An open source GPS multipath simulator in Matlab/Octave

Multipath is detrimental for both GPS positioning and timing applications. However, the benefits of GPS multipath for reflectometry have become increasingly clear for monitoring soil moisture, snow depth, and vegetation growth. In positioning applications, a simulator can support multipath mitigation efforts in terms of, e.g., site selection, antenna design, receiver performance assessment, and in relating different observations to a common parameterization. For reflectometry, in order to convert observed multipath parameters into useable environmental products, it is important to be able to explicitly link the GPS observables to known characteristics of the GPS receiver/antenna and the reflecting environment. Existing GPS multipath software simulators are generally not readily available for the general scientific community to use and/or modify. Here, a simulator has been implemented in Matlab/Octave and is made available as open source code. It can produce signal-to-noise ratio, carrier phase, and code pseudorange observables, based on L1 and L2 carrier frequencies and C/A, P(Y), and L2C modulations. It couples different surface and antenna types with due consideration for polarization and coherence. In addition to offering predefined material types (water, concrete, soil, etc.), it allows certain dimensional properties to be varied, such as soil moisture and snow density.     

Analysis of early late phase in single-and dual-frequency GPS receivers for multipath detection

Much of the research into multipath detection and mitigation has not considered the carrier phase delay between the line of sight (LOS) and reflected signals. A new variable referred to as early late phase (ELP) has recently been proposed to exploit this phase difference. It has been found that in a receiver tracking the L1 GPS signal, the probability of detecting multipath is lower when the carrier phase difference between the LOS and a reflected signal is an integer multiple of π. Since the pseudorange error caused by the multipath’s presence is the highest in this case, we propose to exploit the coexistence of another GPS civilian signal, the L2C. We present an analysis of ELP for the L1 and L2C signals, and a combination of both, for detecting multipath. The multipath detection performance has been compared using probabilities of false alarm and detection. An ideal algorithm should have lower probability of false alarm and higher probability of detection. However, it has been found that using dual-frequency ELP increases both probabilities. Thus, receiver operator characteristics (ROC) curves, and the area under the ROC curves, have been used for effective comparison. It has been found that the L2C signal individually gives worse performance than L1 because of its weaker signal strength. However, the combination of L1 and L2C gives the best overall performance, and thus it can be claimed that ELP using dual-frequency receivers is a more effective approach for detecting multipath.     

L2C码与L2载波数据质量分析

基于IGS的L2C信号跟踪站数据验证了具有L2C码的卫星的L2载波的信噪比高于没有L2C码的卫星的L2载波的信噪比,L2载波恢复的数据质量更好.针对不同的接收机,对比分析了C/A码和L2C码多路径效应及观测噪声水平,发现对TRIMBLE NETRS接收机,L2C码误差水平明显高于C/A码,与期望结果相反.     

Examining the C1-P1 Pseudorange Bias

Use of GPS tracking data from different dual-frequency receiver types (cross-correlating vs. codeless) has revealed satellite-dependent biases in pseudorange observables P1 (Y-code) and C1 (C/A, Clear Acquisition code). These biases can have a direct effect on clock estimates, carrier phase bias fixing, and other parameters estimated in GPS data processing. A set of satellite-specific compensatory pseudorange offsets is calculated, and each is applied to a wee of daily global network analyses in which satlellite, receiver, atmospheric, and Earth rotation parameters are estimated. Results from these analyses are then compared to those from corresponding baseline cases in which no biases were applied. There is also some evidence that suggests that the pseudorange biases differ even among codeless receiver models. Hence, a second set of offsets is computed on a different basis, and compared with the baseline model in a similar manner. A preliminary examination of C1-P1 variations over time is presented. Finally, recommendations are made for the use of the calculated offsets, and consideration is given to a future dissemination of updates to these values as necessary. ? 2001 John Wiley & Sons, Inc.     

Tracking and orbit determination performance of the GRAS instrument on MetOp-A

The global navigation satellite system receiver for atmospheric sounding (GRAS) on MetOp-A is the first European GPS receiver providing dual-frequency navigation and occultation measurements from a spaceborne platform on a routine basis. The receiver is based on ESA’s AGGA-2 correlator chip, which implements a high-quality tracking scheme for semi-codeless P(Y) code tracking on the L1 and L2 frequency. Data collected with the zenith antenna on MetOp-A have been used to perform an in-flight characterization of the GRAS instrument with focus on the tracking and navigation performance. Besides an assessment of the receiver noise and systematic measurement errors, the study addresses the precise orbit determination accuracy achievable with the GRAS receiver. A consistency on the 5 cm level is demonstrated for reduced dynamics orbit solutions computed independently by four different agencies and software packages. With purely kinematic solutions, 10 cm accuracy is obtained. As a part of the analysis, an empirical antenna offset correction and preliminary phase center correction map are derived, which notably reduce the carrier phase residuals and improve the consistency of kinematic orbit determination results.

A multi-technique approach for characterizing the SVN49 signal anomaly, part 2: chip shape analysis

Due to a satellite internal reflection at the L5 test payload, the SVN49 (PRN1) GPS satellite exhibits a static multipath on the L1 and L2 signals, which results in elevation-dependent tracking errors for terrestrial receivers. Using a 30-m high-gain antenna, code and carrier phase measurements as well as raw in-phase and quadrature radio frequency samples have been collected during a series of zenith passes in mid-April 2010 to characterize the SVN49 multipath and its impact on common users. Following an analysis of the receiver tracking data and the IQ constellation provided in Part 1 of this study, the present Part 2 provides an in-depth investigation into chip shapes for the L1 and L2 signals. A single reflection model is found to be compatible with the observed chip shape distortions and key parameters for an elevation dependent multipath model are derived. A good agreement is found between multipath parameters derived independently from raw IQ-samples and measurements of a so-called Vision Correlator. The chip shapes and their observed variation with elevation can be used to predict the multipath response of different correlator types within a tracking receiver. The multipath model itself is suitable for implementation in a signal simulator and thus enables laboratory testing of actual receiver hardware.     

一种改进的载波相位周跳探测与修复方法

本文提出了一种改进的探测和修复小周跳的方法。根据电离层残差值在短周期内的连续性,可以比较准确地用函数来拟合的特点,再根据L1和L2载波相位观测值之间小数部分应该满足的函数关系,来探测和修复第二个载波的周跳。该方法仅使用单星双频载波相位数据,而且每颗卫星之间的数据是独立处理的。由于载波的小数部分不受周跳的影响,所以该方法的有效性能够不受周跳类型的影响。试验表明该方法对于L1上小于9周、L2上小于7周的周跳很有效。     

Use of the L2C signal for inversions of GPS radio occultation data in the neutral atmosphere

Results from processing FORMOSAT-3/COSMIC radio occultations (RO) with the new GPS L2C signal acquired both in phase locked loop (PLL) and open loop (OL) modes are presented. Analysis of L2P, L2C, and L1CA signals acquired in PLL mode shows that in the presence of strong ionospheric scintillation not only L2P tracking, but also L1CA tracking often fails, while L2C tracking is most stable. The use of L2C improves current RO processing in the neutral atmosphere mainly by increasing the number of processed occultations (due to significant reduction in the number of L2 tracking failures) and marginally by a reduction in noise in statistics. The latter is due to the combination of reduced L2C noise (compared to L2P) and increased L1CA noise in those occultations where L2P would have failed. This result suggests application of OL tracking for L1CA and L2C signals throughout an entire occultation to optimally acquire RO data. Two methods of concurrent processing of L1CA and L2C RO signals are considered. Based on testing of individual occultations, these methods allow: (1) reduction in uncertainty of bending angles retrieved by wave optics in the lower troposphere and (2) reduction in small-scale residual errors of the ionospheric correction in the stratosphere.     

Correlation between ROTI and Ionospheric Scintillation Indices using Hong Kong low-latitude GPS data

The correlation between the rate of TEC index (ROTI) and scintillation indices S ₄ and σ _Φ for low-latitude region is analyzed in this study, using data collected from a Global Positioning System (GPS) scintillation monitoring receiver installed at the south of Hong Kong for the periods June–August of 2012 and May 2013 and July–December of 2013. The analysis indicates that the correlation coefficient between ROTI and S ₄/σ _Φ is about 0.6 if data from all GPS satellites are used together. If each individual satellite is considered, the correlation coefficients are above 0.6 on average and sometimes above 0.8. The analysis also shows that the ratio of ROTI and S ₄ varies between 1 and 4. The ratio ROTI/σ _Φ, varies between 2 and 9. In addition, it is also found that there is a good consistency between the temporal variations of ROTI with scintillation activity under different ionospheric conditions. ROTI has a high correlation relationship with scintillation indices on geomagnetically disturbed days or in solar active months. Moreover, the data observed at low elevation angles have weak correlation between ROTI and scintillation indices. These results demonstrate the feasibility of using ROTI derived from GPS observations recorded by common non-scintillation GPS receivers to characterize ionospheric scintillations.     

Evaluation of three ionospheric delay computation methods for ground-based GNSS receivers

GNSS observables for ionospheric estimation are commonly based on carrier-to-code leveling (CCL) and precise point positioning (PPP) methods. The CCL method is a geometry-free method which uses carrier phase to level pseudorange observation for decreasing multipath error and observation noise. However, the ionospheric observable based on the CCL has been proven to be affected by leveling errors. The leveling errors are caused by pseudorange multipath and intraday variation of receiver DCB. To obtain more accurate ionospheric observable, the PPP method takes advantage of precise satellite-to-ground range for retrieving slant total electron content and is less affected by the leveling errors. Previous studies have only proven that the ionospheric observables extracted by the two methods are affected by the leveling errors. The influence on ionospheric observable by the pseudorange inter-receiver satellite bias (IRSB) of the receiver has not been taken into consideration. Also, the magnitude of the differences between the ionospheric observables extracted by the two methods has also not been given. In this work, three methods, namely, the CCL, the conventional ionospheric-free PPP method which uses the ionospheric-free Hatch–Melbourne–Wubbena (HMW) function, and the University of Calgary (UOFC) PPP method, are selected to analyze and compare the differences of ionospheric observables and the global ionospheric maps, using a large number of measured data from international GNSS service global stations. Experimental results show that the accuracy of ionospheric observables obtained by the three methods is not only related to the leveling error, but also pseudorange IRSB. The IRSB of the receiver exerts a major effect on the ionospheric observables obtained by the CCL method and a minor effect on the ionospheric observables obtained by the HMW and UOFC methods. The accuracies in the latter case are similar and superior to those obtained by the CCL. The differences of the ionospheric observables obtained by the CCL and UOFC methods, or the CCL and HMW methods, are at decimeter level, whereas the difference of the ionospheric observables obtained by the UOFC and HMW methods is at centimeter level. The UOFC method presented the highest single-frequency pseudorange positioning accuracy using estimated global ionospheric products, followed by the HMW and the CCL methods which presented the lowest positioning accuracy.     

Performance of the L2C civil GPS signal under various ionospheric scintillation effects

As GPS is modernizing, there are currently fourteen satellites transmitting L2C civil code and seven satellites transmitting L5 signal. While the GPS observables are subject to several sources of errors, the ionosphere is one of the largest error sources affecting GPS signals. Small irregularities in the electrons density along the GPS radio signal propagation path cause ionospheric scintillation that is characterized by rapid fluctuations in the signal amplitude and phase. The ionospheric scintillation effects are stronger in equatorial and high-latitude geomagnetic latitude regions and occur mainly due to equatorial anomaly and solar storms. Several researchers have analyzed the L2C signal quality since becoming available in December, 2005. We analyze the performance of L2C using GPS data from stations in the equatorial region of Brazil, which is subject of weak, moderate and strong ionospheric scintillation conditions. The GPS data were collected by Septentrio PolaRxS–PRO receivers as part of the CIGALA/CALIBRA network. The analysis was performed as a function of scintillations indexes S4 and Phi60, lock time (time interval in seconds that the carrier phase is tracked continuously without cycle slips), multipath RMS and position variation of precise point positioning solutions. The analysis shows that L2C code solutions are less affected by multipath effects than that of P2 when data are collected under weak ionospheric scintillation effects. In terms of analysis of positions, the kinematic PPP results using L2C instead P2 codes show accuracy improvements up to 33 % in periods of weak or strong ionospheric scintillation. When combining phase and code collected under weak scintillation effects, the results by applying L2C against P2 provide improvement in accuracy up to 59 %. However, for data under strong scintillation effects, the use of L2C for PPP with code and phase does not provide improvements in the positioning accuracy.     

A USRP2-based reconfigurable multi-constellation multi-frequency GNSS software receiver front end

We present a multi-constellation multi-band GNSS software receiver front end based on USRP2, a general purpose radio platform. When integrated with appropriate daughter boards, the USRP2 can be used to collect raw intermediate frequency (IF) data covering the entire GNSS family of signals. In this study, C++ class-based software receiver processing functions were developed to process the IF data for GPS L1, L2C, and L5 and GLONASS L1 and L2 signals collected by the USRP2 front end. The front end performance is evaluated against the outputs of a high end custom front end driven by the same local oscillator and two commercial receivers, all using the same real signal sources. The results show that for GPS signals, the USRP2 front end typically generates carrier-to-noise ratio (C/N ₀) at 1–3 and 1–2 dB below that of the high end front end and a NovAtel receiver, respectively. For GLONASS signals, the USRP2 C/N ₀ outputs are comparable to those of a Septentrio receiver. The carrier phase noise from the USRP2 outputs is similar to those of the benchmarking devices. These results demonstrate that the USRP2 is a suitable front end for applications, such as ionosphere scintillation studies.     

GPS双频接收机C/A码与P码伪距精度的分析和比较

本文主要研究GPS双频接收机C/A码和P码伪距的精度问题,首先阐述了保密P(Y)码伪距的测量原理,然后采用自行编制的精密单点定位程序,利用大量IGS跟踪站观测数据进行了试算,对该问题进行了探讨和分析,结论认为C/A码和P1码伪距的精度基本相同,而P2码伪距观测值的精度较低。     

Precision spacecraft navigation using a low-cost GPS receiver

Within the PROBA-2 microsatellite mission, a miniaturized single-frequency GPS receiver based on commercial-off-the-shelf (COTS) technology is employed for onboard navigation and timing. A rapid electronic fuse protects against destructive single-event latch-ups (SEL) and enables a quasi-continuous receiver operation despite the inherent sensitivity to space radiation. While limited to single-frequency C/A-code tracking with a narrow-band frontend, the receiver is able to provide precision navigation services through processing of raw GPS measurements on ground as well as a built-in real-time navigation system. In both cases, ionospheric path delays are eliminated through a combination of L1 pseudorange and carrier phase measurements, which also offers a factor-of-two noise reduction relative to code-only processing. By comparison with satellite laser ranging (SLR) measurements, a 0.3-m (3D rms) accuracy is demonstrated for the PROBA-2 reduced dynamic orbit determinations using post-processed GPS orbit and clock products. Furthermore, the experimental onboard navigation system is shown to provide real-time position information with a 3D rms accuracy of about 1?m, which notably outperforms the specification of the Standard Positioning Service (SPS). In view of their lower hardware complexity, mass budget and power requirements as well as the reduced interference susceptibility, legacy C/A-code receivers can thus provide an attractive alternative to dual-frequency receivers even for demanding navigation applications in low Earth orbit.     

Heading and Pitch Determination Using GPS/GLONASS

This article describes a single difference approach to estimate heading and pitch with a twin global positoning system (GPS)/GLONASS (GG) receiver system. Augmentation of GPS with GLONASS is not straightforward, however, because the latter system employs the frequency division multiple access technique to distinguish the signals form different satellites, rather than the code division multiple access technique used by GPS. The fact that each GLONASS signal has its own slightly different frequency makes the double difference (DD) of carrier phase observables no longer possible without modification. To get around this problem, the use of the between-receiver single difference (SD) of the carrier phase observables is proposed. In this case, however, receiver clock and other errors do not cancel out. The possibility of using a common external oscillator for the two receivers is explored. Remaining time and other biases are estimated using a low-pass averaging filter. The single difference integer ambiguities can then be resolved and the heading and pitch can be determined with a relatively good level of accuracy. Static and kinematic tests conducted with a pair of GPS/GLONASS receivers are used to validate the approach. Under reduced visibility, the combined GPS/GLONASS approach is shown to yield superior availability. ? 2000 John Wiley & Sons, Inc.     

IRNSS-1A: signal and clock characterization of the Indian regional navigation system

An initial characterization of the L5 and S-Band navigation signals transmitted by the first satellite of the Indian regional navigation satellite system (IRNSS) is presented. In the absence of a public signal specification, a 30 m high-gain antenna has been used to record the signal spectrum and the modulated chip sequences. For the IRNSS standard positioning service, use of a Gold ranging code is confirmed and relevant shift register parameters for the two frequencies are identified. Based on a prototype receiver, L5 single-frequency code and phase observations of IRNSS-1A have also been collected. The tracking performance is described, and the measurements are used to characterize the short-term clock stability of IRNSS-1A.     

Coherent GPS reflections from the sea surface

In this letter, we explore a method to obtain accurate ocean heights using measurements of the global positioning system (GPS) carrier phase after reflection on the sea surface. A carrier tracking algorithm is employed in measuring travel path differences between GPS direct and reflected signals collected from antennas suspended over a marine estuary, when roughness guarantees partially coherent reflections at the GPS L1 frequency. This technique proves to be sensitive to surface roughness and able to follow tide variations with a precision better than 5 cm (1-sigma) for sea states with significant wave heights below 10 cm. It is expected that this technique could be further extended to rougher sea states using GPS frequency combinations with longer synthetic wavelengths.     

Methodology for comparing two carrier phase tracking techniques

The carrier phase tracking loop is the primary focus of the current work. In particular, two carrier phase tracking techniques are compared, the standard phase tracking loop, i.e., the phase lock loop (PLL), and the extended Kalman filter (EKF) tracking loop. In order to compare these two different techniques and taking into consideration the different models adopted in each, it is important to bring them to one common ground. In order to accomplish this, the equivalent PLL for a given EKF has to be determined in terms of steady-state response to both thermal noise and signal dynamics. A novel method for experimentally calculating the equivalent bandwidth of the EKF is presented and used to evaluate the performance of the equivalent PLL. Results are shown for both the L1 and L5 signals. Even though the two loops are designed to track equivalent dynamics and to have equivalent carrier phase standard deviations, the EKF outperforms the equivalent PLL in terms of both the transient response and sensitivity.     

An all-pass filter for compensation of ionospheric dispersion effects on wideband GNSS signals

Compared with the traditional GPS L1 C/A BPSK-R(1) signal, wideband global navigation satellite system (GNSS) signals suffer more severe distortion due to ionospheric dispersion. Ionospheric dispersion inevitably introduces additional errors in pseudorange and carrier phase observations that cannot be readily eliminated by traditional methods. Researchers have reported power losses, waveform ripples, correlation peak asymmetries, and carrier phase shifts caused by ionospheric dispersion. We analyze the code tracking bias induced by ionospheric dispersion and propose an efficient all-pass filter to compensate the corresponding nonlinear group delay over the signal bandwidth. The filter is constructed in a cascaded biquad form based on the estimated total electron content (TEC). The effects of TEC accuracy, filter order, and fraction parameter on the filter fitting error are explored. Taking the AltBOC(15,10) signal as an example, we compare the time domain signal waveforms, correlation peaks, code tracking biases, and carrier phase biases with and without this all-pass filter and demonstrate that the proposed delay-equalization all-pass filter is a potential solution to ionospheric dispersion compensation and mitigation of observation biases for wideband GNSS signals.     

Effects of building materials on UHF ranging signals

Positioning and tracking inside buildings using wireless location methods has recently received considerable attention. One method is the estimation of the time of arrival of satellite or terrestrial-based radio frequency signals in order to produce ranges from the transmitter to the receiver. This paper investigates the effect of walls made of various construction materials on the penetration of ranging signals. In the tests conducted, a GPS pseudo-satellite transmits a pseudorandom noise code at a carrier frequency of 1.57542 GHz. After penetrating walls made of plywood, gyprock and cinder blocks, the signal is received by a GPS receiver which measures the carrier phase and the pseudorange between the transmitter and receiver using code correlation. Compared to measurements obtained with no walls present, the effect on the measurements is determined. The results indicate that the carrier phase measurement is superior to the pseudorange in terms of noise, stability, and the magnitude of the effect of the walls on the range obtained. Ranges based on the carrier phase only change by a few centimeters. Given that the integer number of cycles of the phase can be determined and phase lock is maintained, the carrier phase measurement shows considerable promise for indoor ranging.