Tracking comets and satellites with the GBT.

For tracking moving objects such as comets, spacecraft, or satellites, the user must supply a table of positions tabulated at specified UT times. The table may be either in the "User Table" form (see first two examples), or in the "GO Table" form.

User Tables

The coordinates may be given in J2000, B1950, or apparent RA/Dec, as in the following example. For Earth satellites, Azimuth/Elevation may be more convenient. The second example shows a table using Az/El.

The antenna manager makes sure the antenna is at the specified position at the specified time. Interpolation between lines in the table is done by requiring smooth changes of coordinate velocities. It is recommended that the time interval in the table be the time it takes for the object to move by about one HPBW, or less.

The coordinates, in all cases, should be topocentric.

When tracking major planets, there is no need for a user table. GO will generate the ephemeris automatically if you select the planet name under "New Origin".

Example of an RA/DEC User Table

# The first 3 lines are header/comments.
#             UTC        RA___apparent___DEC  
2002 08 18 16:00:00  11:49:29.63  +31:46:18.1
2002 08 18 16:01:00  11:49:15.59  +31:44:23.8
2002 08 18 16:02:00  11:49:01.59  +31:42:29.6
2002 08 18 16:03:00  11:48:47.64  +31:40:35.6
2002 08 18 16:04:00  11:48:33.72  +31:38:41.8
2002 08 18 16:05:00  11:48:19.84  +31:36:48.1
2002 08 18 16:06:00  11:48:06.01  +31:34:54.6
One must select the Coordinate Mode in GO to match what is used in the table (ApparentRaDec in this case).

Example of an Az/El User Table

# PRN14 tracking table (angles in degrees)
# visible 01:30 to 3:00 UT
# UT date and time   Azimuth Elevation
2004 05 16 01:30:06  103.1822   43.0174
2004 05 16 01:30:14  103.2464   42.9721
2004 05 16 01:30:22  103.3105   42.9268
2004 05 16 01:30:30  103.3745   42.8814
2004 05 16 01:30:38  103.4384   42.8360
2004 05 16 01:30:46  103.5023   42.7906
Here we must select the coordinate mode in GO to be AzEl.

To activate a User Table:

Using a GO Table with rates.

An alternate method is to use a GO Table (activated by clicking the "GO Tables" button on the main GO screen, typing in the path to the table, clicking "Load", then clicking "Run"). There may be ten seconds of software overhead when activating each line of the table, meaning that the GBT may not be exactly on the right position for a few seconds as it begins each scan. Using a GO table like this means that each line of the table will start a new scan with a new scan number. (On the other hand, if you use a User Table as described above, the scan length is chosen in GO independently of the table, e.g. the entire time covered by the table can be one scan.)

When running a GO Table like this, the antenna will be at the specified position (ra,dec) at the specified start time, moving with the specified rates.

The GO Table format is quite flexible, and depends on the keywords specified in the header line. One example is as follows:

header \
 sc.starttime   ra         dec      ra_rate  dec_rate   procedure
16:00:00  11:49:29.63  +31:46:18.1   -2.969   -1.900     track
16:05:00  11:48:19.84  +31:36:48.1   -2.931   -1.887     track
16:10:00  11:47:11.06  +31:27:22.0   -2.894   -1.874     track
16:15:00  11:46:03.26  +31:17:59.9   -2.857   -1.860     track
16:20:00  11:44:56.44  +31:08:41.8   -2.821   -1.847     track
16:25:00  11:43:50.57  +30:59:27.6   -2.786   -1.834     track
 NOTE: there must be no blank spaces following the backslash (\)
	in the "header" line !!!  
The start times are in UT, and the ra_rate and dec_rate are in units of arcminutes per minute of time.
Before running one of these tables, set the "New Origin" in GO to "Off", and set the "secant(dec)" to "Yes", set the Primary coordinate mode to match the "ra" and "dec" columns in the table, and set the Offset coordinate mode to match the rates.

Another GO Table example includes tracking velocities:

header \
sc.source_name    ra       dec     ra_rate  dec_rate  lo1.srcvel sc.stoptime proc
encke1205 17:55:22.85 -00:39:12.50 -0.05688 -0.05614  18.052860  12:30:00    Track
encke1205 17:55:20.57 -00:39:46.20 -0.05692 -0.05612  18.060560  12:40:00    Track
encke1205 17:55:18.29 -00:40:19.90 -0.05697 -0.05611  18.068960  12:50:00    Track
encke1205 17:55:16.02 -00:40:53.50 -0.05701 -0.05610  18.078060  13:00:00    Track 
In this example, the antenna will be at the specified position with the specified rate at the specified stoptime. One may select whether the starttime or stoptime refers to UTC or LST by selection of the UTC/LST button in the GO main screen.

For observing wavelength (&lambda) in cm, the half-power beamwidth (HPBW) of the GBT in arcminutes is given by:
   HPBW = 0.42λ   

Modified by F.Ghigo, May 17, 2004