8.4. SPECTRAL COORDINATE SYSTEM REPRESENTATIONS

87

The transformation from the rest frame of the observer to a standard reference frame

requires a specification of the location on Earth1 of the instrument used for the obser-

vation in order to calculate the diurnal Doppler correction due to the Earth's rotation.

The location, if specified, shall be represented as a geocentric Cartesian triple with re-

spect to a standard ellipsoidal geoid at the time of the observation. While the position

can often be specified with an accuracy of 1 meter or better, for most purposes posi-

tional errors of several kilometers will have negligible impact on the computed velocity

correction. For details, see reference [14].

OBSGEO-Xa -- [floating-point; default: none]

X -coordinate (in meters) of a Cartesian triplet that specifies the location, with

respect to a standard, geocentric terrestrial reference frame, where the observation

took place. The coordinate must be valid at the epoch MJD-AVG or DATE-AVG.

OBSGEO-Ya -- [floating-point; default: none]

Y -coordinate (in meters) of a Cartesian triplet that specifies the location, with

respect to a standard, geocentric terrestrial reference frame, where the observation

took place. The coordinate must be valid at the epoch MJD-AVG or DATE-AVG.

OBSGEO-Za -- [floating-point; default: none]

Z -coordinate (in meters) of a Cartesian triplet that specifies the location, with

respect to a standard, geocentric terrestrial reference frame, where the observation

took place. The coordinate must be valid at the epoch MJD-AVG or DATE-AVG.

Information on the relative radial velocity between the observer and the selected

standard of rest in the direction of the celestial reference coordinate may be provided,

and if so shall be given by the VELOSYSa keyword. The frame of rest defined with

respect to the emitting source may be represented in FITS ; for this reference frame it is

necessary to define the velocity with respect to some other frame of rest. The keywords

SPECSYSa and ZSOURCEa are used to document the choice of reference frame and the

value of the systemic velocity of the source, respectively.

SSYSSRCa -- [character; default: none]

Reference frame for the value expressed in the ZSOURCEa keyword to document

the systemic velocity of the observed source. Value must be one of those given in

Table 8.7 except for SOURCE.

VELOSYSa -- [floating-point; default: none]

Relative radial velocity between the observer and the selected standard of rest

in the direction of the celestial reference coordinate. Units must be m s-1. The

1

The specification of location for an instrument on a spacecraft in flight requires an ephemeris;

keywords that might be required in this circumstance are not defined here.

FITS Standard