REALTIME TIME BIAS COMPUTATION AND DISTRIBUTION
               ***********************************************

                               Werner Gurtner

                               March 26, 2002

The following proposal is a result of discussions at the recent
EUROLAS workshop in Herstmonceux.

TIME BIAS FUNCTIONS

The NERC Space Geodesy Facility group at Herstmonceux is automatically
and frequently collecting normal point data to compute updated time
bias functions (with respect to all available IRV sets) for all ILRS
satellites.

FASTER DISTRIBUTION

In order to simplify, and greatly speed up, the distribution of these
updated time biases, I have written a TCP/IP server program that
accesses the latest time bias functions at NERC and computes time
biases for the current epoch, including drag functions (if existing)
for all available satellites and IRV sets. So with one simple call
stations can get the best current estimates of time bias for all
satellites.

USAGE

To download the values use the EUROSTAT program (the program used to
access the EUROLAS status exchange - see SLRMAIL 372) as follows:

  eurostat aiuli3.unibe.ch 7840 1 [>time_bias_file]

The program connects to port 7840, receives the time bias values and
writes them to the standard output.

You can use telnet, too:

  telnet aiuli3.unibe.ch 7840  [>time_bias_file]

On UNIX systems you can also redirect the telnet standard output to a
file.

Or to display the values from a web browser:

  telnet://aiuli3.unibe.ch:7840

or

  http://aiuli3.unibe.ch:8000/cgi-bin/cgi-time_bias

Or you write your own program using socket library routines to connect
to port 7840 and to receive the time bias message. The host
disconnects the tcp/ip connection immediately after having sent the
message.

TIMING

Depending on the time needed for range data post-processing at the
stations, data submission, data handling at the data centers, and
access by the analysis center, the distributed time biases could be
VERY up-to-date.

FEEDBACK FROM STATIONS

For very low satellites (currently Champ, GraceA, GraceB) GFZ has
asked stations to send normal points directly to GFZ as soon as
possible after the pass:  any stations doing this are requested to add
Herstmonceux to the mailing list (slr@slrb.rgo.ac.uk) using the same
subject header as requested by GFZ.

EXAMPLE

!
!      Time biases at 26-Mar-2002 11:11 UT
!
! Satellite   Total IRVset LstObs Passes SIC Drag  Dyn
!            TB[ms]       [hhh:mm] used      [ms] Prio
!
  Ajisai          0 HON084  16:57     5 1500    0  -1
  Ajisai       -233 RGO095  16:57    58 1500    0  -1
  BeaconC         0 HON084   0:00     0 0317    0   0
  BeaconC       217 RGO040  35:26    39 0317    0   0
  Champ        -100 GFZ310   0:00     0 8002 -100  +1
  ERS2            0 GFZ394   0:00     0 6178    0   0
  Etalon1       -32 HON084  34:57     1 0525    0   0
  Etalon1        10 RGO020  34:57    31 0525    0   0
  Etalon2         0 HON084   0:00     0 4146    0   0
  Etalon2        -6 RGO020  40:14    29 4146    0   0
  GFO1            1 HON084  30:52     1 8501    0   0
  GFO1           42 RGO099  30:52    28 8501    0   0
  ...
  ...
! ---------------------------------------------------

The message consists of

- A number of header records
- one line per time bias
- last line: dashed line.

The lines are separated by CR/LF pairs. All non-data lines start with "!".

1st column: Satellite name
2th column: Current time bias, drag function value included (ms)
3rd column: IRV set
4th column: Age of latest np data used for time bias function
            computation
5th column: Number of normal points used to generate the time bias
            update
6nd column: SIC code
7th column: Value of drag function (ms) (ALREADY INCLUDED in TB of
            col 2)
8th column: Dynamic priority

Format:     (2X,A10,1X,I6,1X,A6,1X,I3,1X,I2.2,1X,I5,1X,I4,1X,I4,I3)

NOTE: For program-controlled evaluation of the time bias message
      please skip any line within the message starting with "!".
      The final one (dashes) indicates the end of the message.

SUGGESTIONS ON HOW TO USE THE TIME BIAS SERVER

A) - Compute az/el/range elements immediately before each pass using
     the current time bias from the time bias server

B) - Compute az/el/range elements any time before the pass

     - either computing the predicted time bias using the time bias
       function (and drag function)

     - or downloading the time bias from the time bias server (valid
       at the time of computation)

   - Immediately before the pass: Download the current time bias and
     correct the initial tracking by the difference "actual time bias
     minus time bias used for the computation of the elements"

BACKUP

In order to be prepared for communication problems with the server or
any other unavailability of time biases it might be a good idea to
have the computation of the time bias using the distributed time bias
functions (standard time bias file) ready as a backup.

FUTURE

The NERC colleagues are currently investigating the possibilities to
include also time biases relative to the preceeding ("yesterdailies")
IRV sets to cover the time around IRV updates.

PROGRAM SOURCE AND EXECUTABLES

The C source (eurostat.c), the PC executable (eurostat.exe_doc) and
explanations to the eurostat program (eurostat.txt) can be found on
ftp://ftp.unibe.ch/aiub/slr

Please contact me if you have any problems or suggestions.

ACKNOWLEDGMENTS

I would very much like to thank Roger Wood, Philip Gibbs, and Graham
Appleby of the NERC Space Geodesy Facility and Georg Kirchner, Graz
for their support and many good suggestions.

Best regards,

Werner Gurtner