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The Initial Conditions File

arc input specifying satellite initial conditions at a specific epoch

Overview

The initial conditions file, or g_file as it has been known historically, specifies the satellites' coordinates, velocities and radiation pressure scaling factors at a specific epoch. This set of parameters uniquely defines a satellite's orbit (given the models used in arc) and are equivalent to the keplarian orbital elements. Of course, perturbational forces, e.g. radiation pressure, gravitation attraction of the Moon, and others, modify the orbit so the exact epoch of the initial conditions and models selected to represent those forces are important and emphasized in this discussion. Given that this set of parameters completely defines a satellite's state (i.e. position at any epoch, velocity, energy, etc.) these parameters are often referred as the satellite's state vector. arc, expects these state vectors to be in the inertial frame in which it performs its calculations.

The g_file name is read from arc.inp, can be upper or lower case, but is limited to sixteen characters. This is an ASCII file and easily modified with an editor. The g_file file contains:

a descriptive block and
the satellite state vectors.

Although it was made more flexible, arc remains fairly rigid in reading this file. Some general rules and recommendations for entering this information are given here:

Some parameter settings are specified by character strings. All character strings of this type should be upper case. Although many parameter settings are recognized when entered as either upper or lower case, some are recognized only in upper case and few are recognized when entered as a mix of upper and lower case letters.
All formats should be considered fixed. Required formats and possible variations to those formats will be described.
Defaults settings are selected by entering a blank string. The defaults will be given when available.

The Descriptive Block

The g_file begins with information about the state vectors which follow. This block is terminated by a line beginning with the word "END". The first two lines are required and read in a fixed format. Subsequent lines are purely comment lines and ignored by arc.

The first line gives the epoch and frame of the state vectors to follow. The format used is: (i2, 1x, i3, 1x, i2, 1x, i2, f3.0, 20x, a4, 16x, a5, 1x, a5)
The parameters entered on this line, and the order they must appear are:
1. the two digit year.
2. the day-of-year
3. the hour.
4. the minute.
5. the seconds.
6. the sixth parameter on this line is the time frame of the epoch.
  Options include:
  - UTC
    Note: this parameter is four characters in length. The leading character for the UTC option is a blank.
    DEFAULT
  - GPST
7. the seventh parameter gives the inertial frame of the state vector. Two choices are available:
  - B1950
    Note: 1950.0 is also recognized.
    DEFAULT
  - J2000
8. The last parameter gives the precession model. There is an implication about the precession model in the coordinate frame selection and the default option here is dependent upon the selection of frame.
  Options include:
  - IAU68
    DEFAULT if the B1950 inertial frame is specified.
  - IAU76
    DEFAULT if the J2000 frame is specified.

The next line names the components of the state vectors to follow. There is a one-to-one correspondence with the radiation pressure model specified in the arc.inp file. A mismatch of this list and the radiation pressure model will result in a fatal error. The format for reading this line is:
(i2, 16(1x, a4))

Options for the SPHRC radiation pressure model are:

9 X Y Z XDOT YDOT ZDOT RAD1 RAD2 RAD3

9 X Y Z XDOT YDOT ZDOT DRAD YRAD ZRAD

Options for the ROCK4 radiation pressure model are:

9 X Y Z XDOT YDOT ZDOT RAD1 RAD2 RAD3

9 X Y Z XDOT YDOT ZDOT DRAD YRAD ZRAD

8 X Y Z XDOT YDOT ZDOT DRAD YRAD

Options for the RCKSV radiation pressure model are:

9 X Y Z XDOT YDOT ZDOT XRAD YRAD ZRAD

8 X Y Z XDOT YDOT ZDOT XRAD YRAD

The BERNE radiation pressure model accepts only:

15 X Y Z XDOT YDOT ZDOT DRAD YRAD BRAD DCOS DSIN YCOS YSIN BCOS BSIN

Options for the SRDYX radiation pressure model are:

9 X Y Z XDOT YDOT ZDOT DRAD YRAD BRAD

8 X Y Z XDOT YDOT ZDOT DRAD YRAD

Options for the NOAA radiation pressure model are:

9 X Y Z XDOT YDOT ZDOT DRAD YRAD BRAD

8 X Y Z XDOT YDOT ZDOT DRAD YRAD

All subsequent lines until the first END line are purely comments and ignored by the program. Some suggestions information to include in these comments are the creation time of the file, the name of the program which generated the file, and the sources of the initial conditions.

This block is terminated by a line starting with the word END.

The Satellite State Vectors

Each state vector begins with a line containing the satellite's ID, then the state vector components are given, one per line, in the order specified in the descriptive block. Although the order of the components of the vector is fixed, the order of the satellites included in the file is arbitrary.

The satellite ID must begin in column one and can be given as PRN or Navstar numbers. The corresponding formats are:
- ('PRN', 1x, i2) for PRN numbers or
- ('NS', i2) for the Navstar numbers.
The state vector components, as mentioned, are entered one-per-line. The format is:
- (d20.14)
Coordinates are in units of kilometers, velocities in kilometers/second, and radiation pressure parameters are dimensionless scaling factors.
This section (and therefore the file) must also end with a line starting with the word END.

Example


97 170 11 59 49                     UTC                B1950 IAU68
 9 X    Y    Z    XDOT YDOT ZDOT XRAD YRAD ZRAD
arcic(9703.01) generated on 08/29/97 09:13
01, 02, 03, from igs09104.sp3
END
NS32
 -.1797884635593D+05
  .1663374651058D+05
 -.1014975450382D+05
 -.2505967828868D+01
 -.9812214166500D+00
  .2796297997354D+01
  .9911200000000D+00
 -.0146700000000D-01
  .0000000000000D-00
NS13
  .1087543700687D+05
 -.1999703036324D+05
  .1453127973561D+05
  .1652423429635D+01
  .2557776584308D+01
  .2289015821508D+01
  .9994600000000D+00
 -.0032000000000D-02
  .0000000000000D-00
NS33
 -.5044403972700D+03
  .1677168319635D+05
  .2050850331265D+05
 -.3598719856466D+01
  .1079893266762D+01
 -.9799570951150D+00
  .1008390000000D+00
 -.0053100000000D-02
  .0000000000000D-00
END

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g_file.html
January 14, 2000
Steve Hilla