receivers must include considerable data recording capability.  They may also

include the capability to store additional information about the characteristics of the

surveyed site.

Many surveying receivers have the capability to do a "self survey", that is, develop

a smoothed or averaged position from non differential GPS measurements.  Non 

differential (absolute) surveys require considerably more time than DGPS surveys

to develop the same accuracy.  However, the technique can be useful to establish a

reference point for subsequent DGPS (relative) surveys at locations where a formal

reference point is incon venient or unavailable.  This capability can be especially

valuable for tactical military survey applications where the relative location of the

surveyed sites is more important than the absolute location or where centimeter

accuracy is unnecessary.

Most surveying receivers can also function in some capacity as navigation

receivers, thereby providing guidance for the surveyor to previously surveyed sites.

Additional software functions may also be provided to support datum

transformations, post processing, and other related survey functions.

The signal processing techniques of GPS surveying receivers can be divided into

four categories:

a. Non differential GPS

b. Ranging Code Differential

c. Carrier Phase Differential (Interferometry)

d. Codeless Carrier Phase Differential

As described above, many surveying receivers have a non differential GPS mode

for navigation and self surveys.  The signal processing techniques and accuracies

obtained are similar to other non differential receivers as described in Chapter 1.

Surveying receivers may use RCD to determine an initial survey position that aids

the initialization process.  The more accurate the initial position, the more quickly

initialization can be completed for real time applications.  Even if the final results

are post processed to obtain maximum accuracy, real time outputs can provide

preliminary results that confirm the success of the survey in the field or enable the

surveyor to detect and correct problems that may occur.  The primary surveying

mode of most surveying receivers is CPD.  The carrier phase measurements and

algorithms enable centimeter and sub centimeter accuracies in part due to

significantly lower measurement noise when compared to pseudorange

measurements. As in non differential GPS, iono spheric errors can contribute

significant errors. However, in surveying applications, dual frequency (P code)

measurements are almost essential to achieve surveying accuracies.  Since the P 

code is normally only available as the Y code, most surveying receivers use a

"codeless" technique to perform ionospheric delay meas urements. One technique

uses spectral compressors to compress the GPS signals into audio or subaudio

bands.  A processor is used to extract the frequency and phase of each satellite in

view.  Another technique is to split the received satellite signal and multiply it by

itself to obtain a second harmonic of the carrier phase shift which does not contain

the code modulation.  Codeless tech niques can also be used to make CPD

measurements but the C/A code naviga tion message must also be read to obtain

the satellite ephemeris if real time outputs are desired.

2 6