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GPS antenna: from relative to absolute
Aristidis Fotiou,Christos Pikridas,Miltiadis Chatzinikos

Application to a Greek regional network.

Absolute calibration

The absolute calibration antenna models have been developed by the Geo++ company located in Garsben - Germany and a group of researchers from the University of Hannover (Wübbena et al. 2000, Völksen and Menge 2002). Absolute antenna offsets and PCV values are determined by means of a robotic system which include azimuthal

Fig 2: Differences in the vertical component caused by the transition from relative to absolute PCVS.

values and elevations down to 0?. The robot carries out fast rotations on different axes increasing the efficiency of the method. An advantage of this technique is the determination of a 3D offset and PCVs from 0? to 90? elevation angles with high precision and accuracy. Moreover the whole process takes place in an almost multipath - free environment. A complete set of absolute PCVs for the known tracking antennas is nowadays available.

Studying the impact of absolute versus relative calibration models

In order to estimate the impact of switching from relative to absolute calibrated antennas, a regional network of nine GPS stations was processed and adjusted. Eight of the selected stations are located in Greece and one station in Italy: AUT1 in Thessaloniki, NOA1 in Athens, TUC2 in Crete island, LEMN in Limnos island, PRKV in Lesvos island, RLS in Achaia, VLSM in the island of Cephalonia, SPAN in Lefkada island and the IGS/EPN station MATE in Matera (Italy). Four of the above stations (AUT1, NOA1, TUC2, MATE) are part of the EUREF GPS Permanent Network (EPN), (Bruyninx, 2004) while the rest five stations belong to the National Observatory of Athens Permanent GPS Network. Itshould be noted that the antennas types of the test network were LEIAT504 (AUT1, NOA1, TUC2), LEIAX1202 (RLS, VSLM), LEIAX1202GG (LEM, PRKV, SPAN) of Leica Geosystems and TRM29659.0 (MATE) of Trimble.

The Bernese GPS Software v5.0 (Dach et al., 2007) was used to process the data. The standard method was based on the script file RNX2SNX with slight modifications (Chatzinikos et al., 2007). Some basic options of the used processing strategy should be pointed out:

IGS orbits and pole information
10o of elevation mask
Niel mapping function for the tropospheric refraction
One tropospheric parameter for each hour
MATE station kept fixed
Strategy baseline selection: shortest
Fix all phase ambiguities

The time period of GPS observations covers seven days: 190 GPS day to 196 GPS day, 2007. Daily solutions of the GPS network were processed first and the multi-session solution for the whole period was followed.

The impact on point positioning from relative to absolute calibration antenna models was studied by coordinate comparisons carried out by two different network solutions where MATE station kept fixed:

In the first network solution the IGS05 reference frame and the absolute phase centre variation values (file: IGS05_1455.atx) for the GPS antennas were used.
In the second solution the IGS05 reference frame and the relative phase centre variation model (file: IGS_01. atx) for the GPS antennas were used.

In figures 1 and 2 differences caused by the transition from relative to absolute models in the horizontal and vertical component are shown accordingly. In figure 1 it is clear that latitude is more affected than longitude. For the horizontal component, the difference between the coordinates of the two solutions varied from 1 to 5.5 mm. Almost all GPS stations showed the same horizontal displacement, a strong indication of a systematic error between the two models.

By contrast with the horizontal component the vertical component showed larger displacements, in the range of 8 to 17mm. The largest vertical displacements were found in NOA1, LEMN and PRKV, being of the order of 16mm to 17mm. Although the antenna types of LEMN, PRKV and SPAN were the same (LEIAX1202GG) the respective displacements were not of the same magnitude. Another point that should be underlined is that moving away from the fixed point (in our case MATE) the difference on the vertical component is increasing.

A second comparison should examine the character of the coordinate differences between the two solutions. The similarity transformation model, known as the Helmert transformation, was proved a good choice. The estimations of the seven transformation parameters and the aposteriori rms are given below: It is obvious that the two solutions reflect a systematic difference and define different coordinate frames with a significant translation.

Tx = – 39.0 mm	Rx = 0.0020 sec
Ty = 116.5 mm	Ry = 0.0014 sec	Scale factor = – 1.6 ppb
Tz = 25.0 mm	Rz = 0.0030 sec	rms = 2.9 mm

The presented comparisons show that the use of the more accurate absolute antenna models induce systematic differences with respect to the previous relative models. Therefore the epoch - comparison of GPS campaigns in regional and or global scale must take into account similar systematic effects.

Conclusions

The impact of the conversion from relative to absolute PCV antenna models is of great importance in regional and global GPS networks where high accuracy standards are required.

Solutions derived from relative and absolute models reflect a meaningful systematic difference which could be eliminated by proper transformation models like the 3-d Helmert model.

An application to a Greek regional GPS network with baselines up to about 500 km and specific antenna types showed that the difference is mainly a translation, the height component is greater than the horizontal and the displacement grows with the distance from the fixed point. Similar results have also been reported by other researches.

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