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Observing Solar System Objects

Solar system objects differ from most astronomical sources in that the proper motions are large enough that the object position changes in the course of an observation. CAOBS has the ability to phase track an object with a non-sidereal rate. Note, however, that the motion during a scan is approximated as linear. Thus your scans must be short enough that there is no significant deviation from linear motion of the source during a scan (SCHED does not check this).

Major Planets, Sun and Moon

Scheduling the Sun, Moon and major planets is straightforward: you simply give the name of the object as the source name (e.g. a source name of ``jupiter''). No other special steps are required. When it is time to observe this scan in the schedule, CAOBS looks up the position and proper motion of the object in its ephemeris. Note that the planets positions that you enter into SCHED are ignored by CAOBS. SCHED only recognises these sources as special when it is in absolute time mode (this doesn't work for standalone versions of sched). Consequently you will probably want to specify absolute times in sched (TimeCode UTC) and write the schedule to disk. Sched then computes the correct positions for the objects. Do a listing after writing the schedule to check az and el. You can then change back to relative time (timecode 'REL') before using the schedule in caobs, so you are not tied to a specific start time.

Other Solar System Objects

To handle other solar system objects, you will need to supply an ephemeris file (generating an ephemeris file is described below; its format is described elsewhere). Ephemeris files are expected in the directory /atca/ephem/. The ephemeris file is included in the schedule via the source name: SCHED treats source names beginning with the @ symbol as ephemeris files. For example, use a source name of @hbopp as a pointer to the external ephemeris file /atca/ephem/hbopp.eph. CAOBS will recognize source names starting with @ as well, in both absolute and relative time mode. It is recommended you test and check your ephemeris file using absolute time mode in SCHED before your observation, even if you intend to use a relative time schedule for the actual observation.

An ephemeris file is readily created using JPL's Horizons on-line system. This contains ephemerides for more than 15000 asteroids, comets, natural satellites, several dynamical points and some spacecraft (e.g., Mars Reconnaissance Orbiter). Horizons is available via telnet access:

%telnet 6775
More information on Horizons is available from JPL. Horizons is terminal-based, with it asking the user a series of questions. To generate an ephemeris file for ATCA observations,
  • First select an object by giving the source name (followed by a semi-colon for minor bodies) or by source number.
  • In response to prompts, select

    Ephemeris, then

    Observe, then


    to select geocentric RA/DEC ephemerides.

  • Then give the start time of interest, in UTC, in the format suggested by the prompt. For example,
      1997-May-07 00:00
    Note SCHED requires times in UTC (not TT or any other time system). Then give the end time, and then the time increment. Typically 1 hour is more than adequate for outer solar system objects. Inner solar system objects and the Moon might require shorter intervals. Note that SCHED uses simple linear interpolation of ephemeris values. The ephemeris you produce must start at least two time increments before your observation, and go to beyond your end time.
  • Accept default output, and select ``table quantities'' 1,20. This gives RA, DEC, distance and velocity of the object.
  • Horizons will then list the ephemeris. You can then have this ephemeris e-mailed to you, or you can access it via FTP (you could also capture the screen output).
Having retrieved Horizon's ephemeris file, this needs to be massaged into the format that SCHED requires. To do this, on xbones, use the ephformat command. This prompts for the name of the input (Horizons) and output (SCHED) ephemeris files. By default, the output is written into /atca/ephem (the place expected by SCHED). You do not have to strip mail headers, etc, from the input file (ephformat skips these). For example
  xbones> ephformat
  Give name of Horizons ephemeris file: horizons_mail.txt
  Give name of output ephemeris file: hbopp

Tracking surface points on extended, rotating target bodies (Moon, Sun, planets, natural satellites).

When selecting such a target (those with an IAU rotational model), use this form (units are degrees and km):
              g: E.Long, latitude, h @ BODY  [geodetic/planetographic coords.]
              c: E.Long,     DXY, DZ @ BODY  [cylindrical coordinates]
For example,
              g: 348.8, -43.3, 0 @ 301
... specifies the crater Tycho on the Moon (body 301), at geodetic (planetographic) coordinates 348.8 degrees east longitude, -43.3 degrees latitude (south), and zero km altitude with respect to the IAU reference triaxial ellipsoid.

Observing Earth orbiting satellites

SCHED and CAOBS also support observations of satellites. This can be useful for holography or beam measurements using e.g., geostationairy satellites (which still move slightly) or other antenna investigations. To track a satellite specify a source name starting with '&' followed by the name of the TLE (two-line-element) file for the satellite(s) and an optional '-' and number. For instance, to track the 5th GPS satellite from file /atca/ephem/gps.tle specify &gps-5.

Download the TLE file for the satellite you want to track from the celestrak website and rename and copy it to the /atca/ephem/ area on xbones. The TLE files are frequently updated (daily) so make sure you have a recent one in place at the start of observing.

Some TLE files contain parameters for many satellites and it may be easier to select just those lines you are interested in using an editor. The lines come in groups of 3, with the satellite name on the first line and the orbital elements on the next two lines. If you omit the trailing number on the source name, the first satellite in the file is used.

Note this mode has not been tested much and the tracking accuracy is not known yet, it may not be better than a few tenths of a degree. Many near Earth satellites move so fast that the linear interpolation approach used for tracking may not be very accurate. Using short scans can improve the accuracy. You may also run into problems with slew speed limits for some satellites.

Original: Bob Sault (16-MAR-2001)
Modified: Mark Wieringa (07-MAY-2008)