Abstract
The Ground-based Radio Navigation System (GRNS) is an alternative/backup navigation system based on time synchronized pseudolites. It has been studied for some years due to the potential vulnerability issue of satellite navigation systems (e.g., GPS or Galileo). In the framework of our study, a periodic pulsed sequence was used instead of the randomized pulse sequence recommended as the RTCM (radio technical commission for maritime services) SC (special committee)-104 pseudolite signal, as a randomized pulse sequence with a long dwell time is not suitable for applications requiring high dynamics. This paper introduces a mathematical model of the post-correlation output in a navigation sensor, showing that the aliasing caused by the additional frequency term of a periodic pulsed signal leads to a false lock (i.e., Doppler frequency bias) during the signal acquisition process or in the carrier tracking loop of the navigation sensor. We suggest algorithms to resolve the frequency false lock issue in this paper, relying on the use of a multi-correlator. A flight test with an unmanned helicopter was conducted to verify the implemented navigation sensor. The results of this analysis show that there were no false locks during the flight test and that outliers stem from bad dilution of precision (DOP) or fluctuations in the received signal quality.
Highlights
GNSS (Global Navigation Satellite System) receivers are currently the most widely-used space-basedPNT sensors
The ground-based radio navigation system introduced in this paper is an alternative/backup navigation system, which can overcome the vulnerability against intentional interference attacks through its use of a pseudolite network
An analysis of the flight test confirmed that the navigation solutions were reliable and that there were no issues caused by the false lock, there were a few outliers in the solution caused by bad dilution of precision (DOP) at a low elevation and by channel power fluctuations
Summary
GNSS (Global Navigation Satellite System) receivers are currently the most widely-used space-based. The vulnerability of the satellite navigation systems has been an important issue for more than a decade, as GPS sensors must process signals with an extremely low power level. In a CDMA (code division multiple access)-based system, users share the same frequency channel This means that users can fail to acquire weak signals if there are strong signals from closely-located transmitters. To avoid the near-far problem in the pseudolite sensors, previous researchers suggested a scheme that involved frequency offsetting and pulsing [11,12] We applied these two methods to avoid the near-far issue in our study. We designed pseudolite transmitters with fixed pulse positions, for which the duty ratio is defined as 10% We suggested another algorithm that detects possible false locks in the navigation sensor. An analysis of the flight test confirmed that the navigation solutions were reliable and that there were no issues caused by the false lock, there were a few outliers in the solution caused by bad dilution of precision (DOP) at a low elevation and by channel power fluctuations
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