Subject: [IGSMAIL-1456] CSTG GPS Laser Tracking Campaign
From: John J. Degnan
Date: Fri, 11 Oct 1996 6:08:35 PDT

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IGS Electronic Mail      Fri Oct 11  6:08:35 PDT 1996      Message Number 1456
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Author: John J. Degnan
Subject: CSTG GPS Laser Tracking Campaign

CSTG GPS LASER TRACKING CAMPAIGN UPDATE

DURATION OF THE CAMPAIGN

As announced previously in April 1996, the first international CSTG GPS
Laser Tracking Campaign will begin this Fall. A start date of October 20,
1996 has been selected. The full program will continue  through November 20,
1996, but a more limited program addressing  the GPS orbit in eclipse will
continue through December 15 in order to collect laser range data on a
significant number of GPS Earth-eclipse events .

CAMPAIGN GOALS 

The goals of the first GPS tracking campaign are as follows:

(1) SLR-derived Trajectory:  To develop an all-SLR orbital trajectory,
lasting initially for up to 4 weeks, for the GPS 35 and 36 satellites for
the purpose of acquiring an intensive SLR data set which can be used for
direct comparisons to GPS-derived ephemerides and other studies .

(2) Continuous Multistation Tracking during GPS Eclipse Events: To provide
continuous multistation (preferably 3 or more SLR sites) SLR tracking of GPS
35 during any eclipse episode. SLR tracking should begin 15 minutes before
GPS enters the Earth's shadow and continue for 30 minutes after GPS exits
the Earth's shadow.

(3) GPS Phase Antenna Offset: To provide improved estimates of the GPS
antenna phase center/laser retroreflector offset through "near-simultaneous"
(defined as over the same time interval)  tracking of GPS 35 and/or 36 by
multiple (four or more) SLR stations for several continuous intervals of one
to two hours each.

SUGGESTED METHODOLOGIES FOR ACHIEVING EACH OF THE GOALS

(1) SLR-derived Trajectories: The first of these  experiments will begin on
October 20, 1996 and end November 20, 1996. This particular schedule has
been chosen because: (1) passes of the GPS-35 and GPS-36 satellites largely
occur at night at most of the stations capable of tracking them (except for
the Australian sites) as shown in Figure 1; and (2) the NRL@SOR facility
near Albuquerque, New Mexico will be online during this period . The latter
station has a strong GPS tracking capability and adds needed redundancy in
the Southwestern US for coverage of other satellites. It is anticipated that
tracking coverage in the Southern Hemisphere will be sparse  due to a lack
of a GPS-capable SLR station in South America  or Africa and the fact that
most GPS-35 and 36 passes over the the two SLR stations in Australia will
occur  in daylight. The Australian and Asian stations are encouraged,
however,  to attempt daylight tracking of GPS. In all geographic regions, 15
minutes of SLR station tracking per hour from at least one SLR station will
be the goal, day or night. In order to maintain an adequate data yield for
other Low Earth Orbiting (LEO) satellites during the campaign, it is
recommended that the stations in each region (Americas, Eurasia, and Western
Pacific) be subdivided into two groups: (1) those stations with a
demonstrated or potential GPS tracking capability and (2) stations which do
not at present have such a capability.  Stations in Group 1 should adjust
their tracking priorities, under the guidance of their local networks, to
meet the goals of the various GPS experiments while maintaining an adequate
level of LAGEOS and lower satellite tracking for orbit maintenance and
quality control activities. During the GPS Tracking Campaign, stations in
Group 2 should carry the primary burden for LEO tracking up to and including
LAGEOS altitudes.  Networks are encouraged  to continue to provide adequate
geographic and longitudinal coverage for all satellites by scheduling and
"teaming" pairs or clusters of stations within a locality or region whenever
possible.

Although every SLR station is invited and encouraged to participate in this
international GPS tracking campaign, the stations in Table 1have been
assigned to Group 1  because of their demonstrated ability (or in some cases
potential) to acquire SLR ranging data to GPS and are especially encouraged
to GPS range data. The aforementioned Group 1 stations are further
subdivided into "regions" irregardless of their sponsorship or affiliation
(i.e. NASA, EUROLAS, or WPLTN). Because of their unique geographic location
which places them in a night tracking mode simultaneously with more than one
"region" during the November-December  campaign period (see Figure 1), two
stations , Maidanak and HOLLAS, have been assigned to more than one region.

(2) Continuous Tracking during a GPS Eclipse: Information on GPS-35  eclipse
events and stations which are in a position to track the satellite during
these periods will be posted on the CSTG SLR/LLR Subcommission Home Page at
the following WWW address:

http://cddis.gsfc.nasa.gov/cstg/slr_cstg.html

The networks (NASA, EUROLAS, and WPLTN) should attempt to provide continuous
"near-simultaneous"  tracking of the event from multiple sites beginning 15
minutes before GPS enters the Earth's shadow and continuing until 30 minutes
after it emerges from the shadow. This portion of the campaign will extend
through December 15 in order to collect data on eclipses which occur in the
late November and early December time frame.

(3) Antenna Offset: This episodic experiment will be highly subject to the
vagaries of local and regional weather and should be coordinated and
performed at an individual network level, i.e. NASA, EUROLAS, and WPLTN. It
will require continuous and lengthy (1 to 2 hours per experiment) tracking
of GPS from four or more sites within a "region" simultaneously, and should
only be scheduled when all or most of the "regional" station passes are at
night and are likely to enjoy good weather and visibility. It is therefore
suggested that each of the aforementioned networks (i.e. NASA in the
Americas, EUROLAS in Eurasia, and WPLTN in the Western Pacific) be
responsible for scheduling the activities for the stations in their primary
region. During this experiment, it is proposed that NASA coordinate the
tracking activities (both GPS and LEO) of NASA sites in the Americas and
that of the German station in Santiago de Cuba, that EUROLAS coordinate the
activities of its own network extending from RGO to Maidenak but also
including the SALRO system in Riyadh, and WPLTN will  coordinate the
stations spanning Maidenak to HOLLAS and including the NASA MOBLAS-5 site in
Australia. 

Comments or suggestions regarding the proposed campaign should be addressed
to the CSTG SLR/LLR Subcommission Chairman, John Degnan, via one of the
following means:

	Address: Code 920.3, NASA Goddard Space Flight Center, Greenbelt, MD 20771 USA
	Phone: 01-301-286-8470
	FAX: 01-301-286-0213
	Email: [email-masked], [email-masked]

This announcement, complete with figures as well as periodically updated
information on the campaign,  will be provided on the CSTG SLR/LLR
Subcommission home page listed above. If you have information of general
interest to the community regarding this campaign and would like to include
it on our Web Site, please submit it to either John Degnan or to Carey Noll
at the CDDIS ([email-masked]). Interim analysis results from the
campaign are also welcome.

 
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Dr. John J. Degnan                      
Head, Geoscience Technology Office
Code 920.3
NASA Goddard Space Flight Center
Greenbelt, MD 20771 USA

Phone: (301) 286-8470
Fax: (301) 286-0213
Email: [email-masked]
Internet: [email-masked]
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[Mailed From: [email-masked] (John Degnan)]