In SPICE terminology, a "coordinate system" is a mathematical means of naming vectors or points in a specified reference frame. Examples are "Cartesian coordinates" or "spherical coordinates." The coordinate systems supported by WebGeocalc are described below.
Note that in all the coordinate descriptions provided below, linear coordinates have units of km, which are the units of all position vectors computed using the SPICE SPK subsystem. In the Pointing Direction calculation, for Direction inputs and outputs preserving the original magnitude, units may be different, e.g. user implied linear units, units of velocity, or even unitless for directions given by vectors.
Rectangular (also called “Cartesian”):
Coordinates are X, Y, Z.
X, Y, Z are, respectively, the components of the point (or
vector head, with the vector tail at the origin) along
the X, Y, and Z axes of a specified reference frame.
Units are km.
Azimuth/Elevation:
webGeocalc supports several definitions of Azimuth/Elevation coordinates,
controlled by the setting of the azimuth (“AZCCW”) and elevation
(“ELPLSZ”) flags in the widget:
AZCCW indicates how azimuth is measured.
If AZCCW is set (true), azimuth increases in the
counterclockwise direction; otherwise it increases in
the clockwise direction.
ELPLSZ indicates how elevation is measured.
If ELPLSZ is set (true), elevation increases from
the XY plane toward +Z; otherwise toward -Z.
Coordinates are azimuth, elevation and range.
azimuth is the azimuth of the point. This is the angle between
the projection onto the XY plane of the vector from the
origin to the point and the +X axis of the reference
frame. `azimuth' is zero at the +X axis.
The way azimuth is measured depends on the value of the
logical flag AZCCW. See the description of the AZCCW above
for details.
`azimuth' is output in degrees. The range of AZ is [0, 360).
elevation is the elevation of the point. This is the angle between
the vector from the origin to the point and the XY
plane. `elevation' is zero at the XY plane.
The way elevation is measured depends on the value of
the logical flag ELPLSZ. See the description of ELPLSZ above
for details.
`elevation' is output in degrees. The range of `elevation' is
[-90, 90].
range is the distance of the point from the origin.
The units associated with `range' are km.
Right ascension/declination (also called “RA/Dec”):
webGeocalc supports computation of RA/Dec relative to any specified
reference frame (not just earth-centered inertial frames).
Coordinates are range, right ascension (“RA”), and declination
(“Dec”).
range is the distance of the point (vector head) from the
origin (vector tail).
The units associated with `range' are km.
ra is the right ascension of the point. This is the angular
distance measured toward the east from the prime meridian
to the meridian containing the point. The direction of
increasing right ascension is from the +X axis of a
specified reference frame towards the +Y axis.
The range of `ra' is [0, 360) degrees.
dec is the declination of the point. This is the angle from
the XY plane of the ray from the origin through the
point.
The range of `dec' is [-90, 90] degrees.
Planetocentric (also called “latitudinal”):
Coordinates are radius, longitude, and latitude.
radius is the distance of the point (vector head) from the origin
(vector tail). The units associated with `radius' are km.
longitude is the longitude of the point. This is angle between the
prime meridian and the meridian containing the input
point. The direction of increasing longitude is from the
+X axis towards the +Y axis.
The range of `longitude' is [-180, 180] degrees.
latitude is the latitude of the point. This is the angle from the
XY plane of the ray from the origin through the point.
The range of `latitude' is [-90, 90] degrees.
Cylindrical:
Coordinates are radius, longitude, and Z.
radius is the distance of the point of interest (vector head, with
the vector tail at the origin) from the Z axis. Units are
km.
longitude is the angle between the prime meridian and the meridian
containing the point.
The range of longitude is [0, 360] degrees.
Z is the height of the point above the XY plane. Units are km.
Spherical:
Coordinates are radius, colatitude, and longitude.
radius is the distance of the point (vector head) from the origin
(vector tail). Units are km.
colatitude is the angle between the point and the positive Z-axis.
longitude is the longitude of the point in radians. This is the angle
between the prime meridian and the meridian containing the
input point. Longitude increases in the counterclockwise
sense about the positive Z-axis.
The range of longitude is [-180, 180] degrees.
Planetodetic (also called “Geodetic”):
Planetodetic coordinates are applicable only to bodies that have a
reference spheroid.
Coordinates are longitude, latitude, and altitude.
longitude is the planetodetic longitude of the point (or vector head,
with the vector tail at the origin). This is the angle
between the prime meridian and the meridian containing
the point. The direction of increasing longitude is from
the +X axis towards the +Y axis.
The range of `longitude' is [-180, 180] degrees.
latitude is the planetodetic latitude of the point. For a point P on
the reference spheroid, this is the angle between the XY
plane and the outward normal vector at P. For a point P
not on the reference spheroid, the planetodetic latitude is
that of the closest point to P on the spheroid.
The range of `latitude' is [-90, 90] degrees.
altitude is the altitude of point above the reference spheroid. Units
are km.
Planetographic:
Planetographic coordinates are applicable only to bodies that have a
reference spheroid and a spin direction.
Coordinates are longitude, latitude, and altitude.
longitude is the planetographic longitude of the point (or vector head,
with the vector tail at the origin). This is the angle
between the prime meridian and the meridian containing
the point. For bodies having prograde (a.k.a. direct)
rotation, the direction of increasing longitude is
positive west: from the +X axis of the specified
reference frame toward the -Y axis. For bodies having
retrograde rotation, the direction of increasing
longitude is positive east: from the +X axis toward the
+Y axis.
The Earth, Moon, and Sun are exceptions: planetographic
longitude is measured positive east for these bodies.
The default sense of planetographic longitude for a given
body may be overridden via text kernel assignments. See
the header of the SPICE routine RECPGR for details.
The range of `longitude' is [0, 360] degrees.
latitude is the planetographic latitude of the point. For a point P
on the reference spheroid, this is the angle between the XY
plane and the outward normal vector at P. For a point P
not on the reference spheroid, the planetographic
latitude is that of the closest point to P on the
spheroid.
The range of `latitude' is [-90, 90] degrees.
altitude is the altitude of point above the reference spheroid.
Units are km.