Chapter 5 NMEA SENTENCES from GPS RECEIVER NMEA0183
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CHAPTER 1 INTRODUCTION The Global Positioning System (GPS) is a space-based satellite navigation system that provides location and time information in all weather conditions, anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. The system provides critical capabilities to military, civil and commercial users around the world. It is maintained by the United States government and is freely accessible to anyone with a GPS receiver. The GPS project was developed in 1973 to overcome the limitations of previous navigation systems,integrating ideas from several predecessors, including a number of classified engineering design studies from the 1960s. GPS was created and realized by the U.S. Department of Defense (DoD) and was originally run with 24 satellites. It became fully operational in 1994. Bradford Parkinson, Roger L. Easton, and Ivan A. Getting are credited for inventing it. It is also called NAVSTAR(Navigation Satellite Timing And Ranging) Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS system and implement the next generation of GPS III satellites and Next Generation Operational Control System (OCX).Announcements from the Vice President and the White House in 1998 initiated these changes. In 2000, U.S. Congress authorized the modernization effort, GPS III. In addition to GPS, other systems are in use or under development. The Russian Global Navigation Satellite System (GLONASS) was developed contemporaneously with GPS, but suffered from incomplete coverage of the globe until the mid-2000s. There are also the planned European Union GALILEO positioning system, Chinese COMPASS navigation system, and Indian Regional Navigational Satellite System. Structure of GPS System The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US).The U.S. Air Force develops, maintains, and operates the space and control segments. GPS satellites broadcast signals from space, and each GPS receiver uses these signals to calculate its three-dimensional location (latitude, longitude, and altitude) and the current time. The space segment is composed of 24 to 32 satellites in medium Earth orbit and also includes the payload adapters to the boosters required to launch them into orbit. The control segment is composed of a master control station, an alternate master control station, and a host of dedicated and shared ground antennas and monitor stations. The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial, and scientific users of the Standard Positioning Services. 1 Space segment A visual example of a 24 satellite GPS constellation in motion with the Earth rotating. The space segment (SS) is composed of the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs, eight each in three approximately circular orbits, but this was modified to six orbital planes with four satellites each. The orbits are centred on the Earth, not rotating with the Earth, but instead fixed with respect to the distant stars. The six orbit planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection). The orbital period is one-half a sidereal day, i.e., 11 hours and 58 minutes.[53] The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface. The result of this objective is that the four satellites are not evenly spaced (90 degrees) apart within each orbit. In general terms, the angular difference between satellites in each orbit is 30, 105, 120, and 105 degrees apart which sum to 360 degrees. As of December 2012, there are 32 satellites in the GPS constellation. The additional satellites improve the precision of GPS receiver calculations by providing redundant 2 measurements. With the increased number of satellites, the constellation was changed to a non-uniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail. About nine satellites are visible from any point on the ground at any one time, ensuring considerable redundancy over the minimum four satellites needed for a position. SOME SPECIFICATION OF SATELLITE : 930 kg.(in orbit) : 5.1 m. : 4 km/sec : 1575.42 MHz and 1227.60 MHz : 1783.74 MHz : 2 Cesium and 2 Rubidium : 7.5 year (later model BlockIIR 10 years) Control segment Ground monitor station used from 1984 to 2007, on display at the Air Force Space & Missile Museum The control segment is composed of 1. a master control station (MCS), 2. an alternate master control station, 3. four dedicated ground antennas and 4. six dedicated monitor stations Master Control Station ( one station ): The master control station is responsible for overall managment of the remote monitoring and transmission sites. As the center for support operations , It calculates any position or clock errors for each individual satellite from monitor stations and then order the appropriate corrective information back to that satellite. Monitor Stations ( four stations ): Each of monitor stations checks the exact altitude , position , speed , and overall of the orbiting of satellites. A station can track up to 11 satellites at a time. This check-up is performed twice a day by each station as the satellites go around the earth. Ground Antennas: Ground antennas monitor and track the satellites from horizon to horizon. They also transmit correction information to individual satellites. 3 Satellite maneuvers are not precise by GPS standards. So to change the orbit of a satellite, the satellite must be marked unhealthy, so receivers will not use it in their calculation. Then the maneuver can be carried out, and the resulting orbit tracked from the ground. Then the new ephemeris is uploaded and the satellite marked healthy again. The Operation Control Segment (OCS) currently serves as the control segment of record. It provides the operational capability that supports global GPS users and keeps the GPS system operational and performing within specification. User segment The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly stable clock (often a crystal oscillator). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2007, receivers typically have between 12 and 20 channels. GPS receivers may include an input for differential corrections, using the RTCM SC-104 format. This is typically in the form of an RS-232 port at 4,800 bit/s speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM.Receivers with internal DGPS receivers can outperform those using external RTCM data. As of 2006, even low-cost units commonly include Wide Area Augmentation System (WAAS) receivers. Other Navigation System GLONASS The Russian government has developed a system, similar to GPS, called GLONASS. The first GLONASS satellite launch was in October 1982. The full constellation consists of 24 satellites in 3 orbit planes, which have a 64.8 degree inclination to the earth's equator. The GLONASS system now consists of 12 healthy satellites. GLONASS uses the same code for 4 each satellite and many frequencies, whereas GPS which uses two frequencies and a different code for each satellite. Some GPS receiver manufacturers have incorporated the capability to receive both GPS and GLONASS signals. This increases the availability of satellites and the integrity of combined system. Failure of GLONASS Three Russian navigation satellites crashed into the Pacific Ocean Sunday during the launch of a Proton rocket from Kazakhstan. Officials have established a board of inquiry to investigate the cause of the mishap. The Proton rocket blasted off at 1025 GMT (5:25 a.m. EST) from the Baikonur Cosmodrome in Kazakhstan. Live video from the launch showed a normal ascent into a clear early afternoon sky. The Proton's three core stages were supposed to deliver the rocket's Block DM upper stage and three Glonass navigation satellites to space within about 10 minutes of liftoff. According to Russia's Novosti news agency, the Proton rocket deviated from its course 8 degrees. The outlet quoted an unnamed Russian aerospace industry source as saying the Block DM stage and three payloads crashed into the Pacific Ocean about 1,500 kilometers, or 932 miles, northwest of Honolulu. "According to telemetry, the spacecraft's cluster was lofted into non-targeted orbit," Khrunichev and The Proton rocket lifts off from the Roscosmos said in a joint statement. "Special Board Baikonur Cosmodrome Sunday. has been established to find out the cause of the Credit: Roscosmos contingency and to define next steps." The last failure of a Proton core vehicle was in September 2007, when the rocket's first and second stages did not separate properly, dooming a Japanese commercial communications satellite.The most recent mishap blamed on a Block DM upper stage was a Sea Launch mission in June 2004, in which a commercial payload was placed in a lower-than-planned orbit.