Subject: [SCIGN-0034] phase centers
From: Ken Hurst
Date: 16 Dec 1994 10:11:51

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SCIGN Electronic Mail    Fri Dec 16 10:11:51 PST 1994      Message Number 0034
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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 <[email-masked]>]