DOPPLER FILE TIME TAG OFFSET

			     L. D'Addario
			       97/02/04


	In accord with earlier agreements [1], tracking data from the
NRAO OVLBI Earth Station at Green Bank will be supplied to the JPL
Multi-Mission Navigation Team in files conforming to JPL specification
TRK-2-30 [2], here called "doppler files."  The main observable
contained in each record of such a file is the "integrated doppler"
derived from the measured residual phase in the doppler-compensated
two-way link.  Each record also contains a time tag, but the file
format restricts this to be an integer number of seconds of UTC.
However, measurements made at the NRAO station, when referred to the
time of arrival of the signal at the antenna reference point
(intersection of the azimuth and elevation axes), are not sampled on
the integer UTC second, although the actual time of sampling is
accurately known.  This memo describes the method chosen to ensure
accurate time tagging in spite of this difficutly.

	Measurements of residual phase are made at precise intervals
of 0.1 sec, and the results are sampled at times that include the UTC
second marks.  But this sampling occurs after a processing delay which
is large but accurately known.  Careful accounting [3] causes us to
estimate that the delay between arrival of a wavefront at the
reference point and the (internal, raw) time tag given to the sample
of residual phase associated with that wavefront is:

		T1 = 2.673478892 +- .000001 seconds.

To each sample with raw time tag t, we assign a corrected time tag 
t - T1, representing the arrival time at the reference point.

	Not every residual phase sample is used to compute an
integrated doppler sample.  The TRK-2-30 format prevents reporting a
value more often than once per second, and it is expected that values
every 60 seconds will be sufficient for VSOP.  The time tag given to
the integrated doppler sample in the doppler file is the integer UTC
second *nearest* to the corrected time tag of the corresponding
residual phase sample.  No interpolation is done.  To minimize the
difference between the doppler file time tag and the arrival time at
the reference point, we select 0.1 second samples whose corrected time
tags are nearest to an integer second; with the above value of T1,
this means that the raw time tags will be of the form (n+0.7) seconds,
for integer n.  It follows that the arrival time can be obtained by
adding

		T2 = +0.026521108 sec

to the doppler file's time tag.  Note that, regardless of the value of
T1, the magnitude of T2 should never exceed 0.05 sec.

	The corrected time tag (t-T1) is always used in the
computations for converting residual phase to integrated doppler; in
particular, these calculations involve evaluating the predicted orbit
at appropriate times.  It is the tracking station's responsibility to
ensure that the predicted orbit used to drive the doppler compensation
during each tracking pass is identical with that used to compute
integrated doppler; this includes the times at which the orbit is
evaluated for each purpose.  Additional details are given in [4].

	It is expected that the value of T1, and hence of T2, will be
stable.  However, two kinds of change could occur: (a) a station
design change or modification might cause the actual value of the
processing delay to change; or (b) the actual delay is unchanged, but
an improved estimate of its value might be obtained.  If either type
of change occurs, it is the responsibility of NRAO to notify specific
persons at JPL via electronic mail.  In addition, a log of such
changes will be maintained as an ASCII file (name TBS) in the same
directory as the doppler files.  Each line of the log will give the
effective date and time of each change, the new values of T1 and T2,
and whether this represents a change in the actual delay value or not.
All doppler files *computed* after the given time use the new value,
regardless of the time of the affected tracking pass.


[1] G. Langston, "JPL doppler file format description."  Memo dated
93/08/23.  [ftp.gb.nrao.edu:ovlbi/doc/JPLDopMemo.tex].

[2] K. M. Liewer, "TRK-2-30, DSN Tracking System, DSN Tracking Data
Interface."  JPL Document No. 820-13, Rev. A, dated 95/02/28.

[3] L. D'Addario, "Details of post-pass processing of time correction
data," Appendix A: Calculation of the phase processing delay.
Document in preparation, draft available from the author; future
OVLBI-ES Memo.  See also 'ftp.gb.nrao.edu:ovlbi/doc/signalDelay.txt'.

[4] A. Minter, "Conversion of two-way timing data to doppler data."
OVLBI-ES Memo No. 66, 96/11/26.  
[ftp.gb.nrao.edu:ovlbi/memoseries/es66_dopplerConversion.tex]