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; the spots are approximately 25 meters apart in  the nominal 50-km-high mapping orbit in the form of a cross canted by 26  degrees counterclockwise, repeating approximately every 57 meters  along-track. These spots provide up to five adjacent profiles whose  separation depends on spacecraft altitude. The data set consists of uncalibrated observations, also known as EDRs. It is a time series collection of science and housekeeping data from LOLA, aggregated exactly as they are stored on the LRO spacecraft before being downlinked. Except where noted, they are complete and free from duplicates or errors. The LOLA EDR product is a single data set. It includes the LOLA Instrument Housekeeping data as a subset of the record. Each LOLA EDR data product consists of two files. One contains the data itself, and is arranged in a PDS compliant binary table file. The other is a PDS label file that describes the content of the table file. The label file defines the start time and end of the observation, product creation time, mission phase, etc. The label file also contains pointers to descriptions of the different fields within the table. Processing : The data are pushed to the LOLA Science Operations Center computer from the LRO Mission Operations Center as a combined housekeeping and science telemetry file, one per orbit, starting at each ascending equator crossing. The instrument housekeeping data are sent in a separate real- time stream during each low-rate tracking pass and provided to the Laser Ranging Experiment website for feedback to participating laser stations. Apart from removing transmission headers and eliminating duplicate files, no processing is performed on the telemetry, and no revisions will be made to the EDR data set. The real-time data are not archived, since they are eventually replaced by the combined telemetry data files.

Overview : The data presented in the LOLA EDR is an uncalibrated data set. It is the source for all the higher level data products. Each 1-second packet constitutes a complete data record of the laser altimetry for that second, together with housekeeping data that may include, in some cases, instrument settings collected during the previous second. Data Coverage and Quality : LOLA operates virtually continuously except when commanded to stand by for spacecraft or instrument safety. During such intervals the laser will not fire but data packets will be generated and Earth ranges may be received. Parameter changes 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 one ascending and descending track approximately 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 greater  than 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 ranges 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 in the 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 sun-facing orbit plane geometry, and very few returns were obtained. Over the sunlit face of the Moon the altimeter is 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 terms of coverage density at the poles and is on track to meet its requiremen 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 show trends in 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 adequate margin. Range data are believed to be accurate to 1 meter overall, with better than 10 cm precision (1 sigma) shot-to-shot under nominal conditions, depending on surface slope and roughness. At altitudes 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. 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 differences between the five individual detectors are calibrated in the higher level products. Limitations : These data are uncalibrated instrument counts, as received from the spacecraft telemetry system and the Mission Operations database. Transmission from the spacecraft to ground and ground processing is verified by the Data Management System, while the LOLA SOC institutes further checks. Any remaining gaps or errors are due to known anomalies. Further information concerning mission operations and telemetry handling may be found in [SAYLOR2006A], [SAYLOR2006B]. Instrument calibrations are provided in [RIRISETAL2010]. An instrument overview for LOLA is given in [SMITHETAL2009]. The LRO mission is described in [CHINETAL2007].