Data Set Overview: LOLA is a pulse detection, time-of-flight laser altimeter. LOLA transmits a 5-spot pattern that measures the precise distance to the lunar surface at multiple points simultaneously, thus providing 5 profiles across the lunar surface. Each spot within the five-spot pattern has a diameter of approximately five meters and are ~25 meters apart in the nominal 50-km-high mapping orbit. They take the form of a cross canted by 26 degree counterclockwise, repeating every ~57 meters along-track. These spots provide up to five adjacent profiles whose separation depends spacecraft altitude. The RDR data set is a time-ordered collection of measurement data from LOLA, calibrated, geolocated, and aggregated by orbit. Each topographic return is located with respect to the lunar center of mass in a body- fixed (rotating) coordinate frame. The LOLA RDR data product consists of two files. One contains the data itself, and is arranged in a PDS compliant binary table. The other is a PDS label file which describes the content of the table: the start and end times of the observation, product creation time, mission phase, software algorithms, and parameter files used to generate the data. The label file also contains pointers to description of the different fields within the table.Processing: A full description of the LOLA data processing can be found in the RDR SIS located in the document folder that accompanies this archive. The following paragraphs are a summary of how the data are processed from data receipt through averaging. The LOLA instrument data packets consist of time-ordered, round-trip, time-of-flight ranges to the lunar surface, preceded by housekeeping and ancillary data. LOLA outputs these 3424-byte packets over the spacecraft IEEE-1553 data bus at 1-s intervals. The LRO Solid State Recorder aggregate packets into files at commanded intervals. Each file, stripped of telemetry headers, becomes an Experiment Data Record product (EDR). The LRO Ground Data System receives science data from each downlink tracking pass and transmits them promptly to the LOLA Science Operation Center (SOC) computer. Processing calibrates EDRs spanning a single mission orbit to time-of-flight ranges and other measurements using an automated pipeline. The geolocation processing cycle merges the data records with spacecraft housekeeping and tracking data solutions, transforming them to geolocated altimetric returns together with ancillary data.   Geolocation is further refined with orbit reconstruction by the LOLA  Precision Orbit Determination team, including, most recently,  reconstruction based on a lunar gravity field derived from the NASA  Grail Discovery Mission. The field GRGM660PRIM was developed by GRAIL  team members at the Goddard Space Flight Center using the primary  mission dataset acquired by the twin GRAIL spacecraft. GRGM660PRIM has  coefficient uncertainties several orders of magnitude lower than fields  derived from any previous mission including LRO and allows more precise force modeling over the lunar farside than possible previously. A lunar gravity field is being archived as the GRAIL MOON LGRS RDR - Lunar gravitational field, NASA Level 2 data (Release in October 2013). The field was truncated at degree and order 270 during the Precision Orbit Determination process. Each orbit begin nominally at the spacecraft ascending node as  determined from ephemeris predictions, in one-to-one correspondence  with the EDRs, following the LRO Project orbital numbering convention.  The position of each laser spot, located on the surface using a  spacecraft trajectory, attitude history, and a precise lunar  orientation model, is output as a reduced data record (RDR) product.  Each laser shot generates one record with up to five valid lunar ranges. Ancillary measurements of energies and reflectivities as well as  quality flags are included in each record.  The LRO Ultrastable Oscillator, which is monitored by ground stations, provides timing of LOLA instrument events. LRO employs Coordinated Universal Time (UTC) to correlate spacecraft Mission Elapsed Time (MET)  to ground time with an accuracy of +/- 3 ms. The correlation of MET  time to Barycentric Dynamical Time (TDB) is maintained at much higher  precision by the Laser Ranging system and orbital theories. The LOLA  data analysis use TDB as its primary time system (see http://tycho.usno. navy.mil/systime.html for details). Processing projects the spacecraft  states relative to the Solar System Barycenter (SSB) at the laser  transmit and detector receive times along the instrument boresight and  return path vectors to match the observed time-of-flight times the  speed of light, correcting for the aberration of light and general- relativistic time delays. SSB states are determined in the Earth Mean  Equator of 2000 (J2000) inertial reference frame using lunar spacecraft  trajectories and the DE421 planetary ephemeris [WILLIAMSETAL2008,  FOLKNERETAL2008]. The barycentric coordinates of the laser bounce point are transformed from J2000 to a geodetic coordinate frame about the center of mass of the Moon, using the DE421 Moon Mean-Earth and Rotation Pole (MOON_ME) orientation model. The DE421 model incorporates 37  years of Lunar Laser Ranging (LLR) data, thereby providing the current  geodetic framework for LOLA. The lunar radius and position are then  subjected to orbital crossover analysis to minimize the terrain  mismatch at the intersections of ground tracks. Empirical adjustment  of short-term pointing biases is combined with orbital estimation from  tracking data to bring the ground tracks into agreement, and produce a  dynamically-consistent lunar geodetic coordinate framework. The  crossover adjustment is performed monthly intervals following the  propulsive orbital adjustment maneuvers that break the dynamical orbit  solutions. A byproduct of the tracking and crossover analysis is an  improved lunar potential solution.  Higher-level gridded and transformed data products are produced from the cumulative RDR data set. Binning and interpolation of data values to regularly-spaced intervals are described in the LOLA RDR and higher level data product SIS document, Lunar Reconnaissance Orbiter Lunar Orbiter  Laser Altimeter Reduced Data Record and Derived Products Software  Interface Specification, located in the DOCUMENT directory of this  archive. The values of surface slope and roughness are calculated over  multiple baselines, but there are practically no planetary datasets of  comparable resolution to use as benchmarks for useful derived products. Thus, their format and content may evolve with time. Similarly, the  bidirectional reflectance is likely to be a function of surface  temperature and composition and may not be easily amenable to  averaging.

Overview: Revisions are made to the RDR data set after each orbital adjustmen maneuver. The quality improves with each adjustment by minimizing discrepancies in measurement geometry knowledge between individual  orbital tracks and the accumulating global average.Data Coverage and Quality: LOLA operates virtually continuously except when commanded to stand by for spacecraft safety or instrument safety. During such intervals the laser will not fire but data packets will be generated and Earth ranges may be received. Parameter change to the flight software or operating mode are executed by direct memory writes, during which time the LOLA clock count will not update. This counter is adjusted in subsequent processing. A standby command is also issued to prevent laser firing while laser retroreflectors are in the instrument field of view, to prevent damage to the detectors. The altimetric coverage is a direct function of the duration and geometry of the near-polar LRO orbits. The concept of operation is to remain within two degrees of nadir. Targeting maneuvers are allocated for a small percentage of mission time during which the error budget for altimetry and geolocation may increase significantly. The coverage is also function of the probability of detection of each individual laser spot. During a given month approximately 348 full orbits are performed, providing approximately one ascending and descending track for every  degree of longitude. Monthly station-keeping maneuvers adjust the  orbital period slightly so as to maintain a 35-65 km altitude and avoid  exact repeats of the ground track. The probability of detection of each spot was anticipated to be > 95% at altitudes between 20 and 80 km, but during commissioning, spacecraft altitude reached more than 200 km at apoapse. Sensitivity of the altimeter is inversely proportional to the square of the altitude. Nevertheless some ranges were acquired at more than 120 km while at times no range were acquired around the 30-km periapse. Investigation of this anomalous behavior concluded that the most likely cause is thermal distortion of the alignment of the laser beam expander telescope due to the thermal contraction of the multi-layer insulation encasing the instrument. These blankets were mechanically attached to the telescope in such a way that cold temperatures they were no longer able to flex to accomodate thermal contraction and they pulled the laser spots out of alignment with the detector field of view. The misalignment was confirmed during an Earth- pointing raster scan, whereby the laser spots were imaged at a ground station while the detectors registered pulses fired from a ground laser. The LOLA On-orbit Signal Anomaly Final Report (LRO-LOLA-RPT-00200) describes the loss of signal. At this time there is no plan to correct it. Under the cold conditions of the lunar night side the distortion places sufficient laser energy in two of the detector fields of view to perform ranging, while secondary spots from the laser will occasionally illuminate the other detectors with enough energy to make a measurement. Transitions in alignment occur almost immediately upon approaching the terminator between daylight and night. During the first two days of ranging, LOLA was almost perpetually in twilight owing to the orbit plane geometry, and very few return were obtained, while over the sunlit face of the Moon the altimeter w able to range at distances of nearly 200 km. On average, about 60% of the possible returns were obtained during commissioning and early mapping. Performance is degraded over the poles where the most redundant coverage is obtained, while sufficient data are obtained at mid-latitudes to refine the cross-track coverage during day and night. As of this release LOLA has already met its minimum requirements for mission success in term of coverage density at the poles and is on track to meet its requirement for 1.25-km equatorial track spacing by the end of the nominal mission. Each of the two redundant lasers is operated alternately for one month time to provide ongoing health and engineering trend data. The lasers continue to output full design energy per shot and it is expected that they will complete the mapping mission phase with margin. Under nominal conditions, range data are believed to be accurate to 1  meter overall, with better than 10 cm precision (1 sigma) shot-to-shot. There is a dependance on surface slope and roughness. Additionally, at  altitude greater than 50 km, return signals are weaker and accuracy will degrade somewhat. The Signal Anomaly also results in weaker pulses and somewhat erratic ranges. Direct assessment of range precision is  impossible since there are no known extended targets on the Moon.  Performance of the Laser Ranging signal being measured by identical  hardware paths has shown shot-to-shot precision of better than 20 cm (1  sigma) in one direction, which is equivalent to a two-way measurement  precision of 10 cm. Systematic effects such as clock drift, system  response and range walk under varying signal strength were well under 1  meter overall in pre-flight testing. Systematic difference between the  five individual detectors are calibrated in the higher level products.Limitation: Performance and calibration constants of the five detectors differ. In particular, detector 1 also receives Earth ranges and solar background through the Laser Ranging telescope. Systematic differences between the five separate detector channels are calibrated as described in RIRISETAL2010. Alignment of the transmitted laser beam is taken from the post-shipment survey, and does not take into account the Signal Anomaly thermal effects. The Signal Anomaly also strongly affects the energy return measurement, one of the requirements for measuring the bidirectional reflectivity of the surface in the permanently shadowed regions of the Moon. Efforts have been made to better characterize the instrument's thermal environment, alignment, and receiver response.