Data Set Overview
   =================
     The data set consists of uncalibrated observations, also known as
     EDRs.
 
     Each XRS observation results in four X-ray spectra. When an X-ray
     interacts with one of the four detectors, a charge or voltage pulse
     is generated. This signal is converted into one of 2^8 (256)
     channels, which are correlated to energy. Over a commanded
     integration time period a histogram of counts as a function of
     energy (channel number) is recorded. The EDRs are the number of
     events in each channel of the four detectors accumulated over the
     integration period. Channels above or below the useful energy range
     of the detectors are not saved or transmitted. The result is three
     244- channel GPC histograms and one 231-channel solar monitor
     histogram, each of which is designated as a single X-ray spectrum.
 
     In addition to the science data, associated instrument parameters
     are included.
 
 
   Instrument Overview
   ===================
     The X-Ray Spectrometer (XRS) experiment is comprised of three
     identical gas proportional counters (GPC) that measure X-rays
     emitted from the surface of Mercury in the energy range from about
     1 to 10 keV.  X-rays in this energy range sample the planetary
     surface to depths of a few tens of microns.
 
     See the XRSINST.CAT file for more information and [SCHLEMMETAL2007]
     for full details.
 
 
   Calibration Overview
   ====================
     The supernova remnant X-ray source Cassiopeia-A (Cas-A) is being
     used for periodic in-flight calibration because of its relatively
     high intensity in the XRS energy range. Once or twice a year XRS is
     pointed to Cas-A for about 48 hours and then away from that source
     for another 48 hours to accumulate background measurements. These
     measurements provide an evaluation and calibration of the XRS anti-
     coincidence and rise-time discrimination background rejection
     systems.
 
     This data set is NOT calibrated; it only provides the uncalibrated
     sensor measurements.
 
 
   Parameters
   ==========
     The principal parameters when observing with the XRS are as
     follows:
 
 
     * Integration Period: Normal (quiet Sun) or Flare. In Normal mode
     three different integration intervals are used, depending on
     distance to the planet. These may be set over a wide range, but the
     default values are:
 
                  Integration  Collection Time
       Closest:        40 s       2000 s
       Mid-Range:     200 s      10000 s
       Greatest:      450 s      31200 s
 
     In flare mode integration periods are automatically shortened to a
     pre-determined level, typically 20 s, for 60 minutes or less.
 
     * Veto Event: Accept or reject events with a simultaneous signal
     from the anti-coincidence wires.
 
     * Rise-Time Validation: Accept or reject events with an invalid
     rise time measurement.
 
 
   Data
   ====
     The XRS instrument telemetry conveys science data, configuration
     data and status (housekeeping) data. Several telemetry packets are
     used to downlink this information:
 
        Status Housekeeping Packets
        Uncompressed Science Packets
        Compressed Science Packets
        Raw Sensor Data Packets
 
     The useful science and housekeeping measurements from the telemetry
     are decompressed and decomutated and are correlated and accumulated
     into one binary table, which constitutes the primary experimental
     data record.
 
     Each record of the table contains four energy spectra,
     housekeeping, status, and rate data for one integration period. The
     spectra correspond to the Mg-filtered detector (GPC1), the
     Al-filtered detector (GPC2), the unfiltered detector (GPC3), and
     the solar monitor (SAX). All the spectra have 256 bins, but only
     the highest 244 bins for the GPC and the highest 231 bins for the
     SAX are telemetered. All bins are 16 bits deep. There are five
     32-bit rate counters for SAX and nine 24-bit rate counters for each
     of the three gas proportional counters.
 
     Rate counters for all four detectors include raw, valid event,
     analyzed, pileup, and high-energy rates. The gas detectors also
     have counters for the veto-anode, veto, rise-pileup, and
     rise-rejection rates.
 
     Housekeeping data provide voltage, current, and temperature
     readings measured at the end of each integration period. Also
     reported are record and software status, hardware settings and
     integration and reporting times.

Confidence Level Overview
=========================
The XRS EDR data are the least processed data set released for the XRS.
Data presented here are an accurate representation of the XRS data as
received from the spacecraft, with minimal timing and no spatial processing.
 
 
Review
======
The XRS EDR was reviewed internally by the XRS team prior to release to the
PDS. PDS also performed an external review of the XRS EDR.
 
 
Data Coverage and Quality
=========================
Data reported are the minimally processed data received from the spacecraft
during the following mission phases: Launch, Earth Cruise, Earth Flyby, Venus
1 Cruise, Venus 1 Flyby, Venus 2 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.  These mission phases are
defined as:
 
                     Start time             End time
Phase Name              Date (DOY)            Date (DOY)
-----------------    ----------------    ----------------
Launch                  3 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)    9 Oct 2006 (282)
Venus 1 Flyby           10 Oct 2006 (283)    7 Nov 2006 (311)
Venus 2 Cruise          8 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 2007 (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)    3 Mar 2011 (062)
Mercury Orbit           4 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)
 
No XRS data were collected during Earth Flyby or Venus 1 Flyby phases.  The
XRS was on for some of each of the other fifteen mission phases.  During
these planned operational periods, the XRS functioned nominally and the data
quality was good.   Specific XRS operational periods were:
 
                       Start time          End time
Phase Name               Date (DOY)          Date (DOY)
-----------------    ----------------    ----------------
Launch                  30 Aug 2004 (243)     30 Aug 2004 (243)
Earth Cruise            07 Feb 2005 (038)     11 Feb 2005 (042)
                        14 Apr 2005 (104)     14 Apr 2005 (104)
                        09 July 2005 (190)    13 July 2005 (194)
Earth Flyby             No Data
Venus 1 Cruise          16 Jan 2006 (016)     13 Feb 2006 (044)
                        06 Sep 2006 (249)     10 Sep 2006 (253)
Venus 1 Flyby           No Data
Venus 2 Cruise          27 Jan 2007 (027)     31 Jan 2007 (031)
Venus 2 Flyby           26 May 2007 (146)     20 Jun 2007 (171)
Mercury 1 Cruise        21 Jun 2007 (172)     08 Aug 2007 (220)
                        17 Aug 2007 (229)     25 Aug 2007 (237)
                        20 Dec 2007 (355)     30 Dec 2007 (364)
Mercury 1 Flyby         31 Dec 2007 (365)     21 Jan 2007 (021)
Mercury 2 Cruise        28 Mar 2008 (088)      21 Sep 2008 (265)
Mercury 2 Flyby         22 Sep 2008 (266)     20 Oct 2008 (294)
Mercury 3 Cruise        21 Oct 2008 (295)     19 Nov 2008 (324)
                        15 Jan 2009 (015)      17 May 2009 (137)
                        28 Aug 2009 (240)     01 Sep 2009 (244)
                        04 Sep 2009 (247)     15 Sep 2009 (258)
Mercury 3 Flyby         16 Sep 2009 (259)     29 Sep 2009 (272)
Mercury 4 Cruise        15 Oct 2009 (288)     3 Mar 2011 (062)
Mercury Orbit           4 Mar 2011 (063)       17 Mar 2012 (077)
Mercury Orbit Year 2    23 Mar 2012 (083)      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 Sep 2014 (260)
 
The XRS gas proportional counters were intentionally powered down on 7 June
2007, following the Venus 2 flyby.  The solar monitor remained on throughout
the Venus 2 Flyby phase.
 
During Mercury 1 Cruise the gas proportional counters were on from August
4-8, August 17-24, and December 20-30.  The solar monitor was on for all of
the indicated time periods.
 
The XRS gas proportional counters were intentionally powered down on 15
January 2008, following the Mercury 1 flyby.  The XRS was powered down on 21
January 2008.
 
The XRS was powered up on 28 March 2008 and remained on throughout the
remainder of the Mercury 2 Cruise phase and the entire Mercury 2 Flyby phase.
During the Mercury 2 Cruise phase the aluminum filtered gas proportional
counter safed itself for 1 hour on two separate occasions, 28 August (241)
and 7 September (251).  These two safing events were caused by excessive
count rates in the anti-coincidence wires of the counter.  The gas
proportional counters were intentionally powered down on 7 October,
following the Mercury 2 flyby.  The solar monitor remained on throughout the
Mercury 2 Flyby phase.
 
The solar monitor remained on during Mercury 3 Cruise until 19 November 2008.
The entire XRS was powered up again on 15 January 2009.  The gas proportional
counters were intentionally turned off on 22 January 2009 and then powered up
 again on 7 May 2009.  The entire instrument was powered off on 17 May 2009.
 
The XRS was powered up on 28 August 2009 and was powered down on 1 September
2009 due to spacecraft safing.  After spacecraft recovery, XRS was powered up
on 4 September 2009 and remained on until 29 September 2009.  Power down was
again due to spacecraft safing.
 
During the Mercury 4 Cruise phase of the mission, the GPCs were powered on
primarily for Cas-A observations, 22 January 2010 to 29 January 2010, 30
March 2010 to 5 April 2010, 13 July 2010 to 19 July 2010, and 22 October 2010
 to 29 October 2010.  Other time periods with GPC HV on were 23 July 2010 to
 29 August 2010, and 6 December 2010 to 23 December 2010.  The solar monitor
 was on throughout this mission phase.
 
Once in orbit, the XRS detectors were on continuously except where spacecraft
 operations (eclipses) and instrument health and safety (temperature and
 count rate) required some or all of the detectors to be turned off.
 
The XRS GPCs experienced a number of safing events during early orbit.
During these safing events one or more of the GPCs safed due to high count
rate and remained off for 1 hour until autonomous recovery.  On 25 April the
safing limit for the GPCs was increased from 5000 s-1 to 10000 s-1 and the
autonomous time to recover was reduced from 60 minutes to 5 minutes.
 
MESSENGER experiences a hot planet/long eclipse season every ~88 days.
During these times spacecraft periapse is over a sunlit portion of the planet
and eclipses exceed 15 minutes.  Initially, long eclipses required instrument
power off for ~2 hours for each orbit due to spacecraft power limitations.
Beginning in August 2012, this restriction was lifted and the GPCs remained
on during eclipse, but the solar monitor is still powered off when thermal
modeling shows that the temperature is likely to exceed the red limit
(30 degrees C). In addition, when the solar monitor temperature exceeds ~19
degrees C, spectra are dominated by electronic noise and are generally of
little scientific value.  Hot planet/long eclipse seasons are typically about
2 weeks in duration.  The fourteen seasons experienced so far began on 25 May
2011, 21 August 2011, 15 November 2011, 11 February 2012, 8 May 2012, 13
August 2012, 14 November 2012, 23 February 2013, 20 May 2013, 15 August 2013,
14 November 2013, 1 February 2014, May 7, and August 1.
 
On 5 March 2012 a large solar particle event produced severe radiation damage
in the XRS solar monitor.  Above -10 degrees C leakage current in the solar
monitor was so high that spectra were of no scientific value.  A two day
anneal at 100 degrees C, beginning on 20 March 2012, improved the situation
significantly so that good solar monitor spectra could be collected up to
~+5 degrees C.  A second two day anneal, beginning on 3 April 2012, produced
only marginal improvement.  No other anneals were planned.   After 22 March
2012, only solar spectra collected when the detector temperature is ~3
degrees C or less should be used for science analysis.
 
On 1 April 2012, the high voltage was increased on all three of the gas
proportional counters to adjust for loss of gain in the detectors.  The new
high voltage settings are Mg filtered: 1437 volts, Al filtered: 1426 volts,
and unfiltered: 1436 volts.  The GPC voltages have remained at these levels
except for a period from 17 June 2014 to 2 October 2014, when the voltages
were set to the old values.
 
On 28 February 2013, 11 May 2013, 4 June 2013, 29 August 2013, and 25
November 2013 the solar monitor temperature exceeded its red limit and the
detector safed itself.  In each case the solar monitor remained off for
several days.  Also, on 26 August 2013, the XRS was safed by a spacecraft
autonomy rule.  XRS remained off for about 1 day.
 
On 20 August 2013 a large solar particle event caused the GPCs to safe and
remain off for about 3 days.  This solar particle event also affected
operation of the solar monitor.  The solar monitor noise level increased and
the detector no longer provided solar spectra above ~0 degrees C.  A three
day anneal was performed 9-12 September 2013.  This anneal increased the
useful upper level operating temperature of the solar monitor back to where
it had been prior to the 20 August 2013 solar event.
 
On 1 September 2014 a large solar event caused the GPCs to safe.  They
remained off for about 1 day.
 
Limitations
===========
This data set is minimally processed data.  The data are received from the
spacecraft telemetry and ingested into the MESSENGER Science Operations
Center (SOC).  No data gaps have been identified for any of the XRS
operational periods.