GPS III Arrived – An Initial Analysis of Signal Payload and Achieved User Performance

Steffen Thoelert, Peter Steigenberger, Oliver Montenbruck, Michael Meurer

Abstract:	The global positioning system (GPS) is the pioneer under the present and developing global navigation satellite systems (GNSS). In the beginning of this year a new milestone has been reached with the start of the signal transmission of the first GPS Block III space craft, which was launched with a Falcon 9 rocket from Cape Canaveral, Florida, on December 23, 2018. The satellites of this new GPS generation are built by Lockheed Martin and contain a wide range of technological improvements and new features compared to their predecessors, while maintaining compatibility with other satellites in the constellation for navigation users. The transmission of the L1 C/A, L1 P(Y) and L2 P(Y) signals has been started on January 9, 2019 [Steigenberger 2019]. One of the new features of this GPS generation is the new legacy signal L1C in the L1 frequency band. Figure 1 shows the measured L1 signal spectra captured with a high gain antenna. Clearly visible are the two lobes at +/- 1MHz around the center frequency of 1575.42 MHz. They are the result of BOC(1,1) components of the L1C signal. Further details of the new L1C signal can be found in the GPS interface control document (ICD) IS-GPS-800 or the open literature. The paper will analyze the quality of the signal-in-space based on measurement data captured with a 30m dish antenna at Weilheim, Germany, operated by the German Aerospace Center (DLR) as well as based on data from GNSS receivers. The big dish including the measurement system is precisely calibrated [Thoelert 2009], which is a pre-requisite for an accurate signal analysis. Starting from aspects, like spectral symmetry and modulation quality, the analysis will proceed to a detailed view on the GPS III-1 satellite payload characteristic including the so called analogue and digital distortions. We will analyze the signal distortions of the new components of L1C as well as of the legacy signals L1 C/A and L5 and compare the results to the previous generations of GPS satellites for the legacy signals. Since one of the technical improvements of this third GPS satellite generation is the use of a fully digital signal generation unit one can expect a negligible amount of digital distortions which will be confirmed the first time in the paper. The signal distortion section will end with the presentation of differential code bias maps quantifying the achievable user tracking performance as a function of user receiver parameters. Based on the results recommendations are provided for suitable receiver configurations in differential GNSS applications, like safety critical applications, e.g. ground based augmentation system (GBAS). Using our highly calibrated measurement facility, the paper will also present an analysis of the observable transmitted satellite signal power as a function of satellite elevation, including for each frequency band the estimates about the power sharing among individual signal components within each band. These kinds of analysis provide furthermore the opportunity to assess (in part) the transmit antenna pattern of the satellite. Considering the measured power in relation to the boresight angle of the satellite one get a cut through the antenna pattern of the satellite and can assess the antenna symmetry properties. A further interesting aspect of the new GPS satellite generation is the novel interplexing method used for the L1 band compared to the previous block types organizing the joint transmission of five signals namely L1 C/A, L1 P(Y), L1C data, L1C pilot and L1 M-code within a single frequency band. As an important finding presented in the paper it is shown that in contrast to previous GPS generations the M-code signal is no longer transmitted by the same antenna transmission chain as the other signal components in the L1 band. The presence of a separate antenna transmission chain for the M-code with different transmit antenna phase center for M-code as for the other L1 signal components was one of the first findings of the analysis of the in-phase and quadrature (IQ) measurement data. An IQ constellation plot is presented in Figure 2. The L1C data and pilot signals, the L1 C/A as well as the P(Y) signal are transmitted by a common antenna transmission chain. L1 C/A and the combination of the other three signals mentioned before are transmitted as orthogonal components of an in-phase and quadrature (IQ) modulation. For the combination, i.e. the interplexing of the three signals L1C data, L1C pilot and P(Y) in one of the two IQ channels the so-called majority voting method [Spilker 1998, Allen 2019] is used in order to ensure a signal transmission with constant envelope. Although the basic principle of majority voting is well-known, only few pieces of information are available in open literature about the specific form of majority voting and interlacing being applied on the new GPS III satellite. Based on the IQ data the paper will show estimates of the parameters of the majority voting, which means the time and power sharing among voting signal, L1C pilot and P(Y) as presented in Allen (2019), including estimates of the so-called interlacing frequency. All these parameters are not part of the public domain yet and have never been published in open literature so far. Additionally to the analysis of the measurement of the high gain antenna also various analyses of commercial GNSS receivers will be presented. User relevant information about, e.g. C/No as well as noise and multipath analysis results will be shown. The results complete the picture for the assessment of the achievable user performance based on the signals of the new GPS III satellite. The paper will end with a conclusion regarding the determined signal quality compared to previously assessed GNSS satellites of other constellations like Galileo and BeiDou and will assess its usability for safety-critical applications. References Allen DW, Arredondo A, Barnes DR, Betz JW, Cerruti AP, Davidson B, Kovach KL, Utter A (2019) Effect of GPS III Weighted Voting on P(Y) Receiver Processing Performance. ION ITM, January, 2019, Reston, Virginia, USA Steigenberger P, Montenbruck O, Thoelert S, Langley RB (2019) First Light – Broadcast of L1C by GPSIII. GPSWORLD, March, 2019 Spilker JJ and Orr RS (1998) Code Multiplexing via Majority Logic for GPS Modernization. Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998), Nashville, TN, USA, September 1998, pp. 265-273. Thoelert S, Erker S, Meurer M (2009) GNSS signal verification with a high-gain antenna – calibration strategies and high quality signal assessment. ION ITM 2009, Institute of Navigation, Anaheim, California, USA, January 26–28, pp 289–300
Published in:	Proceedings of the 32nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2019) September 16 - 20, 2019 Hyatt Regency Miami Miami, Florida
Pages:	1059 - 1075
Cite this article:	Thoelert, Steffen, Steigenberger, Peter, Montenbruck, Oliver, Meurer, Michael, "GPS III Arrived – An Initial Analysis of Signal Payload and Achieved User Performance," Proceedings of the 32nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2019), Miami, Florida, September 2019, pp. 1059-1075. https://doi.org/10.33012/2019.17043
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