1. Space Geodesy Facility, Herstmonceux – HERL and HERS

2. SGF: view from HERT GPS/GLONASS antenna, 100m distant

3. Site Survey History 1983 measurement of SLR fiducial point and distances to calibration targets’ markers, within a local frame; 1988 re-measured; set of witness marks set up; 1991 Re-measured, plus inclusion of new HERS (Rogue) antenna marker. Station marker, on SLR pier, established. All three surveys carried out by UK Ordnance Survey; OS conclusion: in all cases, distances from SLR to calibration targets stable to better than 1mm over this time period.

4. Schematic – SLR axis and monument

5. Schematic – calibration targets

6. Schematic – GPS network

7. More recent surveys 2 surveys in 2002: (UCL MSc projects) –1: novel centre-of-rotation of SLR telescope via EDM and angle measurements from GPS-surveyed locations; –2: standard GPS survey of calibration targets and monument on SLR Pillar. 1: discrepancy cf OS survey in vectors from SLR to targets and to HERS GPS marker at level of up to 8mm 2: agreement of vector from Pillar to HERS with OS survey at level of 1mm.

8. More recent surveys Further survey in 2003 specifically to look at distance between SLR and calibration targets; Confirmed OS (1991) distance to primary target; Highlighted some problems with distances to two other calibration targets at ~4mm level To Do – re-determine local tie from SLR to HERT (GPS/GLONASS).

9. SLR – GPS (HERS) tie No evidence for error >1mm in site-log value of tie vector from 1991, 2002 surveys; However: problems in HERS data from April 1999 – June 2001 caused IGS to determine new coordinates with 14mm East shift –> Two coordinates for HERS in ITRF 2000; Second coordinate is spurious, resulting from antenna fault; Automated HERS (and HERT) daily QC via network and differential site solutions now removes risk of erroneous data being deposited at data centres.

10. Site stability or reflection of global geocentre motion?