| CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
This RDR data release covers the period of on-orbit altimetry science
measurements of a solid body (Mercury). The laser altimeter range is
limited to distances less than 1800 km. When enabled, the detector
continuously triggers on optical signals passing through the receiver
telescope at a roughly exponentially-increasing rate with optical power,
at any given threshold, making it useful as a 'one-pixel camera' with a
very narrow spectral bandwidth and a 400-microradian field of view. The
Range Measurement Unit operates at 8 Hz, so that scanning across the
illuminated surface of a target in a raster pattern provides a
boresight calibration. In addition, triggers may be received from
Earth-based lasers within a 14-ms subinterval of each 8-Hz cycle, so
that the time-of-flight may be measured repeatedly.
The MESSENGER spacecraft employs an ovenized, quartz-crystal-based
oscillator whose frequency is stable to a few parts per trillion over
the course of an hour. The MLA acquires its time base from the
spacecraft via a one-pulse-per-second (1PPS) tick along with the
corresponding mission elapsed time (MET) message over the data bus. The
1PPS signal uncertainty during ground testing was 0.021 ms. The 1 PPS
offset, and the offset between the MLA event time reference and the
1PPS, are very stable over short intervals of time. The latter is
monitored by the instrument at 125-ns resolution. While the spacecraft
clock can be related to the MLA timing only to tens of microseconds in
an absolute sense, over intervals of an hour or so they are precisely
coupled. When laser firing is enabled, the time of fire is recorded and
may be matched with pulses received at at an earth station. Transmit
and receive times may be correlated on the ground to measure the
effective 2-way time-of-flight and clock drift. The resolution of the
MLA timing measurement is roughly 400 ps, and the demonstrated overall
precision of an individual time-of- flight measurement between MLA and
Earth is approximately 0.65 ns (20 cm) root-mean-square, owing to
signal variations and atmospheric delays. Ranging to planetary surfaces
entails additional error sources related to terrain effects acting
on a finite-sized laser footprint, but approaches 20 cm precision
for triggers on Channel 1 under optimal conditions.
To ensure accurate altimetric measurements, the absolute time
correlation of the spacecraft clock is maintained to better than 1 ms
via radio tracking, while the spacecraft position relative to Mercury
center of mass should be known to better than a few tens of meters
while in orbit. With repeated altimetry crossing tracks, the orbit was
be further constrained, limited by the roughness of the surface at the
shot-to-shot scale. A further source of uncertainty in geolocation is
the MLA boresight vector, since geolocation multiplies the optical
range by the direction cosines of this vector with respect to an
inertial reference system. The spacecraft attitude control and
knowledge is derived from an inertial measurement unit and star
trackers, whose performance is monitored by instrument calibrations.
The MLA scans of Earth and Venus have characterized the boresight
alignment repeatedly during cruise, and on two occasions, the MLA laser
beam has been observed on Earth, providing an improved laser boresight
vector. Further tests during cruise confirmed the current system
attitude knowledge, which at present is known to be repeatable to
within 50 microradians from day to day.
The radiometric measurements are produced from the threshold crossing
times of the received pulses at two discriminator voltages on channel
one simultaneously, a low threshold for maximum sensitivity, and a
threshold roughly twice as high, to give four sample points of the
received pulse waveform. A laser pulse may result in triggers at one or
both thresholds or not at all. Ranging with low threshold detections is
possible at ranges up to 1500 km, but steady returns that cross both
the low and high thresholds are best obtained at altitudes less than
~600 km with near-nadir (<20 degree) incidence angles. When a pulse is
detected by a pair of discriminators, its energy and duration may be
inferred assuming a model waveform that accounts for the dispersion in
time of return pulses due to surface slope and/or roughness. We adopt a
simple triangular model that fits the rising and falling edges of the
trigger at each threshold. This model generates values nearly equal to
the ideal Gaussian waveform model for well-constrained pulses. The
reflectance is derived using a direct measurement of the outgoing pulse
energy and instrument calibrations from ground test data, via the lidar
link equation. The measurement of radiance factor (relative to an ideal
diffusive reflector) has an associated uncertainty of 30% for each
measurement at low altitude, with an approximately lognormal
distribution.
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: Mercury Orbit,
Mercury Orbit Year 2, Mercury Orbit Year 3, Mercury Orbit Year 4, and
Mercury Orbit Year 5. The mission phases are defined as:
Phase Name Date Start Date (DOY) End Date (DOY)
-------------------- ----------------- -----------------
Launch 03 Aug 2004 (216) 12 Sep 2004 (256)
Earth Cruise 13 Sep 2004 (257) 18 Jul 2005 (199)
Earth Flyby 19 Jul 2005 (200) 16 Aug 2005 (228)
Venus 1 Cruise 17 Aug 2005 (229) 09 Oct 2006 (282)
Venus 1 Flyby 10 Oct 2006 (283) 07 Nov 2006 (311)
Venus 2 Cruise 08 Nov 2006 (312) 22 May 2007 (142)
Venus 2 Flyby 23 May 2007 (143) 20 Jun 2007 (171)
Mercury 1 Cruise 21 Jun 2007 (172) 30 Dec 2007 (364)
Mercury 1 Flyby 31 Dec 2007 (365) 28 Jan 2008 (028)
Mercury 2 Cruise 29 Jan 2008 (029) 21 Sep 2008 (265)
Mercury 2 Flyby 22 Sep 2008 (266) 20 Oct 2008 (294)
Mercury 3 Cruise 21 Oct 2008 (295) 15 Sep 2009 (258)
Mercury 3 Flyby 16 Sep 2009 (259) 14 Oct 2009 (287)
Mercury 4 Cruise 15 Oct 2009 (288) 03 Mar 2011 (062)
Mercury Orbit 04 Mar 2011 (063) 17 Mar 2012 (077)
Mercury Orbit Year 2 18 Mar 2012 (078) 17 Mar 2013 (076)
Mercury Orbit Year 3 18 Mar 2013 (077) 17 Mar 2014 (076)
Mercury Orbit Year 4 18 Mar 2014 (077) 17 Mar 2015 (076)
Mercury Orbit Year 5 18 Mar 2015 (077) 30 APR 2015 (120)
Operational periods of the MLA dictated by orbital geometry were:
Start time (DOY) End time (DOY) Purpose
----------------- ----------------- ----------------------
2004-232T17:09:06 2004-233T20:09:43 Checkout
2005-129T15:10:53 2005-154T23:32:00 Earth ranging
2006-249T13:40:54 2006-250T12:26:30 Venus scan
2007-073T00:20:55 2007-073T23:15:02 FSW upload
2007-146T00:00:53 2007-158T06:51:11 Venus flyby
2007-168T06:05:47 2007-176T02:16:46 Earth ranging
2008-012T12:00:54 2008-023T12:40:11 Mercury Flyby 1
2008-168T19:26:08 2008-189T18:39:50 Cruise test
2008-269T23:01:54 2008-290T11:42:57 Mercury Flyby 2
2009-259T23:01:00 2009-283T22:50:00 Mercury Flyby 3
Mercury Orbit:
2011-088T02:04:05 2011-144T10:37:39 Mercury Orbit cycle 1
2011-158T00:05:12 2011-231T21:16:56 Mercury Orbit cycle 2
2011-246T20:47:32 2011-319T10:22:37 Mercury Orbit cycle 3
2011-335T21:48:47 2012-042T16:46:29 Mercury Orbit cycle 4
2012-058T21:19:24 2012-107T07:38:15 Mercury Orbit cycle 5
2012-114T22:46:30 2012-132T15:20:25 Mercury Orbit cycle 6
2012-146T06:58:01 2012-225T23:38:29 Mercury Orbit cycle 7
2012-233T15:15:23 2012-313T23:56:04 Mercury Orbit cycle 8
2012-321T15:34:12 2012-334T23:44:55 Mercury Orbit cycle 9
2012-342T15:43:05 2013-057T00:18:54 Mercury Orbit cycle 10
2013-064T16:05:05 2013-144T00:30:38 Mercury Orbit cycle 11
2013-152T16:29:53 2013-219T17:08:14 Mercury Orbit cycle 12
2013-243T00:57:35 2013-317T17:30:33 Mercury Orbit cycle 13
2013-330T17:34:15 2014-040T18:00:22 Mercury Orbit cycle 14
2014-058T01:58:46 2014-128T10:37:06 Mercury Orbit cycle 15
2014-146T02:37:45 - continuous operation through year 5 in the
low-altitude campaign except for OCM-10 on September 12, 2014.
During the lowest altitude phases, when the altimeter range was as low
as 25 km distance at periapse, ranges from altitudes lower than the
design range of 200 km were obtained successfully but with some unusual
responses. A ghost of topography appeared in some of the profiles below
50 km, at varying altitudes. It is believed that the laser emits weak
pulses 1-3 microseconds prior to the main pulse, that produce surface
returns at close range but are not strong enough to trigger the start
pulse detector electronics. Some indication that this occurs was seen
during ground testing but it was not documented at that time and it
is still unclear whether this is the cause of ghosts. In any case the
main altimetric pulse provides ranges at the expected altitude.
At very low altitudes, signal strength causes pulse widths measured at
the three applicable electronic thresholds to exceed values for which
a symmetric pulse waveform may be assumed. To avoid excessive range
walk, i.e. biases due to variations in signal strength, the ranges are
calculated assuming that the leading-edge receive time is reliable and
that the pulse centroid occurs 15 ns following the leading edge trigger.
Equivalently, the pulse width is limited to 30 ns when calculating the
centroid (midpoint of leading and trailing edges), while the measured
width on channel 1 is 90 ns or greater.
Unlike passive remote-sensing instruments, an altimeter is limited to
observations within range of a visible reflective surface.
Opportunities for such observations did not occur until the first
Mercury flyby on 14 January 2008 at a velocity of approximately 7 km/s,
at which time the first-ever observations of the equatorial region of
Mercury were obtained along a single, sparsely-sampled profile. Noise
returns may outnumber ground signal at the limits of instrument range,
especially at high emission angles. Since the data are essentially
single independent observations, dropouts or corruption of individual
packets will not have a significant scientific impact. No such gaps
have been detected.
On 2012-04-16 (day 107) a transition to an 8-hour orbit was
accomplished, following which MLA was scheduled to range to Mercury
three times per day when constraints permit. Ranging was performed from
23 May up to 11 May 2012 when it was paused for power reasons during
eclipse, and resumed 25 May 2012. A fault protection rule was
implemented that prevents ranging when the instrument housing
temperature exceeds 30C, to extend the longevity of the instrument, and
power cycling was implemented during the hottest portion of the orbital
cycle to further mitigate the higher average temperatures experienced
during the 8-hour orbit. Altimetric analysis tools are being used to
refine the pre-launch and in-flight calibrations. All of the instrument
data obtained to date are of satisfactory quality. MLA instrument
performance fully meets expectations for the mission science
objectives. Quality issues during the orbital phase are addressed in the
final release of Reduced Data Products.
The EDR files listed below were corrupted for several minutes due to a
lack of detection and timing of the laser start pulse, the origin for
MLA time-of-flight measurements. As a result, the start pulse time
defaulted to the diode pump switchout time. This was later than that
of the pulses themselves, which were too weak to trigger the start
pulse detector.
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
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 was modified
to flag repeated timing values by setting the STARTPLS_WIDTH to 99.9 ns
and to assume that the laser fire occurred within 30 ns of the previous
recorded time. The time of flight data, derived from the difference of
the start pulse and return pulse times, may be useful in spite of the
uncertain origin and drifts only slightly from the true value, but the
accuracy is typically no better than about 200 ns or 30 m in range.
The following field is recorded in the calibrated data product:
NAME = STARTPLS_WIDTH
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 RDR data product entails classification, or editing, of
the Science data to distinguish noise from ground returns. The accuracy
of the data relies on the quality of the Precision Orbit Determination
procedures employed, as well as internal crossover analysis and
correlation with other datasets, and is expected to improve as the
data are reprocessed with more accurate geometry.
Further refinement and resampling of the RDR product produces the
Gridded Data Record (GDR) data products. Note that during the low-
altitude periapse periods, the detector gain settings were adjusted
dynamically to mitigate saturation effects. One of these effects was the
appearance of cloud-like returns above the surface, mimicking the
topography. These artifacts are the result of pre-lasing prior to the
timed laser fire of the main pulse, resulting in an apparent higher-than-
normal topographic surface, and are manually excluded in the RDR
processing pipeline.
Erratum
=======
In the MLA Calibration document SUN&NEUMANN2014, the last
coefficient in Equation 14 is in error. The correct equation should read
Etx(T)= 38.873 - 5.3897*T + 0.49348*T^2 + 0.017589*T^3 + 0.00020891*T^4
This error does not affect any of the calibrations applied to the MLA
datasets.
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