Model 5601MSC
Model 5601MSC Master SPG/Master Clock System
Page - 52
Revision 2.2
OPERATION
Obtaining a position requires the receiver to track at least four satellites. Satellites which do not meet
the minimum quality criteria (elevation, signal strength, dilution of precision) cannot be used during a
self-survey. The status display will read “GPS Poor Signal” if four healthy satellite signals cannot be
found. These requirements are more stringent when performing a self-survey than they are when in
normal operation.
When the self-survey is complete, the GPS receiver will begin to send out a 1pps pulse to the
5601MSC. The 5601MSC will frequency-lock its ovenized oscillator to the pulse edges, and phase-lock
to GPS time. Once the GPS receiver knows its precise location, it can substitute its position into the
equation and solve only for clock error and clock bias. In this mode, it can operate with just one satellite
and still provide accurate timing to the 5601MSC.
The status of the GPS lock progress can be monitored in the
Lock Status
and
Inputs
status screens
(see section 4.1.2). The messages are shown in Table 2-9 along with a description.
Status Display
Description
GPS Disconnected
The 5601MSC cannot communicate with the GPS receiver.
GPS No Almanac
The receiver is updating its GPS almanac. Can take up to 15 minutes
to complete.
GPS Searching
The receiver has not yet detected any satellites and is searching.
GPS No Sat Found
No satellites are being tracked
GPS Only 1 Sat
The receiver is waiting until four satellites are detected to complete a
self-survey of its position.
GPS Only 2 Sats
GPS Only 3 Sats
GPS Poor Signal
One or more satellites do not meet the elevation, SNR, or PDOP
criteria and cannot be used. Attempting to detect more satellites.
GPS Ok. Sig OK
The receiver has completed a self-survey and as operating normally.
GPS 1 Good Sat
The receiver has previously completed a self-survey and is
maintaining synchronization while tracking just one satellite.
GPS 8 Good Sats
The receiver is operating in over determined clock mode, weighting
the results from 8 different satellites that meet the minimum signal
strength and position requirements to obtain maximum accuracy.
GPS 0 Good Sats
The receiver has lost contact with all satellites. This will trigger a
reference fault condition as timing accuracy cannot be guaranteed.
Table 2-9: GPS Status Messages
As more satellite signals are detected, the receiver will enter over determined clock mode. Up to 12
satellites can be tracked, using their signal strength and position in the sky to give them more or less
weight in contributing to the solution. The quality criteria are more relaxed than when performing a self-
survey and each available satellite will improve timing precision. If the receiver is moving, such as in a
mobile application, the receiver will enter dynamic mode and still provide reasonably high precision.
After the 5601MSC has locked to the GPS receiver and is in a steady state condition for longer than 20
minutes the master oscillator will be phase locked to within 15ns to 90ns of the GPS system time
reference, depending on the number of healthy satellites that are tracked. When using multiple
5601MSC units referenced to GPS, it is important to make sure both receivers are mounted equally so
that they detect a similar number of satellites, and that the antenna cable lengths are matched as
closely as possible to limit signal path propagation delays to match their phases as closely as possible.