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SLR-Mail No.151

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Date:1998-09-06 14:00:00
Sender:Roger Wood and Andrew Sinclair <rw@gxvf.rgo.ac.uk (Roger Wood, SLR Herstmonceux)>
Subject:[SLR-Mail] No. 151: Daily GLONASS IRVs for IGEX-98
Author:Roger Wood and Andrew Sinclair
Content:********************************************************************************
SLR Electronic Mail 1998-09-06 14:00:00 UTC Message No. 151
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Author: Roger Wood and Andrew Sinclair
Subject: Daily GLONASS IRVs for IGEX-98



Daily GLONASS IRVs for IGEX-98

1. Introduction

This message announces an extension, to include all active GLONASS
satellites, to the scheme for daily computation and distribution of
IRVs for GPS 35 and 36 described by Andrew Sinclair in SLRMAIL 36. The
GPS predictions are derived from the precise orbits computed daily at
the Centre for Orbit Determination (CODE) at the University of Berne.
The GLONASS predictions are of lower precision since they are formed
from positional data derived by CODE from the satellites´ broadcast
ephemerides. But in both cases the predictions require no timebias
corrections and allow the use of much narrower range gates, typically
+/-50 ns or better. It is hoped that the availability of these new
predictions for GLONASS satellites will greatly increase the quantity
of SLR data obtained during the forthcoming IGEX-98 campaign, in
particular for daylight passes. (A request for SLR tracking will be
issued soon by the IGEX-98 steering committee). The new predictions
have undergone a short period of evaluation at Herstmonceux and are
being made available to the community.

The IRVs for each satellite will be produced and distributed in two
forms: at 6 hourly intervals, the same as the GPS IRVs; and at the
´standard´ interval of 24 hours. Format and file naming are described
in the sections which follow - previous users of the 6 hourly IRVs for
GPS should note changes to filenames.

2. IRV format

The IRV files sent out each day covers 72 hours so as to guard against
network failures on any one day. The header line for each day gives
information on the origin of the IRVs, the date of formation, the
identity of the satellite and the multiplicity (number of IRV sets per
day - see SLRMAIL 36 for a more detailed description) of the IRVs which
follow. Thus:

COD09732 GLO71 4

indicates that the IRVs for GLONASS 71 were formed from the CODE orbit
for day 2 of GPS week 973 and that there are 4 sets of IRVs per day,
one for each interval of 6 hours. The header:

COD09732 GLO71 1

introduces the more familiar IRVs for the same day but with one set
covering the whole 24 hours.

3. Distribution

The IRV files for GPS and GLONASS are formed at Herstmonceux as soon as
the relevant CODE files are available by FTP. This is usually between
10:00 and 12:00 UT but can on occasion be much later. The files are
then deposited at the European Data Centre (EDC) in subdirectory:
pub/laser/predictions/RGO_PRED
and at RGO in subdirectory:
pub/slrirv/current
from where they are available by anonymous FTP. The files can also be
mailed directly to stations on request.

It will only be possible to finalise the EDC distribution arrangements
after September 10, but files are available at the RGO FTP site now.

4. File naming

To make it easier to distinguish GPS from GLONASS files the naming
convention (defined in SLRMAIL 36) has been amended to identify the
satellites explicitly and to indicate both the origin of the orbits and
the multiplicity, as well as the date. Examples of two files formed on
1998 September 1 are:
glo67_cod4_9809.01 - GLONASS 67 with 6 hourly IRVs,
glo71_cod1_9809.01 - GLONASS 71 with 24 hourly IRVS.
The same naming convention will also be applied to GPS files.

5. GLONASS satellite identification

In order to minimise any possible confusion concerning the names of
GLONASS satellites the following paragraphs provide a detailed
description of the naming scheme adopted here, a list cross-referencing
the main identifiers and the sources from which the information was
taken. The cross-reference list is available in the EDC and RGO FTP
directories with the name:
glo_identifiers.lst
the present version of which appears below.

The SLR community uses a 2-digit GLONASS number for each satellite as
the principal identifier, e.g., GLONASS 71. This 2-digit number is used
in the header line of each set of IRVs, and is also used to form the
4-digit SIC number that appears in each IRV data message by adding
9000, e.g., 9071 for GLONASS 71. The satellite identifier used in the
SLR normal point data format is the COSPAR 7-digit number, e.g., for
GLONASS 71, 9500901.

The identifier used in the navigational data broadcast by the GLONASS
satellites themselves is the slot number which gives the location in
the orbit plane. The orbital position data supplied by the Swiss use
this identifier, and it is likely that the IGS community will use it
too. Hence this is also included in the cross-reference list.

The Russians use the slot number as an identifier, and also assign a
Cosmos number and a 3-digit GLONASS number of their own. These are not
the same as the GLONASS numbers used by the SLR community, and bear no
simple relationship to them. A list of the Russian identifiers plus
up-to-date status information, can be found in:
http://www.rssi.ru/SFCSIC/NAGU.TXT.
The Cosmos number and 3-digit GLONASS number are not included in the
cross-reference list.

The source used for cross-referencing the SLR GLONASS numbers against
COSPAR ID and Russian GLONASS numbers is SpaceWarn Bulletin No. 513
(1996 July), which is available at:
http://nssdc.gsfc.nasa.gov/spacewarn/spacewarn.html.
Later bulletins do not give the GLONASS numbers, and it seems to be a
dying numbering system. It is likely that the SLR community will have
to assign these numbers themselves for any future launches.

The COSPAR numbers are the identifiers in the NORAD two-line orbital
elements. These elements can be used to determine the position of a
satellite, and hence its slot number. Unfortunately there are some
contradictions in relating COSPAR number to slot number by this route,
compared with using the SpaceWarn Bulletins and the Russian NAGU
notices. Where there is a contradiction we have chosen the
identification that gives agreement in orbital position. In the
cross-reference list the satellites for which the GLONASS number is
questionable in this sense are GLONASS 71, 72, 77 and 79.

6. Acknowledgements

Thanks are due to Tim Springer of CODE for depositing the daily orbit
files for the GLONASS satellites in addition to those for GPS, to
Werner Gurtner of EuroLas for helpful comments on early drafts and to
Wolfgang Seemueller of EDC for providing database facilities for these
new files.

------------- glo_identifiers.lst as at 1998 September 1 --------------
!
! List of GLONASS satellite identifiers as used in RGO/CODE IRVs
! See SLRMAIL 163 for an explanation of the use of this file
! Slot occupation may change from time to time
!
! Col 1 = Glonass slot number
! 2 = 2-digit Glonass identifier
! 3 = COSPAR number
! 4 = Glonass plane
! 5 = Currently active (Y = Yes; N = No)
! 6 = Notes (* = currently tracked by SLR network)
! (? = cross-referencing of number has been questioned -
! see discussion in SLRMAIL 163)
!
! +-------------------------+
! | Last updated 1998 Sep 1 |
! +-----------+-------------+
! V
! 1 2 3 4 5 6
!-! ---------! ------! ! ! ------------------------!
3 Glonass 68 9407601 1 Y SLR candidate for IGEX-98
4 Glonass 70 9407603 1 Y SLR candidate for IGEX-98
6 Glonass 69 9407602 1 Y SLR candidate for IGEX-98
9 Glonass 79 9506803 2 Y SLR candidate for IGEX-98 ?
10 Glonass 75 9503702 2 Y
11 Glonass 76 9503703 2 Y
12 Glonass 65 9405001 2 Y SLR candidate for IGEX-98 *
13 Glonass 77 9506801 2 Y ?
14 Glonass 67 9405003 2 N *
15 Glonass 74 9503701 2 Y
16 Glonass 66 9405002 2 Y SLR candidate for IGEX-98 *
17 Glonass 62 9402101 3 Y SLR candidate for IGEX-98
18 Glonass 64 9402103 3 Y
20 Glonass 71 9500901 3 Y SLR candidate for IGEX-98 * ?
22 Glonass 72 9500902 3 Y SLR candidate for IGEX-98 ?
--------------------- End of glo_identifiers.lst ----------------------

=======================================================================
Roger Wood, NERC Satellite Laser Ranger Facility, Herstmonceux Castle
Hailsham, East Sussex, UK BN27 1RP
Tel: +44 (01323) 833888
Fax: +44 (01323) 833929 Email: rw@gxvf.rgo.ac.uk
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From: rw@gxvf.rgo.ac.uk (Roger Wood, SLR Herstmonceux)

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