Effects of radome installation

At stations HLID, HVER, OLKE, SOHO and VOGS, offsets in the vertical component can be seen in the time series, marked with vertical dashed lines in Figures 8 to 20. These jumps are apparent offsets due to the installation or removal of plastic radomes (Figure 3). This is a known phenomenon in operation of permanent stations and different offsets are observed for different kinds of radomes and antennas [UNAVCO (2001a)].

In the ISGPS network we use hemispherically shaped radomes from SCIGN with part numbers 0010-1 and 0010-2 for Ashtech and Trimble antennas respectively. Usually the radome is installed at the same time as the station is installed, but the first stations were operated without radomes at the start of measurements. The offsets due to radome installation in the ISGPS network are around 20 mm downwards and are shown in Table 5. There are no significant offsets due to radome installation in the horizontal components. The offsets were estimated by comparing the average coordinates 10 days before and after radome installation, where data were available. SOHO was not recording at the time of radome installation so a longer period (30 days before and after radome installation) was used to estimate the average coordinates. Similar results were observed in preliminary tests made on the roof at IMO. The offsets due to radome installation are larger than the manufacturer states for this specific type of radomes (less than 2 mm) [SCIGN (2001)], [Braun et al. (1997)]. Similar offsets on the order of 20 mm are observed when processing data from the ISGPS network with the GIPSY/OASIS II software (C. Völksen, personal communication 2002).

 

Table 5: Offsets due to radome installation in horizontal and vertical components obtained by comparing the average coordinates 10 days before and after radome installation. The radome at HVER was removed on day 309, 1999 (marked with "OFF" in column 3). The formal errors of the coordinates were scaled by a factor 4 in the horizontal components and by a factor 2.5 in the vertical. The uncertainties given in the table are at the 2$\sigma$ level.

Station	Time (year and day)	Radome on/off	East offset (mm)	North offset (mm)	Vertical offset (mm)
HLID	1999 235	ON	-0.7 $\pm$ 1.0	0.4 $\pm$ 1.6	-21 $\pm$ 6
HVER	1999 222	ON	0.0 $\pm$ 0.9	0.8 $\pm$ 1.4	-21 $\pm$ 6
HVER	1999 309	OFF	-0.2 $\pm$ 1.4	1.3 $\pm$ 2.2	15 $\pm$ 8
HVER	1999 328	ON	1.0 $\pm$ 1.3	-0.5 $\pm$ 2.1	-21 $\pm$ 8
OLKE	1999 182	ON	1.0 $\pm$ 1.0	0.7 $\pm$ 1.7	-23 $\pm$ 6
SOHO*	1999 309	ON	2.0 $\pm$ 1.1	1.9 $\pm$ 1.7	-17 $\pm$ 6
VOGS	1999 328	ON	-0.4 $\pm$ 1.2	1.0 $\pm$ 2.0	-17 $\pm$ 7
*: Data from 30 days before and after radome installation were used for the comparision.

 

The Choke Ring antenna from HLID was absolutely calibrated [Wübbena et al. (1997)] in 2001, with and without the radome, by IFE (Institut für Erdmessung) in Hannover, Germany. The vertical offsets for the mean phase centers of the antenna are lower when the radome is on, the differences are 3.3 mm and 1.1 mm for the L₁ and L₂ mean phase centers respectively (F. Menge, personal communication 2001). The differences in mean phase center offsets in the horizontal components are insignificant. The differences in the Phase Center Variation (PCV) pattern, with and without the radome, as a function of elevation and azimuth are further biased at low elevation angles, up to 6 mm for the L₁ PCV pattern at an elevation of 5$^\circ$. The satellite constellation can enhance this bias since satellites are often observed at low elevation angles in Iceland.

The differences in the mean phase center offsets and PCV pattern also propagate in the processing, e.g. with different linear combinations like L₃, L₄ and L₅. This might cause the observed offsets in the time series, but more studies are required to verify if this is the case.