Unveiling Beamforming Strategies of Starlink LEO Satellites
Mohammad Neinavaie and Zak (Zaher) Kassas, University of California, Irvine
Date/Time: Thursday, Sep. 22, 9:43 a.m.
Integrated satellite-terrestrial and satellite only broadband communication systems are currently being pursued to enhance the coverage and provide good wireless channel properties . Due to their relatively smaller propagation delays, LEO constellations seem to be promising for latency-critical applications with requirements within tens of ms. Satellite-based navigation is also witnessing the new era of low Earth orbit (LEO) megaconstellations . The launch of tens of thousands of LEO satellites for broadband communications will revolutionize the future of satellite-based navigation . The potential of utilizing LEO space vehicle (SV) signals for navigation has been the subject of numerous recent studies [4–11]. Broadband communication signals transmitted from LEO SVs contain timing signals, which if one could acquire and track opportunistically, navigation observables (pseudorange, carrier phase, and Doppler) could be extracted .
The first standalone (non-differential) positioning results with Starlink SV signals were presented in [13,14], which show carrier phase and Doppler tracking of six Starlink SVs, achieving a horizontal positioning error of 7.7 and 10 m with known receiver altitude, respectively. To take the SV ephemeris errors into account in these papers, assuming that most of the ephemeris error is along the SV’s track, the time epoch of the TLE files was shifted such that it minimizes the Doppler residuals, knowing the receiver’s location. Alternatively, the ephemeris of the TLE files can be corrected online through a satellite ephemeris estimation framework, such as simultaneous tracking and navigation (STAN) [11, 15]. Differential methods assess measurement errors for each satellite using a stationary surveyed reference antenna and broadcasts error corrections to many users (which may each see a different set of satellites) . Satellite errors removed by differential methods include clock calibration, ephemeris errors, ionospheric delays, and tropospheric delays . A differential carrier-phase navigation system with GPS and LEO SVs was presented in . An opportunistic framework to navigate with differential carrier phase measurements from megaconstellation LEO SV signals was proposed in . The Starlink constellation is used as a specific megaconstellation example to demonstrate the efficacy of the proposed algorithm in . Future applications of navigation with LEO based satellites, including differential navigation methods, may require a knowledge of beam configuration and reconfiguration in the link between the LEO SV and a ground user terminal (UT) . Current LEO constellation device different techniques which may use fix analog beams that illuminate a given area of Earth’s surface, or may steer narrower beams in the user directions . Digital precoding stages are designed independently of the analog beam aiming at minimizing the inter-beam interference between the adjacent beams .
Designing a beamforming codebook at the transmitter side is crucial to (i) guarantee coverage of the Earth’s surface, limit the inter-beam interference, maximize system capacity (throughput), and exhibit compatibility with cellular standards. Recent studies on deploying LEO constellations for communication purposes, do necessarily consider the integration with terrestrial networks. For particular LEO SVs the sizes of the beam footprints are provided but the details of the beam codebook is not disclosed to public. In [19, 20], all feasible beams corresponding to a uniform planar array at the satellite are studied for massive MIMO LEO satellite communications. These studies suggest a beam switching technique to maximize the receive signal to noise ratio (SNR) at a particular beam spot on the ground. As it can be seen in Fig. 1, due to the high dynamics of the satellite, different beams should be selected to illuminate a beam spot on the ground as the satellite moves.
This paper presents an experimental demonstration of beam forming techniques for Starlink satellites. The provided information allows designing navigation techniques that benefit from differential measurements from multiple users that see the same satellite. To guarantee that two users with a give baseline can receive the signal of the same satellite, some information about beamforming configurations should be available for these satellites. To demonstrate preliminary results with Starlink satellites, a stationary National Instrument (NI) universal software radio peripheral (USRP) 2945R was equipped with a consumergrade Ku antenna and low-noise block (LNB) downconverter to receive Starlink signals in the Ku-band. The sampling rate was set to 2.5 MHz and the carrier frequency was set to 11.325 GHz, which is one of the Starlink downlink frequencies.
Fig. 2 demonstrates the estimated Doppler, the normalized SNRs, and the skyplot for six
Starlink satellites passes. For each satellite, multiple peaks can be observed as was expected and each peak corresponds to a beam. The increasing and decreasing behavior of the peaks indicates if the satellite is approaching or moving away. Also, the effect of beam switching to maintain maximum SNR can be observed in the estimated receive SNRs. The maximum receive SNR is achieved when the Starlink LEO SV 3 is passing above the receiver. Fig. 3(a), shows the Starlink LEO SV 3 trajectory and the locations in which the beam is taken place. It can also be seen that the satellite stops illuminating the region at a particular time instant. Fig. 3(b) demonstrates the SNR change of Starlink LEO SV 3 corresponding to different beams illuminating the spot beam in which the receiver is located at. Having a knowledge of the beam configurations of Starlink LEO SVs enables differential navigation by receiving two different beams of the same satellite signals at the two receivers (the base and the rover). Using the previously mentioned hardware at the base and the rover which was around one kilometers apart, an experiment was conducted to show preliminary results for differential navigation with Starlink LEO SVs. Over the course of the experiment, the receivers on-board the base and the rover were listening to three Starlink LEO SVs, namely Starlink 44740, 48295, and 47728. The satellites were visible for 320 seconds. The results are demonstrated in Fig. 4. The 3D position error was found to be 33.4 m. Upon equipping the receiver with an altimeter (to measure its altitude), the 2D position error reduced to 5.6 m.
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