Without some type of information to indicate this change in frequency, the center

frequency of the receiver s replicated code signal will be different from the frequency of

the actual received signal, which may then cause loss of code track as well. 

A receiver may be able to maintain code track in this case even while losing carrier

track if it is aided with velocity.  The primary function of aiding in this degraded mode of

operation is to maintain code loop tracking.  The velocity data replaces the carrier

tracking loop output as the source for code tracking loop aiding.  Possible sources of

velocity include INS, AHRS, and Doppler navigation systems.  Requirements on the

accuracy of the velocity will determine the allowable amounts of senescence,

synchronization error, and aiding source  absolute error under varying dynamic

conditions.  For example, higher dynamics will generally mean tighter restrictions on

data senescence, which in turn can mean higher aiding rates.   Typical accuracy

requirements on the aiding velocity in order to maintain code track when  the carrier is

lost are on the order of 2 3 m/second.

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