). Limited rovers incur calling charges. Routers or mobile phones, but the reliability of range, obstructions and interference banks of modems are needed in order the connections may not always be can be drawbacks. that several rovers can access the quite as high. There are few data from the same station at the restrictions to range at which same time. connections are possible and multi-user access can be arranged.
In order to compare the use of the Internet with the use of mobile phones, the best is to clarify all cost and reliability issues very carefully with the local Telecom Company and Internet Service Provider.
INTERNET
32 19. NTRIP - Networked Transport of RTCM via Internet Protocol
NTRIP is a new protocol, developed by An RTK or DGPS rover that accesses the Federal Agency for Cartography the server IP port can request data and Geodesy of Germany, for from a so-called mount point or streaming Global Navigation Satellite request a list of the available mount System (GNSS) data over the Internet. points. NTRIP has become an RTCM standard. For example the mount points could NTRIP provides certain advantages for be the sources for the RTK and/or a reference station software server DGPS data from the various reference when distributing RTK/DGPS data to stations, the source for the network RTK and GIS rovers. NTRIP can be used data in RTCM V3.0 format and for distributing data in any format, for network correction parameters (see example in standard RTCM 20.3), the source for network V2.1/2.2/2.3/3.0 formats and/or in corrected RTK data (see 20.4), etc. Leica, CMR and CMR+ proprietary formats. If the requested mount point is available, the data from this source With NTRIP, all communication and are transmitted to the rover. If the data passes through a single IP port requested mount point is not at the server. RTK and DGPS rovers available, the rover has to select can only obtain data if they are another mount point (source of data) authorized, thus the network operator from the list. has full control over access to the data. This facilitates billing (charging for data) if this is a requirement.
33 20. Continuous network analysis and calculation of network correction parameters for enhanced RTK
20.1 Accuracy and range of RTK: • At 1km: 10mm +1mm = 11mm rms Various influences, particularly those the influence of distance • At 10km: 10mm +10mm = 20mm rms relating to ionospheric delays, dependent errors • At 30km: 10mm + 30mm = 40mm rms tropospheric delays and satellite orbits, lead to distance dependent When receiving standard RTK data If RTK measurements could be errors and restrict the range at which from a reference station, an RTK rover carried out in an absolutely perfect the rover can resolve the ambiguities. can usually operate successfully (i.e. environment, there would be no It follows that if the distant resolve ambiguities) at ranges up to ppm accuracy component and no dependent errors within the network about 30km in reasonably restriction to range. Unfortunately, can be modelled with a reasonable favourable conditions. In some regions however, the environment is never degree of success and appropriate of the World, where conditions for perfect and it also changes corrections can be computed and RTK are often exceptionally good, continuously. applied, the accuracy and range of ranges of about 40km or more may RTK can be improved. orbits be achieved at times. In other areas, Satellite especially during the afternoon and in periods of high ionospheric activity, the maximum range for RTK may be reduced significantly. Ionosphere
The accuracy of RTK is normally quoted as about 10mm + 1ppm root mean square. Thus the positional Troposphere accuracies that can be expected are approximately as follows:
34 20. Continuous network analysis and calculation of network correction parameters for enhanced RTK (continued)
20.2 Network analysis and There are two ways in which the • The network correction parameters computation of network network analysis and network can be applied at the server and the correction parameters corrections parameters can be used to server transmits “corrected” RTK improve the performance of RTK: data to the rover. The raw data have to be streamed continuously from the receivers at the • The network data in RTCM V3.0 Note that if a network has a very large network reference stations to the format and the network correction number of stations, it will be split into server through permanently open parameters can be transmitted to cells (subsets of the network). communication links. Use of the the rover and the rover applies Internet will generally be preferred, them. as the running costs should be much lower than with telephone connections (see 17 and 15.2).
A network analysis component of the reference station software processes the incoming data continuously in order to analyze the state of the environment within the network. It models the distance dependent errors and computes network correction parameters. The entire process runs continuously and automatically.
35 20. Continuous network analysis and calculation of network correction parameters for enhanced RTK (continued)
20.3 Network data in RTCM V3.0 Depending on how RTK processing is baselines from several stations, format and network implemented in a particular rover resolve the ambiguities and derive correction parameters are receiver, the rover can now process position coordinates. The results are transmitted to the rover and the baseline from one reference largely free of the distance dependent the rover applies them. station (probably from the closest errors described in 20.1. station), or perhaps even the The network analysis software running on the server produces the required RTCM V3.0 data for the net- work. This RTCM V3.0 data includes the network correction parameters.
In order to be able to use the data, RTK rover receivers have to be RTCM RTCM V3.0 for entire network V3.0 compatible. They have to understand the RTCM V3.0 format and with network correction parameters they have to be able to apply the One-way communication network correction parameters. All RTK rovers receive the data stream The RTK rovers operating in the Network corrections applied at the rovers network area receive the RTCM V3.0 network data with the network correction parameters. This data contains all of the required information for the entire network, i.e. for all stations in the network. Server
36 20. Continuous network analysis and calculation of network correction parameters for enhanced RTK (continued)
The accuracy and range are superior If phones are used, all rovers should 20.4 Network correction parameters to those of standard RTK. be able to dial a single number for are applied at the server and the RTCM V3.0 network data stream. the server transmits “corrected” As the RTCM V3.0 data contains the A suitable router will be needed to RTK data to the rover. information for the entire network ensure simultaneous multiple user (i.e. for all stations), all RTK rovers access. The network analysis software operating within the network can running on the server analyzes the receive exactly the same information. If the Internet is used, all rovers network and computes the network Thus only a single RTCM V3.0 data should be able to access the same IP correction parameters for the network stream has to be output by the server. address for the RTCM V3.0 network as explained in 20.2. With this method, data stream. Multiplexing software however, the network correction If a network has a very large number running on the server will allow parameters are applied at the server of stations, it will be split into cells simultaneous multiple user access. and not at the rover. (subsets of the network). In order to obtain the required data, Radios, phones or the Internet can be the RTK rover has to transmit its used to distribute the RTCM V3.0 data position in an NMEA message to the to RTK rovers. The communication server. Using this position information, needs to be one-way only, i.e. server the network analysis software identifies to rover. The rover does not send which reference station is closest to the information to the server. rover.
If radios are used, the RTCM V3.0 The software then applies the network data stream will probably network correction parameters to the have to be redistributed via repeater measurements from this reference station stations or transmission stations in in such a way that a baseline computa- order to ensure full coverage over the tion between the reference station and network area. the rover should be free of the distance dependent errors described in 20.1. 37 20. Continuous network analysis and calculation of network correction parameters for enhanced RTK (continued)
“Corrected” RTK data in the formats As the “corrected” RTK data A suitable telephone access router RTCM V2.3/3.0 and Leica, can then be transmitted to the rover depends on will be needed to ensure output for transmission to the rover. the position of the rover, all roving simultaneous multiple user access. The RTK rover receives the data and receivers operating within the processes the baseline from the network area will receive different If the Internet is used, all rovers will reference station, resolves the individual data steams. access the same IP address in order to ambiguities, and derives position obtain “corrected” RTK data. The coordinates. If phones are used, all rovers will dial software ensures that each rover a single number in order to obtain receives the RTK data stream that it As the RTK data received by the rover “corrected” RTK data. The software requires. Multiplexing software are already “corrected”, the accuracy ensures that each rover receives the running on the server permits and range are superior to those of RTK data stream that it requires. simultaneous multiple user access. standard RTK.
Note that as the “corrected” RTK data can be transmitted in RTCM V2.3 and Leica formats, rover receivers do not its position to server have to be RTCM V3.0 compatible. Rover transm over itted to r TK data transm As the RTK rover has first to send its Corrected R munication position to the server, two-way Two way com communication is needed. Thus phones and the Internet can be used but not radios.
Each rover receives its own Network correction parameters individual corrected data stream applied at the server
38 20. Continuous network analysis and calculation of network correction parameters for enhanced RTK (continued)
20.5 Comparison of the two • Accuracy will be improved, as the • The maximum range will be improved. methods ppm accuracy component - i.e. the distance dependent component - As the range of RTK is increased, With the first method described in will be reduced significantly. reference stations can be further apart. 20.3, the rover receivers have to be • Thus RTK accuracy will be much Thus fewer stations are needed to RTCM V3.0 compatible in order to be more uniform at different distances. provide RTK coverage for a given area. able to use the transmitted data. • The reliability of RTK position fixes Radios as well as phones and the (ambiguity resolution) will be Note that the reference stations should Internet can be used. The software improved, particularly under be reasonably evenly distributed needed at the server will be difficult ionospheric conditions and throughout the area in order that the somewhat less complex than for the at longer ranges. software can effectively analyze the second method described in 20.4. • The time needed for RTK position state of the environment within the fixes will be reduced when network and compute the appropriate With the second method described in operating at long ranges and under network correction parameters. 20.4, the RTK data can be output in difficult ionospheric conditions. any required format. Rover receivers do not need to be RTCM V3.0 compatible. Phones and the Internet can be used but not radios. The Standard RTK software needed at the server will be slightly more complex than for the first method described in 20.3.
20.6 Improved RTK performance Longer range Both of the methods described above RTK with network correction parameters Higher accuracy will improve the accuracy, reliability and range of RTK. 39 21. Using an FTP server to distribute RINEX data
RINEX (Receiver INdependent There is also almost no practical range FTP server software can usually be EXhange) is a standard data format limit to post processing. With configured for public and/or restricted that is universally accepted by the GPS sufficient data, very long baselines can access. It can be arranged that surveying community and that can be be processed successfully. anyone can obtain data or - which will used by all commercially available post often be preferable - that users have processing software. In order to make RINEX data to be registered and enter passwords readily available for the GPS user in order to access the RINEX files. The reference station software community, the reference station running on the server downloads the software can transfer the RINEX files Although the reference station server raw data files logged in the receivers, automatically to an FTP server. FTP software and the FTP server software converts the data to RINEX, and stands for File Transfer Protocol and is could run on the same computer, the produces RINEX files of the required used to facilitate the distribution of need for a firewall to protect the length (e.g. 1 hour, 2 hours, 3 hours files via the Internet. reference station server will usually etc.). The software can also create mean that it is preferable to use RINEX files from the raw data that are There is a range of FTP server separate computers. streamed continuously from the software available and your IT receivers to the server. support personnel will certainly be able to suggest a suitable product. Using RINEX files of reference station data, users of GPS field equipment can post process baselines from reference stations and determine RINEX files positions to very high accuracy. Post INTERNET processing is often used to determine Raw data files high order geodetic control points.
Server running reference Firewall FTP Server Internet GPS User station software community 40 22. Processing baselines between stations to check antenna positions and monitor movements of natural and man-made structures
22.1 Processing the baselines The reference station software passes 22.2 Checking the positions of the between the stations of a the files to a post processing software antennas network that can be configured to process the baselines automatically as the files are Although antennas that are set up in Baselines between the reference received. With sufficient data - i.e. a stable manner at safe locations stations of a network can be computed from sufficiently long observation should not move, some organizations in real time or by post processing. periods - post processing can that are responsible for reference successfully compute baselines of stations still prefer to be assured at For real time processing, raw data almost any length and achieve higher regular intervals that the antennas have to be streamed continuously accuracy than real-time processing. have not been disturbed. from the receivers to the server running the reference station software. Coordinate results can be output for A software component computes the further analysis. baselines between the reference stations continuously in real time as the data are received. The computation process resolves the ambiguities continuously and is similar to the real time processing in an RTK receiver. The accuracy that can be expected and the length of baselines that can be processed are also essentially the same as for RTK receivers.
For post processing, data files logged in the receivers and downloaded at regular intervals are normally used.
41 22. Processing baselines between stations to check antenna positions and monitor movements of natural and man-made structures (continued)
The positions (coordinates) of the the positions and movements of If the movements that have to be antennas can be determined by real man-made structures such as dams, monitored are known to be slow and time processing or by post bridges, buildings, oilrigs etc. to occur over a relatively long period processing as explained in 22.1. of time, it may be more economical If rapid, short-term movements, and and more suitable to use post As the requirement is usually one of even vibrations, have to be detected processing. periodic quality control, it is often and monitored, real time processing sufficient to post process the as described in 22.1 will usually be Using data files of appropriate lengths, baselines between stations once a needed. Real time processing baselines between reference stations day, or once a week, or perhaps even produces a continuous stream of can be processed at required time once a month. Depending on the independent position fixes intervals, perhaps every 12 hours, 24 software, processing can often be (coordinates). Movements can be hours or even 7 days. If required, the organized to run automatically at the identified and analyzed. results could be passed to an adjust- required time intervals. ment package for automatic least • Real time processing uses raw data squares network adjustments. 22.3 Monitoring movements of the that are streamed continuously Earth's crust, and of natural through permanently open Post processing will certainly be and man-made structures communication links. Running costs preferred if the baselines between for permanently open links can be stations are very long. Networks with Networks of GPS reference stations high. stations that are far apart and that provide the most effective method for cover very large areas are often used measuring movements between • Post processing can use data files for monitoring the movements of tectonic plates, in earthquake zones, that are logged internally in the tectonic plates. along major fault lines, and in areas of receivers and communication links high volcanic activity. that need to be opened only when files have to be downloaded. Thus Small networks with short baselines running costs may be lower than are often very suitable for monitoring with permanently open links. 42 23. Determining the WGS84 coordinates of the reference stations
GPS baseline processing - whether in For successful baseline processing, the There is a global network of over 300 post processing or in real time mode - WGS84 coordinates of the reference IGS (International GPS Service) solves a baseline between a known should be known accurately, to at stations. The coordinates of these reference and an unknown rover least about + or - 10m. As the stations are given in an International position. The baseline computation is purpose of reference stations is to Terrestrial Reference Frame (ITRF). always carried out in the WGS84 serve as the basis for determining the For simplicity one can say that the coordinate system. Thus WGS84 positions of rover receivers, it follows ITRF coordinates of the IGS stations coordinates of the rover are that accurate WGS84 coordinates are are just about the most accurate determined relative to the WGS84 needed for the reference stations. WGS84 coordinates available. coordinates of the reference. Any transformation to local coordinates is carried out afterwards.
43 23. Determining the WGS84 coordinates of the reference stations (continued)
Many countries already have a • Note that post processing software • This will provide very accurate national network of GPS reference may provide links to the closest IGS WGS84 coordinates for the master stations. Such a network will usually stations and facilitate the download and link the master to the IGS, or have been connected to the IGS of RINEX data files. national, network. network and the stations will have To determine the WGS84 coordinates of accurate WGS84 coordinates. a new stand-alone station or the The WGS84 coordinates of the stations “master” station of a new network, of the new network have to be When establishing a new stand-alone proceed as follows: fiducially accurate, i.e. the network has reference station or a new network of to have a very high degree of relative stations, it will usually be preferable to • Select one station of the new accuracy between the stations. The connect the new station or the new network as the “master”. easiest is to proceed as follows: network to the national network. • At the master log data (1 minute Contact the national survey rate is sufficient) for at least 24 • At all stations log data (1 minute organization for information on the hours, and possibly for up to 7 days. rate is sufficient) for at least 24 national network and how to obtain • Download RINEX data from the hours, and possibly for several days. data from the national stations. closest IGS station or from the • Using the WGS84 coordinates of the closest national station. master as the starting point, process If there is no suitable national • Process the baseline from the IGS all the baselines of the network using network, the best is to connect the station, or from the national station, 24 hours of data. Possibly repeat for new station or the new network of to the master using 24 hours of data. another one or two 24-hour periods. stations to the IGS network. If data have been logged for several Adjust the network if adjustment • For general information on the IGS days, process the baseline for software is available. network go to the IGS web site (see different 24-hour periods and take • This will provide WGS84 coordinates 30. Useful links). the mean. for all stations. The coordinates will • For information on IGS stations and • Perhaps process the baselines to the have a high degree of relative how to obtain RINEX data files master from an additional one or two accuracy. consult the SOPAC web site IGS, or national, stations. Take the (see 30. Useful links). mean. 44 24. Where should the stations be positioned? What should be the distances between the stations?
If the prime purpose of a network is to • The number of GPS rover receivers Depending on the requirement, the monitor a natural or man-made operating in the area and that will range of RTK that is typically achieved structure - e.g. a fault line, volcano, use the services. in the area, whether standard RTK dam, bridge etc. - the positions of the • The available budget. data is transmitted, or whether the stations will be determined largely by reference station software provides the structure that has to be monitored GPS reference stations should be set network correction parameters for and the movements that are expected. up where they are needed most and enhanced RTK (see 20), one could where they can provide services to the imagine that the separation distance If a network is to be a geodetic largest number of users. This will between the reference stations could control network that has to supply usually mean establishing more be between about 30km and 70km. services to the GPS surveying stations in highly developed areas and community, careful thought should be less stations in under developed In very large countries, with immense given to the positioning of the areas. open spaces of undeveloped land and stations and the separation distances where few RTK rovers are operating, it between the stations. Consider the For relatively small highly developed will usually be pointless and following: areas, such as in the regions of major uneconomical to try to achieve RTK conurbations where many RTK rovers coverage. A suitable approach could • The extent of the area to be covered. are operating, GPS reference stations be to set up reference stations only at • The areas of high development and will usually have to transmit RTK data. the major population centers and high population. The range limit at which an RTK rover those areas where there are major • The areas of low development and can operate successfully (i.e. resolve development projects. If sufficient RTK low population. ambiguities in real time) will have to rovers are being used, these • The services that have to be be taken into account when stations could transmit RTK data to provided: determining the separation distance provide RTK coverage within about a Perhaps only RINEX data for post between stations. 20km to 30km radius. processing. Perhaps also RTK data. Perhaps also DGPS data. 45 24. (continued) 25. Charging for products such as RINEX and RTK/DGPS data
Reference stations and networks Many organizations that operate A surveyor carrying out a project at a require significant investments. reference stations and networks are long distance from the reference Running costs, particularly for interested in recovering at least part stations would have to set up a networks, have also to be considered. of their investment and covering their temporary field reference, log data for running costs. They would like to a sufficient period of time, download Some organizations establish stand- charge for the data and the services the RINEX files from the reference alone reference stations or networks that they provide. stations, and post process to obtain purely for their own use. an accurate position. Other organizations provide products The temporary reference station would and services for the GPS user then transmit RTK data and provide community: they allow access to RINEX coverage for the project area. files and possibly raw data files on an FTP server, they distribute RTK and Using several hours of data, post DGPS data, and they may even processing will provide good results provide transformation parameters in (5mm + 0.5ppm or better) with order that RTK and GIS rovers can baselines of 100km, 200km, 300km easily transform WGS84 values into and even longer. Thus, if necessary, the local coordinate system. for very large undeveloped areas, the distances between GPS reference If users of GPS rover receivers are able stations could even be up to several to obtain the required data easily and hundred kilometres. reliably from permanent reference stations, they derive benefits from the services and do not need to invest in additional receivers for use as temporary field references.
46 25. Charging for products such as RINEX and RTK/DGPS data (continued)
If users have to be registered and If RTK and GIS rovers use mobile tele- Another possibility would be that enter passwords to access an FTP phone modems to obtain RTK/DGPS users of RTK and GIS rovers, with server and obtain RINEX files and data, it may be possible to arrange mobile telephone or Internet possibly raw data files, it should be with the telephone company that part connections, would have to enter relatively easy to generate a log of of the charges are credited to the passwords. Only those rovers able who has logged in and obtained data. authority operating the reference to authenticate themselves Charges could then be made for stations. (i.e. make connections with downloaded data. It may also be registered passwords) would be possible to ask for a registration fee. Another possibility would be that able to obtain data. Software at the RTK and GIS rovers, with mobile control server could create a log If RTK/DGPS data are distributed by telephone modems or Internet IP showing the duration during which radio, it is almost impossible to addresses, would have to register each password (i.e. user) had arrange a charging system. A solution their telephone numbers or IP obtained data. Charging could then could be that the authority running addresses with the authority be arranged. the reference stations sells or rents operating the reference stations. the required radio modems to users of Only telephone and Internet RTK and GIS equipment. connections from registered devices would be able to obtain data. It is easier to create a charging system Software at the control server could if users of RTK and GIS rovers obtain create a log showing the duration the required data by telephone or during which each registered device Internet. (i.e. user) had obtained data. Charging could then be arranged.
47 26. Some examples of stand-alone reference stations and networks
From the preceding chapters it will be required. In most cases it will be download and archive data. With a obvious that reference stations and preferable to have reference station PC connected, it is also possible to networks can be set up and configu- software running on a PC to control distribute RTK/DGPS data to a number red in many different ways, for many the receiver, and to download and of users at the same time via the different applications, and with many archive logged data. Internet. different levels of complexity. The solutions will vary according to the A single stand-alone reference station For civil engineering and construction requirements, the areas to be may also be perfectly sufficient for a sites where very high accuracy is covered, the services to be provided, small community. Radio and/or phone required, for example for a bridge, it and the available budgets. could be used to transmit RTK/DGPS may be desirable to set up a second data. A router or bank of phone independent reference station in order The following examples are intended modems would be needed in order that RTK rovers can obtain two only as general guides and provide that several users could phone in at independent position fixes at critical suggestions for possible solutions. the same time. points. A single PC (server) running They should not be taken as complete the reference station software could recommendations. Reference station software running control the two reference station on a PC will control the receiver, receivers; this would then become a 26.1 Stand-alone reference station two-station network. for a small area such as an opencast mine, construction ge R era TK site, or local community cov cov K er RT ag e GPS survey work on construction sites and in opencast mines is usually about 30 km carried out with RTK. Thus a single stand-alone reference station transmitting RTK data by radio directly from the receiver may be all that is 48 26. Some examples of stand-alone reference stations and networks (continued)
26.2 A simple network of a few RTK/DGPS data could be transmitted Such a network would be relatively stations providing complete directly from the receivers by radio simple and easy to set up and, as RTK coverage for a relatively and/or phone. The radios at the there would be no permanently open small well developed area. stations would have to be set to lines, the running costs should be low. different channels (frequencies). A In developed areas with high router or bank of phone modems populations, such as around would be needed at each station in metropolitan areas, most GPS survey order that several users could dial work will be carried out with RTK. in at the same time. Thus the main requirement will be to provide complete RTK coverage.
Purely as an example, imagine that RTK/DGPS services have to be provided throughout an area of about about 60 km 60km x 60km. A relatively simple solution would be to have 4 or 5 reference stations spaced about 30km apart. This would provide good RTK coverage throughout the area. about 60 km Dial-up lines could connect the stations to a central server running the reference station software. The software would control the receivers, download logged data files, and push RINEX files to an FTP server.
49 26. Some examples of stand-alone reference stations and networks (continued)
26.3 A more complex network reference stations and a separation Permanently open communication providing complete RTK distance between stations of about links between the receivers and the coverage for a relatively large 70km, RTK rovers will usually be server are required. The best is to use highly developed area. within about 35km of a station and the Internet, as running costs the maximum distance from a station (charges) will be lower than with If the area for which RTK coverage is should not be more than about 50 km. permanently open telephone lines. required is relatively large, there are two possibilities: For continuous network analysis and The software running on the server i) As outlined in 26.2 but with more calculation of network correction would control the receivers, stations. However, this will mean parameters, raw data have to be download logged data files, and increased costs for equipment and streamed continuously from the push RINEX files to an FTP server. infrastructure. receivers to a central server running ii) Place the stations further apart and the reference station software. use software for continuous network analysis and calculation of network e ag er ov about 200 km correction parameters. This will allow c TK R
RTK at longer ranges (see 20.). –
a re a The following example is based on rk o w possibility ii), i.e. using software for t e continuous network analysis and N Continuous calculation of network correction about 200 km network parameters. analysis Fewer Imagine that RTK/DGPS services have stations to be provided throughout an area of needed about 200km x 200km. With about 9
50 26. Some examples of stand-alone reference stations and networks (continued)
In order to increase the range at 26.4 A network of a few stations As an example, imagine that a which RTK rovers can operate providing RINEX data and geodetic control network is needed successfully, the software would also: limited RTK/DGPS for a very for a region of about 1000km x • Transmit the network data in RTCM large undeveloped area 1000km where there are only a few V3.0 format and the network small centers of population. correction parameters to RTK rover Some parts of the world have low receivers. Transmission could be by populations and/or are undeveloped. The most economical solution would radio, phone or Internet. As in such regions there will be few be to set up a reference station at • And/or RTK and DGPS rovers operating, it will each of the important population • Apply the network correction usually be uneconomical to attempt to centers. If there is sufficient demand, parameters at the server and achieve RTK coverage. A few reference these stations could transmit transmit “corrected” RTK data to the stations that are widely separated may RTK/DGPS data by radio and/or phone rover receivers. Transmission could be quite sufficient. to provide local RTK coverage (e.g. be by phone or Internet. within a radius of about 30km). See section 20 for full details.
As the distribution of RTK/DGPS data About 1000 km and logged data is all controlled from a central server, it should also be possible to set up a system for charging for data if this is a requirement. About 1000 km
Such a network would be more complex than that outlined in 26.2 but it would require fewer stations for the area to be covered.
51 26. Some examples of stand-alone reference stations and networks (continued)
With large distances (perhaps up to 26.5 A combined approach: net- Networks of more closely spaced several hundred kilometres) separating works of stations providing stations providing full RTK coverage the reference stations, most of the RTK/DGPS coverage for highly will be needed for the highly region would have no RTK coverage developed areas, widely developed zones surrounding the at all. spaced stations providing major cities. RINEX data throughout Although, if necessary, the reference undeveloped areas. Depending on the areas to be stations could be single stand-alone covered, such networks could be as stations, the best would be to connect Some countries or regions have one described in 26.2 (without the receivers by dial-up lines to a or more large, highly developed continuous network analysis and central server running the reference conurbations, in which millions of calculation of network correction station software. The software would people live and work, separated by parameters) or as described in 26.3 control the receivers, download vast areas of thinly populated land. (with continuous network analysis logged data files, and push RINEX files and calculation of network correction to an FTP server. Whereas RTK/DGPS services will be parameters). essential for the developed zones it A survey crew operating in a remote will often not be economical to A single server could handle the area would have to set up a tempo- provide RTK coverage for the thinly combined network. rary field reference station, log data populated areas. for a sufficient period (perhaps In some cases, however, it may be several hours), download the RINEX A network of widely spaced reference preferable to have separate servers for files from one or more reference stations supplying RINEX data as each of the smaller “RTK” networks stations, and post process the outlined in 26.4 will usually be and for the larger “RINEX” network. baselines from these stations. This sufficient for the less developed The smaller “RTK” networks will of would provide position coordinates areas with low population. course be linked to and form part of with an accuracy of a few centimetres the larger “RINEX” network. that could then be used as the starting point for the new survey. 52 27. Meteorological and tilt sensors
27.1 Meteorological sensors 27.2 Tilt sensors software running on the server down- loads the data and provides the tilt The delay in the GPS signals as they If the GPS antenna at a reference data in RINEX format. The tilt data can pass through the atmosphere station is set up on a high column or then be analyzed in order to monitor provides valuable information for pillar, some organizations prefer to any inclination of the column or pillar. meteorologists. Some organizations attach a tilt sensor to the column or that operate GPS reference stations pillar. The tilt sensor is connected to When studying whether to install a tilt supply data to meteorological the GPS receiver. sensor, it is worth considering the authorities. The data is used to assist following: weather forecasting and studying A twin-axis tilt sensor will measure tilt climatic change. in two directions. The data are •The higher the pillar or column, the recorded in the receiver together with more likely it is to tilt. For such applications it is usually the GPS data. The reference station • High metal pillars or columns may required to connect a meteorological undergo daily deformation due to sensor to the GPS receiver at the heating by the Sun. reference station. The meteorological • A low well-constructed pillar with a sensor delivers temperature, good foundation is unlikely to tilt. humidity and pressure data, which are • If any tilt occurs it will often be slow recorded together with the GPS data. and over a long period of time. • Movement of the antenna can be The reference station software determined to a high degree of running on the server downloads the accuracy by post processing the data and provides the meteorology baselines between the stations as and GPS data in RINEX format. Tilt sensor outlined in 22.2. • Checking the positions of the antennas by post processing will Met sensor often be all that is required by many organisations. 53 28. Protection against lightning
If a reference station is in an exposed If the antenna is on a building, the If the antenna is on a pillar, the light- area and in a region where lightning protector should be ning protector will need a copper strip thunderstorms are likely to occur, it connected to the lightning protection that is correctly earthed. Always is advisable to protect the equipment system of the building and thus to the employ or consult a specialist. against lightning. Protection can be earth (this should be done by a provided by a lightning conductor specialist). and/or by a lightning surge arrestor.
28.1 Lightning conductor
A tall building will usually have a lightning conductor. This usually comprises one or more metal rods connected by a thick copper strip, the lower end of which is firmly embedded in the ground below. Specialized firms install lightning conductors for buildings.
A lightning conductor can be placed close to or around the GPS antenna. In order not to obstruct the satellite signals, the rod or rods should not be too high. One solution is to have four small rods placed around the antenna in such a way that they do not rise above 10° above the horizon of the antenna. 54 28. Protection against lightning (continued)
28.2 Lightning surge arrestor The manufacturers will advise which products are the most suitable. A surge arrestor is a small device that has to be placed within the antenna Employ or consult a specialist to cable between the GPS antenna and ensure that the connection to earth is the receiver. It has also to be correctly carried out correctly. earthed. Many organizations consider that it is If the antenna is on a building, the sufficient to install only a lightning surge arrestor should be connected to surge arrestor and not a lightning the lightning protection system of the conductor. building and thus to the earth (this should be done by a specialist). If the antenna is on a pillar, the lightning surge arrestor has to be connected to a copper strip that is correctly earthed. Always employ or consult a specialist.
The surge arrestor protects the receiver and any equipment connected to the receiver (e.g. a computer) from electrical surges caused by lightning strikes. But the GPS antenna is NOT protected.
There are several products on the market and they are not expensive. 55 29. Points to take into account when deciding what type of stations and/or networks are needed
• What is required? • The most suitable methods of only the stations and services that are • What applications have to be communication between the really needed. Afterwards, as the supported? receivers and the server requirements increase, the number of • What will the stations and/or net- • The most suitable methods for users grows and additional funds are work be used for? distributing RTK/DGPS data available, new stations and features • What is needed today? • The most suitable communication can be added and the services that • What will be needed in future? for distributing RTK/DGPS data are provided can be improved and • The area to be covered • The cost of establishing the stations enlarged. • Where to establish the stations? and/or network • What can be the separation distance • The cost of running the stations The initial investment is never lost. between stations? and/or network • The need for suitable sites with an • The running costs for RTK and GIS open view of the sky rovers • Power, communication, security • Computing the baselines between • The infrastructure that is readily stations to check the positions of available and can be used the antennas • The new infrastructure that has to • The budget that is available today be added • The budget that will be available in • The type of users that have to be future supported • Charging for services and data • The number of users that have to be supported GPS reference stations and networks • Is it sufficient simply to log data and are readily scalable. They can be easily provide RINEX files? enhanced and upgraded as require- • Is it also required to transmit ments change and the number of RTK/DGPS data? users increases. Thus, initially, it will often be quite sufficient to establish
56 30. Useful links
Leica Geosystems AG NGS http://www.leica-geosystems.com http://www.ngs.noaa.gov/CORS/instructions2/ For further information on GPS reference For information on RINEX. stations and networks
UNAVCO RINEX http://www.unavco.org/facility/ Receiver Independent software/preprocessing/ Exchange Format preprocessing.html http://igscb.jpl.nasa.gov/igscb/data/ For TEQC tool. format/rinex210.txt
IGS RTCM (International GPS Service) http://www.rtcm.org http://igscb.jpl.nasa.gov/ For information on RTCM formats. For general information on the IGS network ITRF International Terrestrial Reference EUREF (RINEX data) Frame http://www.epncb.oma.be/_ http://lareg.ensg.ign.fr/ITRF/ dataproducts/datacentres/index.html For information on the UREF network. NTRIP http://igs.ifag.de/NTRIP.htm SOPAC http://sopac.ucsd.edu/ NMEA cgi-bin/dbShowArraySitesMap.cgi National Marine Electronics For information on IGS stations and how Association to obtain RINEX data files http://www.nmea.org
57 58 Leica SmartStation Total Station with integrated GPS
Illustrations, descriptions and technical specifications are not binding and may change. Leica GPS Spider Leica GRX1200 Leica GPS1200 Leica Printed in Switzerland – Copyright Leica Geosystems AG, Heerbrugg, Product brochure Product brochure Product brochure SmartStation Switzerland, 2005. ut labore et. ut labore et. Product brochure 747352en – IV.05 – RVA
Whether providing corrections from just a single reference station, or an extensive range of services from a nationwide RTK network – innovative reference station solutions from Leica Geosystems offer tailor-made yet scalable systems, designed for minimum operator interaction whilst providing maximum user benefit. In full compliance with international standards, Leica's proven and reliable solutions are based on the latest technology.
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