GNSS Markets and Applications - Understanding GPS: Principles and Applications - page 655

be military use of GALILEO signals. Though conceived as a civil system, GALILEO
contains an encrypted PRS that could be used militarily.
The first military applications were with man-operated receivers on ships and
other vehicles. As coverage increased with every new satellite launch, additional
applications emerged until GPS became not only a useful tool; it became an essential
capability for modern, network-centered warfare.

12.9.1 Military User Equipment—Aviation, Shipboard, and Land
The original development of GPS receivers was accomplished at the Magnavox
Research Laboratories (later acquired by Hughes Aircraft and subsequently by
Raytheon). Some typical receivers were produced for aircraft first in a standard avi-
onics package known as a 3/4 ATR size (Collins 3A) shrinking its width in half later
to a 3/8 ATR (Collins and Raytheon MAGR) and to man-portable units like the
Collins PLGR and finally to today's DAGR, which approaches the size and weight
of a commercial handheld receiver. Figure 12.4 shows an airborne military receiver.
At the time of this writing, prototype M code receivers were under development by
these companies.
As many military aircraft already had inertial navigation systems installed,
work began to marry the long-term stability of GPS by virtue of its atomic timing to
the short-term stability of the inertial system to create integrated navigation systems
that could maintain very accurate solutions regardless of short outages to GPS
caused by signal interference, vehicle dynamics, or antenna shading. Similar but far
simpler integrations were performed for today's car navigation systems using head-
ing sensors, wheel counters, and map matching. The integrations became even more

Figure 12.4 Raytheon MAGR2000 airborne GPS receiver. (Courtesy of Raytheon.)