MEDLL On-Strobe Correlator: A Combined Anti-multipath Technique for GNSS Signal Tracking

To reduce the operational complexity of the multipath estimating delay lock loop (MEDLL) and improve its anti-multipath performance for strobe correlators, a combination anti-multipath scheme, namely, the MEDLL on-strobe correlation technique, is proposed for global navigation satellite system (GNSS) signal processing. Short-delay multipath rays are separated from the strobe correlation function by the MEDLL mechanism; the dot product between the estimation residue and the standard correlation function or the BOC-PRN correlation function is then computed to eliminate the potential tracking ambiguity. Finally, this non-coherent combination result is sent to the loop filter to obtain anti-multipath code tracking. The proposed method is analysed via simulator data with a software receiver under different front-end bandwidth conditions. The results corroborate the better multipath mitigation capability and lower computational burden, although it is still difficult to eliminate all multipath interference, especially when the front-end bandwidth is insufficient.

Design an Adjustable Narrow Correlator to Track GPS Signals

Multipath signals problem is still the main challenge for the global positioning system (GPS) receiver to obtain an accurate localization measurement. However, many techniques have been developed to mitigate the impact of these undesired signals at the expense of adding a burden to the GPS receiver design. Even though, none of the techniques in the works of literature was able to solve the loose signal lock issue efficiently. In this piece of work, we used a new approach to maintain the tracking synchronization when the receiver moves from outdoors to indoors /harsh environment. We design a dynamic narrow correlator in the delay lock loop, which made the space between the early and late correlators adjusted according to the receiver’s location. Consequently, this will enable the receiver to keep tracking the received GPS signal in a challenging environment and reduce the thrashing the available resources (power/processing) to acquire the GPS signal again. The results show that the ranging error has been reduced in comparison with a traditional narrow correlator, as well as the bit transition in the in-phase arm has been also maintained.

SATELLITE ALTIMETER DATA FILTERING AND SMOOTHING IN THE COURSE OF GROUND-BASED RETRACKING

Introduction. Satellite radar altimeter is an essential part of the Earth remote sensing space missions. Satellite altimeter on-board delay-lock loop, by a widely shared concept, is operationally just a tool of a reliable retaining of received echo-signal within the tracking window, while “fine” altimetric parameter (orbit height, significant wave height, scattering cross section per unit of a probed surface, etc.) measuring is committed to the ground-based retracking of data. In particular, in the course of retracking altimeter data are being filtered and/or smoothed.Objective. The paper subject is study of retracking algorithms of altimeter data transmitted from the space vehicle to the ground segment.Methods and materials. It is known that data filtering already presents on-board the space vehicle and is implemented in delay-lock loop based on the α–β-filter. However, at the stage of ground-based retracking it seems more appropriate to use the Kalman filter, which possesses a number of theoretical optimal features and is efficient as for utilization of the available computational resource.Results and conclusions. In the paper implementation of filtering and smoothing via Kalman algorithm is described. On the ground of computer simulation data it is stated that Kalman filtering and smoothing make estimate accuracy two and more times higher depending on significant wave height.

ESTIMATION OF SATELLITE ALTIMETER ECHO-SIGNAL PARAMETERS BY STATISTICAL FITTING METHODS IN THE COURSE OF RETRACKING

Satellite radar altimeters play a key role in numerous space missions for the remote Earth sensing. The data they provide are used in solving various fundamental and applied problems of geophysics, oceanography, meteorology, etc. In many modern measuring systems the altimeter data is processed in several stages. One of them is the ground-based retracking of the information streamed from the spacecraft. The goal of this work is to study altimeter parameter estimators in the course of the satellite altimeter data retracking by ground-based complex. The main task of delay-lock loop onboard a satellite carrier is a reliable keeping of received echo-signal within the tracking window. More accurate estimates of information parameters are worked out by the ground segment where data from the satellite is delivered via telemetry. Retracking can be performed either without using any physical echo model, or relying on some mechanism under generation of response of an illuminated surface to the probing signal. In the latter case, the measuring results become more trustworthy. The paper deliberations are based on the model close to the classical Brown’s one, they include algorithms of its parameters statistical fitting to the observation and computer simulation of fitting according to the maximal likelihood (ML) and the least squares (LS) methods. The results obtained are compared to the potential attainable and show that while LS fitting yields noticeably to the potential, experimental accuracy of ML-fitting practically coincides with the potential one.

Spoofing Attack Results Determination in Code Domain Using a Spoofing Process Equation

When a user receiver is tracking an authentic signal, a spoofing signal can be transmitted to the user antenna. The question is under what conditions does the tracking point of the receiver move from the authentic signal to the spoofing signal? In this study, we develop a spoofing process equation (SPE) that can be used to calculate the tracking point of the delay lock loop (DLL) at regular chip intervals for the entire spoofing process. The condition for a successful spoofing signal is analyzed using the SPE. To derive the SPE, parameters, such as the signal strength, sweep velocity, loop filter order, and DLL bandwidth are considered. The success or failure of a spoofing attack is determined for a specific spoofing signal using the SPE. In addition, a correlation between each parameter for a successful spoofing attack could be obtained through the SPE. The simulation results show that the SPE performance is largely consistent with that of general DLL methods, even though the computational load of SPE is very low.