positioning accuracies can be achieved.  Assuming the UERE error budget is

maintained, this generally means DOP conditions of PDOP < 6, HDOP < 4, VDOP <

4.5, and TDOP < 2.  URE and DOP are best measured during tests by a calibrated

reference receiver.  Computer programs which use the broadcast almanac to

predict periods of favorable DOP can assist field test scheduling.  The GPS System

Effectiveness Model (SEM) is one such program developed for the GPS JPO and

has been distributed to all NATO nations.  Other similar programs are commercially

available.

3.3  RECEIVER VELOCITY ACCURACY

GPS receivers typically calculate velocity by measuring the frequency shift (Doppler

shift) of the GPS D band carrier(s).  Velocity accuracy can be scenario dependent,

but 0.2 m/sec per axis (95%) is achievable for PPS receivers.  SPS velocity

accuracy is the same as PPS when SA is off.  When SA is on, SPS velocity

accuracy is degraded.  The amount of degra dation of the velocity is classified. 

However, although not guaranteed, SPS velocity accuracies around 0.4 m/sec 95%

have been observed by civilian users for the typical level of SA associated with

normal peacetime operations and 100 metres 95% horizontal position ing accuracy.

Velocity accuracy can be effectively tested in a laboratory environment, but field

testing can be difficult since a tracking system with 0.05 m/sec or better accuracy is

required.  The reader is urged to carefully consider the methods of testing if

velocity accuracy is an important mission requirement.

3.4  RECEIVER TIME ACCURACY

A dedicated PTTI port should normally be used for precise time output from a GPS

receiver. Significant time delays and uncertainties from microseconds to

milliseconds can be introduced if time output is accomplished via a digital data

interface.  For a PPS P code GPS receiver, tracking 4 satellites, an absolute time

accuracy of better than 200 nanoseconds (95%) relative to UTC is possible in a

stationary or low dynamic situation at an unsurveyed location.  Equivalent SPS C/A 

code accuracy is 340 nanoseconds (95%).  Higher dynamics will increase time

error.  Errors in the PTTI output result from errors in the GPS receiver as well as

the Control and Space segments.  The system time transfer error budget is shown

in Table 3 2. 

Processing errors in the GPS receiver and unaccounted time delays to propagate

the timing pulses to the PTTI port can add another 60 100 nanoseconds (95%),

depending on receiver design.  Therefore, a total (RSS) time error of 209 224

nanoseconds (95%) can be expected.

Typical 95% time accuracies expected for precise time dissemination for different

categories of GPS receivers are shown in Tables 3 3 and 3 4, assuming an RSS of

88 ns for the Control and Space Segment errors, and 78 ns for the PTTI error.

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