PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "2009-11-11 MESS:Neumann/Reid, initial version; 2011-06-07 MESS:Neumann, added text for release 6; 2012-03-15 MESS:Neumann, added text for release 7; 2012-06-15 MESS:Neumann, added text for release 8; 2012-06-15 MESS:Reid, formatting; 2012-07-11 GEO:Ward, formatting; 2012-10-01 MESS:Neumann/Reid, revisions from the RDR peer review; 2012-10-18 MESS:Jha, PDS lien response; 2012-12-20 MESS:Jha, updates for release 9; 2013-01-11 MESS:Jha, Dataset Overview revised; 2013-02-04 GEO:Ward, formatting; 2013-07-08 MESS:K. Jha, GEO:Ward, updated non-operational/operational period tables, added text to Data Coverage and Quality; 2013-12-28 MESS:Reid, updates for release 11. 2014-01-28 MESS:K. Jha, updated Confidence Level Overview; 2014-07-03 MESS:G. Neumann, added text to Data Coverage and Quality, updated non-operational periods. 2014-12-15 MESS:G. Neumann, minor edits and updated non-operational periods. 2015-02-12 MESS:G. Neumann/K. Jha,/M.Reid updates for RDR and RADR delivery. " RECORD_TYPE = STREAM OBJECT = DATA_SET DATA_SET_ID = "MESS-E/V/H-MLA-3/4-CDR/RDR-DATA-V1.0" OBJECT = DATA_SET_INFORMATION DATA_SET_NAME = "MESSENGER E/V/H MLA 3/4 CDR/RDR DATA V1.0" DATA_SET_COLLECTION_MEMBER_FLG = "N" DATA_OBJECT_TYPE = TABLE START_TIME = 2004-257T00:00:00 STOP_TIME = "NULL" DATA_SET_RELEASE_DATE = 2015-03-06 ARCHIVE_STATUS = "ARCHIVED - ACCUMULATING" PRODUCER_FULL_NAME = "GREGORY A. NEUMANN" DETAILED_CATALOG_FLAG = "N" CITATION_DESC = "G. A. Neumann (GSFC), MESSENGER E/V/H MLA 3/4 CDR/RDR DATA V1.0, NASA Planetary Data System, 2010." DATA_SET_TERSE_DESC = "The MESSENGER MLA calibrated and reduced 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." ABSTRACT_DESC = " Abstract ======== This data set consists of the MESSENGER Mercury Laser Altimeter (MLA) Calibrated Data Record (CDR) and 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 CDR products contain the science and auxiliary data products calibrated to physical units. The RDR products contain the calibrated, geolocated range data as profile measurements of the planetary radius." DATA_SET_DESC = " Data Set Overview ================= The data set consists of Calibrated Data Records (CDR), Reduced Data Records (RDR) (profile data), and Radiometric Active Data Records (RADR). The CDRs are calibrated science, status, and hardware diagnostics data. The RDRs contain time-of-flight profile data and the RADRs are radiometric data. The RDRs and RADRs 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 three MLA CDR products correspond to the three EDR data products delivered in the accompanying EDR data set. Each MLA CDR data product consists of two files. One contains the data itself, and is arranged in a PDS compliant ASCII table file. The other is a PDS label file that describes the content of the table file. The MLA RDR and RADR products correspond to the Science CDR products. The RADR is a profile data product that gives active radiometric measurements of the Mercury surface using the laser as a source and is produced with the same geometric information used in the RDR, but contains only those pulses f or which radiometry can be measured. The reflectance at 1064 nm measures surface reflectivity in permanently-shadowed regions as well as sunlit regions. During the Mercury-Orbit mission phase, a single data file contains the observations obtained in one orbit of the spacecraft around Mercury. Prior to the Mercury-Orbit mission phase, a single data file aggregated 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 included. 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 ==================== This data set contains calibrated and reduced sensor measurements. The raw (EDR) products are provided in a separate accompanying data set. The final geolocated altimetry product provides geolocated measurements of the Mercury radius in a planetocentric coordinate system as described in the MLA CDR/RDR Software Interface Specification (MLA CDR/RDR SIS). Parameters ========== 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. Data ==== 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." CONFIDENCE_LEVEL_NOTE = " Confidence Level Overview ========================= This CDR/RDR data release covers the whole of MLA data from the initial TEST data set through the altimetry science measurements of a solid body (Mercury). The laser altimeter range is limited to distances less than 1800 km. Neither the Earth nor Venus were suitable targets during cruise. Earlier data were primarily used 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 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 distribution. Calibration Observations ========================= 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. These offsets are not generated when operating in the Science mode. 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, 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 constants. During the week of June 17-24, 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 valid 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 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 DATA_TYPE = ASCII_INTEGER 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 MISSING_CONSTANT = 99.9 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.' Review ====== 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: 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 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 will be 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. MLASCI1206030658.DAT MLASCI1206051512.DAT MLASCI1206061513.DAT MLASCI1206062302.DAT MLASCI1206070702.DAT MLASCI1206080703.DAT MLASCI1206111505.DAT MLASCI1206112306.DAT MLASCI1207130719.DAT MLASCI1207160711.DAT MLASCI1207231506.DAT MLASCI1207252307.DAT MLASCI1208231543.DAT MLASCI1208222342.DAT MLASCI1208242348.DAT The affected data have been edited as noise triggers in the RDR CHANID column. In the file below the laser amplifier never reached operating temperature owing to a late instrument turnon. MLASCICDR1207160711.TAB Other CDRs with no associated RDRs include: MLASCICDR1206061513.TAB MLASCICDR1207130719.TAB MLASCICDR1208242348.TAB There are fewer RADR files than there are RDR files as in some passes, no suitable pairs of channel 1 low/high threshold return pairs were obtained. As well, RADR records whose height values differ by more than 0.5 km from polar and cylindrical digital elevation models are removed as noise. For these reasons, the following RADR files that might otherwise be expected are not included in the data delivery: MLARADR1206030658.TAB MLARADR1206051512.TAB MLARADR1206070702.TAB MLARADR1206080703.TAB MLARADR1206100704.TAB MLARADR1206112306.TAB MLARADR1208241546.TAB MLARADR1308080043.TAB This 'filtering' for noise has been performed on all RADRs from the start of the mission and all previously published RADRs are replaced in this delivery. 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 zero. 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 has been 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 MISSING_CONSTANT = 99.9 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. Limitations =========== The first Mercury flyby used the spacecraft coarse oscillator, whose accuracy is estimated to be within a few parts per million of nominal rate. Range measurement accuracy is therefore uncertain by 2-10 meters, depending on range. Subsequent observations use the more precise USO time reference, accurate to 10 parts per billion or better. OCX-B is now selected by instrument command. A limitation of this data set is that it is preliminarily calibrated 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. Calibration of the range and housekeeping data are performed in a subsequent processing step to produce the Calibrated Data Record (CDR) data products in scientific units. A final step applies the spacecraft and planetary position and geometry data to the calibrated ranges, to produce geodetic profiles and ancillary data. The Reduced Data Record (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." END_OBJECT = DATA_SET_INFORMATION OBJECT = DATA_SET_MISSION MISSION_NAME = "MESSENGER" END_OBJECT = DATA_SET_MISSION OBJECT = DATA_SET_TARGET TARGET_NAME = {"CALIBRATION", "EARTH", "VENUS", "MERCURY"} END_OBJECT = DATA_SET_TARGET OBJECT = DATA_SET_HOST INSTRUMENT_HOST_ID = MESS INSTRUMENT_ID = "MLA" END_OBJECT = DATA_SET_HOST OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "CAVANAUGHETAL2007" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "SUN&NEUMANN2014" END_OBJECT = DATA_SET_REFERENCE_INFORMATION END_OBJECT = DATA_SET END