Orbsim (gtorbsim) Tutorial

The Orbit Simulator, gtorbsim, is a spacecraft attitude calculator. It has a number of capabilities based on the code already implemented in the general purpose scheduling and planning system TAKO (Timeline Assembler Keyword Oriented) at the FERMI Science Support Center (FSSC).

However, it is anticipated that the typical end user will only need a subset of the overall functionality of this tool. Primarily, a Guest Investigator would use it to generate a spacecraft (or FT2) file to use in conjunction with gtobssim to simulate FERMI LAT observations.

This tutorial provides some examples of how to run the gtorbsim application, which generates a spacecraft data file that is an input for some of the Science Tools. For example, the you may need to run this tool before running gtobssim in order to generate a simulated observation.

The main purpose of this simulator is:

• To calculate spacecraft attitude, i.e., where the local body frame axes are oriented relative to the sky.

• To determine when events such as entry/exit in South Atlantic Anomaly (SAA) will take place.

The above must be accomplished starting with a series of pointing commands. The output of the orbit simulator is a FITS spacecraft data file.

Note: A spacecraft data file is available from the SLAC Data Portal, but in many cases you will probably need to generate that file if you want to perform a particular analysis of simulated data.

Steps

1. Observation Modes

gtorbsim requires that you first select an observation mode strategy (i.e., either survey or pointed mode):

• Survey Mode. The sky survey mode is basically zenith pointed throughout the orbit and has two sub modes:
  • With rocking.
  • Without rocking.

  Rocking provides more uniform sky coverage and allows for complete sky coverage within a shorter period of time. Different rocking profiles may be implemented, (square or sinusoid) with a basic 2-orbit period and a 60-degree maximum amplitude (above and below the orbit plane).

In this mode, you are provided with two options: fixed or profiled.
• In fixed survey mode, the spacecraft does a sky survey with a specified offset with respect to its local zenith for one orbit, and then uses the opposite offset for the next orbit, and so on.

• In profiled survey mode, the spacecraft observes in survey mode according to a specified profile consisting of 17 increasing times and 17 zenith offsets.

  The 17 increasing times (in seconds) are used to indicate the time that it takes during each cycle to go from a corresponding zenith offset to the next.

  The 17 angles (in degrees) are the zenith offsets reached at the end of the corresponding time interval. The first and last of these offsets must be identical in order for the profile to be repeated.

• Pointed Observation Mode. In pointed observation mode, the Z-axis of the observatory is commanded to point at a celestial target.

Note: For more detailed information on either survey mode or pointed mode, see gtorbsim Help; also see the gtorbsim parameter file.

2. Spacecraft Ephemeris

In order to properly calculate the attitude, the orbit simulator needs to know the spacecraft position for the entire interval of interest. Therefore, it must be capable of either reading in a file
that contains the spacecraft ephemeris, or calculating one "on-the-fly".

The orbit simulator can handle three different types of ephemeris files:

• NASA Flight Dynamic Facility (FDF) format, already used for missions such as RXTE.

• Satellite Tool Kit (STK) format, already in use for SWIFT.

• NORAD Two Line Elements, in which case the spacecraft position is calculated on the fly. Users may extract the FERMI TLE into a file from NORAD Two-Line Element Sets Current Data. For example:

Notes:

• For the tool to work, the first line of this file has to be "GLAST" and not "Fermi".

• Keep in mind that the Two Line Elements files are valid only for a few days.

Caution! If the spacecraft data file that you want to create is for a period of time where the ephemeris is not valid, your spacecraft data file may not be accurate.

3. Initial Spacecraft position

Initial spacecraft position in celestial coordinates is provided by the user as an input parameter. (See the gtorbsim parameter file.)

4. The South Atlantic Anomaly region

The instrument high voltage power supplies will be protected when the spacecraft traverses the South Atlantic Anomaly (SAA), which occurs about ~15% of the time.

gtorbsim has the capability to handle SAA constraints. The SAA region is appoximated by a polygon, which is specified by the Longitude and Latitude of its vertices. It is passed to the program as an input file where the specification of the polygon is given. In cases where the file is not available, a default hard-coded table of longitude and latitude pairs of vertices will be used.

Click on L_SAA_2008198.03 to download the SAA file used at the time of this writing.

5. Earth Limb

The Earth Limb Tracing maneuvering is an optional feature that can easily be enabled/disabled using the appropriate input parameter. This maneuvering consists of tracing the Earth Limb if a target is Earth-occulted.

Targets are assumed to be occulted if their Earth angle (Angle between target and the Earth's Limb) is smaller, or – at most – equal to 30 degrees. Once the target is occulted by the Earth, the orbit simulator finds when it is visible again, and where it is coming out from the Earth's Limb. The simulator then finds the angular separation between the in-occult and out-occult position. And finally, the orbit simulator allows the local z-axis to sweep equal angles in equal times during its motion along the Earth's Limb.

6. Run gtorbsim

This section shows different ways to generate survey mode spacecraft data file. To generate a realistic pointed mode observation, a timeline generated by TAKO is needed. This tool is not provided with the Science Tools. If you need this tool to run a realistic pointed mode observation, please contact the FSSC.

There are different ways to enter the parameters in the tool:

• By answering a prompt.
• As a list in a command line.
• By using an input file.

For this reason, the very first input of the simulator is the type of input, i.e., either "console" or "file".

Example - init file:

Note: For the list of inputs refer to the gtorbsim parameters file.

Example - inputs given in a file:

You can also run the tool as follows:

Download FT2 Files

• Click on FT2_FERMI_TLE_09033.78481577.fits to download the spacecraft data file generated with this example. Keep in mind that this spacecraft data file is accurate only for the first few days.

• To download another FT2 file generated with different ephemeris, click on: FT2_GLAST_EPH_2009012_2009068_00.fits.