Carrier Doppler Shift Research Articles

Fast GPU based acquisition with minimum loss and downsampling techniques for weak BeiDou SBAS signals

AbstractFor a receiver of the BeiDou Satellite Based Augmentation System (BDSBAS), the acquisition aims to coarsely estimate the pseudo‐code delays and carrier Doppler shifts of the received signals. The use of downsampling techniques to accelerate this acquisition process for weak signals has been an emerging topic. However, the acquisition probabilities of conventional algorithms with downsampling are inadequate and need to be enhanced to prevent excessive false alarms due to degradation of the peak signal‐to‐noise ratios (PSNRs) during acquisition. To improve signal acquisition performance, we introduce an enhancer function before downsampling to achieve the minimum PSNR loss during acquisition. Then, we use graphic processing unit (GPU) functions to speed up the processing loop, matrix multiplication and fast Fourier transform operations. We thus present a fast GPU‐based acquisition method by using minimum loss and downsampling techniques. Our experimental results show the remarkable effectiveness of our method for receiving weak BDSBAS signals as compared to legacy downsampling algorithms. This work seeks not only to solve the current issue of achieving fast acquisition of weak BDSBAS signals but also to inspire creative new ideas that spur further advances in this promising field in the future.

Navigation Using Doppler Shift From LEO Constellations and INS Data

Absolute navigation using the downlink communication signals of massive low Earth orbit (LEO) constellations augmented by data from an inertial navigation system (INS) is considered. Lack of atomic clocks on-board LEO satellites requires the use of Doppler shift as a navigation observable. Furthermore, footprint limits of downlink signals’ beams necessitate the use of a tightly coupled INS/radio-navigation filter in order to compensate for the fact that it is not possible to measure carrier Doppler shift simultaneously from eight or more satellites. The navigation solution error is shown for a simulation, which demonstrates the feasibility of combining measurements from LEO constellations and INS data to enable global navigation satellite systems-like navigation capabilities. Additional simulations study how navigation solution error is affected by IMU quality, signal availability, downlink spot beam steering protocol, and constellation characteristics.

Navigation using carrier Doppler shift from a LEO constellation: TRANSIT on steroids

A new global navigation concept is studied that relies on carrier Doppler shift measurements from a large LEO constellation. This system could provide an alternative to pseudorange-based GNSS. The concept uses a high-fidelity model of received carrier Doppler shift. This model is used in a point-solution batch filter that simultaneously estimates eight unknowns: the three position vector components, receiver clock offset, three velocity vector components, and receiver clock offset rate. The filter uses eight or more measured Doppler shifts in its least-squares fit. A generalized Geometric Dilution of Precision (GDOP) analysis indicates that absolute position accuracies on the order of 1-5 meters and absolute velocity accuracies on the order of 0.01 m/sec to 0.05 m/sec may be achievable if the range-rate precision of the Doppler shift measurements is 0.01 m/sec. These accuracies are comparable to current pseudorange-based GNSS. Clock offset accuracy is on the order of 0.0001 to 0.0010 sec 1-.

Asynchronous Lift-Off Spoofing on Satellite Navigation Receivers in the Signal Tracking Stage

As Global Navigation Satellite System (GNSS) spoofing techniques are highly stealthy and pose a tremendous risk to targets using GNSS technology, studies on GNSS spoofing techniques have been in the spotlight. If the accurate position and velocity of the target receiver can be obtained, the target receiver can be covertly spoofed during the signal tracking stage using synchronous lift-off spoofing. However, it is often difficult to accurately obtain the position and velocity of a target in real GNSS spoofing scenarios. To address this problem, To study the effects of spoofing signals’ power (relative to the real signal), code pulling rate, carrier Doppler shift, initial code phase difference, and carrier phase difference on the efficacy of spoofing, the intrusion of receiver’s signal tracking loop by spoofing signals is mathematically modeled. Based on the model, an asynchronous lift-off spoofing for GNSS receivers in the signal tracking stage is proposed. Theoretical analysis and experimental results show that the new method resulted in stable Doppler frequency variations, short fluctuations in carrier-to-noise ratio (C/N) and signal lock time, and gentle changes to the receiver’s 3D Earth-Centered Earth Fixed (ECEF) coordinates, when the target’s position and velocity were approximately known during the intrusion period. The proposed spoofing method is highly feasible and could expand the scope of applicability of lift-off spoofing.

GNSS Multipath Mitigation using Antenna Motion

A method is developed for characterizing and compensating specular GNSS multipath by considering signal amplitude and phase variations in response to antenna motion. This method seeks to improve multipath rejection capabilities. A known antenna motion profile is combined with a batch filter to estimate the code phase, carrier phase, and amplitude of the direct signal, the relative code phase, carrier phase, carrier Doppler shift, and amplitude of each significant multipath component, and the directions of arrival of the direct and multipath components. Hardware signal simulator results indicate accurate estimation of all multipath features, and experimental test results demonstrate measurable improvements in estimates of carrier amplitude in all cases, of direct-path carrier-phase in many cases, and of direct-path code phase in some cases. Anomalous results have also been obtained, including specular multipath component estimates that occur in doublets and that may represent attempts by the batch filter to approximate diffuse multipath. Copyright © 2015 Institute of Navigation.

Signal search methods for space-based GNSS receivers. Part 1. Combined search

The paper analyzes the features of GPS and GLONASS signal reception by space-based receivers, which should be considered in designing signal search algorithm. Brief review of major signal search schemes used in GPS/GLONASS receivers is presented. It is demonstrated that none of the existing search schemes meets the requirements on the space-based receivers. A combined approach is proposed, where the most suitable search scheme is selected depending on a priori information on the carrier Doppler shift.

A scheme for weak GPS signal acquisition aided by SINS information

In order to enhance the acquisition performance of global positioning system (GPS) receivers in weak signal conditions, a high-sensitivity acquisition scheme aided by strapdown inertial navigation system (SINS) information is proposed. The carrier Doppler shift and Doppler rate are pre-estimated with SINS aiding and GPS ephemeris, so that the frequency search space is reduced, and the dynamic effect on the acquisition sensitivity is mitigated effectively. Meanwhile, to eliminate the signal-to-noise ratio gain attenuation caused by data bit transitions, an optimal estimation of the unknown data bits is implemented with the Viterbi algorithm. A differential correction method is then utilized to improve the acquisition accuracy of Doppler shift and therefore to meet the requirement of carrier-tracking loop initialization. Finally, the reacquisition experiments of weak GPS signals are implemented in short signal blockage situations. The simulation results show that the proposed scheme can significantly improve the acquisition accuracy and sensitivity and shorten the reacquisition time.

Kalman Filter Tracking of Limb Scan Signal using a Bank of Correlators

A combined phase-locked loop/delay-locked loop (PLL/DLL) algorithm is developed for tracking Global Navigation Satellite System (GNSS) carrier phase and code phase using the output from a large number of correlators. This approach has advantages for limb-scanning applications, in which useful meteorological information, available only at the initial rising time of a GPS satellite, is desired. The technique uses a bank of correlators to span wide ranges of uncertainty in code phase and carrier Doppler shift, thereby avoiding the need for a separate acquisition and the associated loss of an initial span of data. A fusion of optimal estimation methods processes the output from these correlators. A batch optimization of a signal model's fit at a point in time to many accumulations from the correlator bank provides a Kalman filter with measurements of the most likely signal parameters. The Kalman filter utilizes a signal dynamics model to provide estimates that drive the PLL and DLL. The effectiveness of this algorithm is demonstrated by using a truth-model simulation of a limb scan.

Monitoring Water Levels and Currents Using Reflected GPS Carrier Doppler Measurements and Coordinate Rotation Model

This paper describes the development and application of a highly integrated Global Positioning System (GPS) receiver that employs reflected GPS signals to measure the floodwater levels, sea levels, and soil moisture of riverbeds. Both right- and left-hand circular polarization antennas are employed to simultaneously obtain direct and reflected signals. The objective of this paper is to use the carrier-phase Doppler shifts reflectivity of L1 and L2 band reflected signals and direct signals to determine the floodwater and sea levels in riverbeds, as well as to distinguish wet soil from bare soil. In monitoring coastal tidal currents, water levels, and floodwater levels, the reflection heights of this system are accurate to within 29 ~ 31 cm. Furthermore, the currents at reflection points are estimated using differential carrier Doppler shifts and a coordinate rotation correction model, which provide velocity vectors for flows speed of approximately 5 ~ 265 cm/s.