Local Oscillator Contribution to Carrier-Phase Measurements in A GNSS Receiver

Abstract : When performing carrier-phase measurements, the measurement noise affecting the observations reflects two contributions, originating from the thermal noise of the RF signal received at the antenna and from the stability of the local oscillator over the integration time, since normally some form of phase-locked loop (PLL) is used for carrier recovery. The order of the PLL (and the bandwidth) determines the amount of the oscillator contribution to the measurement noise for a given oscillator frequency stability (or, since the integration time is generally less or equal to 1 s, to its phase noise). Therefore, it is important to understand the mechanism by which the local oscillator instability is transferred to the carrier-phase measurement noise and select a proper oscillator to minimize such a contribution. In the paper, we will address these issues, providing examples that guide the selection of the local oscillator. A practical example of implementation will be discussed, where a low-cost, high-stability OCXO has been disciplined to a Rb frequency standard to provide improved stability over the integration times of interest in order to minimize the noise for carrier-phase recovery.