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Unformatted text preview: ge performance in terms of parameters under direct
operational control. Although this standard definition approach has significant advantages in
managing GPS performance, the standards cover only a portion of the total service an SPS user
will experience. To mitigate this limitation in the standards definition, estimates are provided in
Section A-5.5 of end user performance behaviors a single-frequency receiver may experience on
an “average” day during solar maximum.
Two final aspects of accuracy of concern to civil users and in particularly differential service
providers are range rate error and range acceleration error. Previous documents provided
performance standards for these two parameters that reflected limits on the contributions of SA.
With the termination of SA, users experience range rate error and range acceleration error
behaviors that are primarily products of the receiver environment, with infrequent transients due
to satellite navigation uploads or internal satellite design idiosyncrasies (discussed in Section A4.4). The discussion in Section A-5.6 focuses on steady state characteristics of range rate and
range acceleration error. A-5.1 Geographic Variations in SPS SIS Positioning Errors GPS position errors change as a function of latitude primarily due to constellation geometry, as
shown in Figure A-5-1. As seen in Figure A-5-1, performance can be asymmetric between
Southern and Northern latitudes. This asymmetry is caused by variations in URE due to satellite
upload patterns, and variations in constellation geometry due to short-term satellite maintenance
outages. Vertical accuracy is affected significantly more by latitude than horizontal error, due to
the nature of the constellation geometry. Vertical accuracy suffers significantly above
approximately 55° latitude (north or south) due to the lack of satellites at high elevation angles
over the poles. Horizontal accuracy degrades mildly at mid-latitudes due to worsening horizontal
geometry as latitude increases. However, horizontal accuracy begins to improve above
approximately 45° latitude (north or south) as satellites visible from the other side of the Earth
begin to enter into the position solution geometry.
Figure A-5-2 illustrates how accuracy varies as a function of longitude. The primary element in
longitudinal variation of accuracy is the navigation upload patterns of the GPS satellites. A large
percentage of satellites are uploaded over the continental United States or shortly before the
satellites enter visibility over Europe. This trend can be seen very clearly in the vertical
component in Figure A-5-2 and less clearly in the horizontal component. Page A-25 October 2001 GPS SPS Performance Standard Example of GPS SPS Position Accuracy as Function of Latitude
1 June 2000 8 ANALYSIS CONDITIONS 7 - Analysis conducted under
normal constellation operational
- NANUs applied over intervals
defined by 2 SOPS
- Global 2 x 2 degree grid
- 24 hour...
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- Fall '09