GRAIL LGRS Level 1A Calibrated Data Record (CDRL1A)

CDRL1A – Level 1A Calibrated Data Record

Instrument: Lunar Gravity Ranging System

PDS Data Set ID: GRAIL-L-LGRS-3-CDR-V1.0DOI: 10.17189/1519471

For more information about LGRS products, see the Data Product SIS.

The Lunar Gravity Ranging System (LGRS) Level 1A Calibrated Data Record (CDRL1A) data products are calibrated and resampled engineering (e.g., star tracker data and timing) and science data.

GRAIL timing requires coordination of three clocks on each satellite, and two time standards:

LGRS: Lunar Gravity Ranging System clock. Very stable clock for on-board Ka-band ranging (KBR), X-band (RSB), and S-band Time Transfer System (TTS) instruments. Driven by an Ultra-Stable Oscillator (USO). Set to 0 when booted. Produces a pulse per second (pps) signal. LGRS time starts at 0 seconds when powered on. The SDS adds a bias to LGRS time to create an approximate UTC time tag. This time will be referred to as LGRS + bias.
BTC: Base Clock Time. On-board satellite clock, comparable in stability to a wristwatch. Roughly synced to UTC at launch time.
RTC: Real Clock Time. Flight software clock. Set to 0 when booted. Relatively unstable clock.
UTC: Coordinated Universal Time.
TDB: Barycentric Dynamical Time

There are twenty-seven product types in the LGRS CDRL1A dataset. All products are in ASCII format.

Product	Clock	Description
CLK	LGRS time + bias	Time correlation between LGRS time + bias and TDB time
DEL	LGRS time + bias	Inner-satellite LGRS clock offset between spacecraft by TDB time, one file combined for both spacecraft
EHK	UTC	Temperature sensor data for locations near LGRS instruments
IHK	LRGS time + bias	Housekeeping data for the LGRS, including voltage, temperature, and current measurements
IHS	LGRS time + bias	Other LGRS health status data
ILG	LGRS time + bias	Log messages from the LGRS
KBR	LGRS time + bias	Ka-band ranging data, records raw carrier-phase measurements, flagged for phase breaks; gaps of up to 2 seconds are filled in by quadratic interpolation; longer gaps are classified as missing data
LTM	TDB	Position vector and light time between one spacecraft and DSN station, computed from the best ephemeris solution
MAS	UTC	Satellite mass data as a function of UTC time
PCI	TDB	Ka-band phase center to center of mass correction, antenna range corrections, range rate corrections, and range acceleration corrections
PLT	TDB	Position vector and light time between two spacecraft, computed from the best ephemeris solution, one file combined for both spacecraft
PPS	LGRS time + bias	LGRS time of PPS signals
REL	TDB	An approximation of the relativistic time correction from TDB to on-board satellite proper time is calculated in this product, treating the Moon as a point mass
SAE	UTC	Lists solar array short circuit currents and voltages, to identify eclipse events for spacecraft ephemeris models
SBR	LGRS time + bias	S-band ranging data, tracks S-band carrier phase and a modulating range code
SCA	BTC	Because GRAIL-A and GRAIL-B antennas are offset from the spacecraft center of mass, distance between GRAIL-A and GRAIL-B Ka-band antennas depends on spacecraft attitude, an on-board Kalman filter processes Star Tracker attitude data and Inertial Measurement Unit (IMU) angular rotation data, filtered attitudes are saved here
SNV	LGRS time + bias	S-band navigation product that contains ancillary information for the TTS, which primarily serves to tell the ground whether GRAIL-A and GRAIL-B are communicating correctly with each other
TC1	LGRS time + bias	LGRS to BTC time correlation, approximated by flight software
TC2	LGRS time + bias	LGRS to BTC time correlation from BTC clock cycle counts, more accurate mapping than TC11A
TC3	BTC	BTC to RTC time correlation
TC4	LGRS time + bias	LGRS to RTC time correlation
TC5	RTC	RTC to UTC time correlation
TC6	UTC	UTC to TDB time correlation, one product applies to both spacecraft
THR	UTC SCET	Thruster activation data, including time tags, cumulative work cycles by thruster, current thruster on time, and cumulative thruster on time
USO	TDB	Oscillator frequency data, Radio Science Receivers (RSR, in RSSEDR dataset), located at DSN sites, record X-band Radio Science Beacon (RSB) signals; since the LGRS clock drives the RSB, LGRS frequency at TDB can be estimated here
VCM	UTC SCET	Center of mass displacement from the spacecraft mechanical frame origin
WRS	UTC	Rotational wheel speed of each of four Reaction wheels as determined by a digital tachometer

LGRS CDRL1A products have the following file names:

product1A_yyyy_mm_dd_s_vv.ASC (with detached PDS label)

where:

product = product name from above

yyyy = year

mm = month

dd = day of month

s = GRAIL satellite identifier

A = GRAIL-A

B = GRAIL-B

X = combined product of GRAIL-A and GRAIL-B

vv = data product version number (starting from 00)

In ODE, CDRL1A products have the following product IDs:

PRODUCT1A_YYYY_MM_DD_S_VV.ASC