# Solar System Targets Position Levels 2 and 3

The Solar System Targets form is used to specify moving targets in some JWST observations. This article provides a description of the Level 2 and 3 specifications needed for the Astronomer’s Proposal Tool (APT).

# Level 2 Position

For the Level 2 position, you specify either a standard target name (from the list of Solar System Standard Targets) or one of six Target Reference Systems (TRSs), which are:

*PLANETOGRAPHIC* - coordinates relative to Level 1 target

* PLANETOCENTRIC* - coordinates relative to Level 1 target

* POSITION ANGLE* - coordinate offsets from Level 1 target

* MAGNETO* - position in magnetic coordinate system

* TORUS* - line-of-sight projected coordinate system

* SATELLITE *- orbital elements of a satellite

For the * PLANETOGRAPHIC*,

*,*

**PLANETOCENTRIC***, and*

**MAGNETO***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.*

**TORUS****Planetographic Coordinate System **

The following parameters define the * PLANETOGRAPHIC *coordinate frame:

in degrees**LONGITUDE**in degrees (use – to denote south latitude**LATITUDE****)**

The following optional values are available:

above the reference ellipsoid, in kilometers**ALTITUDE**, in degrees/day**LONGITUDE RATE OF CHANGE**, in degrees/day*LATITUDE RATE OF CHANGE*, in kilometers/day**ALTITUDE RATE OF CHANGE**(the reference time for the temporal variation, in**EPOCH**,*TDB*, or*TDT*)*UTC*

Note that if you specify a * LONGITUDE*,

*, or*

**LATITUDE***, you must specify an*

**ALTITUDE RATE OF CHANGE***.*

**EPOCH**The * PLANETOGRAPHIC* 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, and are tracked as the object rotates. (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

*field.)*

**COMMENTS**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 (to the West). For planets with retrograde rotation, the planetographic longitude increases in a **right-handed** direction (to the East). Longitudes should be specified in degrees West for planets with direct rotation and degrees East for planets with retrograde rotation.

If * ALTITUDE* 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. The time-varying coordinate is interpreted as shown in the following example. For

**LONGITUDE = 20**** **

**LATITUDE = -5**

**LONGITUDE RATE OF CHANGE = 45**

and** **

**EPOCH = 5-JAN-2012:15**

the longitude at any time, T, is given by:

longitude = *LONGITUDE *+* LONGITUDE RATE OF CHANGE* * (T – *EPOCH*)

or, numerically,

longitude = *20* + *45* * (T – *5–JAN–2012:00:15:00*)

The same interpretation for time-varying coordinates also applies to the other TRSs described below. |
---|

**Planetocentric Coordinates **

The following parameters define the ** PLANETOCENTRIC **coordinate frame:

in degrees**LONGITUDE**in degrees (use – to denote south latitude)**LATITUDE**

The following optional values are available:

in kilometers**RADIUS**, in degrees/day**LONGITUDE RATE OF CHANGE**, in degrees/day*LATITUDE RATE OF CHANGE*, in kilometers/day**RADIUS RATE OF CHANGE**(the reference time for the temporal variation, in**EPOCH**,*TDB*, or*TDT***UTC**

Note that if you specify a rate of change for the * LONGITUDE*,

*, or*

**LATITUDE***, you must specify an*

**RADIUS RATE OF CHANGE***.*

**EPOCH**The * PLANETOCENTRIC *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, and are tracked as the object rotates. (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

*field.)*

**COMMENTS**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 * RADIUS* is omitted, then

*is assumed to be the equatorial radius of the Level 1 body. Note that in general, if*

**RADIUS***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.*

**RADIUS**For spherical planets, planetographic and planetocentric latitudes are identical. For significantly non-spherical 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.

**Position Angle Coordinate System **

The following parameters define the ** POSITION ANGLE **coordinate frame:

, in arcseconds*RADIUS*relative to the reference axis, in degrees*POSITION ANGLE*–*REFERENCE AXIS*(celestial north) or*NORTH*(the apparent direction to the Sun as projected on the sky)*SUN*

The following optional values are available:

, in arcseconds/sec*RADIUS RATE OF CHANGE*, in degrees/day*ANGLE RATE OF CHANGE*(the reference time for the temporal variation, in*EPOCH*,*TDB*, or*TDT*)*UTC*

Note that if you specify a rate of change for the ** RADIUS **or

**, you must specify an**

*POSITION ANGLE***.**

*EPOCH*The ** POSITION 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

**, angles are measured from celestial north (positive angles are measured in the same sense as rotating from celestial north through east). For**

*REFERENCE AXIS = NORTH***, 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).**

*REFERENCE AXIS = SUN***Magneto Coordinate System **

The following parameters define the ** MAGNETO **coordinate frame:

- Magnetic
*LONGITUDE* - Magnetic
*LATITUDE* - Magnetic
*RADIUS*

The following optional values are available:

- Cartographic
, in degrees*LONGITUDE OF THE POLE* - Cartographic
, in degrees*LATITUDE OF THE POLE* - Cartographic
in degrees*LONGITUDE OF THE ORIGIN* - Cartographic
in degrees*LATITUDE OF THE ORIGIN* - Cartographic
, in kilometers*RADIUS OF THE ORIGIN*

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

**coordinate system is defined in the following manner:**

*MAGNETO*- Define a “cartographic” reference frame identical to the planetographic TRS, except use
**spherical**latitudes.

- Rotate the new coordinate system relative to the cartographic frame so the new pole is located at
and*LATITUDE OF THE POLE*.*LONGITUDE OF THE POLE*

- The final step is to translate the origin of the new system to the specified cartographic latitude, longitude, and radius (
*LATITUDE OF THE ORIGIN**,*, and**LONGITUDE OF THE ORIGIN**, respectively).*RADIUS OF THE ORIGIN*

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, and are tracked as the object rotates.

**Torus Coordinate System **

The following parameters define the ** TORUS **coordinate frame:

- Torus
, in degrees*LONGITUDE* - Torus
*LATITUDE* - Torus
*RADIUS*

The following optional values are available:

- Cartographic
, in degrees*LONGITUDE OF THE POLE* - Cartographic
, in degrees*LATITUDE OF THE POLE* - Cartographic
in degrees*LONGITUDE OF THE ORIGIN* - Cartographic
in degrees*LATITUDE OF THE ORIGIN* - Cartographic
, in kilometers*RADIUS OF THE ORIGIN*

If the optional fields above are left blank, they **default** to the nominal values for the Jupiter magnetic coordinate frame:

*LONGITUDE OF THE POLE = 202*

*LATITUDE OF THE POLE = +83*

*LONGITUDE OF THE ORIGIN = 0*

*LATITUDE OF THE ORIGIN = +0*

*RADIUS OF THE ORIGIN = 0*

The *TORUS** *TRS is defined primarily to support observations of Jupiter’s plasma torus and is closely related to the *MAGNETO** *TRS. 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-observer point.

** 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. If the

**center is defined to be the same as the center of its associated Level 1 body:**

*TORUS**LATITUDE OF THE POLE = +90*

*LONGITUDE OF THE ORIGIN = 0*

*LATITUDE OF THE ORIGIN = +0*

*RADIUS OF THE ORIGIN = 0*

then, in that case, the ** TORUS** TRS will not rotate with the Level 1 body. If the

**center is defined differently from the Level 1 body center, then the polar axis of the**

*TORUS***TRS will precess with the Level 1 body's rotation.**

*TORUS*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, * LONGITUDE = 270* (90 for the west ansa),

*). This coordinate system can also be used to support observations of a planetary ring ansa.*

**LATITUDE = 0, RADIUS = 3.57E05****Satellite Elements Coordinate System **

The following parameters define the ** SATELLITE **coordinate frame:

(Semi-major axis of satellite orbit), in km*A*of the elements (Osculation Time), in*EPOCH*,*TDB*, or*TDT**UTC*(Mean motion of satellite), in degrees/day*N*(Mean longitude at*L***Epoch)**, in degrees

The following optional values are available:

(Eccentricity of satellite orbit)*E*(Inclination of satellite orbit to the planetary equator), in degrees*I*(Longitude of ascending node of the satellite orbit), in degrees*O*(Longitude of periapse), in degrees*W*, in degrees/day*RATE OF CHANGE OF LONGITUDE OF ASCENDING NODE*, in degrees/day*RATE OF CHANGE OF PERIAPSE*at*POSITION OF PARENT BODY POLE*(Right Ascension and Declination)*EPOCH*(B1950 or J2000)*EQUINOX*

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

**) with the pole of the planet. If**

*EQUINOX***,**

*E***,**

*I***,**

*O***,**

*W***, and**

*RATE OF CHANGE OF LONGITUDE OF ASCENDING NODE***are not supplied, then the standard IAU values are used. If**

*POSITION OF THE PARENT PLANET POLE***are supplied, then they should be referred to the standard equator and equinox given by**

*POSITION OF THE PARENT PLANET POLE***. If**

*EQUINOX***is not provided, J2000 Is assumed.**

*EQUINOX*STScI maintains its ephemeris data base with the best available elements, and you should use the ** STANDARD TARGET** option for objects in list of Solar System 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=SATELLITE**.

# Level 3 Position

For Level 3, the TRSs are the same, except that “Level 3” should be substituted wherever “Level 2” occurs, and “Level 2” should be substituted wherever “Level 1” occurs.