intended only for limited use until an international standard can be developed for

precision approach.

10.3  DGPS IMPLEMENTATION TYPES

There are two primary variations of the differential measurements and equations. 

One is based on ranging code measurements and the other based on carrier phase

measurements. There are also several ways to implement the data link function. 

DGPS systems can be designed to serve a limited area from a single reference

station, or can use a network of reference stations and special algorithms to extend

the validity of the DGPS technique over a wide area.  The result is that there is a

large variety of possible DGPS system implementations using combinations of

these design features.

10.3.1  Ranging Code Differential

The ranging code differential technique uses the pseudorange measurements of

the reference station to calculate pseudorange or position corrections for the user

receivers.  The reference station calculates pseudorange corrections for each

visible satellite by subtracting the "true" range, determined by the surveyed position

and the known orbit parameters, from the measured pseudorange.  The user

receiver then selects the appropriate correction for each satellite that it is tracking,

and subtracts the correction from the pseudorange that it has measured.  The

mobile receiver must only use those satellites for which corrections have been

received.

If the reference station provides position corrections rather than pseudorange

corrections, the corrections are simply determined by subtracting the measured

position from the surveyed position. The advantage of using position corrections is

obviously the simplicity of the calculations.  The disadvantage is that the reference

receiver and the user receiver must use the exact same set of satellites.  This can

be accomplished by coordinating the choice of satellites between the reference

receiver and the user receiver, or by having the reference station compute a

position correction for each possible combination of satellites.  For these reasons, it

is usually more flexible and efficient to provide pseudorange corrections rather than

position corrections.  The RTCM SC 104 and RTCA DO 217 formats are all based

on pseudorange rather than position corrections.

The pseudorange or position corrections are time tagged with the time that the

measurements were taken.  In real time systems, the rate of change of the corrections

is also calculated.  This allows the user to propagate the corrections to the time that

they are actually applied to the user position solution.  This reduces the impact of data

latency on the accuracy of the system but does not eliminate it entirely.  SPS

corrections become fully uncorrelated with the user measurements after about 2

minutes.  Corrections used after two minutes may produce solutions which are less

accurate than stand alone SPS GPS.  PPS corrections can remain correlated with the

user measurements for 10 minutes or more under benign (slowly changing) ionospheric

conditions.

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