From jimr@maia.usno.navy.mil Thu Dec 31 11:41:50 EST 1998
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From: Jim Ray (USNO 202-762-1444) 
Message-Id: <199812311640.LAA12713@maia.usno.navy.mil>
Subject: Effect of satellite antenna offsets on clocks
To: gpst@maia.usno.navy.mil
Date: Thu, 31 Dec 1998 11:40:25 EST
X-Mailer: Elm [revision: 212.4]
Status: RO

Effect of Satellite Antenna Phase Center Offsets on Satellite Clock Estimates

The vector offsets between the phase centers of the GPS satellite transmit
antenna arrays and the satellite centers of mass are not well known.  It is
the dynamical motion of the centers of mass which must be modelled in the
data analysis even though the signals originate from the transmit arrays.
Any errors in the radial component of the vector offsets will manifest
themselves almost entirely as biases in the estimated values for the
satellite clocks. Therefore, in order to ensure that satellite clock
determinations from the various IGS Analysis Centers can be compared and
combined consistently, the IGS has adopted a common set of values for the
antenna phase center offset vectors. The rationale and method for
implementing these values is documented at
http://maia.usno.navy.mil/gpst/mail/ant-offs.  This set of offset values
was officially implemented by the IGS starting 29 November 1998. 

In the usual satellite-fixed coordinate system (where the z-axis is directed
from the satellite center of mass towards the Earth center), the IGS offset
values are: 

   Block II & IIA    dx = 0.279 m   dy = 0.000 m   dz = 1.023 m
   Block IIR         dx = 0.000 m   dy = 0.000 m   dz = 0.000 m

Note in particular that no offset is used for Block IIR satellites, of which
PRN13 is currently the only example. 

If a user wishes to compare the IGS satellite clock values with other results
using different antenna offsets, then corrections must be applied:

   Csat_i (user) = Csat_i (IGS) - {[ dz_i (user) - dz_i (IGS) ] / c}

where

   dz_i (IGS)  = IGS value for dz for satellite PRNi
   dz_i (user) = user's value for dz for satellite PRNi
   Csat_i (IGS)  = IGS value for clock of satellite PRNi
   Csat_i (user) = user's value for clock of satellite PRNi
   c  = speed of light = 299792458 m/s

[This information is now posted at the IGS/BIPM Web site
http://maia.usno.navy.mil/gpst.html.]