TRK-2-23
Media Calibration Interface

3.1.1 Command Verbs

The media calibrations use just one CSP command verb: ADJUST. The calibrations are ADJUST commands that specify corrections to one or more data types; e.g., ADJUST(DOPRNG).

3.1.2 Data Types

Data type limiters indicate the specific radiometric data type(s) to be adjusted; e.g., ADJUST(DOPRNG). The data type limiters currently used in the operational media calibrations are: DOPRNG for Doppler and range data, VLBI for Delta Differenced One-way Ranging (ΔDOR) data, and ALL for all radiometric data. The data type limiters DOPPLER (Doppler data) and RANGE (range data) may be used in some experimental calibrations.

3.1.3 Transmission Media

Media calibrations refer to one of three media: ionospheric charged particles (MODEL(CHPART)), tropospheric wet delay (MODEL(WET NUPART)), or tropospheric dry delay (MODEL(DRY NUPART)). Only wet and dry tropospheric delay calibrations are typically mixed together in a single calibration file.

3.1.4 Time Spans

Each calibration applies to the particular time span that it specifies in the form FROM(YY/MM/DD,HH:MM:00.001) TO(YY/MM/DD,HH:MM). All times refer to Universal Time Coordinated (UTC). The year, month, day, hour, and minute values are two-digit integers. Years 69 through 99 refer to 1969 through 1999, and years 00 through 68 refer to 2000 through 2068. The TO time has no seconds field; all calibration time spans end on the minute. The FROM time seconds field is always 00.001, in order to prevent the ODP from interpreting time-contiguous calibrations as overlapping, except in the troposphere seasonal model calibrations (see Figure 3-2). Since the seasonal models never include time-contiguous calibrations, they dispense with the seconds field in the FROM time.

The time span limiter AT(YY/MM/DD,HH:MM:SS.SSS) may be used in experimental calibrations. Here the seconds field is a floating point number and the applicable time span is from one millisecond before the given time to one millisecond after it.

3.1.5 Network Complex or Station

The applicable tracking sites for media calibrations may be specified by DSCC number or by DSS number. DSCC specifications have the form DSN(Cnn) where nn is the complex number; e.g., DSN(C10). These are the more common, since the media effects are essentially the same for all stations at a given complex. Individual stations are specified in the form DSN(nn) where nn is the DSS number; e.g., DSN(15). See module 810-047 for the DSS identifiers.

3.1.6 Data Source

Applicable data sources for media calibrations are specified by spacecraft or quasar number. Spacecraft are identified in the form SCID(nn) where nn is the decimal spacecraft number given in 820-013. Quasar sources of ΔDOR data are identified in the form QUASAR(nn) where nn is the quasar number. If no data source is specified, the calibration is applicable to any data source. Troposphere calibrations use no source specification since they do not refer to a specific spacecraft or quasar line-ofsight.

3.1.7 Computation Specifiers

Computation specifiers indicate how the data corrections are expressed in ADJUST commands. Media calibrations may be expressed by a constant (computation specifier BY CONST), by a normalized power series (BY NRMPOW), or by a Fourier series (BY TRIG). If the constant or the coefficients are expressed in double precision, the computation specifier is preceded by the letter "D" (e.g., BY DNRMPOW).

The computation specifier is followed by a parenthesized, comma-separated list of numerical coefficients. Up to 24 single-precision or 12 double-precision coefficients are allowed in one series. Their interpretation is described below for each type of computation that may be used. In these descriptions S and E refer to the start and end of the applicable time span, which are generally specified by the FROM and TO time span limiters.

A constant correction of size C is expressed in the form BY CONST(C).

A normalized power series is expressed in the form BY NRMPOW(CO, C1, ... ,CN). The calibration at time T is computed as CO + C1 * X + C2 * X² + … + CN * X^N, where T is replaced by the normalized, dimensionless argument X = 2 * ( (T - S) / (E - S) ) - 1, which is -1 at T = S and +1 at T = E.

A Fourier series is expressed in the form BY TRIG (P, AO, Al, B1, A2, B2, …, AN, BN). The calibration at time T is A0 + AlcosX + B1sinX + A2cos2X + B2sin2X +...+ ANcosNX + BNsinNX, where X = 2π * (T - S) / P and P is the period of the fundamental mode, in seconds.

3.1.8 Sign of Calibration

Positive media calibrations indicate positive range delays. Media calibrations are (effectively) subtracted from ODP range observables. ODP Doppler observables are corrected by (effectively) subtracting troposphere calibrations while adding ionosphere calibrations, since the charged particle effect advances the Doppler phase.

3.1.9 Number Formats

The formats for single- and double-precision floating-point numbers are generally the same as the standard FORTRAN G format [Fortran]. However the sign of the exponent of a floating-point number may replace the letter D or E; for example, 1.234E-3 may be written 1.234-3.

3.1.10 Comments

Additional commentary may appear at the end of a CSP command, following the period (“.”), if it is preceded by a number sign (#). All characters from a number sign to the end of the text line are ignored by the ODP.

3.2.1 Ionosphere Calibrations

The distribution of charged particles in the ionosphere is highly inhomogeneous. The magnitude of the ionospheric range delay at a given site depends on the spacecraft line-of-sight and on the observing frequency, time of day, season, and time since the start of the current solar cycle. At S-band, the zenith ionospheric delay ranges from less than 1 meter to more than 10 meters, and the delay at 0 degrees elevation is about 3.5 times as large.

Ionospheric calibrations are given by a normalized polynomial for each tracking pass at each DSCC or DSS. An example for a Doppler/range track is shown in Figure 3-1. The time span of the calibration is from spacecraft rise (0 degrees) to set (0 degrees) at the receiving station or complex. For ΔDOR tracks, the data type is VLBI, the data source is SCID(nn) for spacecraft scans and QUASAR(nn) for quasar scans, and the time spans correspond to the ΔDOR tracking schedule. The calibration represents the one-way line-of-sight ionospheric range delay to the spacecraft, in meters at a nominal S-band frequency of 2295 Mhz. The ODP adjusts the calibration to the tracking frequency that is used.

3.2.2 Troposphere Calibrations

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 ODP uses mapping functions [Niell] 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 DSCC and apply to all data types and spacecraft by specifying data type ALL and omitting a spacecraft scope limiter.

First order calibration of the tropospheric delays is accomplished with seasonal models [Chao] 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 as shown in Figure 3-2. 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 70m antenna at each DSCC. The seasonal models include adjustments for the altitudes of each of the other DSN antennas.