aircraft, as well as the original survey applications.  Near real time and real time

range implementations can achieve centimeter accuracies (fractions of a carrier

wavelength) and post processing surveying techniques can achieve millimeter

range accuracies.  Surveying receivers are described in more detail in paragraph

2.6.

The accuracy of differential corrections developed at a single site will degrade with

distance from the site due to increasing difference between the reference and

mobile receiver ephemeris, ionospheric, and tropo spheric errors.  Such systems are

usually called local area differential GPS systems (LADGPS).  The accuracy of the

corrections can be extended over a larger area by using a network of reference

receivers to develop the corrections, and by modifying the correction algorithms in

the user receiver.  RCD systems which compensate for distance degradations are

usually called wide area differential GPS (WADGPS) systems. CPD systems which

compensate for distance degradations are usually called very long baseline

interferometry (VLBI) systems.

CPD techniques (interferometry) can also be used to determine platform attitude.

In this case, the processing can be contained within one receiver using multiple

antennas.  The distinction is lost between which antenna is the "reference" and

which is "mobile," since all are located at fixed positions on the platform and none

are located at surveyed positions with respect to the earth.  Since the antennas are

separated by fixed distances, and since their relationship to the center of mass of

the platform is known, it is possible to convert the carrier phase differences into

angular differences between the antenna locations and the line of sight to a

satellite.  By using measurements from multiple satellites, or the position of the

platform from a GPS position fix, these angular differ ences can then be transformed

to represent the attitude of the platform with respect to the local vertical axis.

There are several standard (and numerous proprietary) broadcast protocols,

receiver interfaces, data formats, data sets, and sets of algorithms that have been

developed for DGPS applications.  Consequently DGPS receivers are typically

designed with a particular application in mind and may not be suitable for a

different application.  Similarly, proprietary systems may not be compatible for the

same application.  Therefore, DGPS requirements should be investigated

thoroughly and candidate DGPS receivers or systems should be evaluated for

suitability and compatibility.

2.6  SURVEYING RECEIVERS

Formal surveys are typically conducted with one surveying receiver located in a

previously surveyed location and a second receiver at the new location to be

surveyed.  The receiver at the previously surveyed location acts as a DGPS

reference receiver and the receiver at the new location acts as a DGPS "mobile"

receiver.  The "mobile" receiver is usually fixed at the new location for a period of

time to collect redundant measurements and further improve the accuracy of the

survey by post processing to remove or reduce residual errors such as receiver

measurement noise.  The period of time can range from seconds to days depending

on the survey accuracy required.  Consequently, surveying

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