Data Set Information
DATA_SET_TERSE_DESCRIPTION The MESSENGER MLA reduced observations consist of laser ranges and instrument data collected by the MLA instrument during fly-by and orbital operations of Mercury.
Data Set Overview
     The is V2.0 of the data set and contains only on-orbit Reduced Data
     Records (RDR) (profile data).

     The RDRs contain time-of-flight profile data and are produced by merging
     spacecraft geometry and attitude data with range data and a planetary
     orientation model.

     The label file that accompanies each product defines the start time and
     end time of the observations, the product creation time and release
     information, and describes the different fields within the table. The
     RDRs correspond one-to-one with the on-orbit Science CDR products
     contained in the V1.0 of this data set
     (MESS-E/V/H-MLA-3/4-CDR/RDR-DATA-V1.0), except as noted in the

     During the Mercury-Orbit mission phase, a single data file contains
     the observations obtained in one orbit of the spacecraft around

     The RDRs in this V2.0 data set supersede, but do not supplant those in
     the V1.0 version of the data set as the earlier ones may still be
     scientifically useful. This data set does not contain CDR or RADR

     The user is referred to the version V1.0 of the MLA CDR/RDR/RADR data
     set 'MESSENGER E/V/H MLA 3/4 CDR/RDR DATA V1.0' for the CDRs/RADRs and
     documentation on calibration and events prior to orbital operations. The
     CATALOG/CDR_RDR_DS.CAT file in the V1.0 data set provides additional

   Instrument Overview
     The Mercury Laser Altimeter (MLA) uses a solid-state pulsed laser to
     measure the distance between the spacecraft and the surface of
     Mercury.  This allows the science team to take detailed
     measurements of Mercury's shape and surface structure.  The MLA is a
     bi-static system, meaning that it consists of separate transmitter and
     receiver systems.

     See the MLAINST.CAT file for more information and [CAVANAUGHETAL2007]
     and [SUN&NEUMANN2014] for full details.

   Calibration Overview
     See data set MESSENGER E/V/H MLA 3/4 CDR/RDR DATA V2.0 for the CDRs from
     which these RDRs were generated. The calibration is also described in the
     MLA CDR/RDR Software Interface Specification (MLA CDR/RDR SIS).

     The principal parameters when observing with the MLA are as follows:

     * MLA_GOTO_KEEP_ALIVE: This parameter transitions the instrument to
     low-power mode where only the CPU, the Analog Electronics Module, and
     the laser diode's thermo-electric cooler are powered.

     * MLA_GOTO_STANDBY: This parameter transitions the instrument to a
     state similar to the Keep-Alive mode, except the Range Measurement
     Unit is also powered. Laser firing may also be enabled in Standby mode
     in order to perform calibration and ranging experiments under manual
     control, which would be overridden by the Science mode algorithms.

     * MLA_GOTO_SCIENCE: This parameter transitions the instrument to a
     state similar to the Standby mode, except the laser power supply is on
     and the laser fires. The Science task provides variable-rate,
     partially-compressed RMU data at 1 Hz.

     The default algorithm automatically sets parameters associated with
     acquisition of laser ranges; these parameters can be manually set. The
     MLA also includes modes for testing the instrument and maintenance
     activities. Analog and status telemetry data may be
     generated at a prescribed time interval in any instrument mode.

     The three advanced data products are generated from the EDRs and are
     calibrated to physical units. The science RDR is derived from the
     science EDR, and is used to make the GDR products. Each 1-s Science EDR
     is broken into 8-Hz records, one for each laser shot, while Status and
     Hardware Diagnostic records are calibrated one for one.
DATA_SET_RELEASE_DATE 2017-05-12T00:00:00.000Z
START_TIME 2011-03-29T02:05:11.000Z
STOP_TIME 2015-04-30T11:14:16.000Z
MISSION_START_DATE 2004-08-03T12:00:00.000Z
MISSION_STOP_DATE 2015-04-30T12:00:00.000Z
NODE_NAME Geosciences
Confidence Level Overview
     This RDR data release covers the period of on-orbit altimetry science
     measurements of a solid body (Mercury). The laser altimeter range is
     limited to distances less than 1800 km. When enabled, the detector
     continuously triggers on optical signals passing through the receiver
     telescope at a roughly exponentially-increasing rate with optical power,
     at any given threshold, making it useful as a 'one-pixel camera' with a
     very narrow spectral bandwidth and a 400-microradian field of view. The
     Range Measurement Unit operates at 8 Hz, so that scanning across the
     illuminated surface of a target in a raster pattern provides a
     boresight calibration. In addition, triggers may be received from
     Earth-based lasers within a 14-ms subinterval of each 8-Hz cycle, so
     that the time-of-flight may be measured repeatedly.

     The MESSENGER spacecraft employs an ovenized, quartz-crystal-based
     oscillator whose frequency is stable to a few parts per trillion over
     the course of an hour. The MLA acquires its time base from the
     spacecraft via a one-pulse-per-second (1PPS) tick along with the
     corresponding mission elapsed time (MET) message over the data bus. The
     1PPS signal uncertainty during ground testing was 0.021 ms. The 1 PPS
     offset, and the offset between the MLA event time reference and the
     1PPS, are very stable over short intervals of time. The latter is
     monitored by the instrument at 125-ns resolution. While the spacecraft
     clock can be related to the MLA timing only to tens of microseconds in
     an absolute sense, over intervals of an hour or so they are precisely
     coupled. When laser firing is enabled, the time of fire is recorded and
     may be matched with pulses received at at an earth station. Transmit
     and receive times may be correlated on the ground to measure the
     effective 2-way time-of-flight and clock drift. The resolution of the
     MLA timing measurement is roughly 400 ps, and the demonstrated overall
     precision of an individual time-of- flight measurement between MLA and
     Earth is approximately 0.65 ns (20 cm) root-mean-square, owing to
     signal variations and atmospheric delays. Ranging to planetary surfaces
     entails additional error sources related to terrain effects acting
     on a finite-sized laser footprint, but approaches 20 cm precision
     for triggers on Channel 1 under optimal conditions.

     To ensure accurate altimetric measurements, the absolute time
     correlation of the spacecraft clock is maintained to better than 1 ms
     via radio tracking, while the spacecraft position relative to Mercury
     center of mass should be known to better than a few tens of meters
     while in orbit. With repeated altimetry crossing tracks, the orbit was
     be further constrained, limited by the roughness of the surface at the
     shot-to-shot scale. A further source of uncertainty in geolocation is
     the MLA boresight vector, since geolocation multiplies the optical
     range by the direction cosines of this vector with respect to an
     inertial reference system. The spacecraft attitude control and
     knowledge is derived from an inertial measurement unit and star
     trackers, whose performance is monitored by instrument calibrations.
     The MLA scans of Earth and Venus have characterized the boresight
     alignment repeatedly during cruise, and on two occasions, the MLA laser
     beam has been observed on Earth, providing an improved laser boresight
     vector. Further tests during cruise confirmed the current system
     attitude knowledge, which at present is known to be repeatable to
     within 50 microradians from day to day.

     The radiometric measurements are produced from the threshold crossing
     times of the received pulses at two discriminator voltages on channel
     one simultaneously, a low threshold for maximum sensitivity, and a
     threshold roughly twice as high, to give four sample points of the
     received pulse waveform. A laser pulse may result in triggers at one or
     both thresholds or not at all. Ranging with low threshold detections is
     possible at ranges up to 1500 km, but steady returns that cross both
     the low and high thresholds are best obtained at altitudes less than
     ~600 km with near-nadir (<20 degree) incidence angles. When a pulse is
     detected by a pair of discriminators, its energy and duration may be
     inferred assuming a model waveform that accounts for the dispersion in
     time of return pulses due to surface slope and/or roughness. We adopt a
     simple triangular model that fits the rising and falling edges of the
     trigger at each threshold. This model generates values nearly equal to
     the ideal Gaussian waveform model for well-constrained pulses. The
     reflectance is derived using a direct measurement of the outgoing pulse
     energy and instrument calibrations from ground test data, via the lidar
     link equation. The measurement of radiance factor (relative to an ideal
     diffusive reflector) has an associated uncertainty of 30% for each
     measurement at low altitude, with an approximately lognormal

   Mercury Orbital Observations
     MLA enters Science Mode on each orbit prior to periapse. Configuration
     via stored sequences places the instrument in Standby for 10 minutes,
     switches the Range Measurement Unit (RMU) to Oscillator B to select the
     active precision time source, and transitions to Science Mode 1 when
     the predicted slant range and emission angle fall below a predetermined
     set of values, initially 1800 km at nadir. Within 45 seconds the laser
     commences firing at 8 Hz. The instrument is placed in Keepalive mode
     when predictions exceed the predetermined values, allowing for some
     period of operation in case of orbital error.  Almost all triggers of
     the RMU are noise at that altitude. As the orbital geometry changes from
     noon-midnight to dawn-dusk attitude remains near nadir and in-range for
     up to 2500 s without violating the Sun Keep-In (SKI) constraint. Shorter
     observations occur when the spacecraft is commanded off-nadir for thermal
     protection, observations, or to maintain SKI within + or - 10 degrees.
     Any period of time when predicted range exceeds 1920 km for more than
     255 seconds causes demotion to Science mode 0, a less sensitive
     acquisition configuration.

     Science mode ranges are expressed in nominal counts of a 5 MHz clock,
     or 200 ns coarse ranges, interpolated to ~400 ps by fine counts. A
     single clock may be faster or slower by a few tens of parts per billion,
     an amount smaller than the least significant fine count of the RMU.

     Laser pulses are recorded at leading and trailing edge threshold
     crossings relative to each minor frame start by the RMU. An estimate of
     the fire time is given by the mean of the two crossings. Multiple
     triggers may be sensed by three matched filter channel discriminators,
     with a dead time of a few microseconds between triggers. The RMU sets a
     channel id bit (1, 2, 4) when the range is latched. Some triggers
     set multiple bits owing to the dead time. Such events are assigned
     to the matched filter with the shortest system delay, 10, 60, or 540 ns,
     respectively. If triggered, a single high-threshold crossing is recorded
     (id bit 0) which is often paired with a lower-threshold event. When the
     paired event has id 1, it is usually within 1 ns of the high event and
     provides an estimate of the total return pulse width and energy. Such
     pairs may be considered accurate to 0.2 m if coming from the ground.
     If the channel id cannot be determined (leading and trailing edges are
     inconsistent), the id #3 is assigned and the range is less certain.
     Ranges with ids 1, 2 and 4 are determined with accuracy about 1/3 of
     the corresponding matched filter width or 0.5, 3, and 25 m respectively.
     Calibrations for system delay, spacecraft position, and offset to the
     spacecraft center of mass are applied to accurately determine the
     planetary shape. At the extremes of the orbital range, the size of the
     laser footprint expands and the topographic accuracy is degraded.

     For altimetric performance to fully meet expectations for the mission
     science objectives, it is necessary to indicate the quality of any
     given trigger as a likely ground return or a noise trigger. This
     'quality' can only be provided after geolocation and inspection, so the
     description of quality flag below pertains only to RDR data, but is
     provided here for reference. Quality of range data is indicated by a
     Boolean value that is contained within the Reduced Data Record product
     CHANID column. This column is described as COLUMN_NUMBER = 7.

     BYTES          = 2
     FORMAT         = 'I2'
     START_BYTE     = 64
     DESCRIPTION    = 'Receiver channel of ground or noise trigger -
                       =0 channel 1 high threshold.
                       =1 channel 1 low
                       =2 channel 2 low
                       =3 ambiguous, assigned to channel 1 low
                       =4 channel 3 low.
                       =5 channel 1 high noise trigger
                       =6 channel 1 low noise trigger
                       =7 channel 2 low noise trigger
                       =8 unassigned noise trigger
                       =9 channel 3 low noise trigger.'

     A Boolean value consists of CHANID/5, where 1=noise and 0=ground. The
     uncertainty of each ground return in meters is roughly
     SIGMA_RANGE = 2^CHANID taking into account the reduced precision of the
     higher channels.

     The value below is contained in the Science CDR and RDR tables to
     denote observations whose ranges are imprecise:

      NAME             = STARTPLS_WIDTH
      DESCRIPTION      = 'Width of transmit laser pulse in nanoseconds, used
        to determine centroid time of outgoing pulse. A value of 99.9
        denotes a measurement whose pulse width is invalid.'

     This archival data set has been approved by the Instrument Scientist.
     The final data set, including calibrated and reduced data records, will
     be examined by a peer review panel prior to its acceptance by the
     Planetary Data System (PDS). The peer review will be conducted in
     accordance with PDS procedures.

   Data Coverage and Quality
     Data reported are the full-rate processed data received from the
     spacecraft during the mission phases: Mercury Orbit,
     Mercury Orbit Year 2, Mercury Orbit Year 3, Mercury Orbit Year 4, and
     Mercury Orbit Year 5. The mission phases are defined as:

     Phase Name Date       Start Date (DOY)   End Date (DOY)
     --------------------  -----------------  -----------------
     Launch                03 Aug 2004 (216)  12 Sep 2004 (256)
     Earth Cruise          13 Sep 2004 (257)  18 Jul 2005 (199)
     Earth Flyby           19 Jul 2005 (200)  16 Aug 2005 (228)
     Venus 1 Cruise        17 Aug 2005 (229)  09 Oct 2006 (282)
     Venus 1 Flyby         10 Oct 2006 (283)  07 Nov 2006 (311)
     Venus 2 Cruise        08 Nov 2006 (312)  22 May 2007 (142)
     Venus 2 Flyby         23 May 2007 (143)  20 Jun 2007 (171)
     Mercury 1 Cruise      21 Jun 2007 (172)  30 Dec 2007 (364)
     Mercury 1 Flyby       31 Dec 2007 (365)  28 Jan 2008 (028)
     Mercury 2 Cruise      29 Jan 2008 (029)  21 Sep 2008 (265)
     Mercury 2 Flyby       22 Sep 2008 (266)  20 Oct 2008 (294)
     Mercury 3 Cruise      21 Oct 2008 (295)  15 Sep 2009 (258)
     Mercury 3 Flyby       16 Sep 2009 (259)  14 Oct 2009 (287)
     Mercury 4 Cruise      15 Oct 2009 (288)  03 Mar 2011 (062)
     Mercury Orbit         04 Mar 2011 (063)  17 Mar 2012 (077)
     Mercury Orbit Year 2  18 Mar 2012 (078)  17 Mar 2013 (076)
     Mercury Orbit Year 3  18 Mar 2013 (077)  17 Mar 2014 (076)
     Mercury Orbit Year 4  18 Mar 2014 (077)  17 Mar 2015 (076)
     Mercury Orbit Year 5  18 Mar 2015 (077)  30 APR 2015 (120)

     Operational periods of the MLA dictated by orbital geometry were:

     Start time  (DOY)  End time (DOY)     Purpose
     -----------------  -----------------  ----------------------
     2004-232T17:09:06  2004-233T20:09:43  Checkout
     2005-129T15:10:53  2005-154T23:32:00  Earth ranging
     2006-249T13:40:54  2006-250T12:26:30  Venus scan
     2007-073T00:20:55  2007-073T23:15:02  FSW upload
     2007-146T00:00:53  2007-158T06:51:11  Venus flyby
     2007-168T06:05:47  2007-176T02:16:46  Earth ranging
     2008-012T12:00:54  2008-023T12:40:11  Mercury Flyby 1
     2008-168T19:26:08  2008-189T18:39:50  Cruise test
     2008-269T23:01:54  2008-290T11:42:57  Mercury Flyby 2
     2009-259T23:01:00  2009-283T22:50:00  Mercury Flyby 3
     Mercury Orbit:
     2011-088T02:04:05  2011-144T10:37:39  Mercury Orbit cycle 1
     2011-158T00:05:12  2011-231T21:16:56  Mercury Orbit cycle 2
     2011-246T20:47:32  2011-319T10:22:37  Mercury Orbit cycle 3
     2011-335T21:48:47  2012-042T16:46:29  Mercury Orbit cycle 4
     2012-058T21:19:24  2012-107T07:38:15  Mercury Orbit cycle 5
     2012-114T22:46:30  2012-132T15:20:25  Mercury Orbit cycle 6
     2012-146T06:58:01  2012-225T23:38:29  Mercury Orbit cycle 7
     2012-233T15:15:23  2012-313T23:56:04  Mercury Orbit cycle 8
     2012-321T15:34:12  2012-334T23:44:55  Mercury Orbit cycle 9
     2012-342T15:43:05  2013-057T00:18:54  Mercury Orbit cycle 10
     2013-064T16:05:05  2013-144T00:30:38  Mercury Orbit cycle 11
     2013-152T16:29:53  2013-219T17:08:14  Mercury Orbit cycle 12
     2013-243T00:57:35  2013-317T17:30:33  Mercury Orbit cycle 13
     2013-330T17:34:15  2014-040T18:00:22  Mercury Orbit cycle 14
     2014-058T01:58:46  2014-128T10:37:06  Mercury Orbit cycle 15
     2014-146T02:37:45  - continuous operation through year 5 in the
     low-altitude campaign except for OCM-10 on September 12, 2014.

     During the lowest altitude phases, when the altimeter range was as low
     as 25 km distance at periapse, ranges from altitudes lower than the
     design range of 200 km were obtained successfully but with some unusual
     responses. A ghost of topography appeared in some of the profiles below
     50 km, at varying altitudes. It is believed that the laser emits weak
     pulses 1-3 microseconds prior to the main pulse, that produce surface
     returns at close range but are not strong enough to trigger the start
     pulse detector electronics. Some indication that this occurs was seen
     during ground testing but it was not documented at that time and it
     is still unclear whether this is the cause of ghosts. In any case the
     main altimetric pulse provides ranges at the expected altitude.

     At very low altitudes, signal strength causes pulse widths measured at
     the three applicable electronic thresholds to exceed values for which
     a symmetric pulse waveform may be assumed. To avoid excessive range
     walk, i.e. biases due to variations in signal strength, the ranges are
     calculated assuming that the leading-edge receive time is reliable and
     that the pulse centroid occurs 15 ns following the leading edge trigger.
     Equivalently, the pulse width is limited to 30 ns when calculating the
     centroid (midpoint of leading and trailing edges), while the measured
     width on channel 1 is 90 ns or greater.

     Unlike passive remote-sensing instruments, an altimeter is limited to
     observations within range of a visible reflective surface.
     Opportunities for such observations did not occur until the first
     Mercury flyby on 14 January 2008 at a velocity of approximately 7 km/s,
     at which time the first-ever observations of the equatorial region of
     Mercury were obtained along a single, sparsely-sampled profile. Noise
     returns may outnumber ground signal at the limits of instrument range,
     especially at high emission angles. Since the data are essentially
     single independent observations, dropouts or corruption of individual
     packets will not have a significant scientific impact. No such gaps
     have been detected.

     On 2012-04-16 (day 107) a transition to an 8-hour orbit was
     accomplished, following which MLA was scheduled to range to Mercury
     three times per day when constraints permit. Ranging was performed from
     23 May up to 11 May 2012 when it was paused for power reasons during
     eclipse, and resumed 25 May 2012. A fault protection rule was
     implemented that prevents ranging when the instrument housing
     temperature exceeds 30C, to extend the longevity of the instrument, and
     power cycling was implemented during the hottest portion of the orbital
     cycle to further mitigate the higher average temperatures experienced
     during the 8-hour orbit. Altimetric analysis tools are being used to
     refine the pre-launch and in-flight calibrations. All of the instrument
     data obtained to date are of satisfactory quality. MLA instrument
     performance fully meets expectations for the mission science
     objectives. Quality issues during the orbital phase are addressed in the
     final release of Reduced Data Products.

     The EDR files listed below were corrupted for several minutes due to a
     lack of detection and timing of the laser start pulse, the origin for
     MLA time-of-flight measurements.  As a result, the start pulse time
     defaulted to the diode pump switchout time.  This was later than that
     of the pulses themselves, which were too weak to trigger the start
     pulse detector.

     The affected data have been edited as noise triggers in the RDR CHANID

     In the file below the laser amplifier never reached operating
     temperature owing to a late instrument turnon.

     Other CDRs with no associated RDRs include:

     MLASCICDR1206061513.TAB  MLASCICDR1207130719.TAB  MLASCICDR1208242348.TAB

     In mid-July 2012, it was noticed that the data were corrupted
     for several minutes due to a lack of detection and timing of the laser
     start pulse, the time origin for MLA time-of-flight measurements.
     As a result, the start pulse time defaulted to the previous time
     detected.  A command macro to lower the start pulse detection
     threshold from 15 to 14 counts was requested on July 30, 2012,
     as provided for in the instrument design and concept of operations.

     On August 3, 2012, a command at the end of DPU power-on macro 22 was
     uploaded to load the new threshold value into a FSW table. On
     initialization the science algorithm loads this value into a DAC. This
     table value does not persist between power cycles so it must be added
     to the macro for MLA power-on.

     As noted in the Operational period list of the Data Coverage and
     Quality section, the instrument has had periods during which data have
     not been acquired.  Non-operational periods are due to factors
     including off-nadir passes to accommodate data collection by other
     onboard instruments, as well as instances in which the instrument is
     powered off by command due to environmental concerns.  The
     non-operational periods are as follows:

     Start time         End time
     -----------------  -----------------
     2004-233T20:09:43  2005-129T15:10:53
     2005-154T23:32:00  2006-249T13:40:54
     2006-250T12:26:30  2007-073T00:20:55
     2007-073T23:15:02  2007-146T00:00:53
     2007-158T06:51:11  2007-168T06:05:47
     2007-176T02:16:46  2008-012T12:00:54
     2008-023T12:40:11  2008-168T19:26:08
     2008-189T18:39:50  2008-269T23:01:54
     2008-290T11:42:57  2009-259T23:01:00
     2009-283T22:50:00  2011-088T02:05:11
     2011-144T10:40:00  2011-158T00:05:56
     2011-231T21:16:56  2011-246T20:48:47
     2011-319T10:22:37  2011-335T21:49:10
     2012-043T16:46:29  2012-058T21:20:10
     2012-107T07:38:15  2012-114T22:46:30
     2012-132T15:20:24  2012-146T06:58:01
     2012-225T23:38:28  2012-233T15:15:23
     2012-273T16:01:36  2012-275T05:47:06
     2012-281T23:29:26  2012-283T15:13:38
     2012-289T16:00:13  2012-291T00:29:26
     2012-313T23:56:04  2012-321T15:34:12
     2012-334T00:02:57  2012-342T15:34:04
     2012-347T16:11:43  2012-349T07:14:04
     2012-361T16:13:18  2012-363T04:44:15
     2013-056T00:29:58  2013-064T15:47:49
     2013-069T16:33:43  2013-071T07:35:10
     2013-144T00:30:38  2013-152T16:29:53
     2013-229T17:08:14  2013-243T00:57:52
     2013-317T17:30:33  2013-330T17:34:15
     2014-040T18:00:22  2014-058T01:58:46
     2014-128T10:37:06  2014-146T02:37:45
     2014-255T04:09:16  2014-257T04:02:40 for OCM-10.

     Starting with Mercury Orbit cycle 5, laser performance began to show
     significant degradation due to the rapidly changing thermal environment
     as the spacecraft periapse longitude approaches the Mercury hot pole.
     During this season, the Radio Transmitter system requires protection as
     well, resulting in several days when operation ceases entirely. At
     the peak of the hot pole season, when operating outside of its optimal
     thermal range, the laser sometimes fails to produce a pulse within 255
     microseconds of optical pumping, at which point an internal protection
     timer switches off the pump diode current, the switchout limit.

     The lack of laser fires causes gaps in the RDR time series. Such gaps
     extend from one pulse to several minutes in duration. Thermal
     management of the instrument environment has mitigated this problem
     somewhat, but gaps recur at each hot pole season, typically for a few
     days at the beginning and end of each cycle.

     On September 11, 2013, at the request of the MLA team, the detection
     threshold was again lowered by one count to 13 counts via the DPU
     power-on macro because of further decline in laser output and missing
     start pulses. A final reduction to 7 occurred on September 19, 2014.

     The switchout limit is intermittently exceeded during science passes,
     when the laser amplifier temperature is below 10 deg. C, and toward the
     end of some passes over the day side due to excessive heating. The
     output is becoming less predictable as time progresses, and thermal
     excursions are more common in the 8-hour orbit of the extended mission,
     in which case the laser ceases to fire and the TX_ENERGY data value is

     In the current state of operation, the laser may also fire each shot
     but the start pulse time may not be recorded correctly owing to a lack
     of start pulse trigger. Ranges may still be returned from the surface.

     The symptom of this anomaly is that the TX_ENERGY is within normal
     limits, but the STARTPLS_TIME and STARTPLS_WIDTH, normally varying from
     shot to shot, are not updated. The RMU does not clear these values before
     each shot, so the previous values of both time and width are repeated.
     The lack of a start trigger is detected by the RMU and passed
     to the FSW, but owing to a software error it is not being recorded and
     downlinked correctly. The cause for this is under investigation.

     Excerpts from an email December 23, 2013: 'I did find where we used to
     set the bit in the science packet to indicate a problem. I don't know why
     it was taken out.' Response: '... what you found was conclusive that the
     flag for the start pulse detection was not passed on to the telemetry.
     We did not have to use it since the laser never missed a beat during
     ground testing and we always intended to keep it that way. However, the
     problem has become more serious after MLA passed its designed lifetime.'

     One hypothesis is that the laser undergoes Amplified Spontaneous
     Emission (ASE) if the switchout limit is exceeded. In this case the
     energy is recorded but the peak energy is too low to be detected by the
     timing hardware.

     The STARTPLS_INVALID telemetry point, defined as
       '=0 all start pulses for the second were valid.
        =1 at least one start pulse during the second was invalid.'
     does not respond to the case when neither the leading nor the trailing
     edge of the laser pulse is detected. With this in mind, the MLASCICDR
     flags repeated values by setting the STARTPLS_WIDTH value to 99.9. In
     this case the time of flight data, derived from a difference between
     the start pulse time and a return pulse time, may be invalid.

     As a workaround, the MLASCICDR ground data processing was modified
     to flag repeated timing values by setting the STARTPLS_WIDTH to 99.9 ns
     and to assume that the laser fire occurred within 30 ns of the previous
     recorded time. The time of flight data, derived from the difference of
     the start pulse and return pulse times, may be useful in spite of the
     uncertain origin and drifts only slightly from the true value, but the
     accuracy is typically no better than about 200 ns or 30 m in range.
     The following field is recorded in the calibrated data product:

      NAME             = STARTPLS_WIDTH
      DESCRIPTION      = 'Width of transmit laser pulse in nanoseconds, used
        to determine centroid time of outgoing pulse. A value of 99.9
        denotes a measurement whose pulse width is invalid and for which
        the precision of any associated ground return is degraded.'

     The field is also added to the Reduced Data Record, to indicate shots
     for which the range calculation is uncertain.

     In the first year of operation, repeated start times occurred a few
     times per orbit and did not affect data quality. In July 2012, repeats
     became more frequent, indicating the need to lower the start detection
     threshold as described above. By the third year of operation in orbit
     (March 2013) the laser output energy had further declined, resulting
     in bursts of several seconds where the start pulse time was repeated,
     but with sufficient energy to produce ground returns. Further
     reductions in transmit threshold on September 11, 2013 and September 19,
     2014 did not completely eliminate start pulse repeats. Returns are
     obtained from the planet but such ranges should be interpreted with
     caution as the start time and therefore range is uncertain. In the
     current release all CDRs have been regenerated to modify STARTPLS_WIDTH.
     The validity of such ranges as ground returns is determined in the RDR
     on an individual basis.

     The RDR data product entails classification, or editing, of
     the Science data to distinguish noise from ground returns. The accuracy
     of the data relies on the quality of the Precision Orbit Determination
     procedures employed, as well as internal crossover analysis and
     correlation with other datasets, and is expected to improve as the
     data are reprocessed with more accurate geometry.

     Further refinement and resampling of the RDR product produces the
     Gridded Data Record (GDR) data products. Note that during the low-
     altitude periapse periods, the detector gain settings were adjusted
     dynamically to mitigate saturation effects. One of these effects was the
     appearance of cloud-like returns above the surface, mimicking the
     topography. These artifacts are the result of pre-lasing prior to the
     timed laser fire of the main pulse, resulting in an apparent higher-than-
     normal topographic surface, and are manually excluded in the RDR
     processing pipeline.

     In the MLA Calibration document SUN&NEUMANN2014, the last
     coefficient in Equation 14 is in error. The correct equation should read
     Etx(T)= 38.873 - 5.3897*T + 0.49348*T^2 + 0.017589*T^3 + 0.00020891*T^4
     This error does not affect any of the calibrations applied to the MLA
CITATION_DESCRIPTION G. A. Neumann (GSFC), MESSENGER E/V/H MLA 3/4 CDR/RDR DATA V1.0, NASA Planetary Data System, 2010.
ABSTRACT_TEXT Abstract ======== This data set consists of the MESSENGER Mercury Laser Altimeter (MLA) Reduced Data Record (RDR) products. The MLA is a solid-state pulsed laser that measures the distance between the spacecraft and the surface of Mercury. The RDR products contain the calibrated, geolocated range data as profile measurements of the planetary radius.
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