User`s guide

Chapter 3 - Receiver Descriptions
RF Jamming Immunity (M12+ Timing Receiver Only)
Many precise timing GPS installations require locating the GPS antenna at close range to other
systems. Some of these transmitters may randomly cause the GPS receiver to lose lock on
tracked satellites. This can be very disconcerting to the timing user since the system must rely on
clock coasting until the satellite signals are reacquired. Long coasting times require more
expensive oscillators for the timing electronics in order to meet system specifications for holdover
capability.
Experience has shown that receiver selectivity, or the ability to select only the GPS band of
information and reject all other signals, is an important feature for GPS receivers, especially in
cases such as those often encountered in timing applications.
Adaptive Tracking Loops (M12+ Timing Receiver Only)
Motorola has developed an innovative software technique to further improve the jamming
immunity of the M12+ Oncore timing receiver. The technique takes advantage of the fact that for
precise timing applications, the receiver is not moving. In mobile GPS applications, the receiver
must be able to track satellites under varying dynamics. Vehicle acceleration causes an apparent
frequency shift in the received signal due to Doppler shift. In order to track signals through
acceleration, the tracking loops are wide enough to accommodate the maximum expected vehicle
acceleration and velocity. When the receiver is stationary, the tracking loops do not need to be as
wide in order to track the satellites. In the M12+ timing receiver firmware, the satellite tracking
loops are narrowed once the receiver has acquired the satellites and reached a steady state
condition. This adaptive approach allows the tracking loops to be narrowed for maximum
interference rejection while not unduly compromising the rapid startup and acquisition
characteristics of the receiver.
Test results have demonstrated that this approach is effective at providing an additional 10 dB of
jamming immunity to both in-band and out-of-band signals. The combined results of the additional
filtering and the adaptive tracking loops in the M12+ Oncore combine to provide the user with a
receiver/antenna system effective at improving RF jamming immunity, thus making installation in
timing applications more flexible and robust. The status of the tracking loops (wide-band or
narrow-band) are indicated by status bits in the @@Ha and @@Hb messages.
Time RAIM Algorithm (M12+ Timing Receiver Only)
Time Receiver Autonomous Integrity Monitoring (T-RAIM) is an algorithm in Motorola Oncore
timing receivers (including the M12+T) that uses redundant satellite measurements to confirm the
integrity of the timing solution. The T-RAIM approach is borrowed from the aviation community
where integrity monitoring is safety critical.
Motorola GPS Products - M12+ User's Guide Revision 6.X 09FEB05
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