The VLBI to GPS Tie at Westford

Arthur Niell (Haystack Observatory) and Vaughn Nelson (ATSC)

The ITRF96 height difference between the VLBI and GPS positions at Westford is approximately three centimeters different from the eccentricity, after the IERS eccentricity has been corrected for a numerical error (memo by W. Strange, 1996). This eccentricity is based on GPS measurements. An optical survey in 1996, having an accuracy of about 5 mm, agrees with the corrected eccentricity to one centimeter. In order to check the GPS component of the eccentricity a Dorne-Margolin choke- ring (DMCR) antenna was mounted on a tripod over the reference mark nearest the VLBI antenna. While the DMCR to DMCR height for a minimum elevation of 15 degrees agrees with the optical measurement, the height difference has a sensitivity of approximately -3 mm per degree of minimum elevation. The sensitivity of height to minimum elevation was evaluated for all of the antennas used in the orbit/clock solution for one day. The 5 degree to 30 degree difference ranged from about +1 to -10 cm. Thus the Westford height discrepancy may be due to the distribution of height errors averaged over the global network.