The Soviet Space Station MIR
as seen through the Kitt Peak 90" Telescope
at various stages of its evolution.
GPSII-16 on 6/26/95 from CBA Tucson Station
C-14 @ f/11 - AX-2 CCD camera 15 sec exposure.
In early 1984 a 5th generation telescope control system (designated TCS forTelescope Control System) was completed and installed at two large telescopes operated by Steward Observatory - 90 and 61 inch Ritchey Cretion Reflectors located atop Kitt Peak and Mount Bigelow in Arizona.
The control system is based on a distributed architecture that uses a single IBM PC class 16 bit microprocessor to control the two astronomically orthogonal axes of the telescope in a wide variety of applications. A second dedicated PC system is used to propagate orbital elements and generate tracking ephemeris files for use by TCS in satellite tracking.
The procedures for accomplishing satellite tracking have been developed over the past five years by the Steward Center for Orbital Mechanics (S-COM) which is responsible for providing this service to observers from around the world on a resource available basis.
In the first tests of the systems compliance to high speed tracking commands, an interface program designed to download Right Ascension and Declination rates at 250 millisecond intervals to the ST-TCS was implemented. Our first test was performed on the US Solar Maximum Mission Spacecraft.
The satellite was tracked for approximately 2 minutes and 22 seconds in the 2 x 1.5 arcminute field of the 90 inch telescope operating at F/9 as it passed within 621 kilometers the station. Although the track was extremely successful, the satellite was too small to be resolved with the camera and optical arrangement used. Calculations bear out an angular size for Solar Max of about 1.5 arcseconds at 600 kilometers.
To date over 100 tracks have been made of objects ranging from high geosynchronous orbit to as low as the Soviet Space station MIR and US Space Shuttle . Projects currently being supported by the satellite tracking support team of Steward's Center for Orbital Mechanics (S-COM) include: imaging of LEO's with speckle interferometry and CCD cameras, photopolarimetry and spectroscopy.
The TCS machine consists of a single 486 based PC running DOS and software written by the author called PC-TCS. PC-TCS is responsible for the real-time control of the DC Servo hardware in the telescope mount and the interface to the user for operation of the telescope in both astronomical and satellite tracking applications. The user interface is via either the standard keyboard/mouse and video monitor - or via up to eight independent serial "command" channels which can accept simple formatted ACSII commands at a rates of up to 10 hertz.
The PC-TCS software is responsible for all mount and servo system control functions including the display and calibration of the astronomical pointing/ tracking of the telescope. PC-TCS also monitors the telescope for limit conditions and provides automated slew and speed tracking management via a proprietary modified PI servo loop built in to the system software.
The SATPRO machine is a 486 based PC providing software necessary to reduce satellite orbital elements and produce tracking rate files that are read by PC-TCS for real-time tracking. It should be noted here that the tracks performed by the system are based on equatorial velocities and not the position of the satellite at any given moment. This velocity relative method provides a greater degree of freedom during the track in the event the satellite ephemeris is not completely precise. The PC also provides a real-time display of telescope position and velocity as well as track statistics.
Currently the system operates in an "open-loop" mode and no provision has been made for closed loop operation. The system, therefore relies heavily on accurate orbital elements and ephemerides to provide stable tracking. Current processing power is ample to accommodate closed loop operation via external sensors that would provide "correction rates" to default ephemeris via PC-TCS's serial command channels.
Stage two involves the upload of the rate file from the SATPRO to TCS and the positioning of the telescope at a starting coordinates in preparation for the acquisition phase.
The acquisition phase - stage 3, requires a visual acquisition of the satellite in a wide field (approx. 1 degree) telescope or camera. Once the satellite is observed in the acquisition field, PC-TCS is manually instructed to begin the execution of the uploaded telescope rates. Depending on the starting rates, the control system may take several seconds to stabilize on the rates being commanded. At this point the satellite will be stationary in the acquisition field and the tracking phase begins.
The satellite, once acquired, will usually be offset in the primary telescope observing field and the telescope must be manually centered on the object by the use of a rate joy-stick connected to the TCS Computer designed to assist the final co-alignment. The joy-stick allows the telescope "pilot" to make small corrections to the base line rates being commanded of the servo system. Once centered, the satellite's position is maintained in the observing field via this joy-stick. The track continues until aborted by the pilot or TCS exhausts its store of rates. The rate update interval is typically 0.5 to 1 sec for LEO targets and up to once every minute for geosynchronous targets. Since the control system software computes feed forward terms for both velocity and acceleration in its satellite tracking mode, velocity ramps during tracks are smooth and free of jerk.
As of this writing the tracking process involves at least two people to accomplish. In practice however, due to the lack of low light camera equipment, tracking operations usually involve three participants - a telescope pilot to perform phase 4, a acquisition pilot to perform phase 1,2 and 3, and a dome pilot to assist in phase 4. If a wide field acquisition camera is being used, the telescope pilot can perform the duties of the acquisition pilot without assistance. The dome pilot is still essential since the dome rotation is currently unautomated.
The SAT-PRO program takes standard NORAD two line Elsets as input and produces an crude ephemeris of the pass along with a State Vector. The ephemeris is used in planning tracking schedules. The State Vector is processed by a separate propagator built into SAT-PRO for more precise short term ephemerides which in turn is used to generate the telescope rate files digested by PC-TCS.
PC-TCS Satellite Tracking Rate File Structure
Individuals and organizations wishing more information may contact the author at the address given below.
David A. Harvey
Steward Center for Orbital Mechanics
University of Arizona
Steward Observatory
Tucson, Arizona 85721
Voice: (602) 621-3648
Internet: dharvey@as.arizona.edu.