A CGPS station has been operational at Newlyn tide gauge since September 1998. This case study provides details on the technical issues that were considered during the establishment of the CGPS station and provides some preliminary results.
2. Mean sea level measurements at Newlyn tide gauge
Newlyn
is located on the tip of the Cornish Peninsula in the South-West of England,
about 10 km from Land’s End. The Tidal
Observatory is located at the end of the South Pier in Newlyn harbour.
Woodworth et al (1999) computed trends in MSL for a selection of UK tide gauges, with more than 15 complete years of MSL and TGBM information in the PSMSL data set. Based on 80 complete years of data over the period from 1916-96, a rise in MSL of 1.69 ± 0.12 mm per year was computed for Newlyn. This is consistent with the rise in global sea level of 10 to 20 cm over the past century (IPCC, 1995), which may indicate that there has been no significant uplift or subsidence at the tide gauge.
3. Historical geodetic measurements at Newlyn tide gauge
Historical geodetic measurements at Newlyn tide gauge
can be broadly separated into precise levelling surveys carried out by the
Ordnance Survey and episodic GPS measurements made by the IESSG and POL since
1990.
Ordnance Survey precise levelling surveys
The MSL records from the Newlyn tide gauge for the period 1915-21 were used to define ‘Ordnance Datum Newlyn (ODN)’ and the Tidal Observatory has remained in the same location since that time.
The primary tide gauge benchmark (PTGBM) at Newlyn is a bolt, adjacent to the stilling well, inside the Tidal Observatory. The PTGBM was first connected to the primary levelling network in 1915 and was last verified by precise levelling (line G001) in 1990.
The TGBM network (see Figure 1) is
effectively formed from seven benchmarks, comprising the PTGBM, two benchmarks
on the pier (AUX1 and AUX2), two benchmarks in the village and two fundamental
benchmarks (FBMs). AUX1 is located
about 50 m from the PTGBM and AUX2 is located where the pier meets the land,
about 250 m from the PTGBM. The two
benchmarks in the village are located about 550 m and 750 m from the PGTBM on a
Church and on an old School respectively.
The two FBMs are located at Tolcarne, about 900 m to the North-West of
the PTGBM and at Paul, about 1.4 km South-West of the PTGBM. The FBMs are founded on ‘stable’ ground,
whereas all of the other inland benchmarks are Ordnance Survey flush brackets
set into walls. The TGBM network was
first connected to the PTGBM in 1952 and last verified by precise levelling
(line G001) in 1990.
Figure 1 TGBM network at Newlyn tide gauge
The results of the repeated precise levelling surveys showed no significant changes (ie less than 0.1 mm) in height within the TGBM network over the period from 1952 to 1990. This suggests that the pier on which the tide gauge is located did not experience any uplift or subsidence relative to any of the benchmarks, including the two FBMs founded on ‘stable’ ground.
From 1991 to 1996, a total of nine episodic GPS campaigns were carried out, with two to eight epochal measurements at 16 tide gauges in the UK (Ashkenazi et al, 1998). For these episodic GPS campaigns, two new benchmarks were installed at Newlyn (see Figure 1). The first (NEW1) was installed in 1990, about 700 m from the PTGBM. The second (NEW2) was installed in 1994, about 10 metres from the PTGBM.
The first episodic GPS measurements were made at NEW1 in September 1991, August 1992 and August 1993 as part of the UKGAUGE I project. Subsequent episodic GPS measurements were made at NEW2 in November 1993 and March 1994, as part of the EUROGAUGE project. Further episodic GPS measurements were then made at NEW1 in April 1995, September 1995 and November 1995, as part of the UKGAUGE II project. In all campaigns, GPS data were recorded for between 8 and 24 hours per day, for 5 consecutive days. Precise levelling surveys were also carried out by the Ordnance Survey to connect the new benchmarks to the PTGBM. Surveys were carried out to connect NEW1 to the PTGBM in 1991 and 1993, and to connect NEW2 to the PTGBM in 1994 and 1995.
Using a series of episodic GPS campaigns over such a relatively short, 5 year period, it is only really possible to detect vertical land movements of more than a few centimetres. In the case of NEW1/NEW2, such movements were not observed. Instead, assuming that no vertical land movements had taken place at NEW1/NEW2 over the period from 1991 to 1996, the standard deviation of height from a time averaged-mean was computed to be ± 8 mm (Ashkenazi et al, 1998).
In terms of decoupling land movements from the MSL measurements made by the tide gauge, the results of the precise levelling surveys showed no significant changes (ie less than 0.1 mm) in height between the PTGBM and NEW1/NEW2 over the period from 1991 to 1995. This is consistent with the historical precise levelling surveys, from 1952 to 1990, confirming the ‘stability’ of the pier on which the tide gauge is located.
4. Siting of the CGPS station at Newlyn tide gauge
As stated in the technical recommendations (www.soest.hawaii.edu/cgps_tg), to obtain the best possible vertical accuracies for a CGPS station it is crucial that the GPS antenna has a clear view of the sky in all directions for elevation angles above 15 degrees. Apart from this, the main technical issue centres around whether it is better to build a CGPS station immediately adjacent to the tide gauge or close to one or more TGBMs which are known to be in ‘stable’ ground.
The historical geodetic measurements discussed in the previous section suggest that the pier on which the tide gauge is located is as ‘stable’ as the FBMs that are assumed to be on ‘stable’ ground. For Newlyn tide gauge, therefore, it was a relatively straightforward choice between establishing the CGPS station immediately adjacent to the tide gauge and PTGBM or close to one of the other benchmarks on the pier, ie AUX1. Locating the CGPS station in either location would necessitate a precise levelling connection between the CGPS station and the PTGBM, but this would be restricted to a distance of 10 to 50 m.
The initial possibilities considered were, therefore, to locate the GPS antenna on the roof of the Tidal Observatory (ie immediately above the tide gauge) or directly above AUX1, which is an Ordnance Survey flush bracket set into the sea wall. The latter afforded a clear view of the sky, whereas the former had some restrictions imposed by a 10 m high lighthouse at the end of the pier, next to the Tidal Observatory.
In the end, the main issue was not technical but practical, in terms of the security of the GPS antenna. The South Pier on which the Tidal Observatory is located, forms part of Newlyn harbour which is a traditional fishing port. The North Pier mainly serves commercial fishing, whereas the South Pier is a favoured spot for tourists or locals to fish from.
Unfortunately, this means that the roof of the Tidal Observatory and the sea wall are often used by fishermen, and anything that can provide a means of securing person or rod (such as an antenna monument) will be used and abused.
A third possibility then arose, which was to locate the GPS antenna on the lighthouse itself. This would keep the GPS antenna within a few metres of the tide gauge, whilst raising it above the obstruction caused by the lighthouse and, above all, provide security. The lighthouse is a substantial steel structure fixed to the pier. It has been standing for decades and there is no reason why it should not remain so for decades to come.
Hence, the decision was made to establish the CGPS monument on the observation platform of the lighthouse, which is about 7 m above the pier. In this case, the GPS antenna has a reasonably clear view of the sky, although not perfect due to the proximity of a weather vane on top of the lighthouse.
5. Monumentation and instrumentation at Newlyn tide gauge
As described in the previous section, the decision was made to establish the CGPS station immediately adjacent to the tide gauge. This would enable the GPS receiver to be housed in the tide gauge building, with mains power, telecommunications and security. Meanwhile, the GPS antenna could be located on a monument fixed to the observation platform of the lighthouse adjacent to the Tidal Observatory.
Instrumentation
The instrumentation installed at the Newlyn tide gauge followed the guidelines of the IGS in terms of using a dual-frequency code and phase measuring GPS receiver and an IGS standard Dorne Margolin choke ring antenna. The actual instrumentation used comprises an Ashtech Z-XII GPS receiver and an Ashtech Dorne Margolin choke ring antenna (p/n ASH700936-02).
The receiver is connected to a standard telephone line using a US Robotics Courier modem. This enables remote operation and downloading from the IESSG, using the Ashtech CGREMOTE software, running on a PC connected to a standard telephone line using a US Robotics Courier modem.
The antenna monument
The antenna monument installed at the Newlyn tide gauge is cylindrical, with a flat, circular mounting surface that is slightly smaller than the diameter of the base of the antenna. The monument is actually a 3 m high, carbon fibre / stainless steel pipe assembly (see Figure 2), which is mounted on to a stainless steel triangular plate, that is fixed to the floor of the observation platform on the South-West side of the lighthouse.
Figure 2 CGPS station antenna
monument at Newlyn tide gauge
The reference mark on the monument is defined as the top of the 40 mm diameter thread, which is welded on to the triangular plate. The antenna height to the Antenna Reference Point (ARP) was determined by using a steel tape to measure the offset from the triangular plate to the ARP, and then subtracting the known offset from the triangular plate to the top of the 40 mm diameter thread (ie 40 mm). In fact, the measured offset from the triangular plate is 3.005 m, hence, the antenna height to the ARP is 3.005 – 0.040 = 2.965 m.
Vertical ties between the GPS antenna, the tide gauge and the TGBMs
The vertical ties between the GPS antenna, the tide gauge and the TGBMs can be carried out using precise levelling. Mounting a tripod on the roof of the Tidal Observatory and using an inverted staff, a precise levelling connection can be made between the triangular plate and a staff located on the NEW2 benchmark on the pier. Conventional precise levelling can then be used to connect the NEW2 benchmark to the tide gauge contact point, the PTGBM and AUX1 on the pier.
Such a precise levelling survey covers a distance of about 50 m and can be easily accomplished within 1 day using modern equipment, such as the Leica NA2000 digital level. In parallel with the CGPS measurements, it is proposed to repeat the precise levelling surveys at annual intervals.
General aspect
Photographs of the antenna and monument at the Newlyn tide gauge are given as Figure 3 below.
Figure 3 CGPS station antenna and monument at Newlyn tide
gauge
The CGPS station at the Newlyn tide gauge became operational on 30 September 1998. Data are downloaded on a daily basis to the IESSG, in Ashtech (r file) binary format. The 24 hour files are then converted to RINEX format and archived at the IESSG as part of the British Isles GPS Facility (Dodson et al, 2000).
Data quality monitoring
The same equipment (receiver, antenna, cables, etc) have been operational at the Newlyn tide gauge since installation. Firmware upgrades have been made to the receiver, but no hardware changes have been made and the antenna cable has remained connected.
Signal-to-noise ratio (SNR) values are not routinely output in the RINEX format data files. However, at six month intervals, samples of binary data are re-run through the RINEX conversion program and SNR values are output. These are then checked for any changes in signal quality, which may indicate a change in the performance of the CGPS station. To date, no degradation has been found.
6. Geodetic data processing and preliminary results for Newlyn tide gauge
The data from the CGPS station at the Newlyn tide gauge are processed by the IESSG on a daily basis using their in-house developed GPS Analysis Software (Stewart et al, 1997). In this processing, the data from several CGPS stations in the UK are combined with data from IGS stations in Europe, as a series of ‘loosely constrained daily GPS network solutions’. In these solutions, the IGS precise ephemeris is held fixed and the coordinates for Kootwijk are tightly constrained to the same ITRF realisation and observation epoch as the IGS precise ephemeris.
For all of the loosely constrained daily GPS network solutions, the GPS data are processed using the L1/L2 ‘ionospherically free’ double difference observable, with integer ambiguities free, and systematic error models for solid Earth tides, ocean tide loading and antenna phase centre variations. The ‘dry’ zenith tropospheric delay is modelled using Saastamoinen and the ‘wet’ zenith tropospheric delay is estimated as a random walk process.
Following the GPS data processing, coordinate time series are formed for each of the CGPS stations. In order to form coordinate time series with no discontinuities, the vectors and associated covariance information output from each loosely constrained daily GPS network solution are transformed to a common reference frame. This is currently carried out by tightly constraining the European IGS stations of Onsala, Kootwijk, Wettzell and Villafranca to their ITRF97 coordinates, motioned to the observation epoch (Boucher et al, 1999).
Figure 4
shows the daily height time series obtained for the CGPS station at the Newlyn
tide gauge. This is represented in the
ITRF97, based on daily coordinate estimates computed in the ITRF94, ITRF96 and
ITRF97.
Figure 4 CGPS station ITRF97 height time series for Newlyn
tide gauge
The estimation of vertical station velocities and their uncertainties, and the effects of periodic variations on CGPS height time series are current topics of research at the IESSG (Bingley et al, 2000) and many other research centres. At this stage, we can only remark about the possible long-term trends in the height time series, which may suggest that the CGPS station is subsiding.
Emerging estimates of vertical station velocities will be used in conjunction with the precise levelling links between the CGPS station, the PTGBM and AUX 1 to consider if and where uplift and subsidence are occurring. Reliable estimates of vertical station velocities will then be compared with modelled estimates of crustal movements due to glacio-isostacy (Lambeck1993a; 1993b; Lambeck and Johnston, 1995) and geological estimates of Holocene crustal movements (Shennan, 1989; Woodworth et al, 1999), which both suggest that the South-West of England is subsiding by about 0.5 to 1.5 mm per year.
Acknowledgements. Research carried out on the use of CGPS at tide gauges in the UK has been funded by the Ministry of Agriculture Fisheries and Food (MAFF), through the long term commission with POL, the Environment Agency and the Natural Environment Research Council. I would like to thank Simon Booth, Alan Dodson, Nigel Penna, Norman Teferle and Samantha Waugh from the IESSG, who have all contributed to the work described in this case study. I would also like to thank Trevor Baker and Philip Woodworth from POL for their comments and suggestions.
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