Revised: 04/01/97
United States authorities have announced a partial solution
to the Global Positioning System (GPS) Selective Availability (SA) problem.
Beginning at the transition between 23:59:59 GPS
Time (GPS Time is currently ahead of UTC by eleven seconds)
on March 31, 1997 and 00:00:00 GPS Time on April 1, 1997, the new Global
Positioning System Availability Function
(GPS AF) will become operational.
SA is the intentional degradation of the GPS Standard
Positioning Service (SPS) through the introduction of slowly varying biases
with correlation times from as few as five seconds to
several hours. Because the SA bias introduced into each GPS
Space Vehicle (SV) signal is controlled separately through an encrypted
pseudo-random noise (PRN) generator, the
simple averaging of positions obtained while tracking GPS SV
signals does not provide a significant reduction in error unless this averaging
is done over periods of several hours.
Differential GPS (DGPS) has provided a solution to the SA
problem for many years. By applying individual corrections, computed through
the use of a DGPS reference station at a
known location, to each individual SV pseudo-range
measurement prior to the position solution , accuracies of 2-3 meters can be
consistently obtained. DGPS requires access to these
correction either through real time radio links or through
computer data files for post-processed application.
GPS SPS users have had to accept the 100 meter horizontal
(156 meter vertical) position error in GPS, or pay for the required equipment
and be within range of a DGPS service. This
has placed severe restrictions on the civil use of GPS. Some
users, requiring accurate positions only occasionally, or time and frequency
users who only need precise GPS timing signals
periodically, have lobbied for some time for a sub-set of
SVs to operate without SA. Others have suggested only emergency implementation
of SA. Most users who already know their
position (through long term averaging or by utilization of a
United States Geological Survey 1:24,000 scale topographic map) have noticed
that GPS position solutions do occasionally
approach a minimum error at least once during each hour.
On April first of this year (1997) what may be a solution
for many GPS SPS users will be implemented. The Global Positioning System
Availability Function (GPS AF) is a method by
which users can compute specific moments in time when SA
reaches a minimum for the combination of SVs tracked by any SPS receiver. The
simple AF algorithm can be implemented
either in real time or in post-processed applications.
AF is a simple algorithm based on the GPS Week Number (the
number of weeks from the GPS epoch of June 5, 1980), the GPS Second (the
seconds in GPS Time from the beginning
of the week (Saturday midnight GPS Time), and the PRN (the
satellite C/A code identification number ) numbers of each of the SVs tracked
by the receiver. The GPS week number
(899 for the week of March 30 through April 5, 1997), is
first added to the sum of each of the PRN numbers of the tracked SVs. The
result modulo the GPS Second divided by 1023 is
the time during that hour when the SA terms for those
satellites combines to a minimum value.
Users should be aware that this does not remove ionospheric
delays, multipath errors, receiver noise, or tropospheric delays in the GPS SPS
signals. The Availability Function only
reduces the effects of SA for those users who apply the
algorithm. The AF algorithm does not provide a means of removing the effects of
SA except for those specific moments of time
(+/-5 seconds) predicted by the AF algorithm. The AF
algorithm is designed to allow prediction of SA minimums only and does not
provide a means of removing SA between these
predicted SA nulls.
Users should cautiously apply the GPS AF algorithm and it is
suggested that they always compare results with a DGPS-derived solution or a
map. Timing users can simply check the
GPS time solution by comparing to a known 1PPS (one-pulse
per second) standard. Frequency users can compare the special purpose GPS
frequency control receiver output to an
inexpensive rubidium standard. After a period of initial operational testing (not specified) AF may be considered as a part of the full operational capability of GPS.