3. Discussion of Absolute PCV -  Effects, Large Networks, Engineering Applications


The consideration of absolute PCV corrections plays an important role for precise GPS coordinate determination in several fields of application. Within large networks, simultaneously received satellite signals differ in their directions, because of the baseline length and the different orientation of the antennas. Absolute PCV are needed for this observation constellation. Relative PCV information is not sufficient (relative PCV are zero for identical antenna types, but no corrections are obviously insufficient for differently orientated antennas). The other main advantage of absolute PCV is their high resolution (-1- repeatability, -2- significant results for azimuthal PCV, -3- PCV information down to elevation zero). This information is needed for a variety of precise GPS applications, as for example in engineering surveys (e.g. inclined antennas in railroad surveying or machine guidance), in reference networks with mixed antenna types of user and provider and also for "standard" processing of regional networks (observations lower than 10 degree elevation contribute to a better separation of troposphere and height).

The AOAD/M_T choke ring antenna is the most common antenna type within the global network of the International GPS Service (IGS). Furthermore, it serves as the reference antenna for relative PCV calibrations. This group already presented some examples with absolute PCV for the AOAD/M_T (beside other mixed antenna applications) verifying the functionality and the effects of absolute PCV (e.g. ION GPS-98, see publications).

Separation of absolute PCV from other errors (satellite antenna / troposphere / coordinates)

For the verification of the absolute PCV our group did not carry out direct comparison of network results with ITRF, since the ITRF implies VLBI, Laser AND also GPS results. It is almost impossible to separate the individual error terms from PCV effects and a (sub-) mm true reference is generally not available. Our subsumtive way to verify the absolute PCV - correctness and effects - always avoids most error components and refers to a well known reference:


(AOAD/M_T IGS network, difference of +/- absolute PCV solutions)

(AOAD/M_T IGS network, different lenght of baselines +/- absolute PCV solutions)


Two choke ring antennas, 24h observation, L0+trop-solution,
-pcv (no PCV), +pcv (orientated absolute PCV),
height differences to truth:


az = 030 °, zenith = 85 ° - pcv (difference to truth)
15 mm
+ pcv
 3 mm
az = 345 °, zenith = 95 ° - pcv
19 mm
+ pcv
 4 mm

(inclined and rotated AOAD/M_T on antenna mount)

The results of all these tests underline the correctness of the absolute PCV, show their influence on GPS network solutions and verify the succesful separation of the absolute PCV antenna effect.

More tests and examples for diverse areas of applications with new absolute PCV results will be shown and published soon.
 

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Last update: 17.05.2001 by Geo++