Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 461

Start

Beacon St.

Commonwealth Ave.

Actual Route
Navigated Route

Longitude

71.105

Figure 9.1 Effects of signal blockage on GPS receiver operation.

in an urban environment. The periods of shading (i.e., less than three-satellite avail-
ability) are caused by buildings and are denoted by the black lines in the lower por-
tion of Figure 9.1. (This experiment was conducted when five to six satellites above
a 5º mask angle were available for ranging.) When only three usable satellite signals
are available, most receivers revert to a two-dimensional navigation mode by utiliz-
ing either the last known height or a height obtained from an external source. If the
number of usable satellites is less than three, some receivers have the option of not
producing a solution or extrapolating the last position and velocity solution for-
ward in what is called dead-reckoning (DR) navigation. Inertial navigation systems
(INSs) can be used as a flywheel to provide navigation during shading outages.
The discrete-time nature of the GPS solution in some equipment is also of con-
cern in real-time applications, especially those related to vehicle control. As shown
in Figure 9.2, if a vehicle's path changes between updates, the extrapolation of the
last GPS measurement produces an error in the estimated and true position. This is
particularly true for high-dynamic platforms, such as fighter aircraft. In applica-
tions where continuous precision navigation is required, inertial sensors can be
employed. An alternative solution is the use of a GPS receiver that provides higher
rate measurement outputs. In principle, rates on the order of 100 Hz are possible.
In addition to providing navigation continuity during short GPS shading out-
ages and between GPS sensor position outputs, an INS, when calibrated using a
Kalman filter (see Section 9.2.3), can be used to improve the GPS receiver perfor-
mance in two other ways. First, the information that is maintained by the integra-