By Steffen Thoelert, Johann Furthner, and Michael Meurer
Future positioning and navigation applications of modernizing and newly established GNSSs will require a higher degree of signal accuracy and precision. Thus, rigorous and detailed analysis of the signal quality of recently launched satellites, including the discovery of any possible imperfections in their performance, will have important implications for future users.
Global navigation satellite systems achieved amazing progress in 2012, with major milestones reached by the various navigation and augmentation systems, bringing new satellites and satellite generations into orbit. Since the complexity of the satellites and also the requirements for a precise and robust navigation increase consistently, all of the newly available signals of the existing or emerging navigation satellite systems must be analyzed in detail 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, we use a specifically developed GNSS monitoring facility to characterize the signals. The core element of this monitoring facility is a 30-meter high-gain antenna at the German Aerospace Center (DLR) in Weilheim that raises GNSS signals well above the noise floor, permitting detailed analysis. In the course of this analysis, we found differences in the signal quality in the various generations of the Chinese navigation satellite system BeiDou, differences which influence the navigation performance.
This article gives an overview of new navigation satellites in orbit. For selected satellites, a first signal analysis reveals important characteristics of these signals. The data acquisition of these space vehicles was performed shortly after the start of their signal transmission to get a first hint about the quality and behavior of the satellites.
For more detailed analysis, these measurements should be repeated after the satellites become operational. Then the acquired high-gain antenna raw data in combination with a precise calibration could be used for a wider range of analyses: signal power, spectra, constellation diagrams, sample analysis, correlation functions, and codes to detect anomalies and assess the signal quality and consequently the impact at the user performance.
In the early 1970s, DLR built a 30-meter dish (Figure 1) for the HELIOS-A/B satellite mission at the DLR site Weilheim. These satellite missions were the first U.S./German interplanetary project. The two German-built space probes, HELIOS 1 (December 1974–March 1986) and HELIOS 2 (January 1976–January 1981), approached the Sun closer than the planet Mercury and closer than any space probe ever. Later, the antenna supported space missions Giotto, AMPTE, Equator-S, and other scientific experiments.
In 2005, the Institute of Communications and Navigation of the DLR established an independent monitoring station for analysis of GNSS signals. The 30-meter antenna was adapted with a newly developed broadband circular polarized feed. During preparation for the GIOVE-B in-orbit validation campaign in 2008, a new receiving chain including a new calibration system was installed at the antenna. Based on successful campaigns and new satellite of modernizing GPS and GLONASS, and GNSSs under construction — Galileo and COMPASS — the facility was renewed and updated again in 2011/2012.
This renewal included not only an upgrade of the measurement system itself, but also refurbishment of parts of the high-gain antenna were refurbished.
The antenna is a shaped Cassegrain system with an elevation over azimuth mount. The antenna has a parabolic reflector of 30 meters in diameter and a hyperbolic sub-reflector with a diameter of 4 meters. A significant benefit of this antenna is the direct access to the feed, which is located within an adjacent cabin (Figure 2). The L-band gain of this high-gain antenna is around 50 dB, the beam width is less than 0.5°. The position accuracy in azimuth and elevation direction is 0.001°. The maximum rotational speed of the whole antenna is 1.5°/second in azimuth and 1.0°/second in elevation direction.
The antenna offers another significant advantage in the possibility to have very short electrical and high-frequency connection between the L-band feeder and the measurement equipment. As mentioned earlier, the challenge for future GNSS applications is the high accuracy of the navigation solution. Therefore, it is necessary to measure and then analyze the signals very accurately and precisely. To achieve an uncertainty of less than 1 dB for the measurement results required a complete redesign of the setup, which consists of two main parts:
- paths for signal receiving and acquiring the measurement data;
- calibration elements for different calibration issues.
The path for receiving the signal and acquiring the measurement data consists of two signal chains, each equipped with two low-noise amplifiers (LNAs) with a total gain of around 70 dB, a set of filters for the individual GNSS navigation frequency bands, and isolators to suppress reflections in the measurement system. With this setup it is possible to measure right-hand circular polarized (RHCP) and left-hand circular polarized (LHCP) signals in parallel.
This provides the capability to perform axial ratio analysis of the satellite signal, and consequently an assessment of the antenna of the satellite. Using the switches SP01 and SP02, the measurement system is also able to acquire data from two different bands at the same time. This enabless investigations concerning the coherence between the signals in post-processing.
The signals are measured and recorded using two real-time vector signal analyzers with up to 120 MHz signal bandwidth. Both analyzers are connected to a computer capable of post-processing and storing the data. Additional equipment like digitizers or receivers can be connected to the system using the splitter III outputs, where the unfiltered RHCP signals are coupled out after the first LNA. A high-performance rubidium clock is used as reference signal for the whole measurement equipment. In front of the first LNA of each chain, a signal can be coupled in for calibration issues.
Control Software. Due to the distance of the antenna location from the Institute at Oberpfaffenhofen (around 40 kilometers) it was necessary to perform all measurement and calibration procedures during a measurement campaign via remote control. A software tool was developed which can control any component of the setup remotely. In addition, this software can perform a complete autonomous operation of the whole system by a free pre-definable sequence over any period of time. This includes, for example, the selection of the different band-pass filters, the polarization output of the feed, and the control of the calibration routines.
After the measurement sequence, the system automatically copies all data via LAN onto the processing facility, starts basic analysis based on spectral data, and generates a report. Sophisticated analysis based on IQ raw data is performed manually at this time.
To fulfill the challenge of highly accurate measurements, it is necessary to completely characterize all elements of the measurement system, which comprises the antenna itself and the measurement system within the cabin after the feed. An absolutely necessary precondition of the calibration of the high-gain antenna is a very accurate pointing capability. The pointing error should be less than 0.01° concerning antennas of this diameter.
Furthermore, it is important to check long-term stability of these characterizations and the influences of different interference types and other possible error sources. This has to be taken in to account, when it comes to a point where the value of the absolute calibration has the same range as the summed measurement uncertainties of the equipment in use.
Antenna Calibration. High-accuracy measurements require not only the correct antenna alignment but also accurate power calibration of the antenna. To determine the antenna gain, well known reference sources are needed. These could be natural sources like radio stars or artificial sources like geostationary satellites.
Standard reference signal sources for the calibration of high-gain antennas are the radio sources Cassiopeia A, Cygnus, and Taurus. All these radio sources are circumpolar relative to our ground station, and therefore usable for calibrations at all times of the year. A further advantage of these calibration sources is the wide frequency range of the emitted signals. Thus, contrary to other signal sources (like ARTEMIS satellite L band pilot signal) the antenna gain can be calibrated in a wide bandwidth. With the help of the well-known flux density of the celestial radio sources and using the Y-method, the relation between the gain of the antenna and the noise temperature of the receiving system, or G/T, can be measured. Measuring the noise figure of the receiving system, the antenna gain can finally be calculated.
System Calibration. The measurement system calibration behind the feed is performed using wideband chirp signals. The chirp is injected into the signal chains via coupler I and II (Figure 3). The calibration signal is captured by the two vector signal analyzers. In the next step, the signal is linked via the switches directly to the analyzers, and the chirp signals are recorded as reference again. It has to be taken into account that more elements are in the loop during the chirp recordings compared to the receiving chain. These are the link between the signal generator and the couplers and the direct path to the analyzers.
To separate the receiving chain from the additional elements within the wideband calibration loop, two more measurements are needed. The injection path from the signal generator to the couplers and the direct paths are characterized by network analyzer (NWA) measurements. Based on the chirp and NWA measurements, the transfer function of the system is calculated to derive the gain and phase information. To determine the calibration curve over the frequency range from 1.0 GHz to 1.8 GHz, a set of overlaying chirps with different center frequencies is injected into the signal paths and combined within the analysis. Figure 4 and Figure 5 show the results of the wideband calibration of gain and phase.
Is it enough to determine the gain only once? If we assume that there is no aging effect of the elements, and the ambient conditions like temperature are constant, the gain should not change. In reality the behavior of the system is not constant. Figure 6 shows the temperature within the cabin during a failure of its air conditioning system. Figure 7 shows the corresponding gain of the measurement system during the temperature change in the cabin of about 5° Celsius. Clearly, it can be seen that the gain changed around 0.2 dB.
This example shows the sensitivity of the system to changes in environmental conditions. Usually the measurement system is temperature-stabilized and controlled, and the system will not change during data acquisition. But every control system can be broken, or an element changes its behavior. For this reason, the calibration is performed at least at the beginning and at the end of a satellite path (maximum 8 hours).
Here we present selected results from the European Galileo and the Chinese BeiDou navigation systems.
Galileo FM3 and FM4. In October 2012, the third and fourth operational Galileo satellites, FM3 and FM4, were launched into orbit. Signal transmissions started in November and in December, respectively. Both satellites provide fully operational signals on all three frequency bands, E1, E5, and E6. The measurement data of both satellites were captured in December 2012, shortly after the beginning of the signal transmission. Figure 8 shows the spectra of both satellites for El, E5, and E6 bands. The quality of the transmitted signals seems to be good, but for the El signal of FM4 satellite, minor deformations of the spectra are visible.
Figure 9 shows the results of the IQ constellations both for FM3 and FM4 concerning each transmitted signal band. The constellations and consequently the modulation quality of each signal are nearly perfect for the FM3 satellite. The IQ constellation diagrams of FM4 show minor deformations in each band. What impact these imperfections create for future users has yet to be analyzed. Both satellites were at the time of measurement campaign still in the in-orbit test phase and did not transmit the final CBOC signal in the E1 band. It could be expected that especially the signals of the FM4 will be adjusted to become more perfect.
BeiDou M6. BeiDou satellites transmit navigation signals in three different frequency bands, all are located adjacent to or even inside currently employed GPS or Galileo frequency bands. The center frequencies are for the B1 band 1561.1 MHz, B3 band 1268.52 MHz, and B2 band 1207.14 MHz.
In 2012, China launched six satellites: two inclined geostationary space vehicles and four medium-Earth orbit ones, concluding in September (M5 and M6) and October 2012 (IGSO6). There have been further BeiDou launches in 2013, but these satellites’ signals are not analyzed here.
Figure 10 displays calibrated measurement results from the Beidou M6 satellite. The spectra of the B2 and B3 band of the Beidou M6 satellite are clean and show no major deformation. Within the B1 spectra, some spurious results, especially on top of the side lobes, are obvious. This behavior has to be investigated more in detail to determine their origin. The IQ diagrams, which visualize the modulation quality, show also no major deformation. Only within the B3 signal, a marginal compression of the constellation points can be seen, which points to a large-signal operation at the beginning of the saturation of the amplifier of the satellite.
Reviewing the quality of the presented measurements, signal analysis, and verification on GNSS satellites, the use of the 30-meter high-gain antenna offers excellent possibilities and results. Regarding the calibration measurements of the antenna gain and measurement system, the variances are in the range of measurement uncertainty of the equipment.
The sensitivity of the measurement system concerning ambient conditions was exemplarily shown based on the gain drift caused by a temperature drift. But the solution is simple: stabilize the ambient conditions or perform calibration in a short regular cycle to detect changes within the system behavior to be able to correct them.
Based on this absolute calibration, a first impression of the signal quality of Galileo FM3 and FM4 and the BeiDou M6 satellites were presented using spectral plots and IQ diagrams. Only minor distortion could be detected within the Galileo FM4 and Beidou M6 signal; these distortions may be negligible for most users. Concerning FM4 and FM3, both satellites were in the in-orbit test phase during the data acquisition. The signal quality may have been changed during their stabilization process in orbit, or the signals have been adjusted in the meantime. Thus, it would be interesting and worthwhile to repeat the measurements and perform detailed analysis to assess the final satellite quality and consequently the user performance.
The authors wish to thank the German Space Operation Centre for the opportunity to use the high-gain antenna. The support of colleagues at the DLR ground station Weilheim for the operational and maintenance service over recent years is highly appreciated. This work was partly performed within the project “Galileo SEIOT (50 NA 1005)” of the German Space Agency, funded by the Federal Ministry of Economics and Technology and based on a resolution by the German Bundestag. Finally, the support of DLR’s Centre of Excellence for Satellite Navigation is highly appreciated.
This article is based on the paper “GNSS Survey – Signal Quality Assessment of the Latest GNSS Satellites” presented at The Institute of Navigation International Technical Meeting 2013, held in San Diego, California, January 28–30, 2013.
Steffen Thoelert received his diploma degree in electrical engineering at the University of Magdeburg. He works in the Department of Navigation at German Aerospace Centre (DLR), on signal quality assessment, calibration, and automation of technical processes.
Johann Furthner received his Ph.D. in laser physics at the University of Regensburg. He works in the DLR Institute of Communication and Navigation on the development of navigation systems in a number of areas (systems simulation, timing aspects, GNSS analysis, signal verification, calibration processes).
Michael Meurer received a Ph.D. in electrical engineering from the University of Kaiserslautern, where he is now an associate professor, as well as director of the Department of Navigation at DLR.