A Graphical Approach to GPS Software-Defined Receiver Implementation

Global positioning system (GPS) software-defined receivers (SDRs) offer many advantages over their hardwarebased counterparts, such as flexibility, modularity, and upgradability. A typical GPS receiver is readily expressible as a block diagram, making a graphical approach a natural choice for implementing GPS SDRs. This paper presents a real-time, graphical implementation of a GPS SDR, consisting of two modes: acquisition and tracking. The acquisition mode performs a twodimensional fast Fourier transform (FFT)-based search over code offsets and Doppler frequencies. The carrier-aided code tracking mode consists of the following main building blocks: correlators, code and carrier phase detectors, code and carrier phase filters, a code generator, and a numerically-controlled oscillator. The presented GPS SDR provides an abstraction level that enables future research endeavors.Aerospace Engineering and Engineering Mechanic

Analysis of Ionospheric Scintillations using Wideband GPS L1 C/A Signal Data

A non-real-time GPS receiver has been developed and tested for use in scintillation analysis. The receiver consists of a digital storage receiver and non-real-time software acquisition and tracking algorithms. The goal of this work is to shed light on the behavior of strongly scintillating signals: signals which cause conventional GPS receivers to lose carrier lock. The receiver collects wideband GPS L1 digital data sampled at 5.7 MHz using an RF front-end and stores it on disk for post-processing. It processes the data off-line to determine carrier signal amplitude and phase variations during scintillations. The main processing algorithms are traditional code delay and carrier frequency acquisition algorithms and special signal processing algorithms that effectively function as a delay-locked loop and phase-locked loop. The tracking algorithms use non-causal smoothing techniques in order to optimally reconstruct the phase and amplitude variations of a scintillating signal. These techniques are robust against the deep power fades and strong phase fluctuations characteristic of scintillating signals. To test the receiver, scintillation data were collected in Cauchoeira Paulista, Brazil, from December 4 to 6, 2003. The data set spans several hours and includes times when one or more satellite signals are scintillating. The smoothing algorithm has been used to determine the carrier amplitude and phase time histories of the scintillating signals along with the distortion of the pseudorandom noise (PRN) code’s autocorrelation function. These quantities provide a characterization of scintillation that can be used to study the physics of scintillations or to provide off-line test cases to evaluate a tracking algorithm’s ability to maintain signal lock during scintillations.Aerospace Engineering and Engineering Mechanic

Phoenix-XNS - A Miniature Real-Time Navigation System for LEO Satellites

The paper describes the development of a miniature GPS receiver with integrated real-time navigation system for orbit determination of satellites in low Earth orbit (LEO). The Phoenix-XNS receiver is based on a commercial-off-the-shelf (COTS) single-frequency GPS receiver board that has been qualified for use in a moderate space environment. Its firmware is specifically designed for space applications and accounts for the high signal dynamics in the acquisition and tracking process. The supplementary eXtended Navigation System (XNS) employs an elaborate force model and a 24-state Kalman filter to provide a smooth and continuous reduced-dynamics navigation solution even in case of restricted GPS availability. Through the use of the GRAPHIC code-carrier combination, ionospheric path delays can be fully eliminated in the filter, which overcomes the main limitation of conventional single-frequency receivers. Tests conducted in a signal simulator test bed have demonstrated a filtered navigation solution accuracy of better than 1 m (3D rms)

Scintillation on global navigation satellite signals and its mitigation

PhD ThesisThe scintillation effects on the Global Positioning system (GPS) or other GNSS (global navigation satellite system) receivers have been investigated by many researchers and several mitigation strategies have been proposed in this regard but the problem is not yet fully solved. This thesis covers the investigation of scintillation effects on GPS receivers and developing a mitigation approach which can play an important role in mitigating the effects of scintillation on these and other GNSS receivers. Firstly, a new GPS signal acquisition method known as the repetitive block acquisition (RBA) is presented which can be used to speed up the GPS signal acquisition in case fast acquisition is required. This acquisition method is implemented using coarse-acquisition (C/A) codes and tested by collecting real GPS data. The RBA method can also be used for other codes as well. It is rather difficult to show that how scintillation affects the acquisition process in a GPS receiver because mostly it results in tracking loop loss of lock due to cycle slip. However, during strong amplitude scintillation which is usually most important at low or near-equatorial latitudes, deep power fades resulting from amplitude scintillation result in the selection of long data records which leads to slow acquisition due to long acquisition times. It is shown in this thesis that, by using the RBA method, the acquisition time can be reduced to a fairly low level by reducing the number of computations involved in acquisition compared to other well-known methods such as the parallel FFT-based method and zero padding method (ZP). Secondly, the scintillation effects on the GPS tracking loop have also been investigated in this thesis and, based on this investigation, a new improved analogous phase scintillation index, σw φa, has been designed to more accurately represent the phase scintillation intensity at European high latitudes. This is then also validated using the real GPS data from Trondheim, Norway (63.41o N, 10.4o E). The σw φa uses dual frequency (L1 & L2) based vi time and spatial variations of total electron contents (TEC) at 1 Hz for estimating the phase scintillation values. For deriving the σw φa, the low frequency TEC fluctuations due to Doppler shift of the satellite/receiver motion and also due to the slowly varying background ionosphere need to be removed in order to consider only the high frequency TEC fluctuations which are responsible for scintillation due to the fast moving electron density irregularities which is done by using the wavelet transform. The σw φa is really an improved version of σφa where, rather than using time-invariant digital high pass filters (HPF), which according to several researchers are in-appropriate for filtering the non-stationary raw GPS signals affected by the ionospheric scintillation, a wavelet-based filtering technique is used. Although, the wavelet transform has been used previously in detrending raw amplitude and phase observations at 50 Hz for estimating the scintillation indices (amplitude and phase), due to the high sample data rate it may not be desirable to use this transform due to its very high computational cost. Since, σw φa uses TEC data at 1 Hz so this problem has been overcome. The performance of the new improved index (σw φa) is investigated and is also compared with the previously proposed σφa and σφ indices using one whole year of data from a GPS receiver at Trondheim, Norway (63.41o N, 10.4o E). The raw TEC observations and the σw φa index are then used in estimating the tracking phase jitter using two different methods. The phase jitter helps in defining the tracking thresholds for the tracking loops in a receiver which is useful in updating the tracking loop parameters during scintillation conditions as required in robust GPS/GNSS receiver designs because the phase jitter decides how wide the tracking (and thus the noise) bandwidth should be allowed in the tracking loop for the tracking to remain efficient. It is shown that if the phase jitter is estimated using the new proposed methods, generally a better estimate can be obtained compared to the previously proposed phase jitter estimation methods which employs σφa and σφ indices. These new phase jitter estimation methods can further be used in GPS/GNSS receivers for updating the tracking loop parameters during scintillation conditions and hence can serve as a good alternative for mitigating the effects of scintillation on GPS/GNSS receivers.Higher Education Commission (HEC) of Pakistan and the Sukkur Institute of Business Administration, Pakistan

Benchmarking CPUs and GPUs on embedded platforms for software receiver usage

Smartphones containing multi-core central processing units (CPUs) and powerful many-core graphics processing units (GPUs) bring supercomputing technology into your pocket (or into our embedded devices). This can be exploited to produce power-efficient, customized receivers with flexible correlation schemes and more advanced positioning techniques. For example, promising techniques such as the Direct Position Estimation paradigm or usage of tracking solutions based on particle filtering, seem to be very appealing in challenging environments but are likewise computationally quite demanding. This article sheds some light onto recent embedded processor developments, benchmarks Fast Fourier Transform (FFT) and correlation algorithms on representative embedded platforms and relates the results to the use in GNSS software radios. The use of embedded CPUs for signal tracking seems to be straight forward, but more research is required to fully achieve the nominal peak performance of an embedded GPU for FFT computation. Also the electrical power consumption is measured in certain load levels.Peer ReviewedPostprint (published version

Assessing the Spoofing Threat: Development of a Portable GPS Civilian Spoofer

A portable civilian GPS spoofer is implemented on a digital signal processor and used to characterize spoofing effects and develop defenses against civilian spoofing. This work is intended to equip GNSS users and receiver manufacturers with authentication methods that are effective against unsophisticated spoofing attacks. The work also serves to refine the civilian spoofing threat assessment by demonstrating the challenges involved in mounting a spoofing attack.Aerospace Engineering and Engineering Mechanic