of incidence of the signals improve GNSS their penetration (i.e., less reflection); thus, reflection losses are highest with low elevation satellites and satellites Solutions: upslope of an avalanche slide. Next, the signal that penetrates the snow surface is attenuated by eLoran How well can GPS dielectric losses, scattering, and fading signals penetrate as it passes through the snow. High and signal water content in the snow will cause avalanche snow? significant portion of the signal to reception be lost to scattering as the signal is reflected by particles of water and ice under snow urprisingly, GPS signals within the avalanche debris. penetrate avalanche snow very Overall, then, the amount of signal well. attenuation in avalanche snow relates “GNSS Solutions” is a Because snow is in part directly to its water content. Dry snow S 3 regular column featuring composed of frozen water, many with a density of 0.3 g/cm (33 percent questions and answers people assume that snow has signal ice, 67 percent air) has a penetration about technical aspects of attenuation properties similar to depth (i.e., the depth at which half the liquid water. Tests have shown that power/energy is lost) of approximately GNSS. Readers are invited even a high sensitivity GPS receiver 400 meters at 1.5 GHz. In comparison, to send their questions to cannot track through more than 1 wet snow with a similar density but the columnists, Professor to 2 millimeters of water where the having a liquid water content of 1 Gérard Lachapelle and Dr. majority of the GPS signal is reflected percent by volume (that is, 32 percent at interfaces between materials. Two ice, 1 percent water, 67 percent air) Mark Petovello, Department such interfaces exist: an air-water only has a penetration depth of of Geomatics Engineering, interface at the surface of the water approximately 3 meters. University of Calgary, who and the water-air interface around the For GPS to be used to locate an will find experts to answer submerged receiver antenna, which avalanche victim, the signal must reflects what remains of the signal. penetrate avalanche debris made up of them. Some of the attenuation is due to snow packed to a much higher density dielectric losses, but the majority is due than undisturbed snow. Freshly fallen to the air-water reflection. snow will have a density ranging What happens, however, when the from 1 to 25 percent that of water. In GPS signal encounters snow? recent tests in the Rocky Mountains, In terms of reflection, the loss is measured avalanche approximately 11 dB lower for snow snow debris densities of 45 to 50 than with liquid water. Smaller angles percent that of water were found, indicating a significantly higher ice Mark Petovello is a Senior Professor Gérard Lachapelle content and, therefore, higher signal Research Engineer in the holds a CRC/iCORE Chair attenuation. Department of Geomatics in Wireless Location in the How much avalanche snow does Engineering at the University of Department of Geomatics GPS have to penetrate? To be an Calgary. He has been actively Engineering at the University of effective rescue tool, the GPS signals involved in many aspects of Calgary. He has been involved need to reach a victim buried under positioning and navigation with GNSS since 1980 and has at least 2 meters of snow. For signals since 1997 including GNSS received numerous awards for algorithm development, inertial his contributions in the area of normal to the avalanche slope, this navigation, sensor integration, differential kinematic GPS and means that the signals must penetrate 2 and software development. indoor location. meters. For signals with higher angles of incidence, this can translate into Email: mpetovello@geomatics. Email: lachapel@geomatics. needed snow penetrations of up to 15 ucalgary.ca ucalgary.ca meters to reach a victim.
24 InsideGNSS september/october 2007 www.insidegnss Figure 1 shows the histogram of buried at various L1 carrier-to-noise density ratio (C/ depths beneath
N0) values measured by one surface the avalanche receiver and three subsurface receivers snow along with buried in avalanche snow at depths of values collected by 1.3, 2 and 2.7 meters, as a function of a similar surface snow penetration. The total number of receiver. Values observations were counted in bins of for the subsurface 1 meter by 1 dB-Hz and plotted in the receivers vary three-dimensional graph. widely, from 12 to The high sensitivity GPS receivers 35 dB-Hz, while used during the test were able to receive the surface receiver signals through 15 meters of avalanche can track the same snow, with an average attenuation of 2 satellite with a C/N0 to 3 dB per meter of avalanche snow. of 40 dB-Hz. FIGURE 1 Histogram of C/N0 Measured versus Snow Penetration During avalanche tests in the spring With this level 2006, an average L1 signal attenuation of attenuation, a marked increase in with 9 meters RMS horizontal of 12 dB at a receiver buried under pseudorange measurement noise is positioning accuracies beneath 2 1.3 meters of avalanche snow and 15 observed from 3.2 meters RMS on meters of snow, while the surface dB attenuation under 2 meters was the surface to 6.3 meters RMS at the accuracies are 3 meters. Figure 2 observed. shows the C/N0 for receiver buried 2 meters down. In turn, Now, 9-meter single-point GPS satellite PRN19 measured by three the increased pseudorange noise is positioning accuracies would certainly receivers with high-sensitivity designs reflected in the positioning statistics, slow down the search for avalanche
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as averaging can Performance under Avalanche significantly Deposited Snow. GPS Solutions, improve Springer, Published online, DOI positioning 10.1007/s10291-007-0060-1 accuracies. If GPS were to be used for John Schleppe this application, John Schleppe is team a data link that leader of software also operates receiver development at under avalanche Calgary, Alberta. snow would have Canada–based NovAtel to be used to Inc. Prior to joining report positions to NovAtel in November rescuers. 2006, John spent three years as a senior research engineer in the Department of Manufacturers Geomatics Engineering at the University of The avalanche field Calgary where one of his projects was to perform tests described here GPS tracking experiments in avalanche snow. used GPS receivers John has been active for more than 26 years in FIGURE 2 PRN 19 C/N0 Measured in avalanche debris with SiRFstarIII the navigation industry with 9 years field chipsets from SiRF operations experience along with 17 years victims. Because our victim isn’t Technology, San Jose, California, USA. experience in research and development. moving, however, and the errors are Further Reading Schleppe, J., and fairly random, something as simple G. Lachapelle (2007), “GPS Tracking
What is eLoran and is especially important as many based PNT by limiting the effect of components of critical economic GNSS’s vulnerability. In other words, how is it different and safety infrastructure have come use the modernized equipment and to depend profoundly on position other reasonable changes to Loran-C from Loran-C? navigation and timing (PNT) services to provide a Loran system that can currently provided by GPS and other be a GNSS-independent means of imply, eLoran (or enhanced satellite navigation systems. Future providing many PNT services that are Loran) is the latest generation developments in global navigation vital to global economic and critical of the venerable and time- satellite systems (GNSS) will only infrastructure. Stested Loran navigation sys- mitigate, not eliminate, some of the Although eLoran can provide tem. Enhanced Loran improves upon vulnerabilities. PNT redundancy to GNSS in general, previous Loran systems with updated eLoran is the result its particular focus is on services to equipment, signals, and operating pro- critical systems where no other backup cedures. of a confluence of two is commonly available. Therefore, These changes allow eLoran to development streams: although no detailed specifications provide better performance and the modernization of have been written for eLoran as of May additional services when compared the Loran-C system 2007, the following general definition to Loran-C. Most importantly, the and the realization that of the system, “Enhanced Loran improvements enable eLoran to serve satellite navigation has (eLoran) Definitions Document,” has as a backup to satellite navigation in been written by a panel of international many important applications. At the tangible vulnerabilities. Loran and navigation experts and same time, it still remains compatible presented to the International Loran with most Loran-C receivers. These new systems will likely Association (ILA) for comments: Enhanced Loran is the result of increase our usage and dependency “the accuracy, availability, a confluence of two development on satellite navigation. The vision of integrity, and continuity streams: 1) the modernization of the eLoran’s advocates seeks to use the performance requirements Loran-C system and 2) the realization modernized Loran infrastructure in for aviation non-precision that satellite navigation has tangible such a way as to provide a navigation instrument approaches, vulnerabilities. The second point system that complements satellite- maritime harbor entrance and
26 InsideGNSS september/october 2007 www.insidegnss GNSS SOLUTIONS
approach maneuvers, land- mobile vehicle navigation, and location-based services, and is a precise source of time and frequency for applications such as telecommunications.” Enhanced Loran Definitions Document, version 0.1 Modernization and other changes are necessary because many of the desired applications are not supported by the current performance level of Loran-C. Enhanced Loran is designed to use the existing Loran system, modernized equipment, and a minimal amount of changes to achieve the required performance to support the applications mentioned in the eLoran definitions document. Differences from Loran-C One way of describing the details of eLoran is in terms of its differences from Loran-C. These changes from Loran-C, as outlined in the Federal FIGURE 1 Plot of error in the horizontal plane at Volpe National Transportation System Center using Aviation Administration’s (FAA’s) 2004 University of Rhode Island measurements as corrections (The dotted blue line represents the 95 Loran evaluation report (see editors’ percent accuracy circle.) note at the end of this article), come in four major areas: radionavigation poli- Area Major Change cy, operational doctrine, system equip- Radionavigation policy Airport survey to generate ASF database for NPA/enroute ment, and user equipment. European Harbor entrance survey to generate ASF database for HEA and other international Loran stations conforming to eLoran standards will Operational Doctrine Time of transmission (TOT) control have equivalent equipment performing Off air to indicate out-of-tolerance conditions at station similar functions. Continuous phase changes to correct timing errors at stations Some of the major changes from Long-term synchronization to UTC using at least one non-GNSS dependent means Loran-C and their significance will System Equipment All stations use solid state transmitters (SSX) be briefly discussed. A summary of New uninterruptible power supplies and antenna coupler changes is given in Table 1. Government-Provided New timing and frequency equipment (TFE) to control timing Propagation Delay Information. New cesium clocks (three per station) Loran propagation delay is the time Improved monitor network using existing sites difference due to traversing the Loran data channel (ability to add digital data to the Loran signal) actual, non-uniform terrain rather Installation of transmitter control set (TCS) and remote automated integrated Loran-C than an ideal path. These delays can (RAIL) equipment allows for the monitoring and control of all station equipment be separated into two components: User Equipment Ability to incorporate propagation delay tables for specific applications 1) spatial (often termed additional secondary factor or ASF) and 2) All-in-view capability (use all available stations regardless of chain) temporal delay. Improved cross rate interference mitigation Databases accounting for the spatial Improved impulsive noise mitigation term at various locations throughout Ability to demodulate ninth pulse communications the desired coverage area will be Antennas (H field) to mitigate precipitation static (when necessary) published. Additionally, the Loran data channel (see next item) will provide TABLE 1. Summary of eLoran changes from Loran-C
28 InsideGNSS september/october 2007 www.insidegnss corrections for the temporal component through source verification of the time UTC. This level of performance at certain locations. These are necessary signal. In this United States, the data could not be achieved with acceptable for aviation nonprecision approach channel will use the ninth pulse availability using earlier equipment. As (NPA) and maritime harbor entrance communications (NPC) whereby a result the threshold for declaring the approach (HEA) requirements. They pulse(s) modulated using pulse Loran station out of tolerance was set aren’t needed — but are still beneficial position modulation (PPM) are added at 500 nanoseconds. Hence, although — for other users. to the end of the nominal Loran eight- Loran accuracy is generally better than Loran Data Channel. A data pulse sequence. this, 500 nanoseconds is a bound on channel on Loran will be used to Time of Transmission (TOT) the timing rather than the accuracy. provide differential Loran, station Control. Station broadcast times will The goal for eLoran is to reduce that identification, aviation warnings, and be steered to an international standard, number to 100 nanoseconds. other messages. Differential Loran Coordinated Universal Time (UTC), corrects for the temporal or time instead of using the current system Benefits to the User varying component of propagation area monitors (SAMs). TOT ties the The improvements offered by eLoran delay. Station identification provides transmission of each station back to result in many benefits to users. It information to identify station and UTC allowing for better performance yields many performance enhance- determine precise time. Aviation when using stations from different ments in all the traditional areas of warning messages provide timely Loran chains (“cross chain”). concern for PNT: accuracy, integrity, warnings of propagation hazards that New Cesium Clocks and Timing availability, and continuity. Addition- affect the safe use of Loran for landing Suites. The new equipment and ally, eLoran can enable numerous approach. control upgrade improves nominal applications previously unavailable on Other messages may be broadcast, timing performance of Loran Loran and enhance the robustness of including an authentication message broadcasts to always be within 100 the system with the addition of authen- that provides antispoofing capabilities nanoseconds of the expected broadcast ticated navigation information.
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FIGURE 2 Propagation Delay and Bound at Volpe Center from Seneca, New FIGURE 3 Propagation Delay and Bound at University of Rhode Island from York, transmitter (472 km) Seneca, New York, transmitter (458 km)York, transmitter (472 km)
Significantly improved accuracy, generated by the differential Loran all stations, already showed tangible as needed for the harbor entrance monitors will be broadcast on the benefits for availability and continuity approach (HEA) application, is one Loran data channel. when compared to performance of the most obvious improvements. A less obvious and visible benefit from the early 1990s. Availability An example of the potential accuracy is improved integrity. Some of the and continuity are also improved by benefits can be seen inFigure 1. changes made, such as going to TOT being able to perform cross-chain The figure shows a yearlong plot of control, allow Loran errors to be operations, which are aided by such position error at the Volpe National more easily modeled and predicted. eLoran features as station identification Transportation System Center in For example, TOT control aids error message and TOT control. Cambridge, Massachusetts, USA, if modeling because all transmitted Finally, the eLoran program is differential Loran corrections from signals are tied to a common source pursuing addition of an authentication University of Rhode Island (URI) (UTC). This allows for error bounds message for its data channel. The are used to correct for the temporal that are necessary to guarantee the authentication will allow the receiver component of propagation delay. integrity levels required by aviation. to verify that the source of the signal is These two sites are separated by 104 The integrity bounds for the an actual Loran transmitter. Providing kilometers. The 95 percent accuracy temporal variations are plotted on a means of authenticating the Loran level is less than 10.3 meters. Figure 2 and Figure 3. These bounds signal will significantly improve Several studies are under way are based on improved models derived robustness against spoofing. This testing the capability of differential from U.S. Coast Guard data. These enables applications that depend on a Loran for HEA. An accuracy level integrity bounds clearly over-bound secure and trusted source of navigation of 10 meters or less seems achievable the errors and help result in position information. with a reasonable density of monitors. bounds that limit the position errors at The accuracy is achieved by using the the level required by aviation landing. Conclusion monitor corrections to remove the These bounds would be more difficult eLoran is a significant evolutionary temporal changes in propagation delay to determine without changes such as improvement over Loran-C. It incor- and using the ASF database to remove TOT control. porates new technology and operations the absolute delay of various locations Availability and continuity are to provide a system capable of offering relative to the monitor. improved with the addition of more more services. Figure 2 and Figure 3 show the reliable transmitter equipment, The benefits of eLoran fall in two propagation delay at Volpe and URI uninterruptible power supplies (UPS), primary areas. First, they will allow from the Seneca, New York, signal. The lightning arrestors, and changes to aviators, mariners, and other users temporal correlation is clearly visible transmitter operations. Analysis of with suitable user equipment to retain while the spatial difference between the data from 2000–2002, before these much of the operational capability they two has been removed. The corrections improvements were implemented at had with satellite-based PNT even in
30 InsideGNSS september/october 2007 www.insidegnss GNSS SOLUTIONS We’re doing the the absence of GNSS. Enhanced Loran has the capability of ION GNSS serving applications supported by Loran-C as well as aviation landing, maritime harbor entrance and approach, and many others. Show Daily! Second, eLoran will enable applications previously not supported by GNSS, Loran-C, or other navigation systems. This is partly because eLoran has room for growth if and when additional functionality such as authentication is added. These factors make enhanced Loran of great interest to many sectors that have come to rely on precise navigation or timing for safety and economic efficiency. Editors’ Note For more information on Loran and eLoran, refer to: International Loran Association, “Enhance Loran (eLoran) Definition Document”, version 0.1, January 2007,
Sherman Lo Sherman Lo is a research associate at the Stanford University GPS Research Laboratory managing the assessment of Loran for civil aviation and also GPS | GALILEO | GLONASS | COMPASS works on a variety of GNSS-related issues. He received his Ph.D. in aeronautic and astronautics from Stanford University. He has received the Booth 417-419 Institute of Navigation (ION) Early Achievement Award and the International Loran Association (ILA) President’s Award. Ft.Worth, Texas Convention Center
32 InsideGNSS september/october 2007 www.insidegnss