6.2.3  Geometric Dilution of Precision

As described previously in Chapters 2 and 3, GDOP is an important factor in

determining the accuracy of the position (or time) solution.  The combination of

satellites which gives the lowest DOP value will provide the most accurate solution,

assuming that all satellites have the same pseudorange error.  Depending on the user

mission, best PDOP, HDOP, or TDOP can be used as a satellite selection criterion.

6.2.4  User Range Accuracy

Each satellite broadcasts a user range accuracy (URA) value in subframe 1 of the NAV

msg.  URA is a prediction of the pseudorange accuracy obtainable from the satellite

signal in space.  URA is based on recent historical data and is therefore most accurate

immediately following an upload.  It does not include the UEE and therefore does not

include ionospheric compensation error if the ionospheric model is used instead of dual

frequency measurements.  These additional errors should be added to URA for the

best estimate of pseudorange accuracy, especially if the receiver is capable of

performing dual frequency measurements on some satellites and must use an

ionospheric model for others.  (Refer to "Technical Characteristics of the Navstar GPS"

or ICD GPS 200PR for a more detailed explanation of URA.)  URA can be used in

conjunction with DOP to choose the best combination of satellites when the satellites

have significantly different pseudorange errors.  This is done by using URA as a

weighting factor in the covariance matrix for user position and clock bias errors.  Since

URA is a prediction, it is not a guarantee of range accuracy, however, it can be used to

help deselect satellites with known large pseudorange errors.

6.2.5  Satellite Elevation Angle

Selecting satellites by computing a minimum DOP will favor the use of satellites at low

elevation angles.  However, signals from satellites at a low elevation angle must travel

a longer distance through the ionosphere and troposphere than signals from higher

angles.  They will therefore incur additional pseudorange error due to ionospheric and

tropospheric delay.  Many receivers will not use satellites below an arbitrary elevation

angle.  Five degrees is a typical lower limit.  This also helps to reduce multipath

problems.

6.2.6  External Aids

When an external aid is available to the GPS receiver, it can be incorporated into the

satellite selection algorithm.  It can be incorporated as a fixed mode of operation, an

optional mode of operation when only three satellites are visible, or it can be treated as

an additional "satellite" to be selected when the best combination of satellites includes

the aid. Decision logic for the first two cases is relatively simple.  If the aid is treated as

an additional satellite, the expected error and geometry must be modelled and included

in the satellite selection algorithm.  For example, mean sea level (MSL) aiding can be

considered to be equivalent to a satellite at the center of the earth with a UERE on the

order of a typical satellite (6 7 metres).  Other aiding schemes can be more complex,

depending on the complexity of the integration, error model, and equivalent geometry.

Barometric altimeter

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