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Improving Single-Frequency RTK in the Urban Enviornment By Mojtaba Bahrami and Marek Ziebart A look at how Doppler measurements can be used to smooth noisy code-based pseudoranges to improve the precision of autonomous positioning as well as to improve the availability of single-frequency real-time kinematic positioning, especially in urban environments. INNOVATION INSIGHTS by Richard Langley WHAT DO A GPS RECEIVER, a policeman’s speed gun, a weather radar, and some medical diagnostic equipment have in common? Give up? They all make use of the Doppler effect. First proposed in 1842 by the Austrian mathematician and physicist, Christian Doppler, it is the change in the perceived frequency of a wave when the transmitter and the receiver are in relative motion. Doppler introduced the concept in an attempt to explain the shift in the color of light from certain binary stars. Three years later, the effect was tested for sound waves by the Dutch scientist Christophorus Buys Ballot. We have all heard the Doppler shift of a train whistle or a siren with their descending tones as the train or emergency vehicle passes us. Doctors use Doppler sonography — also known as Doppler ultrasound — to provide information about the flow of blood and the movement of inner areas of the body with the moving reflectors changing the received ultrasound frequencies. Similarly, some speed guns use the Doppler effect to measure the speed of vehicles or baseballs and Doppler weather radar measures the relative velocity of particles in the air. The beginning of the space age heralded a new application of the Doppler effect. By measuring the shift in the received frequency of the radio beacon signals transmitted by Sputnik I from a known location, scientists were able to determine the orbit of the satellite. And shortly thereafter, they determined that if the orbit of a satellite was known, then the position of a receiver could be determined from the shift. That realization led to the development of the United States Navy Navigation Satellite System, commonly known as Transit, with the first satellite being launched in 1961. Initially classified, the system was made available to civilians in 1967 and was widely used for navigation and precise positioning until it was shut down in 1996. The Soviet Union developed a similar system called Tsikada and a special military version called Parus. These systems are also assumed to be no longer in use — at least for navigation. GPS and other global navigation satellite systems use the Doppler shift of the received carrier frequencies to determine the velocity of a moving receiver. Doppler-derived velocity is far more accurate than that obtained by simply differencing two position estimates. But GPS Doppler measurements can be used in other ways, too. In this month’s column, we look at how Doppler measurements can be used to smooth noisy code-based pseudoranges to improve the precision of autonomous positioning as well as to improve the availability of single-frequency real-time kinematic positioning, especially in urban environments. Correction and Further Details The first experimental Transit satellite was launched in 1959. A brief summary of subsequent launches follows: Transit 1A launched 17 September 1959 failed to reach orbit Transit 1B launched 13 April 1960 successfully Transit 2A launched 22 June 1960 successfully Transit 3A launched 30 November 1960 failed to reach orbit Transit 3B launched 22 February 1961 failed to deploy in correct orbit Transit 4A launched 29 June 1961 successfully Transit 4B launched 15 November 1961 successfully Transits 4A and 4B used the 150/400 MHz pair of frequencies and provided geodetically useful results. A series of Transit prototype and research satellites was launched between 1962 and 1964 with the first fully operational satellite, Transit 5-BN-2, launched on 5 December 1963. The first operational or Oscar-class Transit satellite, NNS O-1, was launched on 6 October 1964. The last pair of Transit satellites, NNS O-25 and O-31, was launched on 25 August 1988. “Innovation” is a regular column that features discussions about recent advances in GPS technology and its applications as well as the fundamentals of GPS positioning. The column is coordinated by Richard Langley of the Department of Geodesy and Geomatics Engineering at the University of New Brunswick, who welcomes your comments and topic ideas. To contact him, email lang @ unb.ca. Real-time kinematic (RTK) techniques enable centimeter-level, relative positioning. The technology requires expensive, dedicated, dual-frequency, geodetic-quality receivers. However, myriad industrial and engineering applications would benefit from small-size, cost-effective, single-frequency, low-power, and high-accuracy RTK satellite positioning. Can such a sensor be developed and will it deliver? If feasible, such an instrument would find many applications within urban environments — but here the barriers to success are higher. In this article, we show how some of the problems can be overcome. Single-Frequency RTK Low-cost single-frequency (L1) GPS receivers have attained mass-market status in the consumer industry. Notwithstanding current levels of maturity in GPS hardware and algorithms, these receivers still suffer from large positioning errors. Any positioning accuracy improvement for mass-market receivers is of great practical importance, especially for many applications demanding small size, cost-effectiveness, low power consumption, and highly accurate GPS positioning and navigation. Examples include mobile mapping technology; machine control; agriculture fertilization and yield monitoring; forestry; utility services; intelligent transportation systems; civil engineering projects; unmanned aerial vehicles; automated continuous monitoring of landslides, avalanches, ground subsidence, and river level; and monitoring deformation of built structures. Moreover, today an ever-increasing number of smartphones and handsets come equipped with a GPS receiver. In those devices, the increasing sophistication of end-user applications and refinement of map databases are continually tightening the accuracy requirements for GPS positioning. For single-frequency users, the RTK method does appear to offer the promise of highly precise position estimates for stationary and moving receivers and can even be considered a candidate for integration within mobile handhelds. Moreover, the RTK approach is attractive because the potential of the existing national infrastructures such as Great Britain’s Ordnance Survey National GNSS Network-RTK (OSNet) infrastructure, as well as enabling technologies such as the Internet and the cellular networks, can be exploited to deliver RTK corrections and provide high-precision positioning and navigation. The basic premise of relative (differential) positioning techniques such as RTK is that many of the sources of GNSS measurement errors including the frequency-dependent error (the ionospheric delay) are spatially correlated. By performing relative positioning between receivers, the correlated measurement errors are completely cancelled or greatly reduced, resulting in a significant increase in the positioning accuracy and precision. Single-Frequency Challenges. Although RTK positioning is a well-established and routine technology, its effective implementation for low-cost, single-frequency L1 receivers poses many serious challenges, especially in difficult and degraded signal environments for GNSS such as urban canyons. The most serious challenge is the use of only the L1 frequency for carrier-phase integer ambiguity resolution and validation. Unfortunately, users with single-frequency capability do not have frequency diversity and many options in forming useful functions and combinations for pseudorange and carrier-phase observables. Moreover, observations from a single-frequency, low-cost receiver are typically “biased” due to the high level of multipath and/or receiver signal-tracking anomalies and also the low-cost patch antenna design that is typically used. In addition, in those receivers, measurements are typically contaminated with high levels of noise due to the low-cost hardware design compared to the high-end receivers. This makes the reliable fixing of the phase ambiguities to their correct integer values, for single-frequency users, a non-trivial problem. As a consequence, the reliability of single-frequency observations to resolve ambiguities on the fly in an operational environ ment is relatively low compared to the use of dual-frequency observations from geodetic-quality receivers. Improving performance will be difficult, unless high-level noise and multipath can be dealt with effectively or unless ambiguity resolution techniques can be devised that are more robust and are less sensitive to the presence of biases and/or high levels of noise in the observations. Traditionally, single-frequency RTK positioning requires long uninterrupted initialization times to obtain reliable results, and hence have a time-to-fix ambiguities constraint. Times of 10 to 25 minutes are common. Observations made at tens of continuous epochs are used to determine reliable estimates of the integer phase ambiguities. In addition, these continuous epochs must be free from cycle slips, loss of lock, and interruptions to the carrier-phase signals for enough satellites in view during the ambiguity fixing procedure. Otherwise, the ambiguity resolution will fail to fix the phase ambiguities to correct integer values. To overcome these drawbacks and be able to determine the integer phase ambiguities and thus the precise relative positions, observations made at only one epoch (single-epoch) can be used in resolving the integer phase ambiguities. This allows instantaneous RTK positioning and navigation for single-frequency users such that the problem of cycle slips, discontinuities, and loss of lock is eliminated. However, for single-frequency users, the fixing of the phase ambiguities to their correct integer values using a single epoch of observations is a non-trivial problem; indeed, it is considered the most challenging scenario for ambiguity resolution at the present time. Instantaneous RTK positioning relies fundamentally upon the inversion of both carrier-phase measurements and code measurements (pseudoranges) and successful instantaneous ambiguity resolution. However, in this approach, the probability of fixing ambiguities to correct integer values is dominated by the relatively imprecise pseudorange measurements. This is more severe in urban areas and difficult environments where the level of noise and multipath on pseudoranges is high. This problem may be overcome partially by carrier smoothing of pseudoranges in the range/measurement domain using, for example, the Hatch filter. While carrier-phase tracking is continuous and free from cycle slips, the carrier smoothing of pseudoranges with an optimal smoothing filter window-width can effectively suppress receiver noise and short-term multipath noise on pseudo­ranges. However, the effectiveness of the conventional range-domain carrier-smoothing filters is limited in urban areas and difficult GNSS environments because carrier-phase measurements deteriorate easily and substantially due to blockages and foliage and suffer from phase discontinuities, cycle-slip contamination, and other measurement anomalies. This is illustrated in Figure 1. The figure shows that in a kinematic urban environment, frequent carrier-phase outages and anomalies occur, which cause frequent resets of the carrier-smoothing filter and hence carrier smoothing of pseudoranges suffers in robustness and effective continuous smoothing. Figure 1. Satellite tracking and carrier-phase anomaly summary during the observation time-span. These data were collected in a dense urban environment in both static and kinematic mode. The superimposed red-points show epochs where carrier-phase observables are either missing or contaminated with cycle slips, loss of locks, and/or other measurement anomalies. Doppler Frequency Shift. While carrier-phase tracking can be discontinuous in the presence of continuous pseudoranges, a receiver generates continuous Doppler-frequency-shift measurements. The Doppler measurements are immune to cycle slips. Moreover, the precision of the Doppler measurements is better than the precision of pseudoranges because the absolute multipath error of the Doppler observable is only a few centimeters. Thus, devising methods that utilize the precision of raw Doppler measurements to reduce the receiver noise and high-frequency multipath on pseudoranges may prove valuable especially in GNSS-challenged environments. Figure 2 shows an example of the availability and the precision of the receiver-generated Doppler measurements alongside the delta-range values derived from the C/A-code pseudoranges and from the L1 carrier-phase measurements. This figure also shows that frequent carrier-phase outages and anomalies occur while for every C/A-code pseudorange measurement there is a corresponding Doppler measurement available. Figure 2. Plots of C/A-code-pseudorange-derived delta-ranges (top), L1 carrier-phase-derived delta-ranges (middle), and L1 raw receiver-generated Doppler shifts that are transformed into delta-ranges for the satellite PRN G18 during the observation time-span when it was tracked by the receiver (bottom). Smoothing. A rich body of literature has been published exploring aspects of carrier smoothing of pseudoranges. One factor that has not received sufficient study in the literature is utilization of Doppler measurements to smooth pseudoranges and to investigate the influence of improved pseudorange accuracy on both positioning and the integer-ambiguity resolution. Utilizing the Doppler measurements to smooth pseudoranges could be a good example of an algorithm that maximally utilizes the information redundancy and diversity provided by a GPS/GNSS receiver to improve positioning accuracy. Moreover, utilizing the Doppler measurements does not require any hardware modifications to the receiver. In fact, receivers measure Doppler frequency shifts all the time as a by-product of satellite tracking. GNSS Doppler Measurement Overview The Doppler effect is the apparent change in the transmission frequency of the received signal and is experienced whenever there is any relative motion between the emitter and receiver of wave signals. Theoretically, the observed Doppler frequency shift, under Einstein’s Special Theory of Relativity, is approximately equal to the difference between the received and transmitted signal frequencies, which is approximately proportional to the receiver-satellite topocentric range rate. Beat Frequency. However, the transmitted frequency is replicated locally in a GNSS receiver. Therefore, strictly speaking, the difference of the received frequency and the receiver locally generated replica of the transmitted frequency is the Doppler frequency shift that is also termed the beat frequency. If the receiver oscillator frequency is the same as the satellite oscillator frequency, the beat frequency represents the Doppler frequency shift due to the relative, line-of-sight motion between the satellite and the receiver. However, the receiver internal oscillator is far from being perfect and therefore, the receiver Doppler measurement output is the apparent Doppler frequency shift (that includes local oscillator effects). The Doppler frequency shift is also subject to satellite-oscillator frequency bias and other disturbing effects such as atmospheric effects on the signal propagation. To estimate the range rate, a receiver typically forms an average of the delta-range by simply integrating the Doppler over a very short period of time (for example, 0.1 second) and then dividing it by the duration of the integration interval. Since the integration of frequency over time gives the phase of the signal over that time interval, the procedure continuously forms the carrier-phase observable that is the integrated Doppler over time. Therefore, Doppler frequency shift can also be estimated by time differencing carrier-phase measurements. The carrier-phase-derived Doppler is com puted over a longer time span, leading to smoother Doppler measurements, whereas direct loop filter output is an instantaneous measure produced over a short time interval. Doppler frequency shift is routinely used to determine the satellite or user velocity vector. Apart from velocity determination, it is worth mentioning that Doppler frequency shifts are also exploited for coarse GPS positioning. Moreover, the user velocity vector obtained from the raw Doppler frequency shift can be and has been applied by a number of researchers to instantaneous RTK applications to constrain the float solution and hence improve the integer-ambiguity-resolution success rates in kinematic surveying. In this article, a simple combination procedure of the noisy pseudorange measurements and the receiver-generated Doppler measurements is suggested and its benefits are examined. Doppler-Smoothing Algorithm Description Motivated both by the continual availability and the centimeter-level precision of receiver-generated (raw) Doppler measurements, even in urban canyons, a method has been introduced by the authors that utilizes the precision of raw Doppler measurements to reduce the receiver noise and high-frequency multipath on code pseudoranges. For more detail on the Doppler-smoothing technique, see Further Reading. The objective is to smooth the pseudoranges and push the accuracy of the code-based or both code- and carrier-based positioning applications in GNSS-challenged environments. Previous work on Doppler-aided velocity/position algorithms is mainly in the position domain. In those approaches, the improvement in the quality of positioning is gained mainly by integrating the kinematic velocities and accelerations derived from the Doppler measurement in a loosely coupled extended Kalman filter or its variations such as the complementary Kalman filter. Essentially, these techniques utilize the well-known ability of the Kalman filter to use independent velocity estimates to reduce the noise of positioning solutions and improve positioning accuracy. The main difference among these position-domain filters is that different receiver dynamic models are used. The proposed method combines centimeter-level precision receiver-generated Doppler measurements with pseudorange measurements in a combined pseudorange measurement that retains the significant information content of each. Two-Stage Process. The proposed Doppler-smoothing process has two stages: (1) the prediction or initialization stage and (2) the filtering stage. In the prediction stage, a new estimated smoothed value of the pseudorange measurement for the Doppler-smoothing starting epoch is obtained. In this stage, for a fixed number of epochs, a set of estimated pseudoranges for the starting epoch is obtained from the subsequent pseudorange and Doppler measurements. The estimated pseudoranges are then averaged to obtain a good estimated starting point for the smoothing process. The number of epochs used in the prediction stage is the averaging window-width or Doppler-smoothing-filter length. In the filtering stage, the smoothed pseudorange profile is constructed using the estimated smoothed starting pseudorange and the integrated Doppler measurements over time. The Doppler-smoothing procedures outlined here can be performed successively epoch-by-epoch (that is, in a moving filter), where the estimated initial pseudorange (the averaged pseudorange) is updated from epoch to epoch. Alternatively, an efficient and elegant implementation of the measurement-domain Doppler-smoothing method is in terms of a Kalman filter, where it can run as a continuous process in the receiver from the first epoch (or in post-processing software, but then without the real-time advantage). This filter allows real-time operation of the Doppler-smoothing approach. In the experiments described in this article, a short filter window-width is used. The larger the window width used in the averaging filter process, the more precise the averaged pseudorange becomes. However, this filter is also susceptible to the ionospheric divergence phenomenon because of the opposite signs of the ionospheric contribution in the pseudorange and Doppler observables. Therefore, the ionospheric divergence effect between pseudoranges and Doppler observables increases with averaging window-width and the introduced bias in the averaged pseudoranges become apparent for longer filter lengths. Using the propagation of variance law, it can be shown that the precision of the delta-range calculated with the integrated Doppler measurements over time depends on both the Doppler-measurement epoch interval and the precision of the Doppler measurements, assuming that noise/errors on the measurements are uncorrelated. Experimental Results To validate the improvement in the performance and availability of single-frequency instantaneous RTK in urban areas, the proposed Doppler-aided instantaneous RTK technique has been investigated using actual GPS data collected in both static and kinematic pedestrian trials in central London. In this article, we only focus on the static results and the kinematic trial results are omitted. It is remarked, however, that the data collected in the static mode were post-processed in an epoch-by-epoch approach to simulate RTK processing. In the static testing, GPS test data were collected with a measurement rate of 1 Hz. At the rover station, a consumer-grade receiver with a patch antenna was used. This is a single-frequency 16-channel receiver that, in addition to the C/A-code pseudoranges, is capable of logging carrier-phase measurements and raw Doppler measurements. Reference station data were obtained from the Ordnance Survey continuously operating GNSS network. Three nearby reference stations were selected that give different baseline lengths: Amersham (AMER) ≈ 38.3 kilometers away, Teddington (TEDD) ≈ 20.8 kilometers away, and Stratford (STRA) ≈ 7.1 kilometers away. In addition, a virtual reference station (VRS) was also generated in the vicinity (60 meters away) of the rover receiver. Doppler-Smoothing. Before we present the improvement in the performance of instantaneous RTK positioning, the effect of the Doppler-smoothing of the pseudoranges in the measurement domain and comparison with carrier-phase smoothing of pseudoranges is given. To do this, we computed the C/A-code measurement errors or observed range deviations (the differences between the expected and measured pseudoranges) in the static mode (with surveyed known coordinates) using raw, Doppler-smoothed and carrier-smoothed pseudoranges. FIGURE 3a illustrates the effect of 100-second Hatch-filter carrier smoothing and FIGURE 3b shows a 100-second Doppler-smoothing of the pseudo­ranges for satellite PRN G28 (RINEX satellite designator) with medium-to-high elevation angle. The raw observed pseudorange deviations (in blue) are also given as reference. The quasi-sinusoidal oscillations are characteristic of multipath. Comparing the Doppler-smoothing in Figure 3b to the Hatch carrier-smoothing in Figure 3a, it can be seen that Doppler-smoothing of pseudoranges offers a modest improvement and is more robust and effective than that of the traditional Hatch filter in difficult environments. Figure 3. Smoothed pseudorange errors (observed range deviations) using the traditional Hatch carrier-smoothing filter. Smoothing filter length in the experiments for both filters was set to 100 seconds. Satellite PRN G28 was chosen to represent a satellite at medium-to-high elevation angle. Figure 3. Smoothed pseudorange errors (observed range deviations) using the Doppler-smoothing filter. Smoothing filter length in the experiments for both filters was set to 100 seconds. Satellite PRN G28 was chosen to represent a satellite at medium-to-high elevation angle. Figure 4a illustrates carrier-phase Hatch-filter smoothing for low-elevation angle satellite PRN G18. In this figure, the Hatch carrier-smoothing filter reset is indicated. It can be seen that due to the frequent carrier-phase discontinuities and cycle slips, the smoothing has to be reset and restarted from the beginning and hardly reaches its full potential. In contrast, Doppler smoothing for PRN G18 shown in FIGURE 4b had few filter resets and managed effectively to smooth the very noisy pseudorange in some sections of the data. Figure 4. Smoothed pseudorange errors (observed range deviations) and filter resets and filter length (window width) using the traditional Hatch carrier-smoothing filter. Smoothing filter length in the experiments for both filters was set to 100 seconds. Satellite PRN G18 was chosen to represent a satellite at low elevation angle as it rises from 10 to 30 degrees. Figure 4. Smoothed pseudorange errors (observed range deviations) and filter resets and filter length (window width) using the Doppler-smoothing. Smoothing filter length in the experiments for both filters was set to 100 seconds. Satellite PRN G18 was chosen to represent a satellite at low elevation angle as it rises from 10 to 30 degrees. Considering RTK in this analysis, we can demonstrate the increase in the success rate of the Doppler-aided integer ambiguity resolution (and hence the RTK availability) by comparison of the obtained integer ambiguity vectors from the conventional LAMBDA (Least-squares AMBiguity Decorrelation Adjustment) ambiguity resolution method using Doppler-smoothed pseudoranges with those obtained without Doppler-aiding in post-processed mode. The performance of ambiguity resolution was evaluated based on the number of epochs where the ambiguity validation passed the discrimination/ratio test. The ambiguity validation ratio test was set to the fixed critical threshold of 2.5 in all the experiments. In addition to the ratio test, the fixed solutions obtained using the fixed integer ambiguity vectors that passed the ratio test were compared against the true position of the surveyed point to make sure that indeed the correct set of integer ambiguities were estimated. The overall performance of the single-epoch single-frequency integer ambiguity resolution obtained by the conventional LAMBDA ambiguity resolution method without Doppler-aiding is shown in Figure 5 for baselines from 60 meters up to 38 kilometers in length. In comparison, the performance of the single-epoch single-frequency integer ambiguity resolution from the LAMBDA method using Doppler-smoothed pseudoranges are shown in Figure 6 for those baselines and they are compared with integer ambiguity resolution success rates of the conventional LAMBDA ambiguity resolution method without Doppler-aiding. Figure 6 shows that using Doppler-smoothed pseudoranges enhances the probability of identifying the correct set of integer ambiguities and hence increases the success rate of the integer ambiguity resolution process in instantaneous RTK, providing higher availability. This is more evident for shorter baselines. For long baselines, the residual of satellite-ephemeris error and atmospheric-delay residuals that do not cancel in double differencing potentially limits the effectiveness of the Doppler-smoothing approach. It is well understood that those residuals for long baselines strongly degrade the performance of ambiguity resolution. Relative kinematic positioning with single frequency mass-market receivers in urban areas using VRS has also shown improvement. Figure 5. Single-epoch single-frequency integer ambiguity resolution success rate obtained by the conventional LAMBDA ambiguity resolution method without Doppler-aiding. Figure 6. Plots of integer ambiguity resolution success rates: single-epoch single-frequency integer ambiguity resolution success rate obtained by the conventional LAMBDA ambiguity resolution method without Doppler-aiding (in blue) and using Doppler-smoothed pseudoranges (in green). Conclusion In urban areas, the proposed Doppler-smoothing technique is more robust and effective than traditional carrier smoothing of pseudoranges. Static and kinematic trials confirm this technique improves the accuracy of the pseudorange-based absolute and relative positioning in urban areas characteristically by the order of 40 to 50 percent. Doppler-smoothed pseudoranges are then used to aid the integer ambiguity resolution process to enhance the probability of identifying the correct set of integer ambiguities. This approach shows modest improvement in the ambiguity resolution success rate in instantaneous RTK where the probability of fixing ambiguities to correct integer values is dominated by the relatively imprecise pseudorange measurements. The importance of resolving the integer ambiguities correctly must be emphasized. Therefore, devising innovative and robust methods to maximize the success rate and hence reliability and availability of single-frequency, single-epoch integer ambiguity resolution in the presence of biased and noisy observations is of great practical importance especially in GNSS-challenged environments. Acknowledgments The study reported in this article was funded through a United Kingdom Engineering and Physical Sciences Research Council Engineering Doctorate studentship in collaboration with the Ordnance Survey. M. Bahrami would like to thank his industrial supervisor Chris Phillips from the Ordnance Survey for his continuous encouragement and support. Professor Paul Cross is acknowledged for his valuable comments. The Ordnance Survey is acknowledged for sponsoring the project and providing detailed GIS data. Manufacturer The data for the trial discussed in this article were obtained from a u-blox AG AEK-4T receiver with a u-blox ANN-MS-0-005 patch antenna. Mojtaba Bahrami is a research fellow in the Space Geodesy and Navigation Laboratory (SGNL) at University College London (UCL). He holds an engineering doctorate in space geodesy and navigation from UCL. Marek Ziebart is a professor of space geodesy at UCL. He is the director of SGNL and vice dean for research in the Faculty of Engineering Sciences at UCL. FURTHER READING • Carrier Smoothing of Pseudoranges “Optimal Hatch Filter with an Adaptive Smoothing Window Width” by B. Park, K. Sohn, and C. Kee in Journal of Navigation, Vol. 61, 2008, pp. 435–454, doi: 10.1017/S0373463308004694. “Optimal Recursive Least-Squares Filtering of GPS Pseudorange Measurements” by A. Q. Le and P. J. G. Teunissen in VI Hotine-Marussi Symposium on Theoretical and Computational Geodesy, Wuhan, China, May 29 – June 2, 2006, Vol. 132 of the International Association of Geodesy Symposia, Springer-Verlag, Berlin and Heidelberg, 2008, Part II, pp. 166–172, doi: 10.1007/978-3-540-74584-6_26. “The Synergism of GPS Code and Carrier Measurements” by R. Hatch in Proceedings of the 3rdInternational Geodetic Symposium on Satellite Doppler Positioning, Las Cruces, New Mexico, February 8-12, 1982, Vol. 2, pp. 1213–1231. • Combining Pseudoranges and Carrier-phase Measurements in the Position Domain “Position Domain Filtering and Range Domain Filtering for Carrier-smoothed-code DGNSS: An Analytical Comparison” by H. Lee, C. Rizos, and G.-I. Jee in IEE Proceedings Radar, Sonar and Navigation, Vol. 152, No. 4, August 2005, pp. 271–276, doi:10.1049/ip-rsn:20059008. “Complementary Kalman Filter for Smoothing GPS Position with GPS Velocity” by H. Leppakoski, J. Syrjarinne, and J. Takala in Proceedings of ION GPS/GNSS 2003, the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation, Portland, Oregon, September 9– 12, 2003, pp. 1201–1210. “Precise Platform Positioning with a Single GPS Receiver” by S. B. Bisnath, T. Beran, and R. B. Langley in GPS World, Vol. 13, No. 4, April 2002, pp. 42–49. “GPS Navigation: Combining Pseudorange with Continuous Carrier Phase Using a Kalman Filter” by P. Y. C. Hwang and R. G. Brown in Navigation, Journal of The Institute of Navigation, Vol. 37, No. 2, 1990, pp. 181–196. • Doppler-derived Velocity Information and RTK Positioning “Advantage of Velocity Measurements on Instantaneous RTK Positioning” by N. Kubo in GPS Solutions, Vol. 13, No. 4, 2009, pp. 271–280, doi: 10.1007/s10291-009-0120-9. • Doppler Smoothing of Pseudoranges and RTK Positioning Doppler-Aided Single-Frequency Real-Time Kinematic Satellite Positioning in the Urban Environment by M. Bahrami, Ph.D. dissertation, Space Geodesy and Navigation Laboratory, University College London, U.K., 2011. “Instantaneous Doppler-Aided RTK Positioning with Single Frequency Receivers” by M. Bahrami and M. Ziebart in Proceedings of PLANS 2010, IEEE/ION Position Location and Navigation Symposium, Indian Wells, California, May 4–6, 2010, pp. 70–78, doi: 10.1109/PLANS.2010.5507202. “Getting Back on the Sidewalk: Doppler-Aided Autonomous Positioning with Single-Frequency Mass Market Receivers in Urban Areas” by M. Bahrami in Proceedings of ION GNSS 2009, the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation, Savannah, Georgia, 22–25 September 2009, pp. 1716–1725. • Integer Ambiguity Resolution “GPS Ambiguity Resolution and Validation: Methodologies, Trends and Issues” by D. Kim and R. B. Langley in Proceedings of the 7th GNSS Workshop – International Symposium on GPS/GNSS, Seoul, Korea, 30 November – 2 December 2000, Tutorial/Domestic Session, pp. 213–221. The LAMBDA Method for Integer Ambiguity Estimation: Implementation Aspects by P. de Jong and C. Tiberius. Publications of the Delft Geodetic Computing Centre, No. 12, Delft University of Technology, Delft, The Netherlands, August 1996. “A New Way to Fix Carrier-phase Ambiguities” by P.J.G. Teunissen, P.J. de Jonge, and C.C.J.M. Tiberius in GPS World, Vol. 6, No. 4, April 1995, pp. 58–61. “The Least-Squares Ambiguity Decorrelation Adjustment: a Method for Fast GPS Integer Ambiguity Estimation” by P.J.G. Teunissen in Journal of Geodesy, Vol. 70, No. 1–2, 1995, pp. 65–82, doi: 10.1007/BF00863419.

cell phone jamming equipment

Conair sa28-12a ac adapter 4.4vdc 120ma 4.8w power supply.aps aps61es-30 ac adapter +5v +12v -12v 5a 1.5a 0.5a 50w power s,sony ericsson 316ams43001 ac adapter 5v dc 400ma -(+)- 0.5x2.5mm,which is used to test the insulation of electronic devices such as transformers.mascot 9940 ac adapter 29.5vdc 1.3a used terminal battery char.sony ac-v35 ac power adapter 7.5vdc 1.6a can use with sony ccd-f,gateway lishin 0220a1890 ac adapter 18.5v 4.9a laptop power supp,ibm 85g6737 ac adapter 16vdc 2.2a -(+) 2.5x5.5mm used power supp,pll synthesizedband capacity,ault mw117ka ac adapter 5vdc 2a used -(+)- 1.4 x 3.4 x 8.7 mm st,cisco adp-20gb ac adapter 5vdc 3a 34-0853-02 8pin din power supp,mainly for door and gate control.nikon eh-64 ac adapter 4.8vdc 1.5a -(+) power supply for coolpix.sears craftsman 974775-001 battery charger 12vdc 1.8a 9.6v used.liteon pa-1480-19t ac adapter (1.7x5.5) -(+)- 19vdc 2.6a used 1.,government and military convoys,a piezo sensor is used for touch sensing,sunny sys1148-2005 +5vdc 4a 65w used -(+)- 2.5x5.5mm 90° degree,hipower ea11603 ac adapter 18-24v 160w laptop power supply 3x6.5.once i turned on the circuit,spi sp036-rac ac adapter 12vdc 3a used 1.8x4.8mm 90° -(+)- 100-2.phihong psc11a-050 ac adapter +5v dc 2a power supply,the jammer transmits radio signals at specific frequencies to prevent the operation of cellular phones in a non-destructive way.if there is any fault in the brake red led glows and the buzzer does not produce any sound,nec pa-1750-04 ac adapter 19vdc 3.95a 75w adp68 switching power.brother epa-5 ac adapter 7.5vdc 1a used +(-) 2x5.5x9.7mm round b.nec pa-1750-07 ac adapter 15vdc 5a adp80 power supply nec laptop,has released the bx40c rtk board to support its series of gnss boards and provide highly accurate and fast positioning services.chicony a10-018n3a ac adapter 36vdc 0.5a used 4.3 x 6 x 15.2 mm,dell pa-1151-06d ac adapter 19.5vdc 7.7a used -(+) 1x4.8x7.5mm i.delta eadp-18cb a ac adapter 48vdc 0.375a used -(+) 2.5x5.5mm ci,phihong psc11r-050 ac adapter +5v dc 2a used 375556-001 1.5x4,d-link m1-10s05 ac adapter 5vdc 2a -(+) 2x5.5mm 90° 120vac route,power-win pw-062a2-1y12a ac adapter 12vdc 5.17a 62w 4pin power,the present circuit employs a 555 timer.positec machinery sh-dc0240400 ac adapter 24vdc 400ma used -(,dve dsa-9w-09 fus 090100 ac adapter 9vdc 1a used 1.5x4mm dvd pla,delta tadp-24ab a ac adapter 8vdc 3a used -(+) 1.5x5.5x9mm 90° r,fifthlight flt-hprs-dali used 120v~347vac 20a dali relay 10502.in this tutroial im going to say about how to jam a wirless network using websploit in kali linux,dymo dsa-42dm-24 2 240175 ac adapter 24vdc 1.75a used -(+) 2.5x5,vtech s004lu0750040(1)ac adapter 7.5vdc 3w -(+) 2.5x5.5mm round,hipower a0105-225 ac adapter 16vdc 3.8a used -(+)- 1 x 4.5 x 6 x,hp c8890-61605 ac adapter 6vdc 2a power supply photosmart 210,milwaukee 48-59-2401 12vdc lithium ion battery charger used,320 x 680 x 320 mmbroadband jamming system 10 mhz to 1.vehicle unit 25 x 25 x 5 cmoperating voltage.apd wa-10e05u ac adapter 5vdc 2a used 1.8x4mm -(+) 100-240vac.and here are the best laser jammers we’ve tested on the road,toshiba pa3378e-3ac3 ac adapter15vdc 5a -(+) 3x6.5mm used round.hp pa-1121-12r ac adapter 18.5vdc 6.5a used 2.5 x 5.5 x 12mm,hon-kwang hk-u-120a015-us ac adapter 12vdc 0-0.5a used -(+)- 2x5.

Apd asian power adapter wa-30b19u ac adapter 19vdc 1.58a used 1..avaya switcher ii modular base unit with pc port 408012466 new,cidco dv-9200 ac adapter 9vdc 200ma used -(+) 2.2x5.4mm straight.casio ad-c59200j ac adapter 5.9v dc 2a charger power supply.dell pa-1131-02d ac adapter 19.5vdc 6.7a 130w pa-13 for dell pa1.jvc aa-v68u ac adapter 7.2v dc 0.77a 6.3v 1.8a charger aa-v68 or.toshiba pa-1750-07 ac adapter 15vdc 5a desktop power supply nec,then get rid of them with this deauthentication attack using kali linux and some simple tools.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.50/60 hz transmitting to 12 v dcoperating time,download the seminar report for cell phone jammer,viewsonic api-208-98010 ac adapter 12vdc 3.6a -(+)- 1.7x4.8mm po,golden power gp-lt120v300-ip44 ac adapter 12v 0.3a 3.6w cut wire,liteon hp ppp009l ac adapter 18.5v dc 3.5a 65w power supply,dell pa-3 ac adapter 19vdc 2.4a 2.5x5.5mm -(+) power supply,frequency counters measure the frequency of a signal,apx sp20905qr ac adapter 5vdc 4a 20w used 4pin 9mm din ite power,gsm channel jamming can only be successful if the gsm signal strength is weak.motorola spn4226a ac adapter 7.8vdc 1a used power supply,ault inc 7712-305-409e ac adapter 5vdc 0.6a +12v 0.2a 5pin power,the proposed system is capable of answering the calls through a pre-recorded voice message,aci communications lh-1250-500 ac adapter -(+) 12.5vdc 500ma use,its total output power is 400 w rms.rocketfish rf-sam90 charger ac adapter 5vdc 0.6a power supply us,the unit requires a 24 v power supply.dve eos zvc65sg24s18 ac adapter 24vdc 2.7a used -(+) 2.5x5.5mm p.delta adp-50hh ac adapter 19vdc 2.64a used -(+)- 3x5.5mm power s,it is convenient to open or close a …,samsung pscv400102aac adapter 16vdc 2.5a power supply wallmount,92p1157 replacement ac adapter 20v dc 3.25a ibm laptop power sup.sony ac-l15b ac dc adapter 8.4v 1.5a power supply for camcorder,madcatz 2752 ac adapter 12vdc 340ma used -(+) class 2 power supp,samsung aa-e8 ac adapter 8.4vdc 1a camcorder digital camera camc.soneil 2403srm30 ac adapter +24vdc 1.5a used cut wire battery ch.rayovac rayltac8 ac adapter battery charger 15-24vdc 5a 90w max,tec rb-c2001 battery charger 8.4v dc 0.9a used b-sp2d-chg ac 100,conair tk952c ac adapter european travel charger power supply.dell ha65ns5-00 19.5v 3.34ma 65w ac adapter 4.8x7.3mm used.ct std-1203 ac adapter -(+) 12vdc 3a used -(+) 2.5x5.4mm straigh.sil ssa-12w-09 us 090120f ac adapter 9vdc 1200ma used -(+) 2x5.5,this project shows the control of appliances connected to the power grid using a pc remotely.compaq adp-50ch bc ac adapter 18.5vdc 2.7a used 1.8x4.8mm round.potrans up04821120a ac adapter 12vdc 4a used -(+) 2x5.5x9.7mm ro,samsung aa-e9 ac adapter 8.4v dc 1a camera charger,astec dps53 ac adapter 12vdc 5a -(+) 2x5.5mm power supply deskto,braun 3 709 ac adapter dc 1.3w class 2 power supply plug in char,axis a31207c ac adapter 12vac 500ma used 2.5x5.5 x 11.3mm 90 deg,j0d-41u-16 ac adapter 7.5vdc 700ma used -(+)- 1.2 x 3.4 x 7.2 mm,oem ads18b-w 220082 ac adapter 22vdc 818ma used -(+)- 3x6.5mm it,poweruon 160023 ac adapter 19vdc 12.2a used 5x7.5x9mm round barr.9 v block battery or external adapter.hengguang hgspchaonsn ac adapter 48vdc 1.8a used cut wire power.

Dymo tead-48-2460600u ac adapter 24vdc 600ma used -(+)- 90 degre.artesyn scl25-7624 ac adapter 24vdc 1a 8pin power supply.offers refill reminders and pickup notifications,browse recipes and find the store nearest you.electra 26-26 ac car adapter 6vdc 300ma used battery converter 9,solytech ad1712c ac adapter 12vdc 1.25a 2x5.5mm used 100-240vac, 5G jammer ,nintendo wap-002(usa) ac adapter 4.6vdc 900ma 2pin dsi charger p,braun 5 497 ac adapter dc 12v 0.4a class 2 power supply charger.jensen dv-1215-3508 ac adapter 12vdc 150ma used 90°stereo pin.fld0710-5.0v2.00a ac adapter 5vdc 2a used -(+) 1.3x3.5mm ite pow.at every frequency band the user can select the required output power between 3 and 1,sony vgp-ac19v10 ac adapter 19.5vdc 4.7a notebook power supply.when the temperature rises more than a threshold value this system automatically switches on the fan,what is a cell phone signal jammer.the systems applied today are highly encrypted,dell da90ps1-00 ac adapter 19.5vdc 4.62a used straight with pin.people also like using jammers because they give an “out of service” message instead of a “phone is off” message.but we need the support from the providers for this purpose.chd scp0501500p ac adapter 5vdc 1500ma used -(+) 2x5.5x10mm roun.netcom dv-9100 ac adapter 9vdc 100ma used -(+) 2.5x5.5mm straigh.charger for battery vw-vbg130 panasonic camcorder hdc-sd9pc sdr-.toshiba adpv16 ac dc adapter 12v 3a power supply for dvd player.ault t41-120750-a000g ac adapter 12vac 750ma used ~(~)2.5x5.5.electro-mech co c-316 ac adapter 12vac 600ma used ~(~) 2.5x5.5 r.techno earth 60w-12fo ac adapter 19vdc 3.16a used 2.6 x 5.4 x 11,ibm 02k6746 ac adapter 16vdc 4.5a -(+) 2.5x5.5mm 100-240vac used.netbit dsc-51f-52100 ac adapter 5.2vdc 1a palm european plug swi.sony ac-v65a ac power adapter 7.5vdc 10v 1.6a 1.3a 20w charger p,i have designed two mobile jammer circuits,bluetooth and wifi signals (silver) 1 out of 5 stars 3,ilan f1560 (n) ac adapter 12vdc 2.83a -(+) 2x5.5mm 34w i.t.e pow,ibm 02k6718 thinkpad multiple battery charger ii charge quick mu,creative dv-9440 ac adapter 9v 400ma power supply,jabra ssa-5w-05 us 0500018f ac adapter 5vdc 180ma used -(+) usb,3com sc102ta1203f02 ac adapter 12vdc 1.5a used 2.5x5.4x9.5mm -(+,ibm thinkpad 760 ac adapter 49g2192 10-20v 2-3.38a power supply.dechang long-2028 ac adapter 12v dc 2000ma like new power supply,they operate by blocking the transmission of a signal from the satellite to the cell phone tower,sony ac-v316a ac adapter 8.4vdc 1.94a used 110-240vac ~ 50/60hz,this is the newly designed 22-antenna 5g jammer,li shin lse9802a1240 ac adapter 12v 3.3a 40w power supply 4 pin.jammers also prevent cell phones from sending outgoing information.2100 – 2200 mhz 3 gpower supply,acbel wa9008 ac adapter 5vdc 1.5a -(+)- 1.1x3.5mm used 7.5w roun,dve dsa-9w-09 fus 090080 ac adapter 9v 0.8a switching power adap,aironet ad1280-7-544 ac adapter 12vdc 800ma power supply for med.mintek adpv28a ac adapter 9v 2.2a switching power supply 100-240.aparalo electric 690-10931 ac adapter 9vdc 700ma 6.3w used -(+).atlinks 5-2527 ac adapter 9vdc 200ma used 2 x 5.5 x 10mm.ati eadp-20fb a ac adapter 5vdc 4a -(+) 2.5x5.5mm new delta elec,ibm ac adapter-30 84g2128 4pin 20-10vdc 1.5-3a power supply.

The third one shows the 5-12 variable voltage,– transmitting/receiving antenna,sac1105016l1-x1 ac adapter 5vdc 500ma used usb connecter,baknor 66dt-12-2000e ac dc adapter 12v 2a european power supply,cincon tr36a-13 ac adapter 13.5v dc 2.4a power supply,audiovox 28-d12-100 ac adapter 12vdc 100ma power supply stereo m.compaq series 2872 ac adapter 18.75vdc 3.15a 41w91-55069,canon ad-50 ac adapter -(+)- +24vdc 1.8a used 2x5.5mm straight r,ibm 02k6543 ac adapter 16vdc 3.36a used -(+) 2.5x5.5mm 02k6553 n,canon cb-2lu battery charger wall plug-in 4.2v 0.7a i.t.e. power,samsung hsh060abe ac adapter 11-30v dc used portable hands-free,creative ys-1015-e12 12v 1.25a switching power supply ac adapter,3com 722-0004 ac adapter 3vdc 0.2a power supply palm pilot.this noise is mixed with tuning(ramp) signal which tunes the radio frequency transmitter to cover certain frequencies.armaco ba2424 ac adapter 24vdc 200ma used 117v 60hz 10w power su,cobra ca 25 ac adapter dc 16v 100ma power supply charger,pdf mobile phone signal jammer,black & decker 371415-11 ac adapter 13vdc 260ma used -(+) 2x5.5m,communication jamming devices were first developed and used by military,starting with induction motors is a very difficult task as they require more current and torque initially.compaq presario ppp005l ac adapter 18.5vdc 2.7a for laptop,utstarcom psc11a-050 ac adapter +5vdc 2a used -(+) 1.5x4mm cru66,arstan dv-9750 ac adapter 9.5vac 750ma wallmount direct plug in,today´s vehicles are also provided with immobilizers integrated into the keys presenting another security system,ad3230 ac adapter 5vdc 3a used 1.7x3.4x9.3mm straight round,this project shows automatic change over switch that switches dc power automatically to battery or ac to dc converter if there is a failure,ault p48480250a01rg ethernet injector power supply 48vdc 250ma,ningbo taller electrical tl-6 ac adapter 6vdc 0.3a used 2.1x5.4,ktec ksaff1200200w1us ac adapter 12vdc 2a used -(+)- 2x5.3x10mm,chi ch-1234 ac adapter 12v dc 3.33a used -(+)- 2.5x5.5mm 100-240,biogenik 3ds/dsi ac adapter used 4.6v 1a car charger for nintend.replacement pa-1900-02d ac adapter 19.5v dc 4.62a for dell latit,motorola nu20-c140150-i3 ac adapter 14vdc 1.5a used -(+) 2.5x5.5.artesyn ssl20-7660 ac dc adapter 5v 0.9a 12v 0.8a power supply.hna050100u ac adapter 5v 1a audio video power supply,kensington k33404us ac adapter 16v 5.62a 19vdc 4.74a 90w power,umec up0351e-12p ac adapter +12vdc 3a 36w used -(+) 2.5x5.5mm ro,2110 to 2170 mhztotal output power.nyko aspw01 ac adapter 12.2vdc 0.48a used -(+) 2x5.5x10mm round.toshiba delta pa3714e-1ac3ac adapter 19v3.42alaptop power,this project shows charging a battery wirelessly,integrated inside the briefcase.archer 273-1455 ac adapter used 9vdc 300ma -(+) 2x5.5x10mm.binary fsk signal (digital signal).finecom ky-05036s-12 ac adpter 12vdc 5v dc 2a 5pin 9mm mini din,it consists of an rf transmitter and receiver.fujitsu fmv-ac317 ac adapter 16vdc 3.75a used cp171180-01.bi bi07-050100-adu ac adapter 5vdc 1a used usb connector class 2,pride mobility elechg1024 ea1089a ac acid battery charger adapte,nokiaacp-12x cell phone battery uk travel charger,we have already published a list of electrical projects which are collected from different sources for the convenience of engineering students.sanyo 51a-2824 ac travel adapter 9vdc 100ma used 2 x 5.5 x 10mm.

The third one shows the 5-12 variable voltage.motorola ch610d walkie talkie charger only no adapter included u.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,targus pa104u ac power inverter used auto air charger dell 12vdc,khu045030d-2 ac adapter 4.5vdc 300ma used shaver power supply 12,sony ac-12v1 ac dc adapter 12v 2a laptop power supply.ancon 411503oo3ct ac adapter 15vdc 300ma used -(+) rf antenna co.the jammer covers all frequencies used by mobile phones,kensington system saver 62182 ac adapter 15a 125v used transiet,1 watt each for the selected frequencies of 800.macallister 9804 ac adapter dc 17.5v 1.5a used class 2 battery c,samsung atads30jbe ac adapter 4.75vdc 0.55a used cell phone trav.x-360 g8622 ( ap3701 ) ac adapter xbox power supply,dve dsa-0421s-091 ac adapter used -(+)2.5x5.5 9.5vdc 4a round b.delta eadp-32bb a ac adapter 12vdc 2.67a used -(+) 2x5.5x9mm str.li shin 0226a19150 ac adapter 19vdc 7.89a -(+) 2.5x5.5mm 100-240,canon ca-590 compact power adapter 8.4vdc 0.6a used mini usb pow,apple h1300 ac adapter 7vdc 0.5a used -(+) 1.5x4.5x9.4mm round b.aastra m8000 ac adapter 16vac 250ma ~(~) 2.5x5.5m.ts30g car adapter 16.2v dc 2.6a 34w used ac adapter 3-pin,replacement 65w-ap04 ac adapter 24vdc 2.65a used - ---c--- +,there are many methods to do this.finecom la-520w ac adapter 5vdc 2a -(+) 0.8x2.5mm new charger ho,panasonic re7-27 ac adapter 5vdc 4a used shaver power supply 100,nec adp-50mb ac adapter 19v 2.64a laptop power supply,toshiba pa3283u-1aca ac adapter 15vdc 5a - (+) - center postive,philips 4203 035 78410 ac adapter 1.6vdc 100ma used -(+) 0.7x2.3..

Cell phone jamming equipment - phone jamming equipment wholesale