Inside GNSS Media & Research

JUL-AUG 2019

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62 Inside GNSS J U L Y / A U G U S T 2 0 1 9 e LEO receiver draws pseudorange rate observables from Orbcomm LEO signals on the downlink channel. Satellite radio frequency (RF) downlinks to SCs and GESs are within the 137– 138 MHz VHF band. e downlink channels include 12 chan- nels for transmitting to the SCs and one gateway channel, which is reserved for transmitting to the GESs. Each satellite trans- mits to the SCs on one of the 12 subscriber downlink channels through a frequency-sharing scheme that provides 4-fold chan- nel reuse. e Orbcomm satellites have a subscriber transmitter that provides a continuous 4800 bits-per-second (bps) stream of packet data using symmetric diff erential-quadrature phase shi keying (SD-QPSK). Each satellite also has multiple subscriber receivers that receive short bursts from the SCs at 2400 bps. Figure 17 shows a snapshot of the Orbcomm spectrum. Figure 18 shows some of the internal signals of the receiver used to extract Doppler measurement from Orbcomm signals, mainly: (a) an estimate of the Doppler frequency, (b) the carrier phase tracking error, (c) the demodulated QPSK modulation, and (d) the QPSK symbol phase transitions. e Orbcomm receiver is part of the Multichannel Adaptive TRansceiver Information eXtractor (MATRIX) so ware-defi ned radio (SDR) developed by the Autonomous Systems Perception, Intelligence, and Navigation (ASPIN) Laboratory (see, (Autonomous Systems Perception, Intelligence, and Navigation Laboratory, Additional Resources). e receiver performs car- rier synchronization, extracts pseudorange rate observables, and decodes Orbcomm ephemeris messages. Note that Orbcomm satellites are also equipped with a specially constructed 1-Watt ultra-high frequency (UHF) transmitter that is designed to emit a highly stable signal at 400.1 megahertz. e transmitter is coupled to a UHF anten- na designed to have a peak gain of approximately 2 dB. e UHF signal is used by the Orbcomm system for SC position- ing. However, experimental data shows that the UHF beacon is absent. Moreover, even if the UHF beacon were present, one would need to be a paying subscriber to benefi t from position- ing services. Consequently, in this work, only downlink VHF signals are used in the LEO-aided INS STAN. Ground Vehicle Navigation An experiment was conducted to evaluate the performance of the LEO-aided INS STAN framework on a ground vehicle tra- versing a long trajectory. To this end, a car was equipped with the following hardware and so ware setup: • A custom-built quadrifi lar helix VHF antenna • A universal so ware radio peripheral (USRP) to sample Orbcomm signals. ese samples were then processed by the Orbcomm receiver module of the MATRIX SDR. • An integrated GNSS-IMU, which is equipped with a dual- antenna, multi-frequency GNSS receiver and a microelectro- mechanical system (MEMS) IMU. A post-processing so ware development kit (PP-SDK) was used to process GPS carrier phase observables collected by the GNSS-IMU and by a nearby diff erential GPS base station to obtain a carrier phase-based navigation solution. is integrated GNSS-IMU real-time kinematic (RTK) system was used to produce the ground truth FIGURE 20 (a) Skyplot of the Orbcomm satellite trajectories. (b) Doppler frequency measurement produced by the MATRIX SDR and the expected Doppler according to an SGP4 propagator for the ground vehicle experiment. FIGURE 18 Outputs of Orbcomm receiver: (a) estimated Doppler, (b) carrier phase error, (c) demodulated QPSK symbols, and (d) QPSK symbol phase transitions. FIGURE 19 Hardware and software setup for the ground vehicle experiment. STAN WITH LEO

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