Mission Overview
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Mars Global Surveyor (MGS) was the successor to the failed Mars Observer (MO) spacecraft, which was lost as it approached orbit insertion in August 1993. MGS carried five of the original seven MO investigations; the Gamma Ray Spectrometer (GRS) was reflown on the 2001 Mars Odyssey, and the Pressure Modulated Infrared Radiometer (PMIRR) eventually reached Mars in 2006 aboard Mars Reconnaissance Orbiter (MRO) under the name Mars Climate Sounder (MCS). When the reports on the first year of MGS operations were published in 2001, MGS had already returned more information about Mars than all previous missions to that planet combined [ALBEE2001]. The mission to that point was described in an overview paper [ALBEEETAL2001] and a series of papers submitted by members of the scienceteams which will be cited below.

The Mars Global Surveyor (MGS) spacecraft was launched from the Cape Canaveral Air Station in Florida on 7 November 1996 aboard a Delta-2/7925 rocket. The 1062-kilogram spacecraft, built by Lockheed Martin Astronautics, traveled nearly 750 million kilometers over the course of a 300-day cruise toreach Mars on 12 September 1997 [JPLD-12088].

Upon reaching Mars, MGS fired its main rocket engine for a 25-minute Mars orbit insertion (MOI) burn. This maneuver slowed the spacecraft and allowed the planet's gravity to capture it into orbit. The initial MGS orbit was highlyelliptical and had a period of 45 hours.

After orbit insertion, MGS performed a series of orbit changes to drop the low point of its orbit into the upper fringes ofthe Martian atmosphere at an altitude of about 110 kilometers.
During every atmospheric pass, the spacecraft slowed by a small amount because of air resistance. This slowing caused the spacecraft to lose altitude on its next pass through the orbit's high point. MGS was to use this aerobraking technique over a period of four months to lower the high point of its orbit from 56,000 km to near 400 km in altitude, resulting in a nearly circular orbit for mapping. Aerobraking was complicated by discovery of a broken damper arm on one of the solar panels about three weeks into the procedure. After study, spacecraft engineers concluded that aerobraking could resume, but with less stress on the panel. As a result, MGS reached its mapping orbit about a year later than planned; but scienceobservations were interleaved with orbit adjustments.

The spacecraft began its primary mission (the Mapping Phase) on March 9, 1999; at the same time its orbit counter was resetto 1, meaning that the first 1683 orbit numbers were repeated.
During mapping operations, the spacecraft orbited Mars witha period of 117.65 minutes at an 'index' altitude of 378 km.
The orbit parameters resulted in an 88 revolution near-repeat cycle of approximately 7 martian days. With the true altitude ranging between 368 and 438 km and an inclination of 92.96 degrees, navigators could use interactions between the orbit and the gravity field to maintain equator crossings at approximately 2 AM and 2 PM local time without human intervention or expenditure of fuel. The mapping phase of the mission lasted for approximately one Mars year (687 days),ending January 31, 2001, on orbit 8505 [ALBEEETAL2001].

A series of four extended mission phases began on February 1, 2001, (orbit 8506), and continued until communication with the spacecraft was lost on November 2, 2006 (orbit 34202). During that time operations continued in much the same way as during the primary mission, with ongoing data collection by the science instruments. Failure of an oscillator in the laser altimeter reduced its capabilities, but it continued to function in a radiometry mode. Performance of some other instruments was degraded over time; but the MGS camera system continued to provide high-quality imaging of potential landing sites, the radio science experiment was measuring atmospheric profiles, and the spacecraft was providingbackup relay (communications) functions until the end.

MGS was built of lightweight composite materials and divided into four sub-assemblies: the equipment module, the propulsion module, the solar array support structure, and the high-gain antenna support structure. On board power was provided by the solar arrays; attitude was controlled by gyroscopes and small thrusters working in conjunction withcelestial and sun sensors.

Mars Global Surveyor carried four on-board science instruments.
The Mars Orbiter Camera (MOC) had both a wide-angle mode for global coverage and a narrow-angle mode with resolution of 1.4 meters [MALINETAL1992]. Results from the first Mars year of operation have been summarized by [MALIN&EDGETT2001]. The Thermal Emission Spectrometer (TES) measured infrared radiation [CHRISTENSENETAL1992]. TES was used to determine the general mineral composition of patches of ground as small as 9.0 square kilometers; in addition, TES also scanned the Martian atmosphere to provide data for the study of the clouds and weather [CHRISTENSENETAL2001]. The Magnetometer and Electron Reflectometer (MAG/ER) were used to measure the global magnetic properties of Mars, which provided insight on internalstructure [ACUNAETAL1992] [ACUNAETAL2001] [MITCHELLETAL2001].
The Mars Orbiting Laser Altimeter (MOLA) gathered data that allowed calculation of surface feature heights to accuraciesof 30 meters [ZUBERETAL1992] [SMITHETAL2001B].

An ultra-stable oscillator (USO) in conjunction with the on-board telecommunications equipment and ground equipment at stations of the NASA Deep Space Network (DSN) made up the Radio Science Subsystem (RSS) [TYLERETAL1992]. RSS measurements included radio tracking of the spacecraft to improve the gravity field model of Mars, radio occultation observations to study the structure of the atmosphere and ionosphere, and surface scattering measurements to characterize potentiallanding sites [TYLERETAL2001].

A sixth 'instrument' was the Mars Relay, a cylindrically shaped antenna that was used to collect data transmitted to MGS from landers on the Martian surface. These landers were carried to Mars by later spacecraft and operated aftercompletion of the MGS primary mission.

A seventh instrument was the Accelerometer, which measured the deceleration of the spacecraft as it passed through periapsis during the aerobraking orbits. The decelerationcould be used to infer atmospheric drag and, thereby, density.
From the atmospheric density and altitude, it was then possible to infer pressures and temperatures above 100 km altitude, a region not accessible to other instruments[BOUGHER&KEATING1999].

The MGS Horizon Sensor, originally included for attitude monitoring and control, was also used for study of themartian atmosphere [MARTIN&MURPHY2001].


Mission Phases
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Six mission phases were originally defined for significant spacecraft activity periods. These were the Pre-Launch, Launch, Cruise, Orbit Insertion, Mapping, and Relay Phases. The Cruise Phase included both Inner and Outer Cruise components. Once every seven Martian days during the Mapping Phase, the spacecraft approximately retraced its ground track; these 88-orbit intervals are known as 'repeat cycles.'
The final mission phase, Relay, was intended to support the 1998 Mars Polar Lander and possibly the Mars 2001 Lander. It wasplanned to run from February 1, 2001, through January 1, 2003.
Since the Mars Polar Lander was lost and the 2001 mission was reconfigured without a lander, MGS no longer needed a Relay phase. Instead, a series of Extended Mission phases replaced Relay. Relay support, such as for the Mars Exploration Roversstarting in 2004, was woven into the Extended Mission planning.

PRELAUNCH
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The Prelaunch Phase extended from beginning of the MGS mission until the start of the launch countdown at theKennedy Space Center.

Mission Phase Start Time : 1994-10-12 Mission Phase Stop Time : 1996-11-06

LAUNCH
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The Launch Phase extended from the start of launch countdown until completion of the injection into the Earth-Marstrajectory.

Mission Phase Start Time : 1996-11-06 Mission Phase Stop Time : 1996-11-07

CRUISE
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The Cruise Phase extended from injection into the Earth-Mars trajectory until Mars orbit insertion. During the Inner Cruise sub-phase, MGS aimed its solar panels toward the Sun and communicated through its low-gain antenna; during the Outer Cruise sub-phase, the high-gain antenna could be used while the solar panels generated acceptable levels ofpower. The transition occurred on 1997-01-09.

Mission Phase Start Time : 1996-11-07 Mission Phase Stop Time : 1997-09-12
ORBIT INSERTION
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After orbit insertion, MGS performed a series of orbit changes to drop the low point of its orbit into the upper fringes ofthe Martian atmosphere at an altitude of about 110 kilometers.
During every atmospheric pass, the spacecraft slowed by a small amount because of air resistance. This slowing caused the spacecraft to lose altitude on its next pass through the orbit's high point. MGS was to use this aerobraking technique over a period of four months to lower the high point of its orbit from 56,000 km to near 400 km in altitude, resulting ina nearly circular orbit for mapping.

At the low point of orbit 15, on October 8, 1997, MGS experienced difficulties, later diagnosed as due to excess vibrations of one of the solar panels. The problem was associated with the fracture of a panel damper arm [ALBEEETAL1998]. While an evaluation of the solar array problem was underway, periapsis was raised to about 172 km on October 13, 1997, and remained near that altitude until November 7, 1997 (orbits 19 through 36). During this 26-day period the spacecraft instrument panel was pointed toward Mars during close approaches (i.e., near periapsis) and thefirst extensive set of science observations was conducted.
Orbits 19-36 are known as the Assessment Orbits or the Aerobraking Hiatus. The science observations were acquired during the descending leg of each orbit -- that is, as thespacecraft moved from north to south.

Aerobraking resumed on November 8, 1997 (orbit 37), but with a periapsis approximately 10 km higher. Aerobraking was then conducted at about one-third the rate originally planned, placing the spacecraft in a 2 AM Sun-synchronous mapping orbit by March 1999 rather than the planned 2 PM mapping orbit in March 1998. The 2 PM orbit meant that the spacecraft would have crossed the equator in the descending leg of the orbit -- north to south -- at 2 PM, a desirable observing time for some instruments. This orbit could not be achieved given the new aerobraking constraints. However, a 2 AM orbit was possible because, although the descending leg of the orbit crossed the equator at 2 AM, the ascending leg (south to north) crossed the equator at the desired time of2 PM.

Aerobraking was halted again on March 27, 1998, and resumed on September 24, 1998. The period from March 27 through April 28 was known as Science Phasing Orbit 1 (SPO-1, orbits 202 through 268). There was a break for solar conjunction (May 12) between April 29 and May 27. Then Science Phasing Orbit 2 (SPO-2, orbits 329-573) followed from May 28 through September 23. The two science phasing orbits were needed to synchronize the two-hour circular orbit period with the equatorial crossing time of 2 AM. A final period of aerobraking began September 24, 1998, and ended February 4, 1999. Another month was then used for the Gravity Calibration Orbit, other calibration activities, and final trajectory adjustments to put the spacecraft into its mapping orbit. The period between arrival at Mars and completion of the orbit adjustment activities is known collectively as the Orbit Insertion Phase. It ended on March 9, 1999, withorbit 1683.

Mission Phase Start Time : 1997-09-12 Mission Phase Stop Time : 1999-03-09
Subphases Dates Orbits--------- ----- ---------
Aerobraking Phase 1A 1997-09-12 to 1997-10-12 0001-0018 Aerobraking Hiatus 1997-10-13 to 1997-11-07 0019-0036 Aerobraking Phase 1B 1997-11-08 to 1998-03-27 0037-0201 Science Phasing Orbit 1 1998-03-27 to 1998-04-28 0202-0268(SPO-1)
Solar conjunction 1998-04-29 to 1998-05-27 0269-0328 Science Phasing Orbit 2 1998-05-28 to 1998-09-23 0329-0573(SPO-2)
Aerobraking Phase 2 1998-09-24 to 1999-02-04 0574-1284 Transition to Mapping 1999-02-04 to 1999-03-09 1285-1683

MAPPING
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The Mapping Phase was the period of concentrated science data acquisition. At the beginning of this phase, orbit numbering was restarted at 1. The Mapping Phase lasted for 687 days, approximately one Martian year. Mars was at opposition with the Sun on April 24, 1999, and in conjunction with the Sun onJuly 1, 2000.

As a risk reduction measure against possible problems with the deployment of the High-Gain Antenna, the first 20 days of the Mapping Phase were operated in so-called 'Fixed High-Gain Antenna' or FHGA mode. In this mode, the undeployed HGA was pointed at Earth for four to five orbits out of every twelve to transmit data. During data transmission, the science instruments were not pointed at Mars. The HGA was deployed on March 29, 1999, and the first day of full mapping wasApril 3, 1999.

Soon after the antenna was deployed (April 16, 1999) its azimuth gimbal jammed, causing an entry into contingency mode and interruption in the acquisition of science data. This interruption lasted until April 29, 1999, and then data were acquired in a modified FHGA mode (HGA deployed, but boresight fixed in the spacecraft +x direction) until May 6, 1999, when normal mapping resumed. It was determined that the restricted range of travel on the azimuth gimbal would allow normalmapping operations until early 2000.

So-called 'beta supplement mode' operations, in which the antenna was reoriented to allow Earth tracking during data acquisition, were begun on February 7, 2000. But beta supplement mode required that the antenna be 'rewound' while the spacecraft was being tracked and precluded collection ofegress (exit) radio occultations.

Approximately three days of FHGA operation were inserted into the schedule (March 5, 2000, to March 7, 2000) to mitigate impact on radio science. Unexpected heating of MOLA resulted, and further FHGA operation was suspended pending resolution of the thermal problems. Radio science 'egress campaigns' were eventually resumed at a rate of approximately 24 hours everymonth.

Mission Phase Start Time : 1999-03-09 (orbit 0001) Mission Phase Stop Time : 2001-01-31 (orbit 8505)

EXTENDED MISSION E1
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The first Extended Mission phase (E1) began at the end of the Mapping Phase, 1 February 2001, and continued through 22 April2002. Mars was at opposition with the Sun on June 13, 2001.

Operations were much the same as during Mapping. MGS supported orbit insertion and aerobraking for the 2001 Mars Odyssey spacecraft with rapid release of MOC and TES data inlate 2001 and early 2002.

A new type of spacecraft maneuver was designed for MGS targeted science observations during the extended mission: the Roll Only Targeted Observation (ROTO). The maneuver was constrained to occur primarily in the roll axis and could not exceed +/- 30 degrees off nadir. The spacecraft was rolled during selected orbits to acquire off-nadir contiguous MOC, MOLA, and TES data, which could be used to supportlanding site certification for future missions.

Starting on August 16, 2001, the spacecraft was put into the 'Relay 16' or R16 attitude, in which it was pitched back along the velocity vector by 16 degrees. This reduced gravity-gradient torques, slowing momentum buildup in thespacecraft reaction wheels and reducing fuel consumption.

Mission Phase Start Time : 2001-02-01 (orbit 8506) Mission Phase Stop Time : 2002-04-22 (orbit 13960)

EXTENDED-EXTENDED (E2) MISSION
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The E2 Mission phase began at the end of the E1 Extended Mission Phase, April 22, 2002, and continued until September 26, 2004. Mars was in conjunction with the Sun on August 10, 2002, and on September 15, 2004, and was in opposition with the Sun on August 28, 2003. There was a major maneuver in May 2004 to ensure aphelion power within local sun angleconstraints.

Operations were similar to those in the E1 phase. ROTOs, the R16 attitude, and radio science 'egress campaigns' were continued. MGS life expectancy was increased to 10 years based on the rate of fuel consumption. Support of the Mars Exploration Rover (MER) missions was increased with both imaging of potential landing sites and testing of the UHF communication relay system. The MGS orbit was synchronized to support descent and landing of both MER rovers; over 6 terabits of surface science data were returned via theMGS Relay.

Mission Phase Start Time : 2002-04-22 (orbit 13961) Mission Phase Stop Time : 2004-09-26 (orbit 24836)

EXTENDED MISSION (E3)
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The E3 Mission phase began at the end of the E2 phase,September 27, 2004, and continued until September 26, 2006.
Mars was at opposition with the Sun on November 7, 2005.

Operations were similar to those in the E2 phase except that a safe mode event in August 2006 showed that the high-gain antenna azimuth gimbal obstruction was no longer present and that Beta Supplement operation was no longer required. This saved gimbal life, eased sequencing, reduced fuel consumption, and allowed collection of more radio occultation data. A new estimate of attitude control fuel added 10 kg to the inventory, effectively removing fuel as a limiting factor in extending the mission lifetime. MGS supported MRO orbitinsertion and aerobraking after its arrival in March 2006.

Mission Phase Start Time : 2004-09-27 (orbit 24837) Mission Phase Stop Time : 2006-09-26 (orbit 33815)

EXTENDED MISSION (E4)
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The E4 Mission phase began at the end of the E3 Extended Mission phase, September 27,2006, and continued until contact with the spacecraft was unexpectedly lost in early November 2006. Normal operations were suspended between October 17 and November 2, 2006, while Mars was in conjunction with theSun (October 23).

Operations were similar to those in the previous Extended Mission phases. During this phase the 250000-th image was transmitted to Earth, of which 1750 were collected using ROTOs and 250 were collected using motion compensatedROTOs (CPROTOs) with surface resolution as small as 50 cm.
Spacecraft loss may have resulted from a solar panel hitting a hard stop during an eclipse rewind, leading to a cascade of other events which culminated in catastrophic batteryfailure within 10 hours.

Mission Phase Start Time : 2006-09-27 (orbit 33816) Mission Phase Stop Time : 2006-11-02 (orbit 34202)