MESSENGER RSS Troposphere (media) Calibration Record (TRO)

TRO – Troposphere (media) Calibration Record

Instrument: MESSENGER Radio Science Experiment

PDS Data Set ID: MESS-V/H-RSS-1-EDR-RAWDATA-V1.0

For more information about RSS TRO products, see the TRK-2-23 Product SIS.

The Radio Science Subsystem (RSS) Troposphere (media) Calibration Record (TRO) files are Deep Space Network (DSN) TRK-2-23 Media Calibration files. These provide tropospheric calibration information.

The tropospheric delay is comprised of a wet delay, caused by the permanent dipole in atmospheric water vapor, and a hydrostatic or dry delay, caused by induced dipoles in all atmospheric gasses. The zenith dry delay ranges from about 2 to 2.2 meters at the DSN sites. DSN zenith wet delays are smaller and more variable, ranging from a few centimeters to 25 centimeters or more. The slant-range factor is approximately 1/sin(elevation), which is about 10 at 6 degrees elevation.

To a large degree, the troposphere is azimuthally symmetric. Therefore, tropospheric calibrations represent the zenith wet and dry delays and the Orbit Determination Program (ODP) uses mapping functions to perform the slant-range correction to the spacecraft elevation. Thus, troposphere calibrations are not specific to a particular spacecraft. They simply “cover the clock” at a given Deep Space Communications Complex (DSCC) and apply to all data types and spacecraft.

First order calibration of the tropospheric delays is accomplished with seasonal models which do not depend on real time data and need only be delivered once. These models represent the one-way zenith wet and dry delays, in meters, as Fourier series calibrations. The zenith dry delay decreases about 0.25 mm for each additional meter of altitude. Dry delay calibrations refer to the altitude of the intersection of axes of the 70 m antenna at each DSCC. The seasonal models include adjustments for the altitudes of each of the other DSN antennas.

The seasonal troposphere models are routinely used in all ODP runs. More accurate tropospheric calibrations are derived from real time weather and Global Positioning System (GPS) data. These calibrations are expressed as normalized polynomial corrections, in meters, to the one-way seasonal model delays, in order to avoid double-calibrating. A comment before each calibration indicates the post-fit residual of the fitted polynomial. A comment at the end of each calibration indicates the date and time it was created.

RSS TRO products have the following file names:

MESS_RS_yyyyddd_yyyyddd_TRO.TXT (with detached PDS label)

where:

MESS = mission = MESSENGER

RS = instrument = Radio Science Subsystem

yyyyddd = start date, year and day

yyyyddd = stop date, year and day

TRO = product type = Tropospheric (media) calibration

In ODE, TRO products have the following product IDs:

MESS_RS_YYYYDDD_YYYYDDD_TRO