Coordinate Offsets

Discontinuities in coordinate time series are generally termed coordinate offsets. Such offsets can be known or unknown in their magnitudes, the epochs at which they occurred and their causes. Except in the case of earthquakes, offsets in coordinate time series are generally undesirable and until recently, their effect on station velocities and uncertainties had not been investigated in detail (Williams,2002).

The most obvious reason for coordinate offsets is due to movements of the CGPS stations. Earthquakes, are a common source for offsets in coordinate time series in seismically active areas, such as Japan, the USA or southern Europe, e.g. Feigl et al.

(1993); Sagiya et al. (1995); Hudnut et al. (1996); Miyazaki et al. (1996); Bock et al.

(1997); Langbein and Johnson (1997); Wdowinski et al. (1997); Hurst et al. (2000); Li et al. (2000), andNikolaidis(2002). It is normally possible to correlate coordinate offsets with such events and hence the exact time of an offset can be established. However, besides these crustal movements, there is a range of other events that have been reported to cause coordinate offsets.

Chapter 6. Analysis of the Continuous GPS Network 146

One of the most common causes for artificial offsets in GPS coordinate time series is due to changes of the GPS antenna or receiver configuration. Alterations at the antenna, removing and replacing the antenna temporarily or adding or removing a radome, can introduce coordinate offsets, e.g. Braun et al.(1997); Scherneck et al.(1998); Arnad´ottir´ et al.(2000); Emardson et al. (2000);Schupler and Clark (2001); Bruyninx et al. (2002);

Johansson et al. (2002); Kaniuth and Stuber (2002), and Vespe et al. (2002). Changes in the immediate environment can also introduce coordinate offsets. The most prominent offset in the EUREF weekly coordinate time series for HERS was observed in March/April 1999 and was attributed to partial GPS signal obstructions (Takacs and Bruyninx,2001). More recently, Dong et al. (2002b) reported that the trimming of a tree to less than half height close to a GPS antenna caused an offset of 1 and 4 cm in the horizontal and vertical coordinate components, respectively. Ice or snow accumulation on antenna radomes have also been reported to cause jumps in the height time series of the SWEPOS network at the centimetre level (Jaldehag et al.,1996;Scherneck et al.,1998;Johansson et al.,2002). Additionally, vandalism at unsecure sites or errors in log files can also introduce or lead to discontinuities in coordinate time series (Williams, 2002). Some of these offsets may have small magnitudes and may therefore remain undetected, as it is often difficult to accurately correlate them with any specific event. Williams (2002) shows percentages of offsets with unknown causes of up to 66% for various published data sets and concludes that for these data there is on average one offset every nine years.

Careful logging of any changes at CGPS sites may help the identification of offsets. Therefore, the IGS and EUREF have made great efforts in documenting receiver and antenna changes at their CGPS stations in a consistent and appropriate way. Such IGS Site Information Forms are available for each IGS or EUREF station from any of the IGS or EUREF analysis centres. Such forms have been obtained from the IGN ftp site (ftp:// igs.ensg.ign.fr/pub/igscb/station/log/) for stations BRST, HERS, KOSG, NPLD, ONSA, VILL and WTZR. At the IESSG, similar site information forms, are maintained for all of the UK CGPS stations analysed by the author. All recent site information forms, including those of the IGS and EUREF stations, are contained on the CD–Rom in directory /Site_Information_Forms.

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Table 6.2: Antenna and receiver changes for UK CGPS stations in the period from March 1997 to May 2002

Site Date MJD Detail

ABER 30–04–01 52029 RF interference resolved

HERS 21–03–99 51258 New signal obstruction (Takacs and Bruyninx,2001) 08–08–01 52129 Antenna re–mounted

IESG 09–03–98 50881 Antenna re–mounted

MORP 03–03–00 51606 Receiver changed 12–03–02 52345 Antenna changed 15–04–02 52379 Receiver changed 20–05–02 52414 Receiver changed

NSTG 30–08–98 51055 Antenna and receiver changed 04–12–99 51516 Antenna and receiver changed 12–02–00 51586 Antenna and receiver changed 09–05–01 52038 Antenna and receiver changed 12–06–01 52073 Antenna changed

13–03–02 52346 Antenna changed

05–04–02 52369 Antenna and receiver changed 18–05–02 52412 Receiver changed

From the site information forms of the UK CGPS stations, it can be seen that for most stations no receiver hardware, antenna or radome changes were reported for the observation period since 1997. For many sites, the only entry on the form was a receiver firmware upgrade in August 1999, which at this stage is believed to have not introduced any offsets. A summary of all other identified events that could cause a coordinate offset at a particular CGPS station can be found in Table6.2. This table lists the dates, the Modified Julian Days (MJD) and the details of the changes documented in the site information forms.

Although, no physical change to the antenna at ABER was reported, it was assumed that due to the observed RF interference (§5.3.3) prior to 30 April 2001 the coordinate time series may have been offset. It was therefore decided, to test for a discontinuity in the coordinate time series for ABER on this date. The largest offset in all coordinate time series analysed is apparent at HERS on 21 March 1999, as shown in Figure6.4_{in §}6.3.1on page168. This offset has already been mentioned and was reported to be due to a newly

Chapter 6. Analysis of the Continuous GPS Network 148

introduced obstruction in the sky view of the GPS antenna. From the IGS site information form for HERS, it can be seen that on 18 June 2001 the antenna was mounted on an 8 m high, open lattice, cross–braced steel mast, but had to be sent away for repair after it was discovered that the antenna was faulty. When it was returned on 8 August 2001, it was re–mounted on the same mast. Hence, this date will also be used in this analysis for the estimation of coordinate offsets. For IESG, the site information forms state that on 9 March 1998 the GPS antenna was temporarily removed to enable spirit levelling of the survey marker on the turret of the IESSG as part of the EUVN97 GPS campaign (Ineichen et al.,1999). According to experiences byScherneck et al.(1998) andJohansson et al. (2002) in the BIFROST project, it is very likely that this will have introduced a coordinate offset even though the antenna was re–mounted and orientated with uttermost care. Johansson et al.(2002) reported that offsets have been estimated every time antennas were removed and replaced for local stability surveys of their monuments. Changes of up to 10 mm in the estimated height have been attributed to tiny changes in the antenna orientation. At NSTG and MORP several changes have taken place. However, some details have unfortunately not been recorded and assumptions on the receiver or antenna configuration have had to be made. Some of the entries for NSTG can be associated with the recent investigations into the receiver problems at that site (§5.3.3).

Due to the network processing strategy applied in this research, i.e. the processing of the CGPS stations as radial baselines with respect to IESG, it may be assumed that discontinuities in the coordinates of the European IGS stations would definitely affect the results for CGPS station IESG, and possibly affect the results for all other stations in the network. Therefore, the author has compiled a list of antenna and receiver configuration changes for these IGS stations as documented in their IGS site information forms. Table6.3

lists the date, the Modified Julian Day (MJD) and the details of the changes for the IGS stations KOSG, ONSA, VILL, and WTZR up to 31 May 2002.

Although the antenna and receiver configuration changes indicated in this table pose a possible source for discontinuities in the coordinate time series of these IGS stations, it is not possible to visibly identify them by inspecting the EUREF weekly coordinate time series obtained from http://www.epncb.oma.be/series_sm.html. Kenyeres(2002) reported that apart from the antenna and receiver change at ONSA on 2 February 1999,

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Table 6.3: Antenna and receiver changes for European IGS stations used to define the reference frame in this analysis (KOSG, ONSA, VILL and WTZR) from March 1997 to May 2002

Site Date MJD Detail

KOSG 01–Mar–99 51238 Receiver changed KOSG 14–Jul–99 51373 Receiver changed KOSG 24–Nov–00 51872 Receiver changed KOSG 27–Nov–00 51875 Receiver changed

ONSA 02–Feb–99 51211 Receiver and antenna changed VILL 29–May–01 52058 Receiver changed

WTZR None reported

all offsets in Table 6.3 were small or undetectable and hence were not included in the EUREF analysis. The offset in the height time series for ONSA on 2 February 1999 was estimated to be about 11 ± 2 mm (Kenyeres,2002).

In order to investigate the coordinate time series of the UK CGPS stations for any unknown coordinate offsets, a change detection algorithm (CDA) was used to scan the residual coordinate time series for any changes (see AppendixG). The algorithm assumes the coordinate time series to be a sequence of independent random observations with a certain standard deviation. By comparing the means computed for the coordinate time series prior to and after the investigated epoch, an unknown coordinate offset at this epoch can be estimated using a threshold value based on the standard deviation of the total sequence. Williams (2002) investigated the effectiveness of this algorithm and concluded that best results would be obtained in the middle of coordinate time series as the detection threshold was much lower. Based on simulated time series, he concluded that in order for the CDA to correctly estimate the epoch of an offset at the 95 or 99% significance levels, the discontinuity needed to be four or eight times the threshold value of the coordinate time series. Although, it is easier for the CDA to detect the correct offset epoch in the middle of the coordinate time series, this is the worst position in terms of the velocity uncertainty associated with the offset (Williams,2002).

The Student’s t–test (Eq. D.4 in Appendix D) allows for comparison of the means of two normally distributed data sets with the same variance. This test can be used to investigate, whether there is a statistically significant difference in the means for a period

Chapter 6. Analysis of the Continuous GPS Network 150

before and after a suspected coordinate offset. Tests into whether a model including a parameter for a specific coordinate offset fitted to the coordinate time series, is statistically better than a model not including the offset can be carried out by means of the F –test (Eq.D.5 in AppendixD).

Using the documented epochs for possible coordinate offsets given in Tables 6.2 and

6.3, and those detected by the CDA, in connection with both statistical tests, the aim was to establish a list of epochs at which coordinate offsets should be estimated in the final analysis strategy.