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:
- Mixed short baseline: Mixed short baseline applications, including use of the ionospheric free linear combination and estimation of tropospheric parameter, and the comparison with the precisely known reference coordinates (e.g. ION GPS-98, see publications, additional experiments will be carried out).
- Large network processing: Comparison of two solutions of large network processing (identical antennas AOAD/M_T, precise ephemeris, ionospheric free linear combination, tropospheric parameter). The only difference in the processing (identical options/parameter) are the introduced absolute PCV in one solution. The difference reveals a sort of scale in the order of 0.014 ppm (still, it is a first order effect for long observations; there are also constellation dependencies), which shows the effect of neglected absolute PCV information (e.g. ION GPS-98, see publications, additional experiments will be carried out).
Two choke ring antennas, 24h observation, L0+trop-solution, -pcv (no PCV), +pcv (orientated absolute PCV), height differences to truth:
|az = 30°, zenith = 85°||- pcv (difference to truth)||15 mm|
|+ pcv||3 mm|
|az = 345°, zenith = 95°||- pcv||19 mm|
|+ pcv||4 mm|
- Large network simulation: In order to compare the coordinates of a large network with a true reference, we conducted a simulation with a rotated and inclined antenna on a known short baseline. There are no effects due to other error sources (satellite, atmosphere, coordinates). Therefore, a separation of the effect can be easily done, while comparing the results using absolute PCV with precisely known reference coordinates. Height differences are shown in the next table. The AOAD/M_T antenna was inclined +/- 5 degree (roughly corresponding to about 550 km network extension) and also rotated (using an antenna mount; will be repeated with a more extensive measurement program with the robot). We compared the solution with no corrections (comparable to actual processing using relative PCV in a network like IGS) as well as the one with introduced absolute (orientated!) PCV with known reference coordinates. Errors in the range of 1 cm showed up without absolute PCV. Using the absolute PCV, only differences in the mm-range compared to the reference coordinates can be seen.
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.