A.V. Terentev – Head of Department,
LTD «STC» (St. Petersburg)
Email: avt982@gmail.com
A.A. Borozdin – Software Engineer, LTD «STC» (St. Petersburg)
Email: andrewborozdin@yandex.ru
I.V. Sotnikov – Software Engineer,
LTD «STC» (St. Petersburg)
Email: stn_iv@list.ru
This paper discusses choice of optimal signal modulation parameters for building local positioning systems (LPS) and maximizing its noise immunity. LPS under consideration includes 3 types of modules: ground beacon (GB), active navigational module (ANM) and passive navigational module (PNM). PNM receives signals from both GB and ANM, gets pseudo ranges from that sources and finds its position solving equations.
Current version of LPS (LPS-1) uses combined BPSK and QPSK modulated signal. Each character (2 bits) is a 50 us long BPSK signal, modulated with a pseudorandom noise sequence (PRN). Phase difference between two characters if multiple of π/2, so it uses QPSK.
System operates while moving, so Doppler shift takes place.
For better noise immunity it is desirable to add repetition of each PRN sequence k times, so that receiver could perform coherent addition. The ability of the receiver to perform coherent addition is limited by the Doppler shift, because Doppler shift is not compensated in LPS-1 and too high shift with too long coherent addition will result in phase shift reaching level of π.
This paper offers new signal structure with BPSK and QPSK modulations being swapped. This leads to a shorter PRN with two times lower energy. Several parameters decrease (time of arrival accuracy, value of code division between channels decrease by 3 dB), while noise immunity stays the same, but mush longer coherent addition is possible (14 in LPS-2 instead of 3 in LPS-1). With new signal structure introduced, noise immunity is increased by total amount of about 10 dB.
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