  This paper investigates the impact of non-line-of-sight (NLOS) and multipath
signals on frequency-locked loops (FLLs), which are commonly used to obtain
Doppler shift measurements for velocity estimation in radio navigation. A
salient result is that, in the absence of a direct signal, a single NLOS signal
does not corrupt Doppler observables if the angle-of-arrival (AOA) is known.
Another striking result is that, in a multipath environment involving two
signals, the arctangent discriminator (averaged over the beat frequency) tracks
only the higher amplitude signal. Our investigation has particular significance
for radio navigation in urban environments, either using global navigation
satellite system (GNSS) signals or ground-based cellular signals.
