A Comparative Signal Quality Analysis of the New GPS L1C and Beidou B1C MBOC Modulated Signals
Nikolaos Batzilis, Paolo Crosta, Luciano Musumeci, Xurxo Otero, Jose V. Perello, Rui Sarnadas, European Space Research and Technology Centre (ESA/ESTEC), Netherlands
Location: Pavilion Ballroom East
Alternate Number 1
Modernized GNSS satellites have been successfully launched for both GPS and Beidou constellations. The arrival of the first, modernized, GPS III satellites in 2019 was well expected by the GNSS community, although the satellites are still not in operational mode. Beidou launched 19 Beidou-3 satellites in the timeframe of one year, declaring BDS preliminary system operational before the end of 2018, followed by additional launches of BDS-3 satellites in 2019. Now, three major GNSSs transmit MBOC modulations in L1 band: CBOC for Galileo, QMBOC for Beidou, and TMBOC for GPS. Considering the new payloads and new signals transmitted by both GPS III and BDS-3 satellites, it is of great interest to perform Payload & Signal characterization analysis and comparison both versus the precedent generations of satellites, but also between constellations and modulations.
In-Orbit GNSS satellite payload/signal characterisation is normally performed with a high diameter dish antenna in order to reduce the multipath and increase the received signal to noise ratio levels.
A first characterization of GPS III new payload was presented by DLR using a 30m Dish Antenna in [RD 1]. Here the authors have provided an extensive analysis and comparison of the signal quality between GPS III and previous GPS IIF satellite in terms of transmitted signal power, IQ diagrams, spectrum and satellite antenna gain pattern. Additional assessment on measured power level and signal quality on ground have been provided by using GNSS omnidirectional antenna and a PolaRx5 Receiver.
Small dish antennas can still provide valuable results as demonstrated in 2012 by authors in [RD 2] where results in terms of code-carrier coherency and estimated EIRP have been presented for the modernized satellites block GPS, COMPASS and GLONASS, by using the 3m Dish Antenna available at the Navigation Laboratory of the European Space Research and Technology Centre (ESA-ESTEC).
The objective of this work is to provide an extensive signal quality assessment for the new civil GNSS signals transmitted on L1 by the new GPSIII and Beidou 3 satellites, by using the 3m dish antenna setup of the Radio Navigation Laboratory of ESTEC. The RF path of the 3m Dish Antenna setup has been fully calibrated with a vector network analyser (VNA), in order to minimize the distortion coming from the local environment (e.g. multipath) and the RF reception chain. An USRP N320 from Ettus Research has been used in order to filter on L1 carrier and down convert to baseband the RF signal coming from the 3m dish antenna and stream out IQ data for further processing with an in-house Software Defined Receiver.
SDR based analysis will mainly focus on the impact of receiver front-end and tracking loop parameters (e.g. E-L spacing) to assess:
• Correlation function shape
• S-Curve bias
• Correlation losses expected in case of low-complex receiver architecture correlating the incoming signal with a local BOC(1,1) replica.
The assessment of the above mentioned KPI will provided on a comparative level for the TMBOC and the QMBOC modulating the L1C and the B1C pilot component respectively.
In addition to the IQ data capturing and processing with SDR, a commercial receivers such as the PolaRX5, already capable to track the modernized signals, will be used to carry out signal quality characterization in terms EIRP estimation and antenna pattern characterization based on Code Carrier Coherency assessment from the available observables. Multiple passes of the GPS III and Beidou 3 satellites will be tracked in order to characterize the CCC repeatability. Impact of receiver correlator spacing will be also part of the CCC assessment.
For the signal analysis on ground, IQ data will be collected from a COTS omnidirectional antenna in parallel with the 3m dish Antenna tracking. The analysis will mainly focus on pilot tracking performance comparison for the different MBOC modulated pilot components for L1C and B1C on the basis of different acquisition and tracking strategy. Pros and cons of TMBOC versus QMBOC will be assessed and tracking robustness analysis will be assessed in terms of code and phase tracking jitter and cycle slip probability by exploiting the in-house SDR. Code and phase tracking quality will be also assessed on the basis of different tracking strategy including full MBOC or BOC(1,1) local replica generation for prompt and early-late correlators
[RD 1] S. Thoelert, P. Steigenberger, O. Montentbruck, M. Meurer, GPS III Arrived – An Initial Analysis of Signal Payload and Achieved User Performance, ION GNSS+ 2019, Miami Florida, September 16-20, 2019
[RD 2] J. V. Perello Gisbert, N. Batzilis, G. Lopez Risueno, J. Alegre Rubio, GNSS Payload and Signal Characterization using a 3m Dish Antenna, ION GNSS 2012, Nashville, Tennessee, 17-21-2012