Why would a PRN disappear from the RANGE log?

The RANGE log contains information about all the SVs (Satellite Vehicles, also known as PRN) being tracked by the receiver channels. The simplified process of tracking is: 1) A channel gets initialized and assigned to a particular PRN number 2) Then it starts searching that PRN and locks to it if it is in view 3) Finally the receiver checks the reject code If the reject code is not "Good" i.e. "Bad Health" It will drop the PRN from that channel and keep searching. Before downloading the almanac or ephemeris, the receiver will show all the PRNs that are available in view. Once it gets the ephemeris and indicates that a PRN is not healthy the receiver drops that PRN from range log. After receiver has the almanac information it knows what satellites are in good health so unhealthy PRNs don't even show in the range log.