Data Set Information
DATA_SET_NAME MESSENGER E/V/H MERCURY LASER ALTIMETER 2 EDR RAW DATA V1.0
DATA_SET_ID MESS-E/V/H-MLA-2-EDR-RAWDATA-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION 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 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 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. 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 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. Data : 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_NAME MESSENGER
MISSION_START_DATE 2004-08-03T12:00:00.000Z
MISSION_STOP_DATE 2015-04-30T12:00:00.000Z
TARGET_NAME CALIBRATION
EARTH
MERCURY
VENUS
TARGET_TYPE CALIBRATION
PLANET
INSTRUMENT_HOST_ID MESS
INSTRUMENT_NAME MERCURY LASER ALTIMETER
INSTRUMENT_ID MLA
INSTRUMENT_TYPE LASER ALTIMETER
NODE_NAME Geosciences
ARCHIVE_STATUS ARCHIVED_ACCUMULATING
CONFIDENCE_LEVEL_NOTE 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 constants. 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. 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 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 orbit. 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: MLASCI1206030658.DAT MLASCI1206051512.DAT MLASCI1206061513.DAT MLASCI1206062302.DAT MLASCI1206070702.DAT MLASCI1206080703.DAT MLASCI1206111505.DAT MLASCI1206112306.DAT MLASCI1207130719.DAT MLASCI1207160711.DAT MLASCI1208231543.DAT MLASCI1208222342.DAT MLASCI1208242348.DAT 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: MLASCI1304280822.DAT MLASCI1306060031.DAT 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 strength. 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 follows: '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 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.' 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. Limitations : 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.
PRODUCER_FULL_NAME GREGORY NEUMANN
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