The determination of local geodetic parameters for VLBI (very-long-baseline interferometry) antennas is traditionally done by conventional surveying techniques. These techniques are laborious and error-prone. The objective here was to develop a space geodesy method of measuring the parameters to make it easy to reproduce and provide long-term monitoring of the parameters. Errors are reduced by automating the process.
VLBI antenna calibration by GPS (Global Positioning System) uses GPS to measure: (1) the site-tie vector between the reference point of the VLBI antenna and a nearby GPS reference antenna; (2) VLBI antenna axis-offset and tilt; and (3) gravitational or thermally induced antenna deformation.
The method used is to mount GPS antennas at six locations on the VLBI antenna — four at 90° intervals around the outside of the antenna, and one more at the prime focus. These five antennas are pointed outward from the VLBI antenna. The sixth antenna is also mounted at the prime focus, but looks inward through the antenna optics. An additional reference GPS antenna is located nearby that serves as a geodetic control point. Data from the GPS antenna are recorded using a single GPS receiver (so they have a common clock). Different observing strategies are used depending on what is to be measured. To find the geodetic parameters of the VLBI antenna, the antenna is moved in different azimuth and elevation patterns while the GPS data is recorded. For deformations of the VLBI antenna, the carrier phase of the inward-looking GPS antenna is compared to the outward-looking antennas. After data are collected, they are analyzed to produce the desired results.
The novel feature of this method is that it uses a space geodetic technique — in this case, GPS — to measure parameters of the structure including its location. The data are processed interferometrically. Because of this, many sources of noise “difference out.” For example, since all receivers use the same clock, the clock error is common. In addition to making this easier to automate, it makes a direct determination of the three-dimensional vector between the reference GPS antenna and the VLBI reference point.
This work was done by Daniel MacMillan and Thomas Clark of NVI Inc. for Goddard Space Flight Center. GSC-16365-1