Chandra Ephemeris on JPL Horizons

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				20:09 UTC Fri 10 Sep 99

	The Chandra ephemeris is now available on the JPL Horizons
ephemeris computation service. The current ephemeris for Chandra
extends to 1999-OCT-20 10:41:55.81 UTC. The ephemeris will be 
updated monthly or as necessary.

	Those interested in observing Chandra should find this
service useful. The ephemeris is derived from orbit determination using
radio metric Doppler and range tracking data coherently transponded by the 
spacecraft. The prediction accuracy should be good enough (~1 km for a 
current ephemeris) to eliminate the possibility of ephemeris error in any 
failure to observe Chandra.

	The Chandra ephemeris should be useful for astrometric visual 
observers with adequate optical aid who wish to test their observing 
accuracy by comparing their observed positions with the predicted 
positions available through Horizons. For example, a 1 km positional error 
in the cross line of sight direction at a range of 30,000 km corresponds
to an angular offset of 0.002 degree--quite accurate enough to serve
as a reference for comparison with visual observations of spacecraft
position. To generate a topocentric ephemeris users will need to enter
their geodetic station location coordinates using the "coord" option.
	
	To access the Horizons system, enter "telnet ssd.jpl.nasa.gov 6775". 
The port number 6775 is required. When the system prompt appears type 
"Chandra". Then follow the prompts to provide the desired user inputs. 
On-line help is available by entering "?" or (for extended help) "?!". Full
documentation is also available at:

 (PostScript file)
   ftp://ssd.jpl.nasa.gov/pub/ssd/Horizons_doc.ps

 (indexed HTML)
   http://ssd.jpl.nasa.gov/horizons_doc.html

	Thanks go to the Chandra Project for approving release of the Chandra 
ephemeris using JPL Horizons, to JPL Multimission Navigation for providing the 
Chandra ephemeris and to Jon Giorgini for adding the Chandra ephemeris to 
Horizons.

George Lewis
DISCLAIMER: All opinions represented herein are my own and do not in any way
represent the opinion or policy of JPL. Permission to post from JPL machine 
granted by JPL Ethics Office.

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