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].