Site Surveys
One of the most important data products provided by the space geodesy techniques is the International Terrestrial Reference Frame (ITRF), which is the basis for all of our metric measurements of global change. The ITRF depends on the combination of SLR, VLBI, GNSS, and DORIS. Each of the techniques measures something different and each has a different set of systematic errors. The best products are developed by exploiting the strengths and mitigating the weaknesses of each technique. In order to combine the techniques and put them in the same reference frame, we must determine very accurately the inter-systems ties (vectors) or relative positions of the instruments at collocation sites. Precise geodetic surveys are required to determine these inter-system ties from the invariant reference point of the laser system (the intersection of telescope axes) to local monuments and the invariant points in the other co-located instruments (e.g. VLBI, GNSS, etc.). In SLR, the invariant point (intersection of the axes) is buried inside the mount, so the ties are made up of direct baseline measurements between accessible points plus an extrapolation of the invariant point. The baselines are measured by GPS and conventional ground survey techniques. Extrapolation to the reference points is determined through survey procedures to estimate center of axis rotation, engineering information, and modeling procedures.
Since high accuracy is required, electro-optical distance measurements must be corrected for air temperature, pressure and humidity. This local station control network should be connected to the regional/national geodetic networks through a regional geodetic "footprint", a monument network over a well designed region, that can provide robust ties to better than 1 mm, out to 10-30 km.
At each site local surveys should be made on a regular basis (annually if possible) to verify the stability of the site. These data will be considered part of the ILRS data record. In addition, all data acquired by participating investigators will be formatted and archived with the primary data. For sites of particular interest, historical data from seismograph records, tilt meter recordings, gravity field measurements, survey traverses, etc. may also be archived. This data will help give better interpretation to the local dynamics.
Additional information is available from previous Workshops:
Full site survey reports are available for select stations.
