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Methods for Velocity Measuring of Navigation Receivers
Measurements of velocity in navigation receivers are performed in two stages. At the primary (after-satellite)
processing stage each of received signals is synchronized using a separate PLL, after that an estimation block (EB) estimates nonenergy (phase and frequency) and energy (SNR) parameters of the received signal. Doppler primary estimations can be subject to after-satellite filtration to obtain secondary frequency estimates. A number of Doppler estimates are conversed into primary estimates of velocity vector projections (for example, onto axes of the local Cartesian coordinate system) using the least square method (LSM). Primary estimates of velocity can be
filtered at the secondary (after-coordinate) processing. Secondary velocity vector coordinates are outputted to users.
The present paper considers different methods of measuring velocity, they being different from each other by different
tracking filters of primary and secondary processing and different EB. Primary filters operate at the same control
frequency Fc as PLL (for instance, at Fc= 200 Hz), and LSM and secondary filters – at lower frequency FE < Fc (for example, at FE=100 Hz or FE=10 Hz). To shift from Fc to FE, some samples are rejected (intermediate samples are thrown). EB generates either primary estimates of instantaneous frequency or instantaneous phase of the input signal, or primary estimates of average input phase over control period Tc=Fc^-1. These primary estimates are fed to the filters of primary processing. At the outputs of these filters either secondary estimates of instantaneous frequency or estimates of averaged frequency and it’s derivative over period Tc are outputted which further are recalculated in estimates of instantaneous frequency. Based on thinned instantaneous phase estimates sometimes there are generated increments of these phase estimates over period TE = FE^-1. Primary estimates of either coordinates of the instantaneous velocity vector or averaged over period TE are fed to the input of secondary processing filters. In the first case, secondary estimates of instantaneous coordinates of the velocity vector are obtained at filter outputs at once. In the second case, at the filter outputs there are estimates of averaged velocities and accelerations over period TE which are further calculated in estimates of instantaneous velocity vector coordinates.
It has been shown that frequency estimation typically used in analog systems brings about a biased frequency (and hence, velocity) estimate when a receiver with digital PLLs has constant non-zero acceleration. Various algorithms of non-biased estimation have been also considered.