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propagation can be removed internally by the UE set. Since only the military will be capable of
simultaneously monitoring both of the frequencies, civilian users will be forced to live with this error.
16.30. Survivability and Security. For the GPS system to be successful, it must be able to survive and
operate in spite of hostile actions against it. From the program's outset, GPS development efforts have
emphasized the importance of system survivability. The potential threats to the space, control, and user
segments have been scrutinized and counter threat features have been incorporated into the system's
design.
346 AFPAM11-216 1 MARCH 2001
16.30.1. The main threat to the space segment is direct attack of GPS satellites by lasers, particle beam
weapons, and anti-satellite missiles. To counter these threats, all satellites incorporate nuclear and laser
hardening features. Since GPS employs a large constellation of satellites with three orbiting spares, the
direct attack on a small number of satellites will not render the entire system inoperative. Rather, it
would lead to a gradual degradation of system performance. If a satellite is destroyed, a spare will be
positioned near the failed satellite. If a spare is not located nearby, adjacent satellites will be moved to
provide optimum coverage until a new satellite can be launched and positioned. There will be a period
of a few hours during which a satellite being repositioned will be unavailable for service and up to 24
hours if one of the backup atomic clocks is brought into service. The overall impact on the system will
be minimal.
16.30.2. The control segment, composed of the MCS and the remotely located monitor stations and
ground antennas, could be subject to direct enemy attack, sabotage, or high-energy jammers. To counter
control segment threats, redundant monitor and ground antennas were used so that deactivation of any
one station will not affect the system as a whole. To protect against enemy attack or sabotage, a high
security protection system is employed at the MCS, monitor stations, and ground antennas. To counter
the threat of jamming or spoofing, encrypted data is used in all ground-to-space telemetry command and
control links. In the event the MCS is destroyed or unable to upload data to the satellites, the system will
not collapse. The system will suffer some gradual degradation of performance, but GPS will remain a
viable navigational aid.
16.30.3. The key threats to the user equipment sets include electromagnetic pulse (EMP) from nuclear
detonations and jamming or spoofing from unfriendly sources. Should hostile activity result in shortterm
GPS system outages, the impact on navigation should be minimal since most aircraft have a
navigator as a crewmember, or are equipped with a self-contained navigation system (such as an INS).
To counter the effects of EMP, all military sets are radiation hardened. To reduce or negate the impact of
jamming or spoofing, the spread-spectrum L-band satellite signals provide substantial resistance.
16.30.4. In extremely hostile electronic warfare environments, high dynamic sets, in conjunction with
systems such as INS, have been designed to further counter the jamming threat. Such sets are
sophisticated enough to estimate the approximate ranging data they should receive from each selected
satellite. If the received signal does not match this expected data, it is rejected as being spurious and is
ignored when determining the user's position. The possibility always exists that unfriendly forces could
use the system during national or international conflicts. To preclude this possibility, the selective
availability features will permit authorized users to navigate with full accuracy while unauthorized users
will experience degraded navigational performance. However, recent advances in DGPS and local area
augmentation can now provide equal or better accuracy than the encrypted Y code signal and are
available to virtually everyone.
16.31. Summary. GPS accuracy is unquestionable, producing accuracies within meters. An INS backup
cannot be jammed or intercepted and will provide acceptable accuracy in all weather. Combination INSGPS
units in development now will be standard equipment by the end of the century. New computers
that take up less room than a briefcase can process multiple inputs from several sensors (GPS, INS, and
Doppler) and will also provide wind, heading, course, distance, and time information automatically.
Nevertheless, we always include an aviator as part of a weapon system. This is especially true on
complex operational missions requiring the flexibility that only a person can provide. On such a mission,
these sophisticated pieces of equipment are only tools that an aviator uses to make important decisions.
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