From: Ken Hurst Date: 16 Dec 1994 10:11:51 Subject: [SCIGN-0034] phase centers ****************************************************************************** SCIGN Electronic Mail Fri Dec 16 10:11:51 PST 1994 Message Number 0034 ****************************************************************************** Author: Ken Hurst Subject: phase centers Does anyone have good values for the relative phase centers of the large-ground-plane versus regular Ashtech antennas? I consider "good" in this case to be beter than 0.5 mm horizontally and better than 3.0 mm vertically. If anyone has such values please send them to me. The new TRAK station log claims that the phase center for the new large-ground-plane antenna is at the same place (concentric, 0.064m above the reference point) as the old regular sized antenna. I can demonstrate that this is not correct. A quick look at the time series for SIO3 which had a large-ground-plane antenna from May 28 to Oct 12 and a regular antenna at other times, shows noticeable offsets for both North and Vertical. A quick, rough and ready estimate from the SIO3 data is N=-5.5 +- 1.5 mm E=0 +- 3 mm and V=100 +- 40 mm in going from the regular to the large antenna. If good values are not available now, we may be able to get them in the fullness of time, but unless they are very good, antenna swaps tend to corrupt velocity estimates for the station involved. Without knowing the reasons for the recent antenna swap at TRAK, and therefore not wishing to comment on it directly, I would nonetheless like to urge a conservative approach to swapping antennas once a site has collected several months of data. Ken Hurst [Mailed From: Kenneth J Hurst ]