to be different for each satellite at any instant in time and tends to be at a minimum

following a new navigation message upload.

GDOP is an instantaneous measure of the error contributed by the geometric

relationship of the satellites as seen by the receiver.  GDOP is a dimensionless

multiplicative factor.  For a given value of UERE, small GDOP values mean more

precise position and/or time.  GDOP varies because the satellites are in constant

motion and their geometric relationships are constantly changing.  Consequently

GDOP can vary with time and user location.  The "average" GDOP tends to induce

a circular error distribution in the horizontal plane with the vertical contribution of

error approximately 1.5 times the horizontal contribution.  In real time, GDOP can

be asymmetrical in the three dimensions and vary significantly from the average or

typical case, however, GDOP can be easily measured by the receiver, and is often

used to select optimum combinations of satellites for the position solution or to

develop real time accuracy estimates.

GDOP distributions are not Gaussian, particularly in the tails of the distribution. 

The global distribution of GDOP can vary significantly at the 95% probability level

due to temporary "vacancies" in the GPS constellation, while remaining relatively

constant at the 50% probability level where the GPS PPS system accuracy

requirement (16 metres SEP) is defined.  Therefore, PPS 95% accuracy

specifications derived from this requirement may not be rigorously maintained

through all the possible states of the GPS constellation.  However, although small

variations in accuracy performance are likely with each change in the constellation

state, worst case situations are worst case for all users and by all measures of

system performance, and will therefore be avoided or quickly corrected by the

Control Segment.  (Temporary "vacancies" in the satellite constellation can be

expected over the life of the system due to preventive maintenance, satellite end 

of life failures and delayed replacements, or random satellite failures that are

correctable by the Control Segment.)

UERE and GDOP are explained in more detail in paragraphs 3.1.2 and 3.1.3.  It

should be noted that these errors are constantly present as normal variations in

accuracy, even with a complete GPS constellation and correctly operating

satellites, Control Segment, and receiver.

3.1.2  GPS PPS System Range Error Budget

The GPS PPS system range error budget is presented in Table 3 1.  The budget is

expressed for the 95% probability level of the system UERE.  This is a UERE averaged for

all satellites over a 24 hour period.  Therefore, the long term (greater than 24 hours) one 

sigma UERE for an individual satellite can exceed this value and the system can still meet

the accuracy requirements specified in the previous paragraph.  The instantaneous UERE

of all satellites will typically exceed this value at sometime during a 24 hour period.  From

the user point of view, the important values in this error budget are those allocated to the

User Segment.  These are excellent guidelines for the purchase or development of

receivers because they are independent of the performance of the Space and Control

segments.

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