Instrument Information
IDENTIFIER urn:nasa:pds:context:instrument:grail-a.lgrs-a::1.3
NAME Gravity Recovery and Interior Laboratory A Spacecraft Lunar Gravity Ranging System A
TYPE Radio Science
DESCRIPTION The Lunar Gravity Ranging System (LGRS) A instrument is one of two identical instruments onboard the twin Gravity Recovery and Interior Laboratory (GRAIL) spacecraft generate radio signals at Ka-band, X-band, and S-band. The time and frequency of each signal is referenced to its respective Ultra-Stable Oscillator (USO). The Ka-band and S-band signals are captured by the other spacecraft, and precise measurements of signal properties are recorded for later transmission to Earth. The X-band link is received by stations of the NASA Deep Space Network (DSN). Operations are continuous and simultaneous on both spacecraft except that the X-Band downlink to Earth is only used when a DSN station is available. The sinusoidal Ka-band link is used to determine distance (range) between the two spacecraft from the phase of the received signal. The modulated S-band signal is for exchange of Timing and Time Synchronization (TTS). The X-band link is a one-way transmission to the DSN via the Radio Science Beacon (RSB). Except for mounting differences, LGRS-A and LGRS-B (on GRAIL-A and GRAIL-B, respectively) are identical. Each is responsible for working with its twin to send and receive the signals needed to accurately and precisely measure the changes in range between the two orbiters. Each LGRS consists of an Ultra-Stable Oscillator (USO), Microwave Assembly (MWA), a Time-Transfer Assembly (TTA), and the Gravity Recovery Processor Assembly (GPA). The USO provides a steady reference signal that is used by all of the instrument subsystems. Within the LGRS, the USO provides the reference frequency for the MWA and the TTA. The MWA converts the USO reference signal to the Ka-band frequency, which is transmitted to the other orbiter. The function of the TTA is to provide a two-way time-transfer link between the spacecraft both to synchronize and to measure the clock offset between the two LGRS clocks. The TTA generates an S-band signal from the USO reference frequency and sends a GPS-like ranging code to the other spacecraft. The GPA combines all the inputs received from the MWA and TTA to produce the radiometric data that are downlinked to the ground. Finally, each Radio Science Beacon (RSB) transmits an X-Band signal to the ground based on its USO; the drift of the USO is derived from the one-way Doppler data.
MODEL IDENTIFIER
NAIF INSTRUMENT IDENTIFIER
SERIAL NUMBER not applicable
REFERENCES Asmar, S. W., N. A. Renzetti, The Deep Space Network as an instrument for radio science research, NASA Technical Reports Server, 1993STIN...9521456A, 1993.

Asmar, S.W., R.G. Herrera, and T. Priest, Radio Science Handbook, JPL D-7938, Volume 6, Jet Propulsion Laboratory, Pasadena, CA, 1995.

DSN Telecommunications Link Design Handbook, 301, Rev. H Coverage and Geometry, Rev Oct 17, Jet Propulsion Laboratory, Pasadena, CA.

Deep Space Network / Flight Project Interface Design Book, Document 810-5, Jet Propulsion Laboratory, Pasadena, CA.

DSN Geometry and Spacecraft Visibility, Document 810-5, Rev. D, Vol. 1, DSN/Flight Project Interface Design, Jet Propulsion Laboratory, Pasadena, CA, 1987.

Mars Global Surveyor Project, Telecommunications System Operations Reference Handbook, Version 2.1 (MGS 542-257), JPL Document D-14027, Jet Propulsion Laboratory, Pasadena, CA, 1996.

Deep Space Mission System (DSMS) External Interface Specification (820-013, JPL D-16765), Radio Science Receiver Standard Formatted Data Unit (SFDU), Jet Propulsion Laboratory, Pasadena, CA, 2001.