Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 527

tion that calculates the position of the host vehicle. In determining the vehicle's posi-
tion, altitude information derived from a terrain map of the local area can be used to
improve the accuracy and reliability of the solution.
An early example of ephemeris-aiding was the Motorola EAGLE line of GPS
receivers. Introduced in 1985, it was one of the first commercially available GPS
receivers to offer a form of ephemeris aiding [61] in one of its operational modes.
Inherent in its design, and when two receivers were used in a differential mas-
ter/slave configuration, the master station sent DGPS range and range rate data for
all satellites tracked by the master station. In addition to this information, the mas-
ter station transmitted the ephemeris data for all satellites tracked by the master sta-
tion using a commutated message structure. A few parameters of each ephemeris
were sent with each DGPS correction message, allowing eventual broadcast of all
ephemeris data for all visible satellites to the slave receiver. The master-station
ephemeris information was used by the DGPS slave receiver to:

• Enable the best DGPS position performance by ensuring both master and
slave units were using the same ephemeris set for each satellite (this was prior
to the development of the RTCM-104 DGPS messaging standard discussed in
Chapter 8).

Ensure that the slave unit acquired the ephemeris data for all satellites visible
by the master, maximizing the availability of DGPS solutions.

•

The latter was especially useful when the slave unit was partially blocked from
acquiring the data directly from the satellite because of blocked or reduced signal
power to one or more satellites, which occurs near tall mountains, in canyons,
under trees, or near buildings. Many times in these environments, the signal is
strong enough to detect code phases and track but not strong enough to reliably
demodulate the ephemeris data. Transmitting the ephemeris data from the master
to the slave unit alleviated this problem.
In 1990, a system was patented [62] that transmitted almanac data via an
over-the-air message from a master station to many slave units called pseudorangers .
The pseudorangers would accept the almanac data, use it to acquire and track GPS
satellites, then transmit back to the master station the measured pseudoranges and a
time stamp. The master station, remote from the movers, would then compute posi-
tion of each mover from the pseudoranger unit-measured pseudoranges. This idea is
a precursor of the current MS-assisted method of assisted GPS (A-GPS) for cellular
telephones (the preferred method in the United States in CDMA phones) in which the
handset outputs pseudoranges instead of position, and the network then uses the
measured pseudoranges to compute the location of the cell phone.
Approximate position, ephemeris, almanac, and approximate time-assist infor-
mation was present in a White Sands Missile Range system [63]. The White Sands
system used GPS to measure the performance of missiles. When a missile is fired, it
has little time to acquire and track GPS satellites and cannot tolerate the 30-second
ephemeris acquisition period. Thus, a wireless message was sent from a master sta-
tion to the just-launched missile consisting of approximate position, approximate
time, almanac data, and ephemeris data, all of which was used by the missile to
acquire GPS signals rapidly and produce a string of position reports while in flight.