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arc.inp

arc input controlling the orbit integration

Overview

arc.inp lists the satellites whose orbits are to be computed, specifies the models to be used in that computation, the time span of the output ephemeris, and the names of the output files. arc reads this information from stdin but it is recommended that the user create this file and redirect input to arc from this file using a command like

: arc < arc.inp

thereby assuring that the order and format of this input is correct. arc.inp is an ASCII file and easily modified with an editor.

The arc.inp file contains two sections:

a list of satellites, and
the program control.

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.
File names can be upper or lower case; however, file names are limited to 16 characters in length.
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 Satellite List

arc.inp begins with a list of satellites for which orbits will be computed and written to the ephemeris. These satellites are listed one per line with the list terminated by a line containing the word "END" starting in the first column. Upper case is required. Both PRN and Navstar numbers are recognized and the form used to enter these ID's will be used to write them to the output ephemeris. The formats for these ID's are:

('PRN', 1x, i2) for PRN numbers and
('NS', i2) for the Navstar numbers.

Example


NS32
NS13
NS33
PRN 29
PRN 30
PRN 31
END

Program Control

The first line in this section controls the orbit integration. The format used is:
(a5, 1x, a5, 1x, f6.1, 1x, f8.4, 2x, a4, 2x, a20, a5, 1x, a6)
The parameters entered on this line are, in the order they must appear:
1. the system of physical constants. This includes the speed of light, astronomical unit, constants of gravitation for the Earth, Moon and Sun, the Earth's and Sun's semi-major axes, and the Earth's gravity field model.
  Options include:
  - WGS72
  - WGS84
  - MERIT
  - IGS92
  Note: there is no default option.
2. the radiation pressure model and the decomposition of the resulting forces into components. In the following description, D is along the Sun-satellite line. X, Y and Z components are along the indicated satellite body-centered axes. B is orthogonal to the D and Y axes.
  Options include:
  - SPHRC
    - DEFAULT
    - The satellites are represented by simple spheres.
    - The force is decomposed into D, Y and Z components, each with a scaling factor.
  - ROCK4
    - A satellite is represented by the ROCK4 T10 and T20 models (Fliegel, H., T. Gallini and E. Swift, Global Positioning System radiation force model for geodetic applications, J. Geophys. Res. 97(B1), 559-568, 1992) ; however, the direct, i.e. along the Sun-satellite line, force has been separated from the other, residual forces described by the ROCK4 models. The motivation for this further decomposition is the ambiguity in orientation of the satellite body-centered frame when the Sun, Earth and satellite align. During these periods of alignment the ambiguity in the body-centered frame implies an ambiguity in the direction of the forces described by the ROCK4 model. To compensate for this, the principal force, the force directed along the Sun-satellite line, has been split off from the standard ROCK4 models and is applied separately in a clearly defined manner.
    - The force is decomposed into D, and secondary Y and Z components, each with a scaling factor. The "residual" ROCK4 X and Z components are then added.
  - RCKSV
    - A satellite is represented by the ROCK4 T10 and T20 models.
    - The force is decomposed into X, Y and Z components, each with a scaling factor.
  - BERNE
    - A satellite is represented by the ROCK4 T10 and T20 models; however, the direct force has been removed from the standard ROCK4 models and is applied separately (see ROCK4 description).
    - The direct force is decomposed into D, and secondary Y and B components with additional once per revolution (sine and cosine) terms for each component. Each of these nine (9) terms has a scaling factor. The residual ROCK4 X and Z forces are then added.
  - SRDYX
    - A satellite is represented by the ROCK4 T10 and T20 models; however, the direct force has been removed from the standard ROCK4 models and is applied separately (see ROCK4 model description).
    - The direct force is decomposed into D, and secondary Y and B components All have scaling factors. The residual ROCK4 X and Z forces are then added.
  - NOAA
    - The force is decomposed into D, Y and B components with scaling factors.
3. The third parameter is the step size, in seconds, for the output ephemeris. Step sizes of 1350.0 and 900.0 seconds are specifically tested for. Other values will generate a warning message but the program will continue.
  - 1350.0 is the DEFAULT.
  Note: The output step size is required to be evenly divisible by the integration step size. Any other combination will result in a fatal error.
4. The fourth parameter is the integration step size, in seconds, used internally for the computation. Step sizes of 75.0 and 168.75 are specifically tested for. Other values will generate a warning message but the program will continue.
  - 168.75 is the DEFAULT.
  Note: the integration step size is required to be evenly divisible into the output step size. Any other combination will result in a fatal error.
5. The fifth parameter specifies the time frame to be used both for entering times from this file and for the ephemeris.
  Options include:
  - UTC
    - DEFAULT
  - GPST
6. The sixth parameter selects the coordinate frame for the ephemeris. This parameter specifies a consistent set of precession, nutation and sidereal time models. The output ephemeris will be in this frame.
  Options include:
  - INERTIAL B1950
    - DEFAULT
  - INERTIAL J2000
7. The seventh parameter selects 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 INTERIAL B1950 frame is specified.
  - IAU76
    - DEFAULT if the INTERIAL J2000 frame is specified.
  Note: Mixing the IAU76 precession model and the INTERIAL B1950 frame causes a fatal error.
8. The eighth flag controls the application of solid-earth tides.
  - NOTIDE
  - TIDE
    - DEFAULT

The next line names the file where information generated during the processing is logged. The first 16 characters on this line are used for the name. Historically, this is called the "o_file" or "a_file".

The initial conditions (ICs) file is given next. This file contains the IC state vector for each satellite. In other words, the a priori coordinates, velocity and radiation pressure scale factors at the specified epoch for each satellite. The first 16 characters on the line are used for the name and this string will be converted to lower case when the file is opened. Historically, this is called the "g_file".
Note: This file can hold additional satellites to those listed in the arc.inp file but it must have every satellite in the arc.inp file.

The next line should be blank (a defunct input file name was entered here).

The ephemeris start time appears next. The time given here must be in the time system previously entered and described above. Two formats are recognized:

If the first two characters on this line are blanks or "19", the lines is read as the year, day-of-year, hour, minute and second using the format:
(2x, i2, 1x, i3, 1x, 2(i2, 1x), f8.5).
Otherwise the line is read as as the year, month day, hour, minute and second using the format:
(5(i2, 1x), f8.5).

The ephemeris stop time follows. The the same requirements and formats used by the start time are in applied.

The next line contains a single character controlling whether the first order differential equations are integrated and output to the ephemeris. That is to say, whether the partial derivatives for the components of the IC vector are written as part of the binary ephemeris.

Y
- DEFAULT
N

Finally, the file name of the output binary ephemeris is given. As before, the file name is the first 16 characters of the line. Historically, this has been called the "t_file".

Example


IGS92 RCKSV 1350.0   168.75  UTC   INERTIAL     1950.0
o.b1950
g.b1950
 
1997 169 20  0   .00000
1997 171  4  0   .00000
Y
u.b1950

Note: the precession model is not specifically given but is implied to be the IAU68 model by specifying the INERTIAL 1950.0 frame.

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inp.html
January 14, 2000
Steve Hilla