US-12618978-B2 - Demodulating QZSS signals

US12618978B2US 12618978 B2US12618978 B2US 12618978B2US-12618978-B2

Abstract

A method and apparatus are provided for demodulating an L1S signal from a satellite in the Quasi-Zenith Satellite System (QZSS). The method comprises tracking another L1 signal transmitted by the satellite, and predicting, based on the tracking parameters of the other L1 signal, one or more parameters of the L1S signal. The L1S signal is demodulated based on the one or more predicted parameters.

Inventors

Zhenlan CHENG

Assignees

U-BLOX AG

Dates

Publication Date: 20260505
Application Date: 20221130
Priority Date: 20211207

Claims (11)

1 . A method of demodulating an L1S signal from a satellite in the Quasi-Zenith Satellite System the method comprising: tracking another L1 signal transmitted by the satellite, to estimate one or more parameters of the other L1 signal; predicting, based on the estimated one or more parameters of the other L1 signal, one or more parameters of the L1S signal transmitted by the satellite; demodulating the L1S signal to obtain data bits of a data message modulated on the L1S signal by the satellite, wherein the demodulating is based on the one or more predicted parameters; and calibrating a phase offset, wherein calibrating the phase offset comprises: detecting a bit in the other L1 signal; wiping off the detected bit from in-phase and quadrature (I/Q) samples of the other L1 signal; after wiping off said detected bit, integrating a plurality of the I/Q samples of the other L1 signal; obtaining an estimated carrier phase of the other L1 signal based on a result of integrating the plurality of the I/Q samples of the other L1 signal; detecting one or more bits in the L1S signal; wiping off the detected one or more bits from I/Q samples of the L1S signal; after wiping off said detected one or more bits, integrating a plurality of the I/Q samples of the L1S signal; obtaining an estimated carrier phase of the L1S signal based on a result of integrating the plurality of the I/Q samples of the L1S signal; and comparing the estimated carrier phase of the other L1 signal with the estimated carrier phase of the L1S signal to calibrate the phase offset.
2 . The method of claim 1 , wherein: the estimated one or more parameters of the other L1 signal comprise an estimated code phase of the other L1 signal; the one or more parameters of the L1S signal comprise a code phase of the L1S signal; and the method comprises predicting, based on the estimated code phase of the other L1 signal, the code phase of the L1S signal.
3 . The method of claim 1 , wherein the method comprises predicting, based on the estimated carrier phase of the other L1 signal, a reference carrier phase of the L1S signal.
4 . The method of claim 3 , wherein predicting the reference carrier phase of the L1S signal comprises adding or subtracting the phase offset to or from the estimated carrier phase of the other L1 signal.
5 . The method of claim 1 , further comprising low-pass filtering the phase offset.
6 . The method of claim 1 , wherein calibrating the phase offset is performed when a carrier-to-noise ratio of the L1S signal is greater than a predetermined threshold.
7 . The method of claim 6 , wherein the demodulating the L1S signal based on the one or more predicted parameters of the L1S signal is performed when the carrier-to-noise ratio of the L1S signal is less than a predetermined threshold.
8 . The method of claim 1 , wherein demodulating the L1S signal comprises comparing a carrier phase of the L1S signal with a reference carrier phase to detect data bits in the L1S signal.
9 . One or more tangible, non-transitory, computer-readable media storing instructions that, when executed by one or more processors, cause the one or more processors to demodulate an L1S signal from a satellite in the Quasi-Zenith Satellite System, wherein the demodulating includes performing operations comprising: tracking another L1 signal transmitted by the satellite, to estimate one or more parameters of the other L1 signal; predicting, based on the estimated one or more parameters of the other L1 signal, one or more parameters of the L1S signal transmitted by the satellite; demodulating the L1S signal to obtain data bits of a data message modulated on the L1S signal by the satellite, wherein the demodulating is based on the one or more predicted parameters; and calibrating a phase offset, wherein calibrating the phase offset comprises: detecting a bit in the other L1 signal; wiping off the detected bit from in-phase and quadrature (I/Q) samples of the other L1 signal; after wiping off said detected bit, integrating a plurality of the I/Q samples of the other L1 signal; obtaining an estimated carrier phase of the other L1 signal based on a result of integrating the plurality of the I/Q samples of the other L1 signal; detecting one or more bits in the L1S signal; wiping off the detected one or more bits from I/Q samples of the L1S signal; after wiping off said detected one or more bits, integrating a plurality of the I/Q samples of the L1S signal; obtaining an estimated carrier phase of the L1S signal based on a result of integrating the plurality of the I/Q samples of the L1S signal; and comparing the estimated carrier phase of the other L1 signal with the estimated carrier phase of the L1S signal to calibrate the phase offset.
10 . A Global Navigation Satellite System (GNSS) receiver configured to demodulate an L1S signal from a satellite in the Quasi-Zenith Satellite System, the GNSS receiver comprising: at least one tracking loop configured to track another L1 signal transmitted by the satellite, to estimate one or more parameters of the other L1 signal; a prediction subsystem configured to predict, based on the estimated one or more parameters of the other L1 signal, one or more parameters of the L1S signal transmitted by the satellite; and an L1S bit detector, configured to demodulate the L1S signal to obtain data bits of a data message modulated on the L1S signal by the satellite, wherein the L1S bit detector is configured to demodulate the L1S signal based on the one or more predicted parameters, the one or more parameters of the other L1 signal comprise a carrier phase of the other L1 signal; the one or more parameters of the L1S signal comprise a reference carrier phase of the L1S signal; and the prediction subsystem comprises an adder or a subtractor, configured to predict the reference carrier phase of the L1S signal by adding or subtracting a phase offset to or from an estimated carrier phase of the other L1 signal, wherein the prediction subsystem comprises a comparison unit, configured to compare the estimated carrier phase of the other L1 signal with an estimated carrier phase of the L1S signal, to calibrate the phase offset, wherein the GNSS receiver comprises a calibration mode in which it is configured to calibrate the phase offset, and comprises an aiding mode in which it is configured to use the calibrated phase offset to predict the reference carrier phase of the L1S signal.
11 . The GNSS receiver of claim 10 , wherein: the one or more parameters of the other L1 signal comprise a code phase of the other L1 signal; the one or more parameters of the L1S signal comprise a code phase of the L1S signal; the at least one tracking loop comprises a code-phase feedback controller configured to estimate the code phase of the other L1 signal; the GNSS receiver further comprises an L1S code generator, configured to generate a replica spreading code for de-spreading the L1S signal; and the L1S code generator is controlled by the estimated code phase.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority to European Application No. 21212919.1, filed on Dec. 7, 2021, the contents of which are hereby incorporated by reference. FIELD OF THE INVENTION The present invention relates to Global Navigation Satellite Systems (GNSS). In particular, it relates to a method for demodulating signals of the Quasi-Zenith Satellite System (QZSS) and a GNSS receiver configured to demodulate QZSS signals. BACKGROUND OF THE INVENTION Techniques for GNSS positioning are well known in the art. Existing GNSS include the Global Positioning System (GPS), Galileo, GLONASS, and BeiDou Navigation Satellite System (BDS), also referred to herein as simply “BeiDou”. Each GNSS comprises a constellation of satellites, also known in the art as “space vehicles” (SVs), which orbit the earth. Typically, each SV transmits a number of satellite signals. These are received by a GNSS receiver whose position it is desired to calculate. The GNSS receiver can make a number of ranging measurements using the signals, to derive information about the distance between the receiver and respective satellites. When a sufficient number of measurements can be made, the receiver's position can then be calculated by multilateration. In addition to the signals transmitted for ranging purposes (on which data in the form of a navigation message may be modulated), the SVs may transmit one or more data signals. These are not intended to be used directly for ranging measurements. Instead, they may be used to provide data messages, which may support one or more purposes. One possible purpose is to provide a satellite-based augmentation system (SBAS). The aim of such an augmentation system is to improve the integrity-assurance, accuracy, reliability, and/or availability of the GNSS, by incorporating additional external information into the positioning calculations. An SBAS uses ground infrastructure to measure signals from the SVs. Based on these measurements, corrections are calculated, which compensate for sources of error (including effects such as clock drift and ionospheric delay, for example). The corrections are transmitted to the SVs, and broadcast by the SVs as messages in the one or more data signals. In this way, the SVs can transmit correction data that allows a receiver to correct positioning measurements that it makes based on the other signal(s) of that satellite. An SBAS typically operates at a regional or continental scale. The Quasi-Zenith Satellite System (QZSS), developed by the Japanese government, provides a regional satellite-based augmentation system. Its aim is to enhance GPS in the Asia-Oceania region, with a focus on Japan. The QZSS satellite orbits are chosen so that at least one satellite is almost directly overhead in Japan, at any given moment. The signals broadcast are compatible with GPS. In this way, QZSS increases the availability of GPS in urban canyons in Japan, where the signals of satellites at low elevations would not be received or may be subject to severe multipath. The QZSS includes an L1S signal which is used to transmit data messages to support an SBAS called the sub-meter level augmentation service (SLAS). The QZSS L1S signal is also referred to as the L1 sub-meter accuracy service or the L1-SAIF (submeter-class augmentation with integrity function) signal. The L1S signals are transmitted in the same band as the GPS L1 signals centred at 1575.42 MHz. SUMMARY OF THE INVENTION It would be desirable to demodulate L1S signals as efficiently and accurately as possible, so that a GNSS receiver can best make use of the services supported by these signals. According to one aspect, there is provided a method of demodulating an L1S signal from a satellite in the Quasi-Zenith Satellite System, the method comprising: tracking another L1 signal transmitted by the satellite, to estimate one or more parameters of the other L1 signal;predicting, based on the estimated one or more parameters of the other L1 signal, one or more parameters of the L1S signal transmitted by the satellite; anddemodulating the L1S signal to obtain data bits of a data message modulated on the L1S signal by the satellite,wherein the demodulating is based on the one or more predicted parameters. Conventionally, the L1S signal is demodulated without any aiding. However, the data message modulated on the L1S signal has a channel bit-duration of 2 ms. This can make it prone to bit-detection errors, especially in circumstances in which the carrier-to-noise ratio is low. The other L1 signal is an L1 signal other than the L1S signal. It may comprise an L1 positioning, navigation and timing (PNT) signal, optionally an L1 ranging signal. The present inventor, noting that demodulation may be more reliable for the other L1 signal (which may have a longer bit duration) has recognised that it may be beneficial to demodulate the L1S signal aided by the tracking parameters of the other L1 signal