Keywords | Collocation Node File Formats

Collocation Node File Format (*.nod)

A collocation node file can be used with Astrogator's Optimal Maneuver capability. It contains the initial guess of the nodes of the optimal trajectory. Each node contains the time, position, velocity, mass, and thrust direction.

Keywords

Each collocation node table, regardless of the type of data in the table, contains some common elements called keywords. Keywords and their associated values must precede the specification of the data and the actual data points.

Keyword	Required	Description
stk.v.<major release number>.<minor release number>	Yes	The version of STK software for which the file is formatted to be used. Files can be created in and imported to STK software versions consistent with the file version stamp or higher. The version stamp must be the first line in the data file. Example: stk.v.11.0
BEGIN CollocationNodes END CollocationNodes	Yes	Sets off the beginning and end of the collocation node file table, including all other keyword phrases and data point specification (except the version stamp). Refer to any of the sample files included in the Collocation Node File Format section for examples.
FormatVersion	Yes	The version associated with this file format. Currently, the only supported version is 1.0.0
ScenarioEpoch	Yes*	Required unless the TimeFormat (see below) specifies time values that completely specify the date and time. The reference epoch time for the time values of the collocation node data in the table. Specify the scenario epoch using Gregorian UTC time (`dd mmm yyyy hh:mm:ss.s`). There is no relationship between the scenario epoch specified in the collocation node table and the actual scenario epoch in your STK scenario. Example: ScenarioEpoch 1 Jan 2003 00:00:00.0 In this case, a time of `5.5` for a particular collocation node point would correspond to a time of `1 Jan 2003 00:00:05.5`.
CentralBody	No	The central body associated with the specified CoordinateSystem (see below). The keyword value that completes the phrase can be the name of any registered central body. Registered central bodies can be found in the `STKData\CentralBodies` directory. Example: CentralBody Earth Default is the central body for the vehicle, and Earth by default.
ThrustAxes	Yes	The coordinate axes in which the thrust direction in the data table is expressed. Normally, the thrust axes is the name of a valid coordinate axes defined in the Vector Geometry Tool (VGT) or a coordinate system for the central body specified above (see Central Body Coordinate Systems). Typically, each central body supports Fixed, J2000, ICRF, Inertial, TrueOfDate, and MeanOfDate though certain bodies support more systems. VGT-created axes use the keyword ThrustAxes AWB. Examples: ThrustAxes ICRF ThrustAxes AWB RIC
ThrustAxesEpoch	See Note	The epoch time for the thrust axes in Gregorian UTC time (`dd mmm yyyy hh:mm:ss.s`). Required with thrust axes that reference an Epoch: MeanOfEpoch TrueOfEpoch TEMEOfEpoch AlignmentAtEpoch Example: ThrustAxesEpoch 1 Jun 2003 12:00:00.0
CoordinateSystem	Yes	The coordinate system in which the position and velocity components in the data table are expressed. Normally, the coordinate system is the name of a valid coordinate system for the central body specified above (see Central Body Coordinate Systems). Typically, each central body supports Fixed, J2000, ICRF, Inertial, TrueOfDate, and MeanOfDate though certain bodies support more systems. You can also use the Vector Geometry Tool (VGT) by specifying the name of the system and the name of the STK object to which it corresponds. It is required that the VGT-created system use the keyword CoordinateSystem AWB. Examples: CoordinateSystem TrueOfEpoch CoordinateSystem AWB TopoCentric Facility/MyLaunchSite The name of the STK object is optional if the object that is importing the data is the object to which the coordinate system corresponds.
CoordinateSystemEpoch	See Note	The epoch time for the coordinate system in Gregorian UTC time (`dd mmm yyyy hh:mm:ss.s`). Required with coordinate systems that reference an Epoch: MeanOfEpoch TrueOfEpoch TEMEOfEpoch AlignmentAtEpoch Example: CoordinateSystemEpoch 1 Jun 2003 12:00:00.0
DistanceUnit	No	Sets the distance unit to be used for all distance measurements in the data table. By default, STK assumes that all distance measurements in a data table are in meters and velocities are in meters/second. This may be overridden by setting the distance unit to be any valid STK distance unit. To see a list of all available units of measure, open the Units page of the Scenario's Basic properties by double-clicking the scenario in the Object Browser and selecting the Units page. Default for velocity is meters per second. Default for the data table is meters. Example: DistanceUnit Kilometers
TimeFormat	No	Specifies a keyword defining the date format of time tags. If set, each line of data in the file begins with a time in the specific date format followed by the rest of the data. Spaces are valid whenever the format admits them (e.g., 1 Jan 2007 12:00:00.000). Quotes around time strings are invalid. The TimeFormat keyword is the time format abbreviation; e.g., EpSec for Epoch Seconds or JDate for Julian Date. For time format abbreviations, see DateTime Formats. Examples: TimeFormat DD/MM/YYYY TimeFormat GPSZ
NodalTimesType	Yes	Specifies the type of nodal times in the data file. Valid values are: LegendreGaussLobatto - nodal times are the nodes associated with Legendre-Gauss-Lobatto quadrature LegendreGaussRadau - nodal times are the nodes associated with Legendre-Gauss-Radau quadrature. Ephemeris - nodal times have no special properties
InterpolationSamplesM1	No	One less than the number of points used in the interpolation. For the Lagrange interpolation method, this is also the interpolation order. Default is 5. Example: InterpolationSamplesM1 5

Collocation Node Formats

While the previous sections outline the basic format for a data table, the following sections outline the formats used to specify actual data points in the collocation node data table. You must include the format keyword in the file on its own line, after the header keywords and before the actual data point array.

TimePosVelMassRaDec
TimePosVelMassDirCos

The following conventions must be followed when specifying data points in any format:

Each line contains only one data point.
The values on each line must be separated by at least one space.
The lines must be listed in ascending order in time but do not have to be evenly spaced in time.
You cannot have many points at the same time.

TimePosVelMassRaDec Format

Individual data points following the TimePosVelMassRaDec keyword look like this:

where,

`<Time>`	The time value of the node. Either expressed as seconds since the `ScenarioEpoch` reference epoch or formatted according to the TimeFormat specification.
`<X> <Y> <Z>`	The position of the node.
`<Vx> <Vy> <Vz>`	The velocity associated with the node.
`<Mass>`	The mass of the vehicle at the node (in kg).
`<RightAscension>`	Locates the projection of the thrust direction into the XY plane of the ThrustAxes from the X-axis (positive using a right-hand rule).
`<Declination>`	The angle between the thrust direction and the XY plane of the ThrustAxes (positive when the Z-component of the thrust direction is positive).

TimePosVelMassDirCos Format

Individual data points following the TimePosVelMassDirCos keyword look like this:

where,

`<Time>`	The time value of the node. Either expressed as seconds since the `ScenarioEpoch`reference epoch or formatted according to the TimeFormat specification.
`<X> <Y> <Z>`	The position of the node.
`<Vx> <Vy> <Vz>`	The velocity associated with the node.
`<Mass>`	The mass of the vehicle at the node (in kg).
`<Ux>`	The direction cosine of the thrust direction with the X-axis of the ThrustAxes.
`<Uy>`	The direction cosine of the thrust direction with the Y-axis of the ThrustAxes.
`<Uz>`	The direction cosine of the thrust direction with the Z-axis of the ThrustAxes.