GPS System Segments - Understanding GPS: Principles and Applications - page 91

COSPM

CAPEM

KWAJM

HAWAIM

ASCNM

DIEGOM

2

0 1 3 4

Figure 3.16 CS monitor station coverage.

scale disruptions. Meteorological sensors provide surface pressure, temperature,
and dew point measurements to the CS Kalman filter to model the troposphere
delay. However, these meteorological sensors are in disrepair, and their measure-
ments have been replaced by monthly tabular data [13]. The local workstations pro-
vide commands and data collection between the monitor station and the MCS.
The Air Force monitor stations use a 12-channel, survey-grade, all-in-view
receiver. These receivers, developed by Allen Osbourne Associates (AOA), are
based on proven Jet Propulsion Laboratory (JPL) Turbo Rogue technology. The
AOA receiver is designed with complete independence of the L1 and L2 tracking
loops, with each tracking loop commanded by the MCS under various track acqui-
sition strategies. With such a design, the overall receiver tracking performance can
be maintained, even when tracking abnormal satellites (e.g., nonstandard code or
satellite initialization, which require additional acquisition processing). These
all-digital receivers have no detectable interchannel bias errors. (An earlier CS
receiver required external interchannel bias compensation due to its analog design
with separate correlation and data processing cards. Interchannel bias is a
time-delay difference incurred when processing a common satellite signal through
different hardware and data processing paths in a receiver.)
The CS receivers differ from normal receivers in several areas. First, these
receivers require external commands for acquisition. Although most user equip-
ment is only designed to acquire and track GPS signals that are in compliance with
applicable specifications, the CS receiver needs to track signals even when they are
not in compliance. The external commands allow the CS receiver to acquire and
track abnormal signals from unhealthy satellites. Second, all measurements are time
tagged to the satellite X1 epoch (see Section 4.3.1.1 for further details on the X1
epoch), whereas a typical user receiver time tags range measurements relative to the
receiver's X1 epoch. Synchronizing measurements relative to the satellite's X1
epochs facilitates the MCS's processing of data from the entire distributed
CS L-band Monitor Station Network. The CS receivers provide the MCS with 1.5-