Johann Furthner

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.

Kalman filter approaches for a mixed clock ensemble

Clock ensembling is a promising concept for future time scale generation as robustness and stability can be considerably improved compared to master clock approaches. While ensembles consisting of the same clock types were already demonstrated to improve robustness of the generated time scale, ensembles using different clock types can clearly benefit from the advantages of the individual single clocks contributing to the time generation process. In particular, the performance of the ensemble time can be improved over a wide range of averaging times by combining clocks exhibiting superior stabilities for distinct time scales. In future, this approach could be used to integrate newly developed clock types such as e.g. optical clocks into a clock ensemble.

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.