Steffen Thoelert

A multi-technique approach for characterizing the SVN49 signal anomaly, part 1: receiver tracking and IQ constellation

A characterization of the signal anomaly of SVN49 is presented. A mathematical model is developed to relate the observed multipath to the internal signal reflection. The analyses provided are based on measurements, which have been collected during a dedicated tracking campaign with a 30-m dish antenna. Data on the L1 and L2 frequency have been collected with four different receivers. In addition, IQ samples have been recorded directly with a spectrum analyzer. The multipath combination of the receiver measurements on L1 and L2 is analyzed to demonstrate the effect of the signal reflections on different correlator spacing. The capability to suppress the signal reflection with receiver multipath mitigation methods is demonstrated. Finally, preliminary estimates of the attenuation, delay, and phase shift over elevation are obtained from an IQ sample analysis.

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.

GNSS Satellite Transmit Power and its Impact on Orbit Determination

Antenna thrust is a small acceleration acting on Global Navigation Satellite System satellites caused by the transmission of radio navigation signals. Knowledge about the transmit power and the mass of the satellites is required for the computation of this effect. The actual transmit power can be obtained from measurements with a high-gain antenna and knowledge about the properties of the transmit and receive antennas as well as losses along the propagation path. Transmit power measurements for different types of GPS, GLONASS, Galileo, and BeiDou-2 satellites were taken with a 30-m dish antenna of the German Aerospace Center (DLR) located at its ground station in Weilheim. For GPS, total L-band transmit power levels of 50–240 W were obtained, 20–135 W for GLONASS, 95–265 W for Galileo, and 130–185 W for BeiDou-2. The transmit power differs usually only slightly for individual spacecraft within one satellite block. An exception are the GLONASS-M satellites where six subgroups with different transmit power levels could be identified. Considering the antenna thrust in precise orbit determination of GNSS satellites decreases the orbital radius by 1–27 mm depending on the transmit power, the satellite mass, and the orbital period.

Using of spirent GPS/Galileo HW simulator for timing receiver calibration

In this paper the absolute calibration of GPS time receivers with a Spirent GPS/Galileo HW simulator is analyzed and described in detail. The primary step is to calibrate the simulator itself. In this context the most important facts and values which have to be measured are explained. After the calibration of the simulator the receiver is fed with satellite signals generated by the simulator belonging to a standard GPS satellite constellation to determine the absolute offset of the receiver (internal delay) compared to the simulator output. The used GNSS hardware simulator provides GPS and Galileo signals in parallel and therefore additionally can be used for absolute calibration of combined GPS/Galileo or stand alone Galileo time receivers. As there are no such receivers available yet, the precision of the GPS and Galileo pseudoranges determined by a combined GPS/Galileo receiver is analyzed.

Signal in space (SIS) analysis of new GNSS satellites

In the last 12 month a number of major milestones were reached by different satellite navigation systems. New satellites of Galileo, GLONASS and COMPASS are in space and can be used for navigation. Especially for the Galileo system has been started an important phase with the launch of the satellites FM-3 and FM-4 in October 2012. Consequently, there are 4 satellites of the Galileo system in orbit and the navigation performance can be tested in real environments on ground for the first time. Although for most of the systems mentioned above the expected signals according to textbooks or ICDs are known, in reality due to imperfections several deviations can be seen. Therefore and, especially, since the complexity of the satellites and also the requirements for a precise and robust navigation are constantly rising, all of the newly available signals of the existing or emerging navigation satellite systems have to be analysed in detail in order to characterize their performance and imperfections as well as to predict possible consequences for user receivers. Since the signals are well below the noise floor, the authors use a specifically developed GNSS monitoring facility in order to characterize the GNSS signals. The core element of this monitoring facility is a 30m high-gain antenna at DLR / Weilheim which raises the GNSS signals well above the noise floor allowing detailed analysis. Doing this analysis differences in the signal quality were found in the different generations of the Chinese navigation satellite system COMPASS, which show influences on the navigation performance. The paper shows an overview of new navigation satellites in orbit. The fully renewed and modernized DLR GNSS monitoring facility is introduced which now allows coherent capturing of two signals at a time. Doing so inter-frequency analysis or axial ratio behaviour of the transmitted satellite signals are possible. For selected satellites a detailed signal analysis is performed revealing important characteristics of these signals. The acquired high gain antenna raw data in combination with a precise calibration are used for a wide range of analyses i.e. signal power, spectra, constellation diagrams to detect anomalies and assess the signal quality. In addition the signal quality not only of selected new satellite will assessed but also the signal quality development over generations will discussed based on an example of the COMPASS/Beidou system with new results of the satellites M4 and M5.

Ionospheric deformation of broadband GNSS signals and its analysis with a high gain antenna

The ionospheric delay of global navigation satellite systems (GNSS) signals typically is compensated by adding a correction value to the pseudorange measurement. We examine the ionospheric signal distortion beyond a constant delay. These effects become increasingly significant with increasing signal bandwidth and hence more critical for the new broadband navigation signals. By simulation, we first demonstrate that the signal modulation constellation diagram is particularly susceptible to the influence of the ionosphere already at moderate electron content. Using high gain antenna measurements of the Galileo E5 signal, we then verify that the expected influence can indeed be observed and compensated. A new method based on a binned maximum likelihood estimator is derived to estimate the total electron content (TEC) from a single frequency high gain antenna measurement of a broadband GNSS signal. Results of the estimation process are presented and discussed comparing to common TEC products such as TEC maps and dual-frequency receiver estimates.

Multicorrelator signal tracking and signal quality monitoring for GNSS with extended Kalman filter

GNSS signals may present anomalies that degrade the positioning performance of GNSS receivers. Signal Quality Monitoring (SQM) is normally used to detect and to characterize these anomalies. This is required for the GNSS operators and integrity services to determine when a satellite should be considered as faulty and draw conclusions about the type of the fault. In this paper, we present a new SQM algorithm that tracks the GNSS signal and possible channel deformations by using a novel methodology based on the Extended Kalman Filter (EKF). The EKF is designed such that the measurement update is performed in post-correlation and using multiple correlators. After the estimation of the channel response, we add a detection step to determine if the channel deviates from the nominal signal transmission scenario (i.e., the single path propagation). Results suggests that the performance of the delay estimation with the proposed EKF structure outperforms the classical Delay-Locked-Loop (DLL) estimation, especially in the presence of distortions. Furthermore, it can reliably detect anomalous signal deformations as specified by ICAO threat model.

Detection and mitigation of interference in the calibration of high gain antennas for GNSS

In the field of Global Navigation Satellite System (GNSS) a robust and dependable navigation is one of the most challenging task for modern space missions. To accomplish this purpose, DLR uses the high gain ground antenna, a 30 meter dish located in Weilheim. The calibration of the big antenna is performed using the radio sources emission, such as Cassiopea-A, Taurus-A, or Cygnus-A This method has been used by radio astronomers for years, but it shows some impairments when the link is obstructed by the presence of interference. Therefore, the estimation of the final gain of the antenna may be affected by some errors. The aim of this paper is to present a post processing algorithm, which detects the corrupted frequencies, and later on mitigates the effect of the interference. The gain estimation is compared before and after the application of the algorithm, showing some improvements in correspondence of the interference occurrence.