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Three fields are used to describe the target's position, referred to here as the Target Pointing Specification (TPS). The TPS has been defined using a hierarchical structure.
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No more than three levels are allowed. If you believe that your target cannot be described in this form, contact your Program Coordinator. |
If you are using the Text Proposal File, TPS items must be separated by commas.
The targets specified in the target position fields can be described in up to four ways:
Table 4.1 gives the list of valid names for solar system targets. PIs are responsible for obtaining up-to-date orbital elements for bodies not in this table. Objects must be denoted by their IAU-adopted name. A good reference for object names can be found in the Astronomical Almanac, and in the Marsden comet catalog (Marsden, B. G., Catalog of Cometary Orbits, Enslow Publishers, Hillside, NJ, 1983). If you are uncertain whether or not your target can be referenced by name, contact your Program Coordinator.
In those cases where the target's position is given with respect to one of the standard objects, the latest available data from JPL on the bodies' physical dimensions, orientation, and rotation rates are used in calculating the target's position. In those cases where all or part of the TPS for your target can be described using standard names, we strongly recommend that you do so. Generally, this will result in the most accurate ephemeris generation for your target.
Wherever there is an entry involving time, the format for that entry must be:
where DD is the day of the month, MMM is the first three letters of the month name, YYYY are the full four digits of the Gregorian calendar year, hh is the hour, mm is the minute, and ss.s are the decimal seconds. Only the necessary precision need be specified (NOTE: But the time after the colon it must be completely specified or not at all)
Two different systems of time are used in this document. TDB refers to Barycentric Dynamical Time and can be considered synonymous with ET (Ephemeris Time), which was used before 1984. UTC refers to Coordinated Universal Time. The precise interpretation of each time value depends on the context in which it is used.
Specify your target in this field in one of the following ways:
For all cases, the required input data are described below. If the data are valid only over a specific period of time, then specify this time interval in the Window field according to the rules given later.
The elements given above refer to the mean ecliptic and equinox of either B1950 or J2000 depending on which "value" is specified for EQUINOX. An example of TYPE = COMET is shown in the "Example Target List Blocks" below for Example 3.
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It is the responsibility of the observer to supply accurate orbital elements to STScI when specifying TYPE=COMET or TYPE=ASTEROID. |
Five Target Reference Systems (TRSs) are described in the following paragraphs. Please pay careful attention to the definitions of each TRS. Specify your target in one of the following ways:
In this case <object name> is from Table 4.1: Solar System Standard Targets, or the Type is:
PGRAPHIC
planetographic coordinates relative to Level 1 target
polar coordinate offsets from Level 1 target
position in magnetic coordinate system
line-of-sight projected coordinate system
orbital elements of a satellite
planetocentric coordinates relative to Level 1 target
For the PGRAPHIC, MAGNETO, and TORUS coordinate systems, the north pole is defined to be the rotational pole in the northern celestial hemisphere. For planets with direct rotation, the angular momentum vector coincides with the north pole. For planets with retrograde rotation, the angular momentum vector coincides with the south pole.
Table 4.5: Parameters for TYPE = PGRAPHIC
LONG = <value>
planetographic longitude in degrees,
LAT = <value>
planetographic latitude in degrees; use - to denote south latitude.
[ALT = <value>]
planetographic altitude above the reference ellipsoid, in kilometers,
[R_LONG = <value>]1
rate of change of LONG, in degrees/day,
[R_LAT = <value>] 1
rate of change of LAT, in degrees/day,
[R_RAD = <value>] 1
rate of change of RAD, in kilometers/day, and
[EPOCH = <value>]
the reference time for the temporal variation, in UTC (4 digits).
1EPOCH must also be specified with this quantity.
The PGRAPHIC TRS is the IAU planetographic coordinate system. It is a non-spherical coordinate system aligned with and rotating about the rotation axis of the Level 1 body, positive north, whose origin lies at the center of the reference body. Locations within this TRS are specified by longitude, latitude, and altitude above the surface. (The lambda(III) coordinate system defines the prime meridian in this coordinate system; if lambda(I) or lambda(II) coordinate systems are desired, note this in the Comments field.)
Planetographic Latitude is defined as the angle between the equator and the normal to the surface of the reference ellipsoid at the point of interest.
By definition, the planetographic longitude of the sub-Earth point increases with time. For planets with direct rotation, the planetographic longitude increases in a left-handed direction. For planets with retrograde rotation, the planetographic longitude increases in a right-handed direction.
If ALT is omitted, then the surface of the reference ellipsoid is assumed.
If the coordinates are constant in time, then none of the other optional entries should be used. If any coordinate is given as a function of time, then EPOCH is required and the time-varying coordinate is interpreted in the following way.
For this example the longitude at any time, T, is given by:
longitude = LONG + R_LONG * (T - EPOCH)
longitude = 20 + 45 * (t - 5-JAN-1990:00:15:00)
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The same interpretation for time-varying coordinates also applies to the other TRSs described below. |
Table 4.6: Parameters for TYPE = POS_ANGLE
RAD = <value>
Radius, in arcseconds
ANG = <value>
Position angle relative to the reference axis, in degrees
REF = NORTH REF = SUN
Reference axis is celestial north, or Reference axis is the apparent direction to the Sun as projected on the sky.
[R_RAD = <value>]1
Rate of change of RAD, in arcseconds/sec
[R_ANG = <value>] 1
Rate of change of ANG, in degrees/day
[EPOCH = <value>]
the reference time for the temporal variation, in UTC (4 digits).
1EPOCH must also be specified with this quantity.
The POS_ANGLE TRS is a position-angle coordinate system (i.e. a two-dimensional polar-coordinate system). This TRS is useful for pointing at targets whose positions are known only in terms of an offset in projected celestial coordinates from another body. The origin of the system lies at the center of the Level 1 body. Locations are specified by giving the apparent distance from the origin (in projected celestial coordinates as viewed from the Earth) and the position angle from some reference axis to the target point. For REF = NORTH, angles are measured from celestial north (positive angles are measured in the same sense as rotating from celestial north through east). For REF = SUN, angles are measured from the direction to the Sun as projected on the sky (positive angles are measured in the same sense as rotating from celestial north through east).
The MAGNETO TRS is intended to support observations fixed with respect to a planetary magnetic field. It is a spherical coordinate system rotating with the Level 1 body around the rotation axis, with a specified offset of the coordinate origin and inclination of the coordinate pole. The MAGNETO coordinate system is defined in the following manner:
While the origin and coordinate axes may differ from those of the cartographic system, the rotation axis and rotation rate are identical to those of the cartographic system. Locations in the MAGNETO TRS are specified by longitude, latitude, and radius from the origin of the defined coordinate system.
If the optional fields above are left blank, the following default values are used:
The TORUS TRS is defined primarily to support observations of Jupiter's plasma torus and is closely related to the MAGNETO TRS. TORUS is also useful for observers who want to observe in a coordinate system that is fixed relative to the apparent disk of the Level 1 body, e.g. central meridian observations (see special instructions below). The difference between the two systems is in the definition of the prime meridian. For the TORUS TRS, the prime meridian is defined by the instantaneous longitude of the sub-Earth point. Therefore, the TORUS TRS does not rotate with the Level 1 body. A typical observation would be of the east or west ansa (point of maximum elongation) of an equatorial circle whose radius is roughly five times the equatorial radius of Jupiter (in this case, LONG = 270 (90 for the west ansa), LAT = 0, RAD = 3.57E05). As the planet rotates, the target moves up and down in celestial coordinates as Jupiter rotates. This coordinate system can also be used to support observations of a planetary ring ansa.
When the target is a satellite of the object defined in the Level 1 field, but the satellite itself is not among the standard objects, then orbital elements must be specified. These elements refer to the motion of the satellite around the Level 1 object.
The "reference" axis for the angles defined above is the intersection of the Earth's equator at the standard epoch implied by the EQUINOX with the parent planet's equator at the EPOCH of the elements. The positive X-axis for the coordinate system used in the orbit calculation is obtained by taking the cross product of the Z-axis of the standard system (i.e. the system defined by the standard equator and equinox given by EQUINOX) with the pole of the planet. If E, I, O, W, O_RATE, and W_RATE are not supplied, then their values are assumed to be 0. If RAP and DECP are not supplied, then the standard IAU values are used. If RAP and DECP are supplied, then they should be referred to the standard equator and equinox given by EQUINOX. If EQUINOX is not provided, we will assume J2000.
STScI maintains its ephemeris data base with the best available elements, and you should use the STD = form for objects in Table 4.1: Solar System Standard Targets unless there is compelling scientific justification for specifying orbital elements. Note: It is the responsibility of the observer to supply accurate orbital elements to STScI when specifying TYPE=SAT.
PCENTRIC: planetocentric coordinates relative to Level 1 target
For the PCENTRIC coordinate system, the north pole is defined to be the rotational pole in the northern celestial hemisphere. For planets with direct rotation, the angular momentum vector coincides with the north pole. For planets with retrograde rotation, the angular momentum vector coincides with the south pole.
| LONG = <value> |
planetocentric longitude in degrees, |
| LAT = <value> |
planetocentric latitude in degrees; use - to denote south latitude. |
| [RAD = <value>] |
planetocentric radius in kilometers, |
| [R_LONG = <value>]1 |
rate of change of LONG, in degrees/day, |
| [R_LAT = <value>] 1 |
rate of change of LAT, in degrees/day, |
| [R_RAD = <value>] 1 |
rate of change of RAD, in kilometers/day, and |
| [EPOCH = <value>] |
the reference time for the temporal variation, in UTC (4 digits). |
| 1EPOCH must also be specified with this quantity. |
The PCENTRIC TRS is the IAU planetocentric coordinate system. It is a right-handed spherical coordinate system aligned with and rotating about the rotation axis of the Level 1 body, positive north, whose origin lies at the center of the Level 1 body. Locations within this TRS are specified by longitude, latitude, and radius from the origin. (The lambda(III) coordinate system defines the prime meridian in this coordinate system; if lambda(I) or lambda(II) coordinate systems are desired, note this in the Comments field.)
Planetocentric longitude increases in a right-handed direction for all planets. For planets with direct rotation, the planetocentric longitude of the sub-Earth point does not increase with time.
If RAD is omitted, then RAD is assumed to be the equatorial radius of the Level 1 body. Note that in general, if RAD is omitted, the point specified will not necessarily be on the visible surface of the planet. This is of special concern for oblate planets, e.g. Jupiter and Saturn, where a point at high latitude at the equatorial radius can appear above the limb of the planet in projection. When using this coordinate system for surface features on Jovian planets, it is best to specify the radius explicitly.
For spherical planets, planetographic and planetocentric latitudes are identical. For significantly nonspherical objects, there is no simple conversion between the two latitude systems.
For planets with retrograde rotation, the planetocentric and planetographic longitudes of a point are identical. For planets with direct rotation, the planetocentric and planetographic longitudes of a point have opposite sign.
The instructions for this field are identical to those for the Level 2 field except that "Level 3" should be substituted wherever "Level 2" occurs, and "Level 2" should be substituted wherever "Level 1" occurs.
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Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |
