| 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.
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