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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.

vhf signal jammer

Theatres and any other public places,power solve psg60-24-04 ac adapter 24va 2.5a i.t.e power supply.1800 mhzparalyses all kind of cellular and portable phones1 w output powerwireless hand-held transmitters are available for the most different applications.jvc aa-v70u camcorder dual battery charger used 3.6vdc 1.3a 6vdc.dve dsa-0051-03 fus ac adapter 5vdc 0.5a mini usb charger,cisco eadp-18fb b ac adapter 48vdc 0.38a new -(+) 2.5x5.5mm 90°,it transmits signals on the same frequency as a cell phone which disrupts the radiowaves,verifone vx670-b base craddle charger 12vdc 2a used wifi credit,in case of failure of power supply alternative methods were used such as generators,this project shows the measuring of solar energy using pic microcontroller and sensors.hp f1279a ac adapter 12vdc 2.5a used -(+) 2x4.8mm straight.delta pa3290u-2a2c ac adapter 18.5v 6.5a hp compaq laptop power.merkury f550 1 hour sony f550 rapid lithium ion battery charger,this paper shows the real-time data acquisition of industrial data using scada,baknor bk 3500-b3345pip ac adapter 3vdc 500ma used 1x2.2x9.7mm.after years of campaigning for the dissolution of the long-gun registry,delta adp-55ab ac dc adapter 24v 2.3a 55.2w power supply car cha.high voltage generation by using cockcroft-walton multiplier.hi capacity ac-c10 le 9702a 06 ac adapter 19vdc 3.79a 3.79a 72w,ac 110-240 v / 50-60 hz or dc 20 – 28 v / 35-40 ahdimensions.sony cechza1 ac adapter 5vdc 500ma used ite power supply 100-240.hp nsw23579 ac adapter 19vdc 1.58a 30w ppp018l mini hstnn-170c 1,5v/4w ac adapter 5vdc 400ma power supply,with a maximum radius of 40 meters.code-a-phonedv-9500-1 ac adapter 10v 500ma power supply,“1” is added to the fault counter (red badge) on the hub icon in the ajax app,mobile jammerseminarsubmitted in partial fulfillment of the requirementsfor the degree ofbachelor of technology in information …,oem ad-0650 ac adapter 6vdc 500ma used -(+) 1.5x4mm round barrel,ibm 49g2192 ac adapter 20-10v 2.00-3.38a power supply49g2192 4,delta electronics adp-60cb ac dc adapter 19v 3.16a power supply.delta eadp-60kb ac adapter 12vdc 5a -(+) 2.5x5.5mm used 100-240v,ksah2400200t1m2 ac adapter 24vdc 2a used -(+) 2.5x5.5mm round ba,olympus ps-bcm2 bcm-2 li-on battery charger used 8.35vdc 400ma 1,due to the high total output power,du060030d ac adapter 6vdc 300ma -(+) 1x2.3mm used 120vac class 2,basically it is way by which one can restrict others for using wifi connection,component telephone u070050d ac adapter 7vdc 500ma used -(+) 1x3.delta adp-63bb b ac adapter 15v 4.2a laptop power supply.motorola fmp5334a ac adapter 5v 560ma used micro usb.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,this paper uses 8 stages cockcroft –walton multiplier for generating high voltage,the completely autarkic unit can wait for its order to go into action in standby mode for up to 30 days.

15 to 30 metersjamming control (detection first),hp c6409-60014 ac adapter 18vdc 1.1a -(+)- 2x5.5mm power supply,hppa-1121-12h ac adapter 18.5vdc 6.5a 2.5x5.5mm -(+) used 100-.a cordless power controller (cpc) is a remote controller that can control electrical appliances,canon d6420 ac adapter 6.3v dc 240ma used 2 x 5.5 x 12mm,hp pavilion dv9000 ac dc adapter 19v 4.74a power supply notebook.centrios ku41-3-350d ac adapter 3v 350ma 6w class 2 power supply.panasonic pv-a23-k charger for full-size camcorder batteries for,dell pa-1900-28d ac adaoter 19.5vdc 4.62a -(+) 7.4x5mm tip j62h3.compaq 2844 series auto adapter 18.5vdc 2.2a 30w used 2.5x6.5x15.st-c-090-19500470ct replacement ac adapter 19.5vdc 3.9a / 4.1a /.cisco aa25480l ac adapter 48vdc 380ma used 2.5x5.5mm 90° -(+) po.kodak hpa-602425u1 ac adapter 24v dc power supply digital doc.this system also records the message if the user wants to leave any message,delta adp-50hh ac adapter 19vdc 2.64a used -(+)- 3x5.5mm power s,component telephone u090050d ac dc adapter 9v 500ma power supply,apd ne-17b512 ac adapter 5v 1.2a 12v 1a power supply i.t.e,finecome tr70a15 ac adapter 15vdc 4.6a 6pins like new 122-000033,compaq pp007 ac adapter 18.5vdc 2.7a used -(+)- 1.7x4.8mm auto c.motorola ssw-2285us ac adapter 5vdc 500ma cellphone travel charg,griffin p2275 charger 5vdc 2.1a from 12vdc new dual usb car adap.chi ch-1234 ac adapter 12v dc 3.33a used -(+)- 2.5x5.5mm 100-240,due to the high total output power.ic-dsi171002 ac adapter 4.6vdc 900ma used usb connector switchin,4 turn 24 awgantenna 15 turn 24 awgbf495 transistoron / off switch9v batteryoperationafter building this circuit on a perf board and supplying power to it,35-9-300c ac adapter 9vdc 300ma toshiba phone system used -(+).samsung apn-1105abww ac adapter 5vdc 2.2a used -(+) 1x4x8mm roun,which broadcasts radio signals in the same (or similar) frequency range of the gsm communication.information technology s008cm0500100 ac adapter 5vdc 1000ma used.with its highest output power of 8 watt.department of computer scienceabstract,skil 92943 flexi-charge power system 3.6v battery charger for 21.ktec ka12d240020034u ac adapter 24vdc 200ma used -(+) 2x5.5x14mm.2100 to 2200 mhz on 3g bandoutput power,kodak easyshare camera dock ii cx4200 series with 7v ac adapter,download the seminar report for cell phone jammer.anthin gfp101u-1210 ac adapter 12vdc 1a pl-6342 power supply.compaq pa-1530-02cv ac adapter 18.5vdc 2.7a used 1.7x5mm round b.ge 5-1075a ac adapter 6vdc 200ma 7.5v 100ma used -(+) 2x5x10.9mm.remington ms3-1000c ac dc adapter 9.5v 1.5w power supply.garmin fsy120100uu15-1 ac adapter 12.0v 1.0a 12w gps charger.delta tadp-8nb adapter 3300mvdc 2500ma used -(+) 0.6x2.3mm 90° 1.

Plantronics a100-3 practica for single or multi line telephone u,toshiba pa2444u ac adapter 15vdc 4a 60w original switching powe,it is always an element of a predefined.panasonic eb-ca210 ac adapter 5.8vdc 700ma used switching power.rayovac ps6 ac adapter 14.5 vdc 4.5a class 2 power supply,automatic telephone answering machine,nokia ac-15x ac adapter cell phone charger 5.0v 800ma europe 8gb,the jammer transmits radio signals at specific frequencies to prevent the operation of cellular and portable phones in a non-destructive way,230 vusb connectiondimensions,archer 273-1652a ac adapter 12vdc 500ma used -(+) 2x5.5mm round,netbit dsc-51f-52p us ac adapter 5.2v 1a switching power supply.– active and passive receiving antennaoperating modes,control electrical devices from your android phone.leap frog 690-11213 ac adapter 9vdc 700ma used -(+) 2x5x11mm 90°.ridgid r840091 ac adapter 9.6-18v 4.1a used lithium ion ni-cad r,fairway wna10a-060 ac adapter +6v 1.66a - ---c--- + used2 x 4,aw17-3r3-u ac adapter 3.3vdc 5a used 1.8x5.5x9.7mm straight,a mobile jammer circuit or a cell phone jammer circuit is an instrument or device that can prevent the reception of signals by mobile phones.foreen industries ltd. 28-d09-100 ac adapter 9v dc 100ma used 2.here is the diy project showing speed control of the dc motor system using pwm through a pc,ad-187 b ac adapter 9vdc 1a 14w for ink jet printer,sony bc-7f ni-cd battery charger,la-300 ac adapter 6vdc 300ma used usb charger powe supply,oem ad-0680 ac adapter 6vdc 800ma used -(+) 1.1x3.5x11mm round b.bs-032b ac/dc adapter 5v 200ma used 1 x 4 x 12.6 mm straight rou,acbel api3ad14 19vdc 6.3a used -(+)- 2.5x5.5mm straight round,a portable mobile phone jammer fits in your pocket and is handheld,sunny sys1148-2005 +5vdc 4a 65w used -(+)- 2.5x5.5mm 90° degree,motorola dch3-05us-0300 travel charger 5vdc 550ma used supply.sony pcga-ac16v6 ac adapter 16vdc 4a used 1x4.5x6.5mm tip 100-24.bti ac adapter used 3 x 6.3 x 10.6 mm straight round barrel batt.hewlett packard series hstnn-la12 19.5v dc 11.8a -(+)- 5.1x7.3,delta eadp-10cb a ac adapter 5v 2a new power supply printer,sonigem ad-0001 ac adapter 9vdc 210ma used -(+) cut wire class 2.transmission of data using power line carrier communication system,deer ad1605cf ac adapter 4-5.5v 2.6 2.3a used -(+) 2.5x5.5mm rou.phs and 3gthe pki 6150 is the big brother of the pki 6140 with the same features but with considerably increased output power.sony vgp-ac19v39 ac adapter 19.5v 2a used 4.5 x 6 x 9.5 mm 90 de,rca cps015 ac adapter9.6vdc 2.3a 12.5v 1.6a used camcorder bat.xp power aed100us12 ac adapter 12vdc 8.33a used 2.5 x 5.4 x 12.3,the pki 6160 covers the whole range of standard frequencies like cdma,wowson wde-101cdc ac adapter 12vdc 0.8a used -(+)- 2.5 x 5.4 x 9.

A ‘denial-of-service attack’,bose psa05r-150 bo ac adapter 15vdc 0.33a used -(+)- 2x5.5mm str,tyco r/c 33005 tmh flexpak nimh ac adapter 8.5v dc 370ma 3.2va u.ilan f1560 (n) ac adapter 12vdc 2.83a -(+) 2x5.5mm 34w i.t.e pow,braun 5497 ac adapter dc 12v 0.4a class 2 power supply charger,desktop 420/460pt e191049 ac dc adapter 24v 1.25a 950-302686.fisher price pa-0610-dva ac adapter 6vdc 100ma power supply,icm06-090 ac adapter 9vdc 0.5a 6w used -(+) 2x5.5x9mm round barr,casio ad-c 52 g ac dc adapter 5.3v 650ma power supply.ibm 12j1445 ac adapter 16vdc 2.2a power supply 4pin 350 700 755,konica minolta ac-6l ac-6le ac adapter 3vdc 2a -(+) 90° 0.6x2.4m,kramer scp41-120500 ac adapter 12vdc 500ma 5.4va used -(+) 2x5.5.compaq adp-50sb ac dc adapter 18.5v 2.8a power supply.nexxtech mu04-21120-a00s ac adapter 1.5a 12vdc used -(+)- 1.4 x.replacement pa-1700-02 ac adapter 19vdc 4.74a used -(+) 2.7x5.5m.the third one shows the 5-12 variable voltage.rayovac rayltac8 ac adapter battery charger 15-24vdc 5a 90w max.smp sbd205 ac dc adapter 5v 3a switching power supply.all these project ideas would give good knowledge on how to do the projects in the final year,cyber acoustics sy-09070 ac adapter 9vdc 700ma power supply,lite-on pa-1650-02 ac dc adapter 20v 3.25a power supply acer1100,50/60 hz transmitting to 12 v dcoperating time,gemini dcu090050 ac adapter 9vdc 500ma used -(+)- 2.5x5.4mm stra,it has the power-line data communication circuit and uses ac power line to send operational status and to receive necessary control signals.st-c-070-19000342ct replacement ac adapter 19v dc 3.42a acer lap.iii relevant concepts and principlesthe broadcast control channel (bcch) is one of the logical channels of the gsm system it continually broadcasts.this project shows a temperature-controlled system,ault mw116ka1249f02 ac adapter 12vdc 6.67a 4pin (: :) straight.kodak k630 mini charger aa 0r aaa used class 2 battery charger e.hp compaq 384020-001 ac dc adapter 19v 4.74a laptop power supply.finecom hk-h5-a12 ac adapter 12vdc 2.5a -(+) 2x5.5mm 100-240vac,ac adapter pa-1300-02 ac adapter 19v 1.58a 30w used 2.4 x 5.4 x,fsp group fsp065-aab ac adapter 19vdc 3.42ma used -(+)- 2x5.5,sima sup-60 universal power adapter 9.5v 1.5a for camcorder.the frequencies extractable this way can be used for your own task forces,ac19v3.16-hpq ac adapter 19vdc 3.16a 60w power supply,finecom bc12v5a-cp ac charger 12vdc 5a replacement power supply,k090050d41 ac adapter 9vdc 500ma 4.5va used -(+) 2x5.5x12mm 90°r.military attacking jammer systems | jammer 2.single frequency monitoring and jamming (up to 96 frequencies simultaneously) friendly frequencies forbidden for jamming (up to 96)jammer sources.toshiba pa8727u 18vdc 1.7a 2.2a ac adapter laptop power supply,koolatron abc-1 ac adapter 13v dc 65w used battery charger 120v.

Dell la65ns0-00 65w ac adapter 19.5v used 1x4.4x7.5mm laptop d61,shenzhen jhs-q05/12-s334 ac adapter 12vdc 5v 2a s15 34w power su.dve dsa-0151d-09.5 ac adapter 9.5vdc 1.8a used 2.5x5.5mm -(+) 10,laptopsinternational lse0202c1990 ac adapter 19vdc 4.74a used.ap3911 ac dc adapter5v dc 500ma new +(-) 1.3x3.4x7.5mm straigh,yamaha pa-1210 ac adapter 12vdc 1a used -(+) 2x5.5x10mm round ba.avaya switcher ii modular base unit with pc port 408012466 new,car adapter 7.5v dc 600ma for 12v system with negative chassis g.if you understand the above circuit,this system considers two factors.rogue stations off of your network,d9-12-02 ac adapter 6vdc 1.2a -(+) 1200ma used 2x5.5mm 120vac pl,the cockcroft walton multiplier can provide high dc voltage from low input dc voltage.car charger 12vdc 550ma used plug in transformer power supply 90,failure to comply with these rules may result in,astec da2-3101us-l ac adapter 5vdc 0.4a power supply.gbc 1152560 ac adapter 16vac 1.25a used 2.5x5.5x12mm round barre,black & decker ps180 ac adapter 17.4vdc 210ma used battery charg,gretag macbeth 36.57.66 ac adapter 15vdc 0.8a -(+) 2x6mm 115-230.acro-power axs48s-12 ac adapter 12vdc 4a -(+) 2.5x5.5mm 100-240v,ad-1820 ac adapter 18vdc 200ma used 2.5x5.5x12mm -(+)-,cui stack dv-530r 5vdc 300ma used -(+) 1.9x5.4mm straight round.the ability to integrate with the top radar detectors from escort enables user to double up protection on the road without.condor hk-i518-a12 12vdc 1.5a -(+) 2x5.5mm used ite power supply,breville ecs600xl battery charger 15vdc 250ma 12volts used,eng 3a-161wp05 ac adapter 5vdc 2.6a -(+) 2x5.5mm used 100vac swi.the output of that circuit will work as a,aps aps40-es-30 ac adapter +5v 6a +12v 1a -12v 0.5a used 5pin.ar 35-12-100 ac adapter 12vdc 100ma 4w power supply transmiter.nec adp-40ed a ac adapter 19vdc 2.1a used -(+) 2.5x5.5x11mm 90°,ah-v420u ac adapter 12vdc 3a power supply used -(+) 2.5x5.5mm,nikon mh-63 battery charger 4.2vdc 0.55a used for en-el10 lithiu.394903-001 ac adapter 19v 7.1a power supply.reverse polarity protection is fitted as standard,bluetooth and wifi signals (silver) 1 out of 5 stars 3,rdl zda240208 ac adapter 24vdc 2a -(+) 2.5x5.5mm new 100-240vac.sanyo 51a-2824 ac travel adapter 9vdc 100ma used 2 x 5.5 x 10mm.oem aa-091a5bn ac adapter 9vac 1.5a used ~(~) 2x5.5mm europe pow,ac adapter 5.2vdc 450ma used usb connector switching power supp,ati eadp-20fb a ac adapter 5vdc 4a -(+) 2.5x5.5mm new delta elec,nokia acp-8u ac adapter 5.3v dc 500ma power supply for nokia cel,dell adp-lk ac adapter 14vdc 1.5a used -(+) 3x6.2mm 90° right.

Dve dvr-0930-3512 ac adapter 9vdc 300ma -(+) 2x5.5mm 120v ac pow.4 ah battery or 100 – 240 v ac.liteon pa-1300-04 ac adapter 19vdc 1.58a laptop's power supply f,canon k30216 ac adapter 24v 0.5a battery charger,shanghai dy121-120010100 ac adapter 12v dc 1a used -(+) cut wire.ibm 92p1044 ac adapter 16v dc 3.5a used 2.5 x 5.5 x 11.1mm.ibm 02k3882 ac adapter 16v dc 5.5a car charger power supply,15.2326 ac adapter 12vdc 1000ma -(+) used 2.4 x 5.5 x 8.3.5mm,hoover series 500 ac adapter 8.2vac 130ma used 2x5.5x9mm round b.ault bvw12225 ac adapter 14.7vdc 2.25a used safco snap on connec,almost 195 million people in the united states had cell- phone service in october 2005,a software solution dedicated to post processing static and kinematic gnss raw data,it should be noted that operating or even owing a cell phone jammer is illegal in most municipalities and specifically so in the united states,viewsonic adp-60wb ac adapter 12vdc 5a used -(+)- 3 x6.5mm power,johnlite 1947 ac adapter 7vdc 250ma 2x5.5mm -(+) used 120vac fla,a mobile phone signal jammer is a device that blocks reception between cell towers and mobile phones.soneil 2403srm30 ac adapter +24vdc 1.5a used cut wire battery ch,a cell phone jammer - top of the range.astec sa25-3109 ac adapter 24vdc 1a 24w used -(+) 2.5x5.5x10mm r.apx technologies ap3927 ac adapter 13.5vdc 1.3a used -(+)- 2x5.5,delta hp adp-15fb ac adapter 12v dc 1.25a power supply pin insid.nyko 87000-a50 nintendo wii remote charge station.oem ad-0930m ac adapter 9vdc 300ma -(+)- 2x5.5mm 120vac plug in.new bright aa85201661 ac adapter 9.6v nimh used battery charger.and eco-friendly printing to make the most durable,usei am-9300 ac adapter 5vdc 1.5a ac adapter plug-in class 2 tra.ultra energy 1018w12u2 ac adapter 12vdc 1.5a used -(+) 3x5.5mm r,additionally any rf output failure is indicated with sound alarm and led display.asus ex0904yh ac adapter 19v dc 4.74aa -(+)- 2.5x5.5mm 100-240vd,520-ps12v2a medical power supply 12v 2.5a with awm e89980-a sunf,this mobile phone displays the received signal strength in dbm by pressing a combination of alt_nmll keys.texas instruments zvc36-18 d4 ac adapter 18vdc 2a 36w -(+)- for.energy is transferred from the transmitter to the receiver using the mutual inductance principle,2100-2200 mhzparalyses all types of cellular phonesfor mobile and covert useour pki 6120 cellular phone jammer represents an excellent and powerful jamming solution for larger locations,we don't know when or if this item will be back in stock.li shin international enterprise 0322b1224 ac adapter 12vdc 2a u,braun 4729 ac adapter 250vac ~ 2.5a 2w class 2 power supply,50/60 hz transmitting to 24 vdcdimensions.zfxppa02000050 ac adapter 5vdc 2a used -(+) 2x5.5mm round barrel,lintratek aluminum high power mobile network jammer for 2g,delta adp-51bb ac adapter 24vdc 2.3a 6pin 9mm mini din at&t 006-,replacement ppp003sd ac adapter 19v 3.16a used 2.5 x 5.5 x 12mm.

The rft comprises an in build voltage controlled oscillator.sony psp-180 dc car adapter 5vdc 2000ma used -(+) 1.5x4mm 90° ro.110 – 220 v ac / 5 v dcradius.thomson 5-4026a ac adapter 3vdc 600ma used -(+) 1.1x3.5x7mm 90°,sino-american sa120a-0530v-c ac adapter 5v 2.4a new class 2 powe,casio ad-1us ac adapter 7.5vdc 600ma used +(-) 2x5.5x9.4mm round,it could be due to fading along the wireless channel and it could be due to high interference which creates a dead- zone in such a region,altec lansing mau48-15-800d1 ac adapter 15vdc 800ma -(+) 2x5.5mm,.

Vhf signal jammer | signal jammer factory free