THE CONSTANT DEFINITION BLOCK

A VIDF Constant Definition consists of 5 coupled fields which together comprise the definition of a fixed reference value associated with the UDF sensors. In general they are used to define pointing directions, acceptance windows, an the like. Each defined UDF sensor may be given a unique value of the constant within the constant definition block. The constant definition blocks are always placed at the end of the VIDF after the table definitions.

Some constant definitions require predefined reference frames. The UDF uses a generic reference frame in which +Z axis points along the angular momentum vector and X and Y form a set of orthogonal axes. In this reference frame elevation or polar angles are measured from the +Z axis. Azimuth angles run from 0 to 360 degrees with 0 degrees occurring when the +X axis passes through a predefined line of sight (sun, earth, etc).

CONSTANT ID

A scalar field which provides an identification tag to the defined constant. This field allows both the UDF kernel and other UDF based software to easily search the VIDF for required or optional values.

The UDF kernel software makes use of several constants definitions if found but does not require them to be present with the exception of the three constants which define the normal vectors to the UDF sensors. These must exist if the computation of pitch angles has been enabled. If they are not found the pitch angle computation is disabled within UDF kernel and pitch angles are not produced. For a discussion on the VIDF Constant ID's used by the UDF kernel and what they are used for see the section on: UDF KERNEL CONSTANT USAGE

The list of defined constant id's is shown shown in the following table.

The field begins with the line format id b followed by an integer specifying the constant id. An optional comment field may follow.

b  2                                    /* ID is Azimuthal Offset  */

NUMBER OF COMMENT LINES

A scalar field specifying the number of lines of comments in the constant comment field.

The field begins with the line format id s followed by an integer specifying the number of comment lines. An optional comment field may follow.

s  6                                    /* Num Comments        */

COMMENTS

An array field of length Number Of Comment Lines, this field is a set of free-form text lines which make up the Constant Comments. Each element in the array is a line of text not to exceed 79 characters in length. If the number of comment lines has been declared to be 0 then this field is undefined and is entered in the VIDF as a null line.

The constant comment field begins with the array designation line: m followed by the input N 1. N is the total number of entries and is set to the value of the Number Of Comment Lines field and 1 indicates that each element in the array will occupy its own line. Each of the following N lines begins with the line format t followed by a line of text not to exceed 79 characters. An optional comment field may be added to the end of each line.

m 6  1                                                      /* Comments */
t                          CONSTANT 00                           /* C00 */
t                                                                /* C01 */
t This is the instrument azimuthal offset. The instrument is     /* C02 */
t mounted 43.25 degrees clockwise from the satellite +X axis.    /* C03 */
t Zero degrees in the satellite frame is defined to occur when   /* C04 */
t when the +X axis points at the Earth.                          /* C05 */
s  6                                    /* Num Comments        */

CONSTANT VALUE SCALES

An array field of length equal to the Constant Values field to convert them from integer to real values. There is a 1-1 correspondence between the values in this field and the values in the constant value field. The scaling values are power of 10 scaling values. They are applied as

The constant value scale field begins with the array designation line: m followed by the input N M. N is the total number of entries and is set to the value of the number of sensors field and M is the number of entries given on each line. Each of the following N lines begins with the line format s followed by M entries (except the last line which may have less). An optional comment field may be added to the end of each line.

m  5  5                                        /* Constant Scale Factors */
b  -2   -2   -2   -2   -2                                       /* 00-04 */

CONSTANT VALUES

This is an array field with a length equal to the length of the above Constant Value Scale field or to the number of sensors field and M is the the number of entries given on each line. Each of the following N lines begins with the line format s followed by M entries (except the last line which may have less). An optional comment field may be added to the end of each line.

m  5  5                                           /* Constant Values     */
l  4325  4325  4325  4325  4325                                 /* 00-04 */

UDF KERNEL CONSTANT USAGE

The UDF kernel software makes active use of several of the VIDF constant fields if they are present. The constants and how they are used are described below.

AZIMUTH OFFSET INFO (ID 2)

The UDF kernel software tracks the azimuthal position of the UDF generic +X axis and returns its position at each data acquisition. When array or matrix data are being read the azimuthal position is for every array element or matrix row respectively. The computation used is:

If there is no VIDF constant with ID 2 present in the VIDF then A0 is set to 0.0 in the computation and the returned azimuths are relative to the UDF defined +X axis. The offset allows the returned azimuth to be the relative azimuthal position of the measurement in the spin and not that of the +X axis.

ELEVATION INFO (IDs 1, 4, 5 ,9)

The UDF kernel returns the elevation angle range (minimum and maximum) at each data read. When array or matrix data are being accessed the elevation angle range is is returned for each array element or matrix row respectively. The input used to determine these angles is obtained from one or more of the VIDF elevation constants of which there are four. If there is no elevation information present in the VIDF constant block both the minimum and maximum elevation angles are defaulted to -500.0 degrees.

There are two possible senarios.

Single Elevation Constant Defined

When only a single elevation constant is defined the VIDF, the value given for the sensor associated with the returned measurement is used as both the start and stop elevation angle.

Multiple Elevation Constants Defined

When two or more of the elevation constants are listed in the VIDF, the UDF kernel takes the first two encountered and constructs the start and stop elevation angles from these. The formula used depends on the Elevation Constants read. The formula are given below.