GPS Site Continuity

Arthur Niell
MIT Haystack Observatory
Westford, MA 01886 USA

    The use of permanent monuments has reduced the error due to 
the reinstallation of an antenna at a site but does not alleviate 
the possible error due to change of equipment. Antennas and 
receivers might be changed because of component failure or 
because better equipment has become available. Nominally 
identical antennas can have different phase centers at the level 
of a few millimeters, while receiver gains can differ, leading to 
a difference in the elevation distribution of observations. 
Change of model or brand of antenna or receiver is likely to have 
an even larger effect. To obtain the potential accuracy of the 
geodetic measurements, care must be taken to minimize any error 
in position introduced by these actions.
    A possible means to evaluate the effect of altering the GPS 
equipment (when known ahead of time) is to establish an 
independent nearby reference system and make measurements before 
and after the equipment change. The position of the permanent 
antenna relative to the reference antenna provides a very 
accurate record of the effect of the change on the estimated 
position.
    In 1999 December I monitored the addition of a second 
parallel receiver on the primary antenna at a permanent site by 
this method. I will report on the effectiveness of this technique 
by evaluating the effect of the second receiver on observable 
distribution, estimating the difference in the apparent position 
of the permanent antenna, and looking at the relative position of 
the antenna as seen by the two receivers.