Updating Orbital Elements

TeraScan’s ability to automatically capture data from satellites depends on a means of predicting where the satellites will be at any particular time. TeraScan uses an orbital prediction model for this purpose. The input to this model is a set of parameters called the orbital elements.

The Customer Support Group at SeaSpace collects new orbital elements daily from Naval Space Surveillance, Virginia, and distributes this data to their customers. There are four ways of obtaining the orbital elements from SeaSpace Customer Support:

About Orbital Elements

For polar-orbiting satellites, TeraScan uses the orbital elements to determine when each satellite will be in range of the specified site, where the satellite will first come into view, and the path it will follow across the sky.

For geostationary satellites, the orbital elements are used to determine the pointing direction of the geostationary antenna. Orbital elements should be updated at least every two weeks.

For each satellite that TeraScan works with, there are 3 orbital elements files located in the directory $SATDATA/satellite:

orbdata	contains the orbital elements for that satellite in binary form
orbinput	contains one-line orbital elements for that satellite in readable ASCII format
twoinput	contains two-line orbital elements for that satellite in readable ASCII format

Checking the Age of Orbital Elements

To check the age of the orbital elements on your system, run the following command:

A list of satellites will be displayed with the date each satellite was last updated. The age of the orbital elements will be checked only for those satellites whose status is set to active in the file $SATDATA/satellite/Define.

Updating Orbital Elements via FTP

Using the get_elements Command

If your system is connected to the internet, you can download the current orbital elements from SeaSpace onto your system and update them by using the script get_elements. The get_elements script will FTP to a workstation at SeaSpace called califia, log on, get the newest available orbital elements (1-line and 2-line), log out, and then run fixoes to append them to your orbital elements files. A list of the satellites and their corresponding orbital elements will be displayed as the data is downloaded.

If you use 2-line orbital elements, you may need to edit the get_elements script (in $TSCANROOT/bin) to add the function add2line after fixoes if necessary.

The following is an example of running get_elements which updates both 1-line and 2-line elements:

% get_elements

get_elements: Updating 1-line elements

Processing f-14
Adding 1 lines
24753.1723994.8296548.00025.0008467.6819112.8583232.2742360010608   8
/vol1/terascan/pass/satel/f-14/orbdata created

Processing f-15
Adding 1 lines
25991.3082452.8302474.00015.0009961.6135721.8959849.2744949010608   0
/vol1/terascan/pass/satel/f-15/orbdata created
   .
   .
   .
get_elements: Updating 2-line elements

Processing f-14
Adding 2/2 lines
1 24753U 97012  A 01159.15184589  .00000455  00000-0  00000+0 0  8214
2 24753  98.7250 205.5913 0008467 245.0553 115.0356 14.14028349215485

Processing f-15
Adding 2/2 lines
1 25991U 99067  A 01159.14696678  .00000278  00000-0  00000+0 0  7062
2 25991  98.8182 219.1465 0009961 220.4676 139.6373 14.15038309 76834

You can also update elements by manually downloading them via FTP and then running fixoes and add2line. This option may be necessary if your acquisition system is behind a security firewall.

Updating Orbital Elements Automatically

Your system has a daemon called cron that runs jobs at regularly scheduled intervals. The jobs to be run are entered in a file called crontab. Each job is specified on a separate line and consists of the following fields, separated by spaces: minute, hour, day of the month, year, day of the week, and command.

Using the crontab command, you can configure your system to automatically retrieve and update orbital elements on a regular schedule:

Updating Orbital Elements via HTTP

Using the get_web_elements Command

If your system is connected to the internet, you can download the current orbital elements from Celestrak onto your system and update them by using the script get_web_elements, which will use GNU Wget software to download the latest NORAD 2-line orbital elements via HTTP, convert 2-line elements to 1-line elements, and then run fixoes and add2line to append both 1- and 2-line elements, respectively, to your orbital elements files. Notes: Orbital elements for DMSP satellites are not available by this method. Also, GNU Wget software must be installed on your system.

To configure your system to automatically update elements with the get_web_elements script, see Updating Orbital Elements Automatically. In step 1c, substitute the path for the get_web_elements script.

1.	Create a crontab entry that will schedule the script to be run at a specified time each day.
	a.	Set your default editor to vi:
		% setenv EDITOR vi
	b.	Edit the crontab file. If no crontab file exists, a new one will be created:
		% crontab -e
	c.	Enter a line similar to the following in the crontab file:
		*30 6 * * /bin/csh /opt/terascan/bin/get_elements**
		This line will run the get_elements function at 6:30 am each day.
2.	Verify the crontab entry:
	% crontab -l