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Re: SA off at midnight GMT



SA is now definitely off. Beginning at 0400Z, several of us on the
Internet saw dramatically lower range and range rate residuals on most
(but not all) of the GPS satellites in view. However, the URA (User
Range Accuracy) figures in the broadcast ephemerides remained at 7.

For over two years I have been running my Oncore VP receiver in
"position hold" (i.e., DGPS reference station mode), using a position
derived from a month of averaging (with SA on). Last night I released
position hold mode and went to bed.

This morning, here are some of the relevant figures:

URAs: 0 2 0 1 1 0 1

These are the "User Range Accuracy" settings for the 7 satellites I'm
currently tracking. They are included in the broadcast ephemerides,
and represent the control segment's estimates of the accuracy of the
satellites. These figures all represent errors of a few meters max.

Time ref: 229374.0 Tue May  2 (123) 15:42:42 2000 UTC
PRN 24 range -3.86 m range rate 0.016 m/sec IOD 157
PRN 9 range -0.89 m range rate -0.026 m/sec IOD 63
PRN 7 range -4.29 m range rate 0.004 m/sec IOD 211
PRN 2 range 1.11 m range rate -0.014 m/sec IOD 237
PRN 4 range 0.45 m range rate -0.004 m/sec IOD 124
PRN 8 range 1.77 m range rate 0.018 m/sec IOD 25
PRN 26 range 1.33 m range rate -0.022 m/sec IOD 112

These are the range and range-rate residuals derived by the receiver
from a smoothed least-squares fit using 7 satellites and its current
position estimate. These are essentially the values that a
differential GPS reference station computes and sends over the data
link to a user terminal. If there were no errors anywhere (SA or
other), they would be zero. With SA on, the range figures would often
reach values in the tens of meters, with range rates of tenths of a
meter per second.  Now they are clearly all low and due to residual
errors, such as ionospheric.

SA is finally gone. Hallelujah!

Phil



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