OCS Field Descriptions

Start Time
The Earth Receive Time at which the first radio occultation data sample was acquired.

Stop Time
The Earth Receive Time at which the last radio occultation data sample was acquired.

Occultation Time
The time at Mars when the geometrical ray path grazed the limb. This is computed by subtracting the limb-to-Earth light time from the appropriate Earth Receive Time.

Orbit Number
Orbit number from which the data were acquired; set to zero if not known.

DSN Antenna Number
Number of the DSN antenna used to collect the data (e.g., 14, 43, 65, ...).

Ray Path Direction
The angle between local north and the tangent to the ray path at the occultation point, measured positive from local north toward local east. The tangent to the ray path indicates the direction of signal propagation. A signal traveling from west toward east as it grazed the surface would have a ray-path-direction of 90 degrees.

Angle From Diametric
Angle at which the spacecraft rises or sets behind the planet limb, measured clockwise from the planet radial direction as viewed from Earth. An angle-from-diametric equal to zero indicates a diametric occultation with motion of the ray away from the surface (e.g., occultation egress). A value near 180 degrees is a nearly diametric ingress occultation.

Latitude at Surface
Areocentric (north) latitude of the occultation point.

Sigma Latitude
One standard deviation uncertainty in LATITUDE AT SURFACE. Set to -9.999 if not known.

Longitude at Surface
Areocentric east longitude of the occultation point in body fixed coordinates.

Sigma Longitude
One standard deviation uncertainty in LONGITUDE AT SURFACE. Set to -9.999 if not known.

Sub-Solar Latitude
Areocentric (north) latitude of the sun at OCCULTATION TIME.

Sub-Solar Longitude
Areocentric east longitude of the sun in body fixed coordinates at OCCULTATION TIME.

Solar Longitude
Value of the angle between the Mars-Sun line at the time of geometric occultation and the Mars-Sun line at the Mars vernal equinox. Measured positive with increasing time following the vernal equinox. Sometimes denoted L-sub-s.

Radius at Surface
Radius of Mars at the occultation point.

Sigma Radius
One standard deviation uncertainty in RADIUS AT SURFACE. Set to -9999. if not known.

Surface Pressure
Atmospheric pressure at the occultation point -- i.e., at the point defined by RADIUS AT SURFACE, LATITUDE AT SURFACE, and LONGITUDE AT SURFACE. Obtained by extrapolating from the lowest sample in the retrieved atmospheric profile to the estimated RADIUS AT SURFACE.

Sigma Surface Pressure
One standard deviation uncertainty in SURFACE PRESSURE. Set to -9.99 if not known.

Temperature Near Surface
Atmospheric temperature at lowest sample in temperature-pressure profile. Note that the height of this sample above the surface varies among profiles. No attempt has been made to extrapolate to surface because thermal structure of boundary layer varies strongly with local time.

Sigma Temperature Near Surface
One standard deviation uncertainty in TEMPERATURE NEAR SURFACE.

Spacecraft to Limb Distance
Distance from spacecraft to occultation point at time when ray path grazed planetary surface.

Spacecraft to DSN Distance
Distance from spacecraft to DSN receiving antenna at time when ray path grazed planetary surface.

Local True Solar Time of Occultation
Local true solar time (LTST) at the time and location of the occultation measurement. LTST at the sub-solar longitude is defined as 12 solar hours. LTST on other meridians changes by one solar hour for each 15 degree increment in longitude. Thus, LTST for the occultation measurement is given by 12 + (LONGITUDE_AT_SURFACE - SUB-SOLAR_LONGITUDE)/15.

Solar Zenith Angle
Angle between the direction to Sun and local vertical direction on Mars at time and location of the occultation measurement.

Sun-Earth-Spacecraft Angle
Approximate angle between Sun and spacecraft as viewed from Earth during experiment.

DSN Elevation Angle
Approximate angle between direction to spacecraft and local horizon at DSN receiving antenna during experiment.

Gravity Field Model
File name of gravity field spherical harmonic model used in profile retrieval and calculation of geopotential.

Geopotential Reference
Reference value for geopotential. Reference geoid has mean equatorial radius of 3396 kilometers.

PCK File Name
File name of the NAIF Planetary Constants File used in this retrieval and calculation (name of the PCK file as stored in the MGS RST data system).

Trajectory File Name
File name of the spacecraft and planetary ephemeris file used in this retrieval and calculation (name of the file as stored in the MGS RST data system).

Spacecraft Attitude File Name
File name of the spacecraft attitude file used in this retrieval and calculation (name of the file as stored in the MGS RST data system). Left blank if not known or no attitude file was used.

TPS File Name
File name of complete temperature-pressure profile from which these parameters derive.


Last updated: November 04, 1998
Joe Twicken