required in the user receiver when DGPS corrections are utilized in the position

solution.  As described above, the DGPS measurements/corrections ensure the

integrity of the corrected measurements used by the user receiver.  If the reference

station validates the measurements/corrections sufficiently to guarantee the

broadcast UERE, the only other place an integrity problem can occur is the user

receiver.  The user receiver can perform internal validity checks, other than RAIM,

on its measurements and processing to detect any internal integrity problems.  In a

wide area DGPS system, RAIM may be useful to detect extreme ionospheric or

tropospheric errors, if the desired differential accuracy can be degraded by these

effects.

Information provided in a correction message includes the measurement time of

each correction and the "Issue Of Data" (IOD) of the Ephemeris that was used to

determine the corrections.  It should also include a UDRE value for each satellite. 

The significance of these parameters is explained in the following paragraphs.

Most differential user receivers check the measurement times to automatically stop

using reference station measurements (corrections) that exceed a predetermined

age.  DGPS position error increases as the measurement age increases because

the reference station and user errors decorrelate with time.  A user receiver

propagates ranging code corrections to the current time using the range rate

corrections or propagates its own carrier phase measurements to the reference

station measurement time.  This decreases the effect of the measurement age, but

does not eliminate it.  The major change in SPS measurements is due to SA. 

Consequently, the time for SPS measurements to become completely decorrelated

is around two minutes.  The major change in PPS measurements is due to satellite

geometry. Therefore, PPS measurements can remain correlated for 10 minutes or

more.

In order to maintain the integrity of a DGPS position solution, all corrections must

be provided by the same reference station or by time synchronized reference

stations.  This is because the correction values are dependent on the reference

station clock.  Mixing corrections from unsynchronized reference stations can

generate unpredictable clock and range errors in the user receiver.

The corrections must also be based on the same IOD values that the user receiver

is using.  If the ephemeris data used to develop the corrections is different than the

ephemeris data used in the user receiver, then the magnitude of the corrections will

be invalid.

The UDRE is the differential equivalent of the URE for uncorrected satellites.  It is a

statistical measure of the expected residual range error after the corrections are

applied. The correction UDRE values can be used in conjunction with the satellite

geometry to calculate an estimate of the differential position error.  The user

receiver contribution to the  total User Equivalent Differential Range Error (UEDRE)

must also be considered in the  position error calculations. (Different receivers can

have different measurement errors.) If the reference station validates the broadcast

UDRE values, the position error estimates can be used as an integrity check, much

in the same way that RAIM is used by nondifferential  receivers.  That is, estimates

of the user position accuracy can be periodically compared against the minimum

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