Data Set Information
DATA_SET_TERSE_DESCRIPTION The MESSENGER MLA raw observations consist of laser ranges and instrument data collected by the MLA instrument during fly-by and orbital operations of Mercury. Also included are observations of Earth and Venus for calibration purposes.
Data Set Overview
     The data set consists of uncalibrated observations, also known as EDRs.
     The MLA EDR products are grouped together into one data set.  Within
     that data set, there are three EDR data products.  Each MLA 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, etc.  The label file also describes the different fields
     within the table.
     During the Mercury Orbit mission phase, a single data file will contain
     the observations obtained in one orbit of the spacecraft around
     Mercury.  Prior to the Mercury Orbit mission phase, a single data file
     will aggregate the observations such that all data within the file are
     taken during the same year, month, day, and hour, an efficient way to
     archive data resulting from instrument commands that would turn the
     instrument on, generate data for upwards of several hours, and then
     turn the instrument off.
     In addition to the science data, associated instrument parameters are
   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 will allow 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]
     for full details.
   Calibration Overview
     The EDR data set is NOT calibrated; it only provides the uncalibrated
     sensor measurements.
     The principal parameters when observing with the MLA are as
     * 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 otherwise be overridden by the Science
     mode algorithms. The mla_hw_diagnostic_tlm_enable,'LITE' command may
     be used in Standby mode to provide full-rate uncompressed telemetry.
     * 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 Science 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 EDR data products are described as follows:
     Science (RAW) EDR contains the ranging information as collected by the
     instrument in Science mode.  Designated as raw science data as none of
     the values have been calibrated or converted into engineering units.
     Status (STATUS) EDR contains the instrument status information, such as
     voltages, temperatures, and timing parameters.  Note that the
     measurements such as voltage and temperature values are stored in the
     EDR as the original telemetry counts sampled at multi-second intervals.
     Hardware Diagnostic Lite (HAD) EDR contains diagnostic information
     about the instrument and background brightness information from the
     detector at 8-Hz resolution.  It is designated as Lite because the EDR
     is obtained from the lite version of the hardware diagnostic telemetry
     packet.  This differentiates it from the full version of the packet,
     which contains more diagnostic fields, but will not be utilized during
     the course of the mission due to bandwidth limitations.  As with the
     other MLA EDRs, all values are the original telemetry counts. Each 1-s
     Science EDR is broken into 8-Hz records, one for each laser shot.
DATA_SET_RELEASE_DATE 2015-03-06T00:00:00.000Z
START_TIME 2004-09-13T12:00:00.000Z
STOP_TIME N/A (ongoing)
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 EDR release extends the previous TEST data set with the first
     altimetry science measurements of a solid body (Mercury). The laser
     altimeter range is limited to distances less than 1800 km, and neither
     the Earth nor Venus were suitable targets during cruise. Earlier data
     were primarily for use in calibration and monitoring of performance.
     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
     will entail additional error sources related to terrain effects acting
     on a finite-sized laser footprint, but will approach 20 cm precision
     under optimal conditions for triggers on Channel 1.
     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 is known to better
     than a few tens of meters during cruise, and even better while in
     orbit. The main source of uncertainty in targeted observations 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 will confirm the current system attitude
     knowledge, which at present is known to be repeatable to within 50
     microradians from day to day.
   Calibration Observations During Cruise
     On May 27 and 31, 2005, two-way detection of laser pulses was achieved
     at a distance of 24 million km between MESSENGER and Earth. In the
     weeks prior to detection, passive scans of Earth were conducted to
     refine the MLA pointing with respect to the spacecraft reference
     frame. The two-way detection was the first successful end-to-end test
     of the MLA hardware in space.
     A total of 40 MLA downlink pulses were detected at the NASA Goddard
     Geophysical and Astronomical Observatory (GGAO), and 90 uplink
     observations were obtained during observing sessions on 27 and 31 May
     2005. The uplinks were relayed to Earth in the hardware diagnostic
     packets, along with the laser transmit timing. Ranging analysis
     established that these uplinks corresponded to the times of fire of a
     16-mJ laser operating at 240 Hz at GGAO. Although tens of thousands of
     noise triggers were also received, a dozen or more uplink triggers were
     obtained within a 10-second interval on May 27. No clear uplinks were
     seen on May 31. The uplinks on May 27 showed several cases where the
     MLA coarse clock counter recorded the 200-ns clock edge following the
     trigger. After correcting for the 200-ns offset, these triggers match
     the predicted time of arrival of ground pulses.
     During the second Venus flyby, 5 June 2007, the hardware and flight
     software were exercised to produce the first targeted science
     observations of the cloudy atmosphere of Venus. The performance of the
     instrument and flight software were nominal, but no returns from the
     surface were seen, owing to the strong absorption of 1064-nm light by
     the CO2 atmosphere. Although many detector triggers occurred while the
     laser beam was directed at Venus, and possibly significantly greater
     numbers of triggers at altitudes where previous experiments had
     inferred H2SO4 droplets, a clearly-resolved layering of clouds was not
     seen in the data. While laser altimeters can be designed for
     atmospheric studies, the Venus clouds were probably too diffuse for the
     relatively short MLA laser pulses and detector subsystem time
     During the week of 17-24 June 2007, several attempts were made to repeat
     the two-way ranging experiment at a distance greater than 100 million
     kilometers. All instrument data were acquired as planned and there were
     no anomalies. Passive detection of earthshine verified the pointing of
     the MLA instrument, but active detection of a 48-Hz, 250-mJ pulsed
     laser at GGAO was not achieved. Detection of the MLA laser on the
     ground using a photon-counting detector could not be confirmed.
     Alignment problems related to the relatively large velocity aberrations
     for interplanetary trajectories together with problems in the ground
     telescope control systems and poor visibility during part of the week
     hampered this effort.
   Mercury Flyby 1 Observations
     The MLA was turned on two days in advance of the flyby so as to warm up
     to operating temperature and configure instrument parameters. Using a
     stored command to enter Standby Mode, the instrument range measurement
     unit was powered on 45 minutes prior to the flyby closest approach
     (CA). At 2 minutes and 40 seconds prior to CA, MLA entered Science Mode
     and the laser commenced firing 43 seconds later as diode current
     reached operational level. Science data were collected until 9 minutes
     after CA. Altimetric measurements commenced at a range of 600 km and a
     laser incidence/emission angle of 71 degrees. Pointing of the
     spacecraft to nadir was achieved well after CA, by which time ranges
     were increasing above 1000 km. MLA demonstrated ability to range with
     more than 50% probability of detection when operating at nadir below
     1200 km, and usable ranges were acquired at more than 1600 km. The
     precision of measurement is greatest at nadir, where the least
     spreading of the laser footprint and reflected pulse occurs. A pair of
     threshold measurements are made independently for such pulses, which
     allows the estimation of pulse energy. Such paired returns were
     obtained out to 1400 km, after which the 1/R^2 decline in signal
     strength prevented triggers at the higher threshold. A total of 5537
     altimetric ranges were obtained during the flyby.
     Nine days following the flyby, passive scan observations of the
     half-moon illuminated shape of Mercury were performed, as well as a
     dark noise-vs- threshold test. The instrument was then commanded off.
     These observations served to improve the calibration of the detector's
     response to Mercury surface conditions and verified the
     spacecraft-instrument alignment.
   Mercury Flyby 2 Observations
     Several months prior to the flyby, a sequence was tested that commanded
     the MLA to use the MP-B clock signal for its range measurement
     hardware. This successful test corrected the previous use of the coarse
     oscillator signal during flyby 1 and ensured accurate ranging and
     timing. Otherwise, Flyby M2 operations were identical to those of Flyby
     M1. The MLA ranged to the surface successfully for nearly 12 minutes,
     during which period 4388 successful ranges were taken, more than half
     of which triggered on more than one channel. Instrument health and
     sensitivity has been unchanged since launch. Passive scan observations
     of the half-moon illuminated shape of Mercury were performed after
     Flyby 2, with nearly identical results.
   Mercury Flyby 3 Observations
     Mercury Flyby 3 was aborted 21 seconds prior to MLA entering Science
     Mode. While the spacecraft was quickly recovered and the primary goal
     of the flyby was achieved, placing MESSENGER in position for its final
     encounter, the laser did not fire, and no science data were obtained.
     Prior to that time, and during the passive scan that followed a few
     days later, the operation of the instrument was nominal, and the
     alignment of the detector field of view remains close to that of the
     earlier flybys.
   Mercury Orbital Flight tests
     MLA was operated for several days in August 2010 in Science Mode as an
     orbital simulation by the Project, firing into space. No ranges were
     collected. In February 2010, attempts to communicate by firing the MLA
     laser at the 1-m GLAS instrument on the ICESAT mission were
     unsuccessful, owing to difficulties in pointing the GLAS boresight
     toward MLA using the spacecraft inertial reference system. A subsequent
     test was canceled due to more urgent orbital flight test preparations.
   Mercury Orbit Cycle 1
     Spacecraft constraints naturally divide MLA observations into periods
     of operation, or cycles, of approximately one Mercury year or 88 Earth
     days. As the MESSENGER orbital plane is inertially fixed in space, the
     orbit plane rotates with respect to the direction of the Sun. The
     spacecraft maintains its +Y (sunshade) axis within a few degrees of
     the Sun at all times (the Solar Keep-In constraint). Thus the +Z (nadir)
     instrument deck may only maintain nadir attitude during the dawn-dusk
     orbital phase, and may only point nadir during the highest latitude
     portion of the noon-midnight orbital phase. The SKI constraint forces
     a cycle of offnadir attitude and poor equatorial coverage to nadir
     attitude at low latitudes back to offnadir ranging, limiting
     equatorial coverage. When crossing the perihermian sunlit face or
     hot pole of Mercury at closest approach, temperatures spike and
     operation ceases due to fault protection, ending a three-month cycle.
     Orbital altimetric ranging began on March 29, 2011, until eclipse
     conditions precluded instrument operations on May 24. A total of 113
     ranging orbits comprise the first cycle. No data were lost due to
     instrument anomalies, however, much of the time, ranging to
     the planet at large emission angles reduced the probability of
     detection of ground returns substantially. Where conditions are optimal
     (dawn-dusk orbit, no targeting slews), ranges were obtained nearly to
     the 1800-km hardware limit, with a probability of return dependent on
     distance and incidence angle to the surface. Profile data over steep
     features such as craters will have better or worse coverage depending
     on the local slope of the target.
   Mercury Orbit Cycle 2
     Operations resumed on 2011-06-07 and continued to 2011-08-19.
     Because of spacecraft thermal issues as well as the eclipse, MLA
     operations were suspended for two weeks during which nadir attitude
     could not be sustained. At the end of this period the MLA instrument
     deck temperature reached 44.7 degC, only 0.3 degC below the Fault
     Protection threshold. MLA was not at risk as it was not actively
     ranging, but the RF Phased Array temperatures also precluded nadir
     pointing. A total of 147 orbits produced useful ranges, and
     performance was nominally the same as during the previous cycle.
     During the next Orbit Cycle however the laser pump diode switchout
     time, a measure of the time required to produce a laser pulse,
     started to increase. It is believed that extreme temperatures may
     have led to contamination in the laser path. The laser energy output
     measurement did not show an immediate effect but the average pulse
     energy return as a function of distance declined somewhat.
   Mercury Orbit Cycle 3
     Operations resumed on 2011-09-03 and continued through 2011-11-15,
     for a total of 149 ranging orbits, completing the first Mercury year of
     spacecraft orbital operation. PDS delivery 7 contained 67 orbits.
     During this cycle the orbital periapse passes continued to maintain
     nadir attitude, within constraints, with the exception of a few
     high-priority targeted requests by other teams. The nadir attitude and
     orbital inclination limited polar ground track coverage to latitudes
     less than approximately 83.5 degrees N. Offnadir slews toward the pole
     commenced on Sept. 29, 2011, soon reaching to 88 degrees N. Latitude.
   Mercury Orbit Cycle 4
     Operations resumed on 2011-12-11 and continued through 2012-02-11,
     with the orbital height and periapse latitude rising so as to preclude
     any further observations of the southern hemisphere.
   Mercury Orbit Cycle 5
     Operations resumed on 2012-02-27 and continued through 2012-04-16,
     ending the Primary Mission, and extending observations of the northern
     smooth plains into the large Prokofiev and Kandinsky craters. Reflective
     anomalies identified as surficial water ice were discovered in areas
     of permanent shadow by the active radiometric measurement of MLA
     [NEUMANNETAL2013], corroborated by Neutron Spectrometer data.
   Mercury Orbit Cycles 6 onward
     Operations resumed on 2012-04-23 after the orbit was lowered to an
     eight-hour period, resulting in slightly more terrain coming within
     operating range and more frequent observations overall. However the
     periapse altitude and the argument of periapsis increased slowly,
     restricting the latitude of coverage to regions northward of the
     tropics. Heating of the spacecraft in the more frequent crossings of
     the Mercury hot poles precluded some observations. Thermal degradation
     of the MLA laser continued and some adjustments of operation were
     required, such as powering off completely to maximize the cooling of
     the instrument through passive radiation. Thermal extremes, together
     with the declining laser health, caused intermittent laser firing.
     An issue with laser fire time data is discussed below under Events.
     Weak and intermittent laser output degrades ranging accuracy somewhat.
     By the end of Cycle 15, the orbital latitude and altitude at periapse
     decreased owing to solar tidal perturbations. By the end of Mercury
     Orbit Year 3, some altimetric ranging commenced at latitudes of 10
     degrees North. As of June 2014, the minimum spacecraft altitude was
     between 115 and 155 km, controlled by propulsive maneuvers, allowing
     ever-closer observations of the surface and improving MLA link.
     Earth ranging experiments were conducted on the last days of January
     2014 in an attempt to detect pulses at a range approaching 1 AU, and
     provide a further calibration of the MLA boresight. The results of
     this experiment were too inconclusive to report at this time.
     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 minimally processed data received from the
     spacecraft during the mission phases: Launch, Earth Cruise,
     Venus 1 Cruise, Venus 2 Flyby, Mercury 1 Cruise, Mercury 1 Flyby,
     Mercury 2 Cruise, Mercury 2 Flyby, Mercury 3 Cruise, Mercury 3 Flyby,
     Mercury 4 Cruise, Mercury Orbit, Mercury Orbit Year 2,
     Mercury Orbit Year 3, and Mercury Orbit Year 4. 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)
     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 4 in
     low-altitude campaign.
   Significant Operational Events:
     During Mercury 4 Cruise, propellant tank heating was assisted by
     powering the instrument on in keep-alive mode. During Mercury_Orbit
     cycle 1 (first Mercury year) the instrument was powered continuously
     until the first Mercury eclipse period. During this time the most
     temperature-sensitive components, the laser oscillator and laser
     amplifier, remained within operational limits. Fault protection autonomy
     rule 243 powered the MLA off as the external main body sensor reached
     40 C, well after the laser had ceased operation. A change request to
     increase the high limit to 45 C was approved following the first
     perihelion hot pole phase. All other sensors and power monitors
     including the laser remained nominal and trending showed no decline in
     instrument performance. 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. 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) MESSENGER transitioned to an 8-hour orbit,
     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
     In June-August, thermal problems caused the MLA instrument to trigger
     fault protection rules before acquiring signal. Data in these files
     contain almost no usable altimetry or radiometry:
     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 poweron.
     During Mercury Orbit cycles 11 and 12, owing to offnadir operation
     commanded by other instruments, the following data have no usable
     ground returns:
     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 accomodate 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
     Starting in April 2014 orbit maintenance maneuvers were suspended to
     allow the spacecraft to descend naturally to lower altitudes. This
     allowed observations at altitudes lower than the design minimum of
     200 km, while the apoapse altitude remained above 10,000 km.
     Peak temperatures increased, but coverage of lower latitudes was
     obtained as the periapse latitude steadily drifted southward. In
     June, September, October, and January 2015 the altitude was raised
     by means of propulsive maneuvers to delay impact.
     The periapse altitude as low as 24 km produced unusual MLA data at
     times, with the detectors receiving nearly 100 times the signal as was
     received at the beginning of this period. Instances of ghosts from
     channel 2 returns whose pulses were substantially wider than the 60-ns
     matched filter are seen below 50 km. The ghost profiles hover 2-3 km
     above the ground while simultaneous channel 1 returns are suppressed.
     The ghost returns are not fully understood, but sufficient ground
     returns are obtained on the high threshold channel.
     The nonlinear response of detectors and electronics under saturation
     leads to greater uncertainty in derived data such as the energy return
     and normal albedo, whose resolution is best at moderate signal
   Laser Performance
     Starting with Mercury Orbit cycle 5, laser performance began to show
     significant degradation in 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.  Near
     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 the MLA internal
     protection circuits switch off the pumping diodes.
     The lack of laser fires produces gaps in the RDR time series. Such gaps
     are from one pulse to several minutes of seconds 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.
     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 laser
     output has been is 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
     zero. Or, the energy is recorded, but the pulse amplitude is too low
     to be detected by the timing hardware. It is believed that the laser
     undergoes Amplified Spontaneous Emission (ASE) when it fails to trigger.
     In this case the energy is recorded but the pulse amplitude is too low
     to be detected by the timing hardware.
     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.
     The lowered threshold reduced but did not eliminate laser misfires.
     In its degraded state of operation, the laser may fire pulses with a
     TX_ENERGY within normal limits (10-20 mJ), and appear to produce valid
     ranges, but the start pulse time is not recorded correctly.
     The symptom of this anomaly is that the STARTPLS_TIME counts and the
     STARTPLS_WIDTH counts, normally varying from shot to shot, are not
     updated by the RMU from the previous values when the start trigger is
     not detected by the RMU. The anomaly is indicated by a flag generated
     by the RMU that is passed to the FSW. In the HW_Diag_Lite packet,
     the STARTPLS_INVALID flag is recorded correctly. The FSW Science task
     was intended to summarize this infrequent event, i.e., when neither the
     leading nor the trailing edge of the laser pulse is detected, as
       'The STARTPLS_INVALID telemetry point is defined as:
        =0 all start pulses for the second were valid.
        =1 at least one start pulse during the second was invalid.'
     After reporting this anomaly it was determined by the software
     leads that the FSW Science task ignores this flag. It is unclear at
     what point in the software development cycle this error occurred.
     As a workaround, the MLASCICDR ground data processing has been modified
     to flag repeated timing values by setting the STARTPLS_WIDTH to 99.9 ns
     and 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.
     Thus 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.'
     In the first year of operation, invalid start times occurred only a few
     times per orbit and did not affect data quality. In July 2012, they
     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 many seconds where the start pulse time was repeated, but
     with sufficient energy to produce ground returns. Such ranges are
     considered more uncertain than most, possibly more than 30 m in error
     due to the >200 ns variability in start time from shot to shot. The
     lowering of transmit threshold on September 11, 2013 to 13 counts
     reduced invalid triggers below 1% but resulted in wider, more variable
     pulse widths.
     Additional periods of calibration activity occurred 29-31 January 2014,
     during which the Earth was targeted by the MLA boresight and the
     Earth's sunlit face was used to align the detector field of view.
     Hardware diagnostic packets were obtained. As well, a test of the laser
     transmit threshold was performed.
     During Mercury Orbit cycle 15, on 3 March 2014, the transmit threshold
     was lowered to a setting of 9, or about 32 mV, to mitigate further
     decline in transmit power. As a result, the outgoing pulse width
     increased from a typical 15 ns to between 20 and 30 ns, as measured at
     the threshold voltage, now approximately half the original 61 mV. The
     laser energy output, as measured on board, continued to decline and the
     intermittent loss of start pulse triggers continued to increase slowly.
     Following OCM-10, on September 19, 2014 the transmit threshold
     was lowered to a setting of 7, or 22.5 mV. This lowered the rate of loss
     of start pulse triggers to between 0 and 3%. The outgoing pulse width
     increased 5 ns on average. A test on June 24 indicated that
     this is the most sensitive possible setting and that lower thresholds
     will generate mainly noise triggers.
     Cruise data are primarily for use in calibration and understanding the
     quality of data received during Mercury orbital operations. The results
     of the first Mercury flyby used the spacecraft coarse oscillator for
     timing, whose accuracy is estimated to be within a few parts per million
     of nominal rate. Observations on Flyby 2 used the more precise time USO
     reference, correcting an instrument commanding error. A limitation of
     this data set is that it is minimally processed data. The data are
     received from the spacecraft telemetry and ingested into a database,
     whence the instrument data products are extracted and reformatted in a
     reversible fashion. 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 with time.
     Further refinement and resampling of the RDR product will produce
     the Gridded Data Record (GDR) data products.
CITATION_DESCRIPTION G. A. Neumann (GSFC), MLA raw (EDR) DATA E/V/H V1.0, NASA Planetary Data System, 2007.
ABSTRACT_TEXT Abstract ======== This data set consists of the MESSENGER Mercury Laser Altimeter (MLA) uncalibrated observations, also known as Experiment Data Records, or EDRs. The MLA is a solid-state pulsed laser that measures the distance between the spacecraft and the surface of Mercury. There are three EDR data products, MLASCIENCERAW, MLASTATUS, and MLAHDIAGNOSTIC, including the laser ranges, instrument status, and hardware diagnostic information.
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