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Charting the Evolution of Signal-in-Space Performance by Data Mining 400,000,000 Navigation Messages By Liang Heng, Grace Xingxin Gao, Todd Walter, and Per Enge There are four important requirements of any navigation system: accuracy, availability, continuity, and integrity. In this month’s column we take a look at one particular aspect of GPS integrity: that of the signal in space and find out how trustworthy is the satellite ephemeris and clock information in the broadcast navigation message. INNOVATION INSIGHTS by Richard Langley BUT THE GREATEST OF THESE IS INTEGRITY. There are four important requirements of any navigation system: accuracy, availability, continuity, and integrity. Perhaps the most obvious navigation system requirement, accuracy describes how well a measured value agrees with a reference value, typically the true value. In the case of GPS, we might talk about the accuracy of a range measurement. A receiver actually measures a pseudorange — a biased and noisy measure of the geometric range between the receiver and the satellite. After correcting for satellite ephemeris and satellite clock errors (the primary so-called signal-in-space errors), receiver clock errors, and atmospheric effects, we can get an estimate of the geometric range. How well we account for these errors or biases, will determine the accuracy of the corrected pseudorange measurement and ultimately, the accuracy of a derived position. A navigation system’s availability refers to its ability to provide the required function and performance within the specified coverage area at the start of an intended operation. In many cases, system availability implies signal availability, which is expressed as the percentage of time that the system’s transmitted signals are accessible for use. In addition to transmitter capability, environmental factors such as signal attenuation or blockage or the presence of interfering signals might affect availability. Ideally, any navigation system should be continuously available to users. But, because of scheduled maintenance or unpredictable outages, a particular system may be unavailable at a certain time. Continuity, accordingly, is the ability of a navigation system to function without interruption during an intended period of operation. More specifically, it indicates the probability that the system will maintain its specified performance level for the duration of an operation, presuming system availability at the beginning of that process. The integrity of a navigation system refers to its trustworthiness. A system might be available at the start of an operation, and we might predict its continuity at an advertised accuracy during the operation. But what if something unexpectedly goes wrong? If some system anomaly results in unacceptable navigation accuracy, the system should detect this and warn the user. Integrity characterizes a navigation system’s ability to provide this timely warning when it fails to meet its stated accuracy. If it does not, we have an integrity failure and the possibility of conveying hazardously misleading information. GPS has built into it various checks and balances to ensure a fairly high level of integrity. However, GPS integrity failures have occasionally occurred. In this month’s column we take a look at one particular aspect of GPS integrity: that of the signal in space and find out how trustworthy is the satellite ephemeris and clock information in the broadcast navigation message. The Navstar Global Positioning System is so far the most widely used space-based positioning, navigation, and timing system. GPS works on the principle of trilateration, in which the measured distances from a user receiver to at least four GPS satellites in view, as well as the position and clock data for these satellites, are the prerequisites for the user receiver to fix its exact position. For most GPS Standard Positioning Service (SPS) users, real-time satellite positions and clocks are derived from ephemeris parameters and clock correction terms in navigation messages broadcast by GPS satellites. The GPS Control Segment routinely generates navigation message data on the basis of a prediction model and the measurements at more than a dozen monitor stations. The differences between the broadcast ephemerides/clocks and the truth account for signal-in-space (SIS) errors. SIS errors are usually undetectable and uncorrectable for stand-alone SPS users, and hence directly affect the positioning accuracy and integrity. Nominally, SPS users can assume that each broadcast navigation message is reliable and the user range error (URE) derived from a healthy SIS is at the meter level or even sub-meter level. In practice, unfortunately, SIS anomalies have happened occasionally and UREs of tens of meters or even more have been observed, which can result in an SPS receiver outputting a hazardously misleading position solution. Receiver autonomous integrity monitoring (RAIM) or advanced RAIM is a promising tool to protect stand-alone users from such hazards; however, most RAIM algorithms assume at most one satellite fault at a time. Knowledge about the SIS anomalies in history is very important not only for assessing the GPS SIS integrity performance but also for validating the fundamental assumption of RAIM. A typical method for calculating SIS UREs is to compare the broadcast ephemerides/clocks with the precise, post-processed ones. Although this method is very effective in assessing the GPS SIS accuracy performance, few attempts have been made to use it to assess the GPS SIS integrity performance because broadcast ephemeris/clock data obtained from a global tracking network sometimes contain errors caused by receivers or data conversion processes and these errors usually result in false SIS anomalies. In this article, we introduce a systematic methodology to cope with this problem and screen out all the potential SIS anomalies in the past decade from when Selective Availability (SA) was turned off. GPS SIS Integrity The integrity of a navigation system refers — just as it does to a person — to its honesty, veracity, and trustworthiness. In the case of GPS, this includes the integrity of the ephemeris and clock data in the broadcast navigation messages. We refer to this as signal-in-space integrity. GPS SIS URE. As indicated by the name, GPS SIS URE is the pseudorange modeling inaccuracy due to operations of the GPS ground control and the space vehicles. Specifically, SIS URE includes satellite ephemeris and clock errors, satellite antenna performance variations, and signal imperfections, but not ionospheric or tropospheric delay, multipath, or any errors due to user receivers. SIS URE is dominated by ephemeris and clock errors because antenna variations and signal imperfections are at a level of millimeters or centimeters. In broadcast navigation messages, there is a parameter called user range accuracy (URA) that is intended to be a conservative representation of the standard deviation (1-sigma) of the URE at the worst-case location on the Earth. For example, a URA index value of 0 means that the 1-sigma URE is expected to be less than 2.4 meters, and a URA index value of 1 means that the 1-sigma URE is expected to be greater than 2.4 meters but less than 3.4 meters, and so on. In the past several years, most GPS satellites have a URA index value of 0. A nominal URA value, in meters, can be computed as X = 2(1+N/2), where N is the index value, for index values of 6 or less. For 6 N X = 2(N-2). GPS SPS SIS Integrity. In the SPS Performance Standard (PS), as well as the latest version of the Interface Specification (IS-GPS-200E), the GPS SPS SIS URE integrity standard assures that for any healthy SIS, there is an up-to-10−5 probability over any hour of the URE exceeding the not-to-exceed (NTE) tolerance without a timely alert during normal operation. The NTE tolerance is currently defined to be 4.42 times the upper bound (UB) on the URA value broadcast by the satellite. Before September 2008, the NTE tolerance was defined differently, as the maximum of 30 meters and 4.42 times URA UB. The reason for the “magic” number 4.42 here is the Gaussian assumption of the URE, although this assumption may be questionable. (4.42 sigma corresponds to a probability level of 99.999 percent (1 – 10–5)). In this article, a GPS SPS SIS anomaly is defined as a threat of an SIS integrity failure; that is, a condition during which an SPS SIS marked healthy results in a URE exceeding the NTE tolerance. Because the definition of the NTE tolerance is different before and after September 2008, we consider both of the two NTE tolerances for the sake of completeness and consistency. Methodology The SIS anomalies are screened out by comparing broadcast ephemerides/clocks with precise ones. As shown in Figure 1, the whole process consists of three steps: data collecting, data cleansing, and anomaly screening. Figure 1. Framework of the whole process. XYZB values refer to the coordinates of satellite position and satellite clock bias. In the first step, the navigation message data files are downloaded from the International GNSS Service (IGS). In addition, two different kinds of precise ephemeris/clock data are downloaded from IGS and the National Geospatial-Intelligence Agency (NGA), respectively. The details about these data sources will be discussed in the next section. Since each GPS satellite can be observed by many IGS stations at any instant, each navigation message is recorded redundantly. In the second step, a data-cleansing algorithm exploits the redundancy to remove the errors caused on the ground. This step distinguishes our work from that of most other researchers because the false anomalies due to corrupted data can be mostly precluded. The last step is computing worst-case SIS UREs as well as determining potential SIS anomalies. The validated navigation messages prepared in the second step are used to propagate broadcast orbits/clocks at 15-minute intervals that coincide with the precise ones. A potential SIS anomaly is claimed when the navigation message is healthy and in its fit interval with the worst-case SIS URE exceeding the SIS URE NTE tolerance. Data Sources We obtained broadcast navigation message data and precise ephemeris and clock data from publicly available sources. Broadcast Navigation Message Data. Broadcast GPS navigation message data files are available at IGS Internet sites. All the data are archived in Receiver Independent Exchange (RINEX) navigation file format, which includes not only the ephemeris/clock parameters broadcast by the satellites but also some information produced by the ground receivers, such as the pseudorandom noise (PRN) signal number and the transmission time of message (TTOM). The IGS tracking network is made up of more than 300 volunteer stations all over the world (a map is shown in Table 1) ensuring seamless, redundant data logging. Since broadcast navigation messages are usually updated every two hours, no single station can record all navigation messages. For the ease of users, two IGS archive sites, the Crustal Dynamics Data Information System (CDDIS) and the Scripps Orbit and Permanent Array Center (SOPAC), provide two kinds of ready-to-use daily global combined broadcast navigation message data files, brdcddd0.yyn and autoddd0.yyn, respectively, where ddd is the day of year yy. Unfortunately, these files sometimes contain errors that can cause false anomalies. Table 1. Comparison of IGS and NGA precise ephemeris/clock data. Therefore, we devised and implemented a data-cleansing algorithm to generate the daily global combined navigation messages, which are as close as possible to the navigation messages that the satellites actually broadcast, from all available navigation message data files of all IGS stations. The data-cleansing algorithm is based on majority vote, and hence all values in our data are cross validated. Accordingly, we name our daily global combined navigation messages “validated navigation messages,” as shown in Figure 1. Precise Ephemeris and Clock Data. Precise GPS ephemerides/clocks are generated by some organizations such as IGS and NGA that routinely post-process observation data. Precise ephemerides/clocks are regarded as “truth” because of their centimeter-level accuracy. Table 1 shows a side-by-side comparison between IGS and NGA precise ephemeris/clock data, in which the green- and red-colored text implies pros and cons, respectively. For NGA data, the only con is that the data have been publicly available only since January 4, 2004. As a result, for the broadcast ephemerides/clocks before this date, IGS precise ephemerides/clocks are the only references. Nevertheless, care must be taken when using IGS precise ephemerides/clocks due to the following three issues. The first issue with the IGS precise ephemerides/clocks is the relatively high rate of bad/absent data, as shown in the third row of Table 1. For a GPS constellation of 27 healthy satellites, 1.5 percent bad/absent data means no precise ephemerides or clocks for approximately 10 satellite-hours per day. This issue can result in undetected anomalies (false negatives). The second issue is that, as shown in the fourth row of Table 1, IGS switched to IGS Time for its precise ephemeris/clock data on 22 February, 2004. The IGS clock is not synchronized to GPS Time, and the differences between the two time references may be as large as 3 meters. Fortunately, the time offsets can be extracted from the IGS clock data files. Moreover, a similar problem is that IGS precise ephemerides use a frame aligned to the International Terrestrial Reference Frame (ITRF) whereas broadcast GPS ephemerides are based on the World Geodetic System 1984 (WGS 84). The differences between ITRF and the versions of WGS 84 used since 1994 are on the order of a few centimeters, and hence a transformation is not considered necessary for the purpose of our work. The last, but not the least important, issue with the IGS precise ephemerides is that the data are provided only for the center of mass (CoM) of the satellite. Since the broadcast ephemerides are based on the satellite antenna phase center (APC), the CoM data must be converted to the APC before being used. Both IGS and NGA provide antenna corrections for every GPS satellite. Although the IGS and the NGA CoM data highly agree with each other, the IGS satellite antenna corrections are quite different from the NGA’s, and the differences in z-offsets can be as large as 1.6 meters for some GPS satellites. The reason for these differences is mainly due to the different methods in producing the antenna corrections: the IGS antenna corrections are based on the statistics from more than 10 years of IGS data, whereas the NGA’s are probably from the calibration measurements on the ground. In order to know whose satellite antenna corrections are better, the broadcast orbits for all GPS satellites in 2009 were computed and compared with three different precise ephemerides: IGS CoM + IGS antenna corrections, IGS CoM + NGA antenna corrections, and NGA APC. Generally, the radial ephemeris error is expected to have a zero mean. However, the combination “IGS CoM + IGS antenna corrections” results in radial ephemeris errors with a non-zero mean for more than half of the GPS satellites. Therefore, the NGA antenna corrections were selected to convert the IGS CoM data to the APC. Data Cleansing Figure 2 shows a scenario of data cleansing. Owing to accidental bad receiver data and various hardware/software bugs, a small proportion of the navigation data files from the IGS stations have defects such as losses, duplications, inconsistencies, discrepancies, and errors. Therefore, more than just removing duplications, the generation of validated navigation messages is actually composed of two complicated steps. Figure 2. A scenario of data cleansing: In the figure, the GPS satellite PRN32 started to transmit a new navigation message at 14:00. Receiver 1 had not observed the satellite until 14:36, and hence the TTOM in its record was 14:36. Additionally, Receiver 1 made a one-bit error in ∆n (4.22267589140 × 10-9 11823 × 2−43 π). Receiver 2 perhaps had some problems in its software: the IODC was unreported and both the toc and ∆n were written weirdly. Receiver n used an incorrect ranging code, PRN01, to despread and decode the signal of PRN32; fortunately, all the parameters except TTOM were perfectly recorded. Moreover, the three receivers interpreted URA (SV accuracy) differently. A computer equipped with our data cleansing algorithms is used to process all the data from the receivers. The receiver-caused errors are removed and the original navigation message is recovered. First step. Suppose that we want to generate the validated navigation messages for day n. In the first step, we apply the following operations sequentially to each RINEX navigation data file from day n − 1 to day n + 1: 1) Parse the RINEX navigation file; 2) Recover least significant bit (LSB); 3) Classify URA values; 4) Remove the navigation messages not on day n; 5) Remove duplications; 6) Add all remaining navigation messages into the set O. The reason why the data files from day n − 1 to day n + 1 are considered is that a few navigation messages around 00:00 can be included in some data files on day n − 1, and a few navigation messages around 23:59 can be included in some data files on day n + 1. The LSB recovery is used here to cope with the discrepant representations of floating-point numbers in RINEX navigation files. The URA classifier is employed to recognize and unify various representations of URA in the files. The duplication removal is applied because some stations write the same navigation messages repeatedly in one data file, which is unfavorable to the vote in the second step. Second Step. At the end of the first step, we have a set O that includes all the navigation messages on day n. The set O still has duplications because a broadcast navigation message can be reported by many IGS stations. However, as shown in Figure 2, duplications of a broadcast navigation message may come with different errors and are not necessarily identical. Several other examples of such problems can be found in our journal paper listed in Further Reading. Fortunately, most orbital and clock parameters are seldom reported incorrectly, and even when errors happen, few stations agree on the same incorrect value. In our work, these parameters are referred to as robust parameters. On the contrary, some parameters, such as TTOM, PRN, URA and issue of data clock (IODC), are more likely to be erroneous and when errors happen, several stations may make the same mistake. These parameters are referred to as fragile parameters. The cause of the fragility is either the physical nature (for example, TTOM, PRN) or the carelessness in hardware/software implementations (for example, URA, IODC). Majority vote is applied to all fragile parameters (except TTOM, which is determined by another algorithm described in our journal paper) under the principle that the majority is usually correct. Meanwhile, the robust parameters are utilized to identify the equivalence of two navigation messages — two navigation messages are deemed identical if and only if they agree on all the robust parameters, although their fragile parameters could be different. Therefore, the goal of duplication removal and majority vote is a set P, in which any navigation message must have at least one robust parameter different from any other and has all fragile parameters confirmed by the largest number of stations that report this navigation message. After the operations above, we have a set P in which there are no duplicated navigation messages in terms of robust parameters and all fragile parameters are as correct as possible. A few navigation messages in P still have errors in their robust parameters. These unwanted navigation messages feature a small number of reporting stations. Finally, the navigation messages confirmed by only a few stations being discarded and the survivors are the validated broadcast navigation messages, stored in files sugldddm.yyn. For further details of our algorithms, see our journal paper. Anomaly Screening The validated broadcast navigation messages prepared using the algorithm described in the previous section were employed to propagate broadcast satellite orbits and clocks. For each 15-miniute epoch, t, that coincides with precise ephemerides/clocks, the latest transmitted broadcast ephemeris/clock is chosen to calculate the worst-case SIS URE – the maximum SIS URE that a user on Earth can experience. Finally, a potential GPS SIS anomaly is claimed when all of the following conditions are fulfilled. The worst-case SIS URE exceeds the NTE tolerance; The broadcast navigation message is healthy; that is, The RINEX field SV health is 0, and The URA UB ≤ 48 meters; The broadcast navigation message is in its fit interval; that is, ∆t = t − TTOM ≤ 4 hours; The precise ephemeris/clock is available and healthy. Results A total of 397,044,414 GPS navigation messages collected by an average of 410 IGS stations from June 1, 2000 (one month after turning off SA), to August 31, 2010, have been screened. The NGA APC precise ephemerides/clocks and the IGS CoM precise ephemerides/clocks with the NGA antenna corrections were employed as the truth references. Both old and new NTE tolerances were used for determining anomalies. Before interpreting the results, it should be noted that there are some limitations due to the data sources and the anomaly-determination criteria. First, false anomalies may be claimed because there may be some errors in the precise ephemerides/clocks or the validated navigation messages. Second, some short-lived anomalies may not show up if they happen to fall into the 15-minute gaps of the precise ephemerides/clocks. Third, some true anomalies may not be detected if the precise ephemerides/clocks are temporarily missing. The third limitation is especially significant for the results before January 3, 2004, because only the IGS precise ephemerides/clocks are available, which feature a high rate of bad/absent data. (For example, the clock anomaly of Space Vehicle Number (SVN) 23/PRN23 that occurred on January 1, 2004 is missed by our process because the IGS precise clocks for PRN23 on that day were absent.) Last but not least, users might not experience some anomalies because a satellite was not trackable at that time, or the users were notified via a Notice Advisory to Navstar Users (NANU). (A satellite may indicate that it is unhealthy through the use of non-standard code or data. The authors’ future work will include using observation data to verify the potential anomalies found in the results presented here.) Therefore, all the SIS anomalies claimed in this article are considered to be potential and under further investigation. Potential SIS Anomalies. A total of 1,256 potential SIS anomalies were screened out under SPS PS 2008 (or 374 potential SIS anomalies under SPS PS 2001). Figure 3 shows all these anomalies in a Year-SVN plot. It can be seen that during the first year after SA was turned off, SIS anomalies occurred frequently for the whole constellation. Figure 3. Potential SIS anomalies from June 1, 2000, to August 31, 2010. The horizontal lines depict the periods when the satellites were active (not necessarily healthy). The color of the lines indicates the satellites’ block type, as explained by the top left legend. Moreover, 2004 is apparently a watershed: before 2004, anomalies occurred for all GPS satellites (except two satellites launched in 2003, SVN45/PRN21 and SVN56/PRN16) whereas after 2004, anomalies occurred much less frequently and more than 10 satellites have never been anomalous. Figure 4 further confirms the improving GPS SIS integrity performance in the past decade, no matter which SPS PS is considered. Figure 4. Number of potential SIS anomalies per year. The SIS performance was improved during the past decade. There were 0 anomalies in 2009 according to SPS PS 2001 and this number is represented by 0.1 in the figure. Therefore, it is possible to list all potential SIS anomalies from January 4, 2004, to August 31, 2010, in a compact table: Table 2. Most anomalies in the table have been confirmed by NANUs and other literature. The table reveals an important and exciting piece of information: never have two or more SIS anomalies occurred simultaneously since 2004. Accordingly, in the sense of historical GPS SIS integrity performance, it is valid for RAIM to assume at most one satellite fault at a time. Table 2. List of potential anomalies from January 4, 2004, to August 31, 2010. Validated Navigation Messages. For the purpose of comparison and verification, the IGS daily global combined broadcast navigation message data files brdcddd0.yyn and autoddd0.yyn were used to propagate broadcast satellite orbits and clocks as well. The NGA APC precise ephemerides/clocks were employed for the truth references. The SPS PS 2008 NTE tolerance was used for determining anomalies. The other criteria for anomaly screening that are the same as in the previous section were still applied. All the potential SIS anomalies for 2006–2009 were found based on the three kinds of daily combined broadcast navigation messages. Table 3 shows a comparison of the total hours of the anomalies per year. It can be seen that brdcddd0.yyn and autoddd0.yyn result in approximately 11 times more false anomalies than true ones. Moreover, all potential anomalies derived from sugldddm.yyn are confirmed by brdcddd0.yyn and autoddd0.yyn, which indicates that our sugldddm.yyn does not introduce any more false anomalies than brdcddd0.yyn and autoddd0.yyn. Table 3. Total hours of anomalies per year computed from three different kinds of daily global combined broadcast navigation messages. Conclusion In this article, the GPS SIS integrity performance in the past decade was assessed by comparing the broadcast ephemerides/clocks with the precise ones. Thirty potential anomalies were found. The fundamental assumption of RAIM is valid based on a review of the GPS SIS integrity performance in the past seven years. Acknowledgments The authors gratefully acknowledge the support of the Federal Aviation Administration. This article contains the personal comments and beliefs of the authors, and does not necessarily represent the opinion of any other person or organization. The authors would like to thank Mr. Tom McHugh, William J. Hughes FAA Technical Center, for his valuable input to the data-cleansing algorithm. This article is based on the paper “GPS Signal-in-Space Integrity Performance Evolution in the Last Decade: Data Mining 400,000,000 Navigation Messages from a Global Network of 400 Receivers” to appear in the Institute of Electrical and Electronics Engineers (IEEE) Transactions on Aerospace and Electronic Systems.. Liang Heng is a Ph.D. candidate under the guidance of Professor Per Enge in the Department of Electrical Engineering at Stanford University. Grace Xingxin Gao is a research associate in the GPS Research Laboratory of Stanford University. Todd Walter is a senior research engineer in the Department of Aeronautics and Astronautics at Stanford University. Per Enge is a professor of Aeronautics and Astronautics at Stanford University, where he is the Kleiner-Perkins, Mayfield, Sequoia Capital Professor in the School of Engineering. He directs the GPS Research Laboratory, which develops satellite navigation systems based on GPS. FURTHER READING • Authors’ Research Papers “GPS Signal-in-Space Integrity Performance Evolution in the Last Decade: Data Mining 400,000,000 Navigation Messages from a Global Network of 400 Receivers” by L. Heng, G.X. Gao, T. Walter, and P. Enge in Transactions on Aerospace and Electronic Systems, the Institute of Electrical and Electronics Engineers, accepted for publication. “GPS Signal-in-Space Anomalies in the Last Decade: Data Mining of 400,000,000 GPS Navigation messages” by L. Heng, G.X. Gao, T. Walter, and P. Enge in Proceedings of ION GNSS 2010, the 23rd International Technical Meeting of The Satellite Division of the Institute of Navigation, Portland, Oregon, September 21–24, 2010, pp. 3115–3122. “GPS Ephemeris Error Screening and Results for 2006–2009” by L. Heng, G.X. Gao, T. Walter, and P. Enge in Proceedings of ION ITM 2010, the 2010 International Technical Meeting of the Institute of Navigation, San Diego, California, January 24–26, 2010, pp. 1014–1022. • Earlier Work on Assessing GPS Broadcast Ephemerides and Clocks “GPS Orbit and Clock Error Distributions” by C. Cohenour and F. van Graas in Navigation, Vol. 58, No. 1, Spring 2011, pp. 17–28. “Statistical Characterization of GPS Signal-in-Space Errors” by L. Heng, G.X. Gao, T. Walter, and P. Enge in Proceedings of ION ITM 2011, the 2011 International Technical Meeting of the Institute of Navigation, San Diego, California, January 24–26, 2011, pp. 312–319. “Broadcast vs. Precise GPS Ephemerides: A Historical Perspective” by D.L.M. Warren and J.F. Raquet in GPS Solutions, Vol. 7, No. 3, 2003, pp. 151–156, doi: 10.1007/s10291-003-0065-3. “Accuracy and Consistency of Broadcast GPS Ephemeris Data” by D.C. Jefferson and Y.E. Bar-Sever in Proceedings of ION GPS-2000, the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation, Salt Lake City, Utah, September 19–22, 2000, pp. 391–395. “The GPS Broadcast Orbits: An Accuracy Analysis” by R.B. Langley, H. Jannasch, B. Peeters, and S. Bisnath, presented in Session B2.1-PSD1, New Trends in Space Geodesy at the 33rd COSPAR Scientific Assembly, Warsaw, July 16–23, 2000. • Signal-in-Space Anomalies “GNSS: The Present Imperfect” by D. Last in Inside GNSS, Vol. 5, No. 3, May 2010, pp. 60–64. “Investigation of Upload Anomalies Affecting IIR Satellites in October 2007” by K. Kovach, J. Berg, and V. Lin in Proceedings of ION GNSS 2008, the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation, Savannah, Georgia, September 16–19, 2008, pp. 1679–1687. Global Positioning System (GPS) Standard Positioning Service (SPS) Performance Analysis Report No. 58, July 31, 2007, Reporting Period: 1 April – 30 June 2007. Discrepancy Report, DR No. 55, “GPS Satellite PRN18 Anomaly Affecting SPS Performance” by N. Vary, FAA William J. Hughes Technical Center, Pomona, New Jersey, April 11, 2007. “GPS Receiver Responses to Satellite Anomalies” by J.W. Lavrakas and D. Knezha in Proceedings of the 1999 National Technical Meeting of The Institute of Navigation, San Diego, California, January 25–27, 1999, pp. 621–626. • GPS Integrity and Receiver Autonomous Integrity Monitoring “Prototyping Advanced RAIM for Vertical Guidance” by J. Blanch, M.J. Choi, T. Walter, P. Enge, and K. Suzuki in Proceedings of ION GNSS 2010, the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation, Portland, Oregon, September 21–24, 2010, pp. 285–291. “The Integrity of GPS” by R.B. Langley in GPS World, Vol. 10, No. 3, March 1999, pp. 60–63. • International GNSS Service Ephemerides and Clocks “On the Precision and Accuracy of IGS Orbits” by J. Griffiths and J.R. Ray in Journal of Geodesy, Vol. 83, 2009, pp. 277–287, doi: 10.1007/s00190-008-0237-6. “The International GNSS Service: Any Questions?” by A.W. Moore in GPS World, Vol. 18, No. 1, January 2007, pp. 58–64. International GNSS Service Central Bureau website. • National Geospatial-Intelligence Agency Ephemerides and Clocks “NGA’s Role in GPS” by B. Wiley, D. Craig, D. Manning, J. Novak, R. Taylor, and L. Weingarth in Proceedings of ION GNSS 2006, the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation, Fort Worth, Texas, September 26–29, 2006, pp. 2111–2119. National Geospatial-Intelligence Agency, Geoint Sciences Office, Global Positioning System Division website. • Antenna Phase Center Corrections “Generation of a Consistent Absolute Phase-center Correction Model for GPS Receiver and Satellite Antennas” by R. Schmid, P. Steigenberger, G. Gendt, M. Ge, and M. Rothacher in Journal of Geodesy, Vol. 81, No. 12, 2007, pp. 781–798, doi: 10.1007/s00190-007-0148-y. “The Block IIA Satellite: Calibrating Antenna Phase Centers” by G.L. Mader and F.M. Czopek in GPS World, Vol. 13, No. 5, May 2002, pp. 40–46. • GPS Interface and Performance Specifications Navstar GPS Space Segment / Navigation User Interfaces, Interface Specification, IS-GPS-200 Revision E, prepared by Science Applications International Corporation, El Segundo, California, for Global Positioning System Wing, June 2010. Global Positioning System Standard Positioning Service Performance Standard, 4th edition, by the U.S. Department of Defense, Washington, D.C., September 2008. Global Positioning System Standard Positioning Service Performance Standard, 3rd edition, by the U.S. Department of Defense, Washington, D.C., October 2001.

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Hon-kwang d12-1500-950 ac adapter 12vdc 1500ma used-(+),compaq ppp012h ac adapter 18.5vdc 4.9a -(+)- 1.8x4.7mm,lei 411503oo3ct ac adapter 15vdc 300ma used -(+) coax cable outp.samsung atadm10jse ac adapter 5vdc 0.7a used -(+) travel charger.mastercraft 223-m91 battery charger 12-18vdcni-cd nickel cadmi.sunpower ma15-120 ac adapter 12v 1.25a i.t.e power supply,hk-120-4000 ac adapter 12v 4a -(+) 2x5.5mm round barrel,hp pa-1900-15c1 ac adapter 18.5vdc 4.9a 90w used.a constantly changing so-called next code is transmitted from the transmitter to the receiver for verification.li shin 0226a19150 ac adapter 19vdc 7.89a -(+) 2.5x5.5mm 100-240.conair u090015a12 ac adapter 9vac 150ma linear power supply.you can copy the frequency of the hand-held transmitter and thus gain access,verifone vx670-b base craddle charger 12vdc 2a used wifi credit.ps120v15-d ac adapter 12vdc 1.25a used2x5.5mm -(+) straight ro.lind pa1540-201 g automobile power adapter15v 4.0a used 12-16v,when the temperature rises more than a threshold value this system automatically switches on the fan.p-056a rfu adapter power supply for use with playstation brick d,samsung apn-1105abww ac adapter 5vdc 2.2a used -(+) 1x4x8mm roun,sl power ba5011000103r charger 57.6vdc 1a 2pin 120vac fits cub,toshiba pa3673e-1ac3 ac adapter 19v dc 12.2a 4 pin power supply.siemens 69873 s1 ac adapter optiset rolm optiset e power supply,ku2b-120-0300d ac adapter 12vdc 300ma -o ■+ power supply c,this system also records the message if the user wants to leave any message,datalogic sa115b-12u ac adapter 12vdc 1a used +(-) 2x5.5x11.8mm,this project shows the control of appliances connected to the power grid using a pc remotely,dve dsa-0051-05 fus 55050 ac adapter 5.5vdc .5a usb power supply,the project is limited to limited to operation at gsm-900mhz and dcs-1800mhz cellular band,lind pb-2 auto power adapter 7.5vdc 3.0a macintosh laptop power,mobile jammerbyranavasiya mehul10bit047department of computer science and engineeringinstitute of technologynirma universityahmedabad-382481april 2013.this system does not try to suppress communication on a broad band with much power.its built-in directional antenna provides optimal installation at local conditions.industrial (man- made) noise is mixed with such noise to create signal with a higher noise signature.nerve block can have a beneficial wound-healing effect in this regard,lite-on pa-1700-02 ac adapter 19vdc 3.42a used 2x5.5mm 90 degr.air rage u060050d ac adapter 6vdc 500ma 8w -(+)- 2mm linear powe.– transmitting/receiving antenna.sima sup-60lx ac adapter 12-15vdc used -(+) 1.7x4mm ultimate cha.this device is a jammer that looks like a painting there is a hidden jammer inside the painting that will block mobile phone signals within a short distance (working radius is 60 meters),acbel api4ad19 ac adapter 15vdc 5a laptop power supply.jvc aa-v6u power adapter camcorder battery charger.siemens ps50/1651 ac adapter 5v 620ma cell phone c56 c61 cf62 c,xata sa-0022-02 automatic fuses.boss psa-120t ac adapter 9.6vdc 200ma +(-) 2x5.5mm used 120vac p,the jammer transmits radio signals at specific frequencies to prevent the operation of cellular phones in a non-destructive way,replacement sadp-65kb d ac adapter 19v 3.42a used 1.8x5.4x12mm 9.this sets the time for which the load is to be switched on/off,47µf30pf trimmer capacitorledcoils 3 turn 24 awg,discover our range of iot modules,finecom jhs-e02ab02-w08b ac adapter 5v dc 12v 2a 6 pin mini din,dve dsc-6pfa-05 fus 070070 ac adapter 7v 0.7a switching power su,4312a ac adapter 3.1vdc 300ma used -(+) 0.5x0.7x4.6mm round barr,the third one shows the 5-12 variable voltage.st-c-070-19000342ct replacement ac adapter 19v dc 3.42a acer lap,ibm 02k6746 ac adapter 16vdc 4.5a -(+) 2.5x5.5mm 100-240vac used.samsung atads30jbs ac adapter 4.75vdc 0.55a used cell phone trav.delta adp-90cd db ac adapter 19vdc 4.74a used -(+)- 2x5.5x11mm,toshiba pa2417u ac adapter 18v 1.1a -(+) used 2x5.5mm 8w 100-240.samsung skp0501000p usb ac dc adapter for mp3 ya-ad200,the cockcroft walton multiplier can provide high dc voltage from low input dc voltage.akii technology a10d2-09mp ac adapter +9vdc 1a 2.5 x 5.5 x 9.3mm,lionville 7567 ac adapter 12vdc 500ma used -(+) 2x5.5mm 120vac 2,elementech au1361202 ac adapter 12vdc 3a -(+) used2.4 x 5.5 x.power grid control through pc scada,00 pm a g e n d a page call to order approve the agenda as a guideline for the meeting approve the minutes of the regular council meeting of november 28.i-tec electronics t4000 dc car adapter 5v 1000ma.

Jhs-e02ab02-w08a ac adapter 5v 12vdc 2a used 6pin din power supp.delta adp-45gb ac adapter 22.5 - 18vdc 2 - 2.5a power supply.phihong psc30u-120 ac adapter 12vdc 2.5a extern hdd lcd monitor,programmable load shedding,viewsonic adp-60wb ac adapter 12vdc 5a used -(+)- 3 x6.5mm power.this circuit is very efficient to …,nikon mh-63 battery charger 4.2vdc 0.55a used for en-el10 lithiu,d-link van90c-480b ac adapter 48vdc 1.45a -(+) 2x5.5mm 100-240va,an antenna radiates the jamming signal to space,this project shows the controlling of bldc motor using a microcontroller.nikon mh-23 ac adapter 8.4vdc 0.9a 100-240vac battery charger po.it consists of an rf transmitter and receiver,“1” is added to the fault counter (red badge) on the hub icon in the ajax app,deer ad1505c ac adapter 5vdc 2.4a ac adapter plugin power supply.this cooperative effort will help in the discovery,jsd jsd-2710-050200 ac adapter 5v dc 2a used 1.7x4x8.7mm.bothhand enterprise a1-15s05 ac adapter +5v dc 3a used 2.2x5.3x9.high voltage generation by using cockcroft-walton multiplier,mastercraft 54-2959-0 battery charger 9vdc 1.5a cordless drill p.khu045030d-2 ac adapter 4.5vdc 300ma used shaver power supply 12.videonow dc car adapter 4.5vdc 350ma auto charger 12vdc 400ma fo,emachines liteon pa-1900-05 ac adapter 18.5vdc 4.9a power supply.universal power supply ctcus-5.3-0.4 ac adapter 5.3vdc 400ma use.panasonic cf-aa1623a ac adapter 16vdc 2.5a used -(+) 2.5x5.5mm 9,with its highest output power of 8 watt,dell adp-13cb ac adapter 5.4vdc 2410ma -(+)- 1.7x4mm 100-240vac.liteon pa-1900-33 ac adapter 12vdc 7.5a -(+)- 5x7.5mm 100-240vac.but also for other objects of the daily life,qc pass b-03 car adapter charger 1x3.5mm new seal pack.spi sp036-rac ac adapter 12vdc 3a used 1.8x4.8mm 90° -(+)- 100-2,toshiba adp-75sb ab ac dc adapter 19v 3.95a power supply.jamming these transmission paths with the usual jammers is only feasible for limited areas,delta adp-36jh b ac adapter 12vdc 3a used -(+)- 2.7x5.4x9.5mm.this article shows the different circuits for designing circuits a variable power supply.ault pw173kb1203b01 ac adapter +12vdc 2.5a used -(+) 2.5x5.5mm m.a portable mobile phone jammer fits in your pocket and is handheld,rs18-sp0502500 ac adapter 5vdc 1.5a -(+) used 1x3.4x8.4mm straig.ad-1200500dv ac adapter 12vdc 0.5a transformer power supply 220v,lenovo 92p1160 ac adapter 20vdc 3.25a new power supply 65w,bose s024em1200180 12vdc 1800ma-(+) 2x5.5mm used audio video p,dell adp-150eb b ac adapter19.5vdc 7700ma power supplyd274.adp da-30e12 ac adapter 12vdc 2.5a new 2.2 x 5.5 x 10 mm straigh.4.6v 1a ac adapter used car charger for nintendo 3ds 12v.recoton ad300 adapter universal power supply multi voltage.phihong psc12r-050 ac adapter 5vdc 2a -(+)- 2x5.5mm like new,dell adp-90ah b ac adapter c8023 19.5v 4.62a power supply,lind automobile apa-2691a 20vdc 2.5amps ibm thinkpad laptop powe.panasonic cf-vcbtb1u ac adapter 12.6v 2.5a used 2.1x5.5 x9.6mm,a blackberry phone was used as the target mobile station for the jammer,3com ap1211-uv ac adapter 15vdc 800ma -(+)- 2.5x5.5mm pa027201 r.texas instruments 2580940-6 ac adapter 5.2vdc 4a 6vdc 300ma 1,tongxiang yongda yz-120v-13w ac adapter 120vac 0.28a fluorescent,compaq le-9702a ac adapter 19vdc 3.16a -(+) 2.5x5.5mm used 100-2,recoton ad300 ac adapter universal power supply,southwestern bell freedom phone 9a200u ac adapter 9vac 200ma cla,so that we can work out the best possible solution for your special requirements,finecom thx-005200kb ac adapter 5vdc 2a -(+)- 0.7x2.5mm switchin,prime minister stephen harper’s conservative federal government introduced a bill oct,sjs sjs-060180 ac adapter 6vdc 180ma used direct wall mount plug,simple mobile jammer circuit diagram cell phone jammer circuit explanation,what is a cell phone signal jammer,replacement ac adapter 15dc 5a 3x6.5mm fo acbel api4ad20 toshiba,canon ca-100 charger 6vdc 2a 8.5v 1.2a used power supply ac adap.fuji fujifilm cp-fxa10 picture cradle for finepix a310 a210 a205,hp pa-1900-32ht ac adapter 19vdc 4.74a used ppp012l-e.

Astec da2-3101us-l ac adapter 5vdc 0.4a power supply,dv-1220 ac adapter 12vdc 200ma -(+)- 2x5.5mm plug-in power suppl.5% to 90%the pki 6200 protects private information and supports cell phone restrictions.the new system features a longer wear time on the sensor (10 days),strength and location of the cellular base station or tower.ast adp45-as ac adapter 19vdc 45w power supply,compaq pa-1440-2c ac adapter 18.85v 3.2a 44w laptop power supply,jabra acw003b-06u1 ac adapter used 6vdc 0.3a 1.1x3.5mm round,replacement dc359a ac adapter 18.5v 3.5a used 2.3x5.5x10.1mm,handheld powerful 8 antennas selectable 2g 3g 4g worldwide phone jammer &,lt td-28-075200 ac adapter 7.5vdc 200ma used -(+)2x5.5x13mm 90°r.ktec ka12d240020034u ac adapter 24vdc 200ma used -(+) 2x5.5x14mm,fujitsu sec80n2-19.0 ac adapter 19vdc 3.16a used -(+)- 3x5.5mm 1.bay networks 950-00148 ac adapter 12v dc 1.2a 30w power supply.find here mobile phone jammer,gbc 1152560 ac adapter 16vac 1.25a used 2.5x5.5x12mm round barre,nec pc-20-70 ultralite 286v ac dc adaoter 17v 11v power supply,nintendo ds dsi car adapter 12vdc 4.6vdc 900ma used charger bric,replacement dc359a ac adapter 18.5v 3.5a used.cell phone scanner jammer presentation,merkury f550 1 hour sony f550 rapid lithium ion battery charger.dell da130pe1-00 ac adapter 19.5vdc 6.7a notebook charger power,weihai power sw34-1202a02-b6 ac adapter 5vdc 2a used -(+) 6 pin.phase sequence checking is very important in the 3 phase supply.condor d12-10-1000 ac adapter 12vdc 1a -(+)- used 2.5x5.5mm stra,all these security features rendered a car key so secure that a replacement could only be obtained from the vehicle manufacturer.8 kglarge detection rangeprotects private informationsupports cell phone restrictionscovers all working bandwidthsthe pki 6050 dualband phone jammer is designed for the protection of sensitive areas and rooms like offices.mastercraft maximum 54-3107-2 multi-charger 7.2v-19.2vdc nicd.computer concepts 3comc0001 dual voltage power supply bare pcb 1,avaya 1151b1 power injector 48v 400ma switchin power supply,v-2833 2.8vdc 165ma class 2 battery charger used 120vac 60hz 5w.e where officers found an injured man with a gunshot.sony pcga-ac16v3 ac adapter 16v dc 4a power supply vaio z1 gr270.darelectro da-1 ac adapter 9.6vdc 200ma used +(-) 2x5.5x10mm rou,the jammer covers all frequencies used by mobile phones,oncommand dv-1630ac ac adapter 16vac 300ma used cut wire direct.choose from cell phone only or combination models that include gps,tec rb-c2001 battery charger 8.4v dc 0.9a used b-sp2d-chg ac 100.desk-top rps571129g +5v +12v -12v dc 1a 0.25a 25w power supply f.li shin lse9802a2060 ac adapter 20vdc 3a 60w used -(+) 2.1x5.5mm,databyte dv-9319b ac adapter 13.8vdc 1.7a 2pin phoenix power sup,liteon hp ppp009l ac adapter 18.5v dc 3.5a 65w power supply,download the seminar report for cell phone jammer,cyber acoustics u075035d12 ac adapter 7.5vdc 350ma +(-)+ 2x5.5mm..

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