Variable Parameter (*.gdc) File Format

The Variable Parameter (*.vp) file is an ASCII formatted file with a header section. The file specified tabulated values of an identified parameter as a function of the indicated independent variable. Currently, the use of this file format is restricted to the specification of drag or solar pressure area as a function of time or argument of latitude. The file consists of a header section which contains information describing the content of the subsequent data section.

Header information inside the (*.vp) file is presented in terms of keyword-value pairs where the keywords and values are separated by an equals sign. Anywhere in the file, a # symbol in the first column of a line indicates a comment.

Header Information
Keyword Value Description
Version Version of file format (1.0)
ParameterName

Specifies the name of the parameter for which tabulated values will be provided. Valid choice is:

Area - area specified in m2

IndependentVariable

Specifies the independent variable according to which values of the indicated parameter will be tabulated. Valid choices are:

  • ArgumentOfLatitude – The argument of latitude is defined as the angle within the osculating orbit plane from the osculating ascending node to the current location of the spacecraft. The osculating ascending node is defined in the central body Inertial reference frame, which for the Earth is the ICRF. Use of the ArgumentOfLatitude as the independent variable is advantageous when possible since the file is valid at all times.
  • Time - Time specification is described through selection of the TimeScale and TimeFormat options.
TimeScale (Added in Version 2.0)

Specifies the time scale which will be used in the specification of times in the Data sections of the file. Valid choices are: UTC. UTC is the default value.

Note: Applies when IndependentVariable is specified as Time.

TimeFormat (Added in Version 2.0)

Specifies time formatting which will be used in the specification of times in the Data sections of the file. Valid choices are:

ISO - YMD - (ex: 2012-10-15T00:00:00.000)

ISO-YD - (Ex: 2012-232T00:00:00.000)

EpSec – Continuous seconds past the specified ReferenceEpoch (Ex: 86400.0). EpSec is the default.

NOTE: Applies when IndependentVariable is specified as Time.

ReferenceEpoch

Reference epoch used when TimeFormat is specified as EpSec. Must be specified in Gregorian format (Ex: 12 Jun 2020 12:00:00.00) and interpreted as being in the UTC time scale.

NOTE: Applies when IndependentVariable is specified as Time.

CycleRepeats

Valid choices are: Yes, No. If set to Yes, the pattern indicated in tabulated data will repeat for times outside of the specified time span. Since the pattern will repeat, it is recommended that the parameter values at the start and end times of the tabulated data match. If set to No, or not specified, the appropriate end value from the data table will be held as a constant for times outside of the specified span of values.

NOTE: Applies when IndependentVariable is specified as Time.

InterpolationOrder

The order of Lagrange interpolation to be used in determining parameter values for independent variable values between nodes. Default value is 1.

Example File 1

#This is an example VP file using Argument of Latitude as the independent variable.
Version = 1.0
ParameterName = Area
IndependentVariable = ArgumentOfLatitude
InterpolationOrder = 1

Begin Data
000   30
010   30
020   30
030   26
060   14
…
320   26
330   30
340   30
350   30
360   30
End Data

Example File 2

#This is an example VP file using Time as the independent variable
#With CycleRepeats set to Yes, the pattern will repeat for times outside of the data span
#Setting CycleRepeats to No would result in the value of 30 being used for times
#    outside of the data span
Version = 1.0
ParameterName = Area
IndependentVariable = Time
TimeScale = UTC
TimeFormat = EpSec
ReferenceEpoch = 12 Jun 2020 12:00:00.00
CycleRepeats = Yes
InterpolationOrder = 1

Begin Data
00000   30
00060   30
00120   25
00180   25
00240   22
…
05580   22
05640   25
05700   25
05760   30
05820   30
End Data

ODTK 6.5