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LP L RSS LINE OF SIGHT ACCELERATION PROFILES V1.0 (PDS)

NSSDCA ID: PSPG-00802

Availability: Archived at NSSDC, accessible from elsewhere

Description

This description was generated automatically using input from the Planetary Data System.

Data Set Overview ================= Line of Sight Acceleration Profile Data Records (LOSAPDR) consist of data from Doppler tracking of the orbiting spacecraft. The relative motion of the spacecraft and the earth-based radio receiver is measured very precisely, and known motions are removed a priori (i.e. earth rotation, planetary motions, spacecraft orbital motion, solar pressure, drag), leaving small velocity changes caused by variations in the mass distribution of the planet. The residual Doppler frequency shifts are linearly proportional to the component of velocity in the Earth direction. Numerical differentiation of these velocity residuals with respect to time produces line-of-sight (LOS) gravity. These measures are accelerations at spacecraft altitude which can be modeled for geophysical interpretation. For information on Lunar Prospector (LP) gravity investigations see [KONOPLIVETAL1998; KONOPLIV&YUAN1999; CARRANZAETAL1999]. The LOS data have been produced using a 100th degree and order spherical harmonic model LP100J [KONOPLIV&YUAN1999] of the Lunar gravity field. The gravity model and other relevant gravity data and documentation are available via the World Wide Web at: http://pds-geophys.wustl.edu/pds/lunar_prospector/harmonics_maps/

Parameters ========== Users of these data should be aware of particular events and tracking station operations to properly reduce the complex motions inherent in the Doppler signals. Lunar Prospector is a simple spin-stabilized 1.3 meter high by 1.4 meter diameter cylindrical spacecraft with a 191-kg mass after lunar orbit insertion. LP had minimal orientation changes (roughly every two weeks) and very small nongravitational accelerations. The spin of LP results in a bias in the Doppler due to the spacecraft antenna pattern spinning with respect to the Earth station. For the case where the omni antenna is used for both the up-link and down-link, a bias of (1+240/221)xS Hz is the result where S is the spin rate of the spacecraft in revs/sec (0.2 for LP). The bias is thus 0.417 Hz or 27.3 mm/s (1 mm/s = 0.0153 Hz at S-band). For the medium gain antenna which is only used for the down-link or spacecraft-to-Earth, the polarization changes for the down-link and the bias is (1-240/221)xS Hz. For LP the bias is thus 0.0172 Hz or -1.12 mm/s. For the LOS data, the bias is removed for the corresponding antenna configuration.

In addition to the bias and a completely different effect, a sinusoidal signature appears in the high-rate (1 second) Doppler data due to an offset of the antenna phase center from the spacecraft spin axis. Since the spacecraft spin axis is nearly normal to the line-of-sight direction from the Earth station, most of the phase center motion about the spin axis is seen in the high-rate Doppler. Using the omni antenna for the up-link and down-link, the result is a signature with a 5-second period and 8.15 mm/s amplitude due to the spin. This indicates a 6.4 mm offset of the omni antenna phase center from the spin axis. The amplitude reduces to 4.5 mm/s when using the medium gain antenna for down-link. Since the Doppler data are essentially a differenced range measurement, these amplitudes can be mostly removed by using a sample time that is a multiple of 5 seconds. The LOS data is sampled at 5 seconds and so after 5 seconds, the antenna phase center returns to nearly the same location and results in a small remaining signature less than 0.1 mm/s [CARRANZAETAL1999]. This small signature is the result of the spacecraft spin period not being exactly 5 seconds. The transmitter ramp rates and initiation times are listed in the ODFDR (Orbit Data File Data Record) and ATDFDR (Archival Tracking Data File Data Record) data files. The equations to incorporate the ramp data as well as the complete theoretical Doppler observable are given in [MOYER1971; MOYER1987]. The ATDFDR and ODFDR data can be obtained from the Geophysics Subnode of the Planetary Data System (http://pds-geophys.wustl.edu/pds/lunar_prospector). Processing ========== The raw S-Band uplink/downlink Doppler observations have been fit with a model which accounts for theoretical motions and other signal effects. The difference between the observed or measured Doppler and the theoretical model are Doppler residuals. These are small systematic variations and are attributed to unmodeled gravity field perturbations.

The Doppler residuals are then spline fitted and analytically differentiated to produce line-of-sight (LOS) accelerations. The spacecraft position in orbit at each acceleration point is noted at a specific altitude, latitude and longitude.

All the Doppler residuals are two-way (round trip), where the Doppler shift is the shift between the outgoing and returned frequency. The Doppler residuals can be converted into velocity residuals of the spacecraft relative to Earth using:

(Doppler residual * c) / (2 * frequency),

where c is the velocity of light, 2.997925E11 mm/sec, and the frequency is 2273 MHz for the down-link, which reduces to 1 Hz Doppler being equivalent to 65.95 mm/sec (range-rate).

The LOS data during the nominal mission at 100-km average altitude (Jan. 13, 1998 to Dec. 19, 1998) and extended mission data at 40-km (Dec. 19, 1998 to Jan. 29, 1999) and 30-km average altitudes (Jan. 29, 1999 to July 30, 1999) have been produced using a 100th degree and order spherical harmonic model LP100J [KONOPLIV&YUAN1999] of the Lunar gravity field, which causes the Doppler residuals to be much smaller and thus the LOS accelerations to be also small (i.e. on the order of a few milligals as compared to 50-80 milligals with the GM only model, which has only a central term and without any spherical harmonic coefficients).

Ancillary Data ============== N/A

Coordinate System ================= The lunar gravity field was developed using the lunar orientation specified by JPL planetary ephemeris DE403. On the ephemeris, the orientation of the Moon with respect to the Earth Mean Equator of J2000 (EME2000) is given by three Euler angles [NEWHALL&WILLIAMS1997]: (1) the rotation by angle j about the Z-axis from the vernal equinox or X-axis of EME2000 to the intersection of the ascending node of the lunar equator, (2) the tilt up about the X-axis by q to match the lunar equator, and (3) the rotation by y along the lunar equator to the lunar prime meridian. These three angles describe the lunar librations to a very high accuracy (2-3 cm accuracy for the Lunar Laser Ranging, Dickey et. al., 1994) and were determined from numerically integrating the lunar orientation together with the planetary positions. These three angles give a lunar body-fixed coordinate system with axes aligned with the lunar principal axes.

Software ======== N/A

Media/Format ============ The nominal and extended mission LOSAPDR dataset will be available electronically via the World-Wide Web at: http://pds-geophys.wustl.edu/pds/lunar_prospector/los/ The archive will also be delivered to the National Space Sciences Data Center (NSSDC) using compact disk write once (CD-WO) media. Formats will be based on standards established by the Planetary Data System (PDS).

These data are available on-line from the Planetary Data System (PDS) at:

http://pds-geosciences.wustl.edu/LUNAR01/lp-l-rss-5-los-v1/lp_1101/

Alternate Names

  • LP-L-RSS-5-LOS-V1.0

Discipline

  • Planetary Science: Geology and Geophysics

Additional Information

Spacecraft

Experiments

Questions and comments about this data collection can be directed to: Dr. David R. Williams

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. Alexander S. KonoplivGeneral ContactNASA Jet Propulsion Laboratoryask@krait.jpl.nasa.gov
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