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.