Subject: [IGSLEO-8] Analysis Centre coordination for LEO PP
From: [email-masked]
To: [email-masked]
Date: Fri, 18 May 2001 19:02:30 +0200

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IGS LEO Mail      18 May 10:16:34 PDT 2001      Message Number 8
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Author: Henno Boomkamp

Dear Colleagues,

This mail aims to answer some items listed in IGSLEO mail
No. 4 (actions raised during the Potsdam meeting) and No. 5
on data formats. The comments below show the present status
of the Coordination and Analysis tasks assigned to the ESOC
Associate Analysis Centre Coordinator.

A few concrete suggestions are included in these comments.
These are merely intended to provide a starting point, under the
assumption that an incomplete standard is better than no standard
at all. It is expected that many of the points that are now left open can
be made more concrete after the first CHAMP data processing
results become available.

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(1) Identify responsible technical point of contact
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As from May 2001, LEO Pilot Project AAC and AACC activities at ESOC
are handled by Henno Boomkamp, with support from Carlos Garcia
and John Dow.

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(2) Develop processing standards
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The natural way for developing processing standards is to study existing
practice at the various Analysis Centres, and only agree upon a common
standard in cases of identified incompatibilities. For this purpose it would
be useful if the ESOC AACC could obtain some documentation from those
Analysis Centres that are already prepared for routine data processing,
describing their present solution set-up and processing standards.

Of interest are in particular details like:
- Summary of dynamic models (gravity field, drag modelling)
- Use of Earth rotation parameters, reference frame realisation
- Which observables are used (differenced or undifferenced GPS,
  phase data, other tracking data like SLR, DORIS, ...)
- Handling of LEO satellite clock,
- Handling of LEO accelerometer data, if available
- LEO orbits estimated with respect to fixed IGS GPS orbits/clocks,
  or simultaneous solution for LEO and GPS

The following requests are therefore made at this point :
(a) from GFZ Potsdam (C. Reigber) : to forward documentation on CHAMP
      processing standards to ESOC AACC
(b) from JPL and/or CNES : to forward documentation on planned JASON
      data processing standards to ESOC AACC
(c) from any Analysis Centre that plans on processing the CHAMP
      data that will be arriving shortly : a summary on their standards
      and/or solution set-up by e-mail, and if possible some results.

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(3) Exchange format for orbit comparisons
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The SP3 format seems to be adequate. Differences in vector
intervals, arc lengths and starting points can be overcome by
careful N-point interpolation, at least during the initial stages of
the PP when precision will not yet be at cm level. It is suggested
to maintain substantial overlaps between successive solution
arcs to enable elimination of inaccuracies around the start and
end of the arcs.

A matter to be settled as soon as possible would be the assignment
of ID numbers to the various LEO satellites. It has been suggested to
modify the SP3 format to include the ID code Lxx (c.f. Gxx for GPS, Rxx
for GLONASS). In that case, it will be helpful to have agreement on
the numbers 'xx' to be used for each LEO satellite.

At ESOC, we are not aware of any existing ID lists of this type.
Lacking further information, the simple proposal is made here
to assign the numbers in order of launch date, starting with L01 for
TOPEX/Poseidon, etc. With that system, the only remaining discussion
is to decide which LEO satellites are to be included in the numbering
system. At least all LEO satellites with GPS data should be covered,
so at this point the following table is suggested :

 ID        Satellite                             Launch
-------------------------------------------------------
L01      TOPEX/Poseidon         08/1992
L02      GPS-MET                        12/1994
L03      GFO                                   02/1998
L04      Orsted                               01/1999
L05      SUNSAT                         02/1999
L06      CHAMP                            07/2000
L07      SAC-C                               11/2000
L08      JASON-1                                2001
L09       GRACE (*)                             2001
L10       GRACE (*)                             2001
L11       GOCE                                     2004

(*) order of the GRACE satellites to be determined in the future

Any further comments on this point are welcome. In particular, other
satellites of interest that may have been overlooked in the list above
should be reported as soon as possible, so that a more definitive
proposal can be prepared as to which satellite corresponds to which
Lxx number.

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(4) LEO AAC data sheets
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The proposal is to start with the standard IGS data sheet, and see
in practice which additional information would be useful. This point is
closely related to the action (2) on analysing the various processing
standards and modelling details, and can be coordinated by the
ESOC AACC during the course of the Pilot Project.

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(5) Arc length and parametrisation
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In this early stage the natural approach is to let every Analysis Centre
determine a solution set-up that seems to provide the best results in
their processing environment, independent of other AC's.
After analysis of the different solution set-ups and processing standards
by the ESOC AACC (see point 2) it may be useful to standardise certain
aspects of the solutions, in particular modelling aspects that affect
interchange of data products such as reference frame realisations.

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(6) Orbit comparisons and test definitions
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For orbit comparisons, the following methods are proposed:

(a) In case that very few solutions are available, pair-wise comparisons
      of SP3 files can be performed to provide an idea of their relative
      accuracy

(b) Once that sufficient (stable) orbit solutions are available, a combination
      solution can be produced in the style of the IGS GPS orbits.
      Typically this would be a staged process, with a first unweighted
      solution to estimate standard deviations of each input orbit,
      followed by a second, weighted solution.

To avoid reference system problems as much as possible, the
following two recommendations are made here :
(a) to apply a 7-parameter transformation to the LEO orbit in order
      to bring it in line with the IGS / GPS reference frame (this should be
      relevant only in the case of combined solutions GPS/LEO)
(b) to clearly indicate if the GPS orbits were solved together with
       the LEO orbit, or not (this may be one of the issues to be added
       to the LEO AAC data sheets)

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(7) Specify data period for processing
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For the time being, little data is available for routine processing but this
will change in the short term. At present no specific period is indicated
and all available data is analysed to the best possible extent.

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(8) LEO receiver clock solution and comparisons
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This matter forms part of the analysis described under (2).
For the time being no standard will be set, but the LEO Analysis Centres
are requested to inform the ESOC AACC about their models for the LEO
clock, in order to assess the need for standardisation.

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(9) SLR for external validation
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See point (7) - no specific processing has been identified yet.
Centres with the relevant capabilities are encouraged to process
CHAMP laser data, as an independent source of validation.

As will be clear, these comments leave almost total freedom
to the Analysis Centres to set up their preferred processing
environment. The short-term intention of the ESOC AACC is to
collect as much concrete information as possible regarding
the various solution standards and regarding the first CHAMP
processing results.

For any feedback or additional inquiries, you can now contact
me directly, or send a mail to the IGS-LEO mailing list.

Best regards,

Henno Boomkamp