Performance of Stand-Alone GPS - Understanding GPS: Principles and Applications - page 335

which GPS accuracy is to be determined. The availability of the GPS navigation
function to provide a given accuracy level is therefore dependent on the geometry of
the satellites for a specific location and time of day.
In order to determine the availability of GPS for a specific location and time, the
number of visible satellites, as well as the geometry of those satellites, must first be
determined. GPS almanac data, which contains the positions of all satellites in the
constellation at a reference epoch, can easily be obtained from the U.S. Coast Guard
(USCG) Navigation Center (NAVCEN) Web site or as an output from some GPS
receivers. Since the orbits of the GPS satellites are well known, the position of the
satellites at any given point in time can be predicted. The process of determining the
satellite positions at a particular point in time is not intuitive, however, and soft-
ware is needed in order to perform the calculations.
7.4.1 Predicted GPS Availability Using the Nominal 24-Satellite GPS
Constellation
This section examines the availability of the nominal 24-satellite GPS constellation.
The nominal 24-satellite constellation is defined in Section 3.1.1. Worldwide GPS
coverage is evaluated from 90ºN to 90ºS latitude with sample points spaced every 5º
(in latitude) and for a band in longitude circling the globe spaced every 5º. This grid
is sampled every 5 minutes in time over a 12-hour period.
Since the GPS constellation has approximately a 12-hour orbit, the satellite cov-
erage will then repeat itself on the opposite side of the world during the next 12
hours. (The Earth rotates 180º in the 12-hour period, and the satellite coverage
areas will be interchanged.) A total of 386,280 space/time points are evaluated in
this analysis.
GPS availability also is dependent on the mask angle used by the receiver. By
lowering the mask angle, more satellites are visible; hence, a higher availability can
be obtained. However, there may be problems with reducing the mask angle to
include very low elevation angles, which is discussed later in this section. The avail-
ability obtained by applying the following mask angles is examined: 7.5º, 5º, 2.5º,
and 0º.
Figure 7.8 demonstrates GPS availability based on HDOP using an all-in-view
solution. This figure provides the cumulative distribution of HDOP for each of the
mask angles considered. The maximum value of HDOP is 2.55 for a mask angle less
than or equal to 5º.
Figure 7.9 provides the availability of GPS based on PDOP for the same mask
angles. This availability is lower than that for HDOP since unavailability in the ver-
tical dimension is taken into consideration in the calculation of PDOP. The maxi-
mum value of PDOP for a 5º mask angle is 5.15, at 2.5º it is 4.7, and for a 0º mask
angle the maximum value is 3.1.
Although these graphs demonstrate the improvement in availability that can be
obtained when the mask angle is lowered, there is a danger in lowering it too far.
During the mission planning process, signal blockage from buildings or other
objects that extend higher than the set mask angle must be taken into consideration.
There also is a greater potential for atmospheric delay and multipath problems at a
lower mask angle.