GRAIL LGRS Level 1B Calibrated Data Record (CDRL1B)
CDRL1B – Level 1B 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 1B Calibrated Data Record (CDRL1B) data products are calibrated and resampled engineering (e.g., star tracker data and timing) and science data. Products in this dataset have been converted to Barycentric Dynamical Time (TDB).
GRAIL timing requires coordination of three clocks on each satellite, and two time standards:
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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.
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BTC: Base Clock Time. On-board satellite clock, comparable in stability to a wristwatch. Roughly synced to UTC at launch time.
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RTC: Real Clock Time. Flight software clock. Set to 0 when booted. Relatively unstable clock.
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UTC: Coordinated Universal Time.
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TDB: Barycentric Dynamical Time
There are sixteen product types in the LGRS CDRL1B dataset. All products are in ASCII format.
|
Product |
Clock |
Description |
|
CLK |
LGRS time + bias |
Best estimate of LGRS to TDB time correlation |
|
EHK |
TDB |
Temperature sensor data for locations near LGRS instruments |
|
GNI |
TDB |
GRAIL satellite orbit solution in Moon-centered Inertial frame; best estimate of spacecraft ephemeris |
|
GNV |
TDB |
Satellite orbit solution in lunar body fixed frame; best estimate of spacecraft ephemeris |
|
KBR |
TDB |
Biased dual one-way Ka-band ranging data; one file combined for both spacecraft |
|
MAS |
TDB |
Spacecraft mass data as a function of TDB time |
|
SAE |
TDB |
Solar array short circuit currents and voltages, to identify eclipse events for spacecraft ephemeris models |
|
SBR |
TDB |
Dual one-way S-band ranging data, tracks carrier phase and a modulating range code; more accurate range produced by carrier smoothing over each arc; one file combined for both spacecraft |
|
SCA |
TDB |
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 |
|
THR |
TDB |
Thruster activation data, including time tags, cumulative work by thruster, current thruster on time, and cumulative thruster on time |
|
USO |
TDB |
Best estimate of LGRS frequency at TDB |
|
VCM |
TDB |
Center of mass displacement from spacecraft mechanical frame origin |
|
VGS |
TDB |
List of TDB times at which the low-gain antennas (LGAs) are switched, with the mechanical frame vector of the new transmitting antenna phase center |
|
VGX |
TDB |
List of TDB times at which Radio Science Beacons (RSBs) are switched, with the mechanical frame vector of the new transmitting beacon phase center |
|
VKB |
TDB |
Ka-band boresight vector, as a result of Ka-band boresight calibration analysis |
|
WRS |
TDB |
Rotational wheel speed of each of four Reaction wheels as determined by a digital tachometer |
LGRS CDRL1B products have the following file names:
product1B_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, CDRL1B products have the following product IDs:
PRODUCT1B_YYYY_MM_DD_S_VV.ASC