SELECTIVE AVAILABILITY |
As mentioned in section 2.4, due to the surprisingly good Standard Positioning
Service (SPS) accuracy for point position determination, the policy of Selective Availability (SA) was endorsed in order
to artificially widen the gap between the civilian and U.S. military
positioning services (GEORGIADOU
& DOUCET, 1990). SA is an intentional degradation of the accuracy
of GPS horizontal positioning to 100m (at the 95% confidence level), and
height determination to 150-170m (at the 95% confidence level), for SPS
users. SA has been implemented since 25 March, 1990, and consists of two
different components:
) component consists of
the truncation of the orbital information transmitted within the Navigation
Message so that the WGS84 coordinates of the satellites cannot be computed
correctly. There is, however, no evidence to date of this effect on
the broadcast ephemerides.
) component which is achieved
by dithering the satellite clock output frequency. The varying
errors in the satellite clock's fundamental frequency has the same direct
impact (as an artificially induced "satellite clock bias") on
both the pseudo-ranges and carrier phase observations -- see section
6.2.2. The variations in range have amplitudes of as much as 50m, with
periods of several minutes (see Figure below).
Range measurement errors (left) and point position errors (right) at a static
site, before and after SA was implemented.
(After GEORGIADOU & DOUCET,
1990)
It should be emphasised that the situation as far as SA is concerned, is now under almost continuous review due to the proliferation of real-time Differential GPS services which have effectively replaced the SPS for many navigation-type applications. LACHAPELLE (1995) and SANDLIN et al (1995) report that it has been recommended that SA be deactivated within the next few years. For the GPS surveying community however, SA was never a serious problem because it was very effectively nullified through the use of the relative positioning mode.
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© Chris Rizos, SNAP-UNSW, 1999