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)
[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).
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.