SPOT 1 Earth observation satellite deorbiting

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SPOT 1 Earth observation satellite de-orbiting

• Track 3 Sharing the Mission Experience.
• Luc Moliner - CNES DCT/OP/BL

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SPOT 1 Earth observation satellite de-orbiting

• 1. INTRODUCTION
• SPOT Earth observation system presentation
• The SPOT system received French government
  approval in 1978.
• Designed by the French Space Agency CNES, it was
  developed in cooperation with Belgium and Sweden.
• SPOT 1 was successfully launched on 22 February
  1986, with a 3 year expected lifetime.
• SPOT 2 was placed into orbit on 22 January 1990,
  and SPOT 3, identical to its predecessors, on 26
  September 1993.
• SPOT 4 was placed into orbit on 24 March 1998,
  with the secondary mission VEGETATION, and the
  passengers DORIS, POAM III from the Naval
  Research Laboratory (US), PASTEL and ESBT from
  ESA.
• SPOT 3 failed in 1996, consequently to an
  attitude control problem.

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SPOT 1 Earth observation satellite de-orbiting

• The latest satellite in the family, SPOT 5, using
  new instruments and an innovative sampling
  concept called SUPERMODE, was
  successfully launched on 4 May 2002.
• SPOT 2,4,5 are still operationnal.

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SPOT 1 Earth observation satellite de-orbiting

• SPOT system main features are
• high resolution (down to 2.5 m with SPOT 5 in
  supermode).
• large field of view (60 km x 2 instruments).
• excellent geometric and radiometric quality.
• stereoscopy, and revisit flexibility.
• Two operators manage the SPOT system
• CNES, responsible for orbit control operations
  and system performance, is the satellite
  operator, and has developed the Satellite
  Operations Ground Segment (SSOS).
• Spot Image, responsible for the distribution of
  SPOT products, is the commercial operator, and
  has developed the Commercial Operations Ground
  Segment (SSOC) for this purpose.
• Since the first SPOT satellite was launched in
  1986, the SPOT system has sought to provide
  continuity of service and constantly improved
  quality of products for users.

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SPOT 1 Earth observation satellite de-orbiting
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SPOT 1 Earth observation satellite de-orbiting

• SPOT satellites orbit features
• SPOT satellites orbit is circular (altitude 822
  km at the equator),
• near-polar (inclination 98) and Sun-synchronous
  (the equator crossing time is 10h30 a.m.).
• The orbit is also phased (26-day cycle).
• The orbital period is 101 mn.
• Each satellite is phased relative to the others,
  to optimize repeat image acquisition and to allow
  receiving stations and the CMP operations control
  centre to track the satellites without
  introducing additional operational constraints.

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SPOT 1 Earth observation satellite de-orbiting
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SPOT 1 Earth observation satellite de-orbiting

• SPOT 1 desorbitation reasons
• In 2001, the solar panel was partially damaged
  (loss of 10), and in case of a new anomaly, the
  power budget margin will be low to ensure the
  mission.
• We decided to start a study, to be ready to
  de-orbit SPOT 1 before a new damage.
• Unfortunately, a straight atmospheric reentry was
  not possible, but a compromise solution was found
  to use the residual propellant (65 kg of
  hydrazine), to manoeuver into a lower altitude
  disposal orbit, from which reentry could be
  completed within 25 years.
• The main objective of this project, was to
  preserve space environment, in accordance with
  the IADC ( Inter Agency Space Debris Coordination
  Committee ) recommendations.

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SPOT 1 Earth observation satellite de-orbiting

• 2. SATELLITE DESIGN
• SPOT 1 satellite design is based on early
  eighties technology.
• Considering the operational aspect of the
  mission, the satellite equipment is fully
  redundant, without any single point failures
  except elements such as structure.
• The SPOT 1 platform performs the housekeeping
  functions vital to the success of the mission.
• These functions are controled by the onboard
  computer (OBC) flight software, and include
• automatic control of all onboard systems when the
  satellite is not within range of a TTC station,
• autonomous response capability in the event of a
  malfunction,
• and three-axis stabilization (Attitude and Orbit
  Control Subsystem).

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SPOT 1 Earth observation satellite de-orbiting

• 3. Attitude and Orbit Control Subsystem
  description
• AOCS CONSTRAINT (GEOCENTRIC POINTING)
• The most demanding constraints are the specified
  rotation rates which impact directly on image
  geometric quality.
• The local orbital reference system, and the
  satellite axes system.
• With perfect geocentric pointing Xs -T, Ys
  -R and Zs L.

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SPOT 1 Earth observation satellite de-orbiting

• AOCS INPUT AND CONTROL DEVICES
• The AOCS uses rate gyros, digital Earth sensors
  (STD), and digital Sun sensors (SSD) as input
  devices
• and hydrazine thrusters, reaction wheels and
  magnetic torquers as control devices (so called
  actuators).

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SPOT 1 Earth observation satellite de-orbiting

• AOCS MODES
• The fine-pointing mode (MPF), which is the
  nominal mode, uses rate-gyros, digital Earth
  sensors and digital Sun sensors as input devices,
  and reaction wheels and magnetic torquers as
  control devices.
• The fine earth-acquisition mode (MAF), required
  during the attitude acquisition phase, and in the
  event of a temporary failure of the fine pointing
  mode, uses the same sensors, but the actuators
  are a set of small hydrazine thrusters.
• The maneuver modes (MCC or MCO), during orbital
  manoeuvers.
• A safe mode (MSU) is also available. This is
  activated only in the event of an onboard
  hardware or software failure, or a combination of
  the two. This mode ensures that no damage occurs
  to the satellite while awaiting the intervention
  of ground control.

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SPOT 1 Earth observation satellite de-orbiting

• 4. DE-ORBITING PROJECT
• PLANNING
• June, 2001
• Start of the de-orbiting study (solar panel
  partially damaged).
• July to October, 2001
• AOCS feasibility study (ASTRIUM), and internal
  mission analysis.
• 18 October, 2001
• Presentation in SPOT satellites exploitation
  review (REVEX).
• The de-orbiting project follows in 2002/2003,
  with the detailed conception and development
  phases.
• The operational qualification goes on in October,
  ending with a specific review (04/11/03), and
  after a final decision of CNES and Spot-Image
  management (07/11/03), the realization of the
  de-orbiting operations (from 17 to 28 November,
  2003).

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SPOT 1 Earth observation satellite de-orbiting

• STRATEGY POSSIBILITIES
• The mission analysis revealed that two different
  strategies were possible.
• A first strategy using MCC manoeuvers.
• 2 / Earth center symmetrical thrusts (each thrust
  duration 20s).
• in this case, the final orbit will be a circular
  orbit (alt. 600 km).
• great number of manoeuvers, and long duration
  operation.
• A second strategy using MCO manoeuvers.
• only 1 thrust per orbit (duration limited to
  1000s).
• here, the final orbit will be an elliptical orbit
  (800/580 km).
• 10 or 12 manoeuvers, and 15 days duration
  operation.

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SPOT 1 Earth observation satellite de-orbiting

• STRATEGY CHOICE
• We have chosen the second strategy (MCO), for
  several reasons
• technical feasibility.
• better final orbit (inferior time to reentry).
• short duration (low cost aspect).
• Lowering perigee with multiple apogee burns
  (strategy diagram)
• Apogee centered thrust description
• 1000s duration, 5 kg hydrazine consumption, D
  perigee of -25 km.
• apogee rotation 3/day, due to the J2 term of
  the Earth potential.

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SPOT 1 Earth observation satellite de-orbiting

• 5. ONBOARD MODIFICATIONS
• ONBOARD SOFTWARE MODIFICATIONS (AOCS module)
• The MAF2 mode will be the waiting mode between
  two orbital maneuvers, and the MPF mode will be
  used only for MAF2 to MCO transitions.
• This, because the MAF2 mode is more robust than
  the MPF mode, especially at final altitudes (lt
  650 km), where reaction wheels could be
  saturated.
• However, this AOCS modes sequence (MAF2 - MCO -
  MAF2), presents an inconvenience The hydrazine
  consumption in MAF2 mode, evaluated to 0.5 kg/day
  (depending on altitude).

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SPOT 1 Earth observation satellite de-orbiting

• We plan only minor modifications (patches) on the
  onboard software
• avoidance of the residual accelerations in MAF2
  mode, inhibiting accelerative Z thrusters (i.e.
  Z-/Y- and Z/Y- thrusters).
• Inhibition of  Earth diameter  and  Sun
  attendance  surveillance.
• The Earth-pointing surveillance will stay enabled
  (no risk of release).
• The solar panel motor position safety margin,
  will be enlarged to 30.
  

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SPOT 1 Earth observation satellite de-orbiting

• ONBOARD HARDWARE MODIFICATION (propulsion
  subsystem)
• The propulsion subsystem is composed of
• 2 separate propellant tanks.
• 2 independent propulsive branches.
• We plan to interconnect the two tanks before the
  desorbitation operations, and this presents
  several advantages
• better management of propellant, minimizing the
  unusable part.
• avoid pressures rebalancing between two
  manoeuvers.
• better accuracy of pressure measurement, (switch
  redundancy).

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SPOT 1 Earth observation satellite de-orbiting

• 6. GROUND MODIFICATIONS
• FLIGHT DYNAMICS SOFTWARE LIMITS
• The ground flight dynamics software (OMGS) main
  functions are
• orbit determination, using tracking by TTC
  stations.
• orbit control, programming orbital manoeuvers
  (MCC or MCO).
• AOCS management, producing space mechanics
  commands.
• In nominal exploitation, we perform two orbit
  determinations per day, and we program a MCC
  manoeuver (altitude correction) per month,
  (depending of solar activity).
• a MCO manoeuver (inclination correction), is
  planned every year.
• These operations are executed with a high degree
  of automation and security.

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SPOT 1 Earth observation satellite de-orbiting

• The flight dynamics software has been designed
  for orbits closed to the SPOT reference one, and
  consequently, was not suited for the
  desorbitation operations...
• THE SOLUTION
• Use of the generic flight dynamics software CLEO
  (Jason, SPOT 5).
• During this critical phase, we plan to use the
  Orbit Computation Centre (OCC), another
  operational entity of Toulouse Space Centre
  (CST), to perform orbit determinations.
• The OCC will perform also
• TTC stations designation.
• computation of collision risk with other
  satellites, or space debris.
• computation of interference risk with SPOT 2
  (same frequency).

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SPOT 1 Earth observation satellite de-orbiting

• 7. OPERATIONAL SCENARIO
• End of the desorbitation study.
• Operational procedures and onboard modifications
  validation with the satellite simulator (QO).
• Specific review (RQO).
• CNES and Spot-Image management final decision to
  de-orbit SPOT 1.
• Start of the desorbitation operations (17/11/03)
  
• First MCO manoeuver (delta a of -15 km to a
  circular orbit).
• First orbit determination phase.
• Succession of MCO manoeuvers (1000s duration to
  an eliptical orbit), and orbit determination
  phases, up to hydrazine exhaustion.
• Satellite passivation (end of life procedures).

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SPOT 1 Earth observation satellite de-orbiting

• 8. DESORBITATION OPERATIONS
• PRELIMINARY OPERATIONS
• 12/11/03 interconnection of the two hydrazine
  tanks. (bias of 0.3 bar
  between the two pressure sensors)
• 17/11/03 first MCO maneuver (two Earth centre
  symetrical thrust). gt -15 km circular orbit, to
  avoid risk of collision with others SPOT.
• 18/11/03 upload of the on-board software
  modifications (patches).
• 19/11/03 first desorbitation maneuver
  computation, using the generic flight dynamics
  software CLEO, and upload to the satellite, for
  an execution in the evening (see pres. in poster
  session by C. Salcedo).

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SPOT 1 Earth observation satellite de-orbiting

• PARTICULAR OPERATIONS
• 21 and 26/11/03 SPOT 2 telemetry transmitter
  turned off during few hours, to avoid a risk of
  interference announced by OCC.
• 23/11/03 inhibition of  tension BNR 
  surveillance, due to a total eclipse of the Sun
  on the Antartic (risk of release).
• No risk of collision identificated during the
  desorbitation operations.
• ROUTINE OPERATIONS (daily chronology)
• Morning OBC soft. control dumps, and next
  maneuver computation.
• 13h30 UTC Operations Coordination Meeting
  (GCO).
• Afternoom maneuver commands send to the
  satellite.
• Evening maneuver execution in visibility of
  TTC stations, satellite ground check, and
  new orbit determination by OCC.

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SPOT 1 Earth observation satellite de-orbiting

• DESORBITATION GROUND SEGMENT
• For these desorbitation operations we have used
• the nominal SPOT control centre.
• the CNES TTC stations network, and Svalbard
  station in Norway, to have better visibility and
  a back-up in case of a technical problem on
  Kiruna station in Sweden.
• the generic flight dynamics software CLEO, for
  maneuvers computation.
• SYNTHESIS OF DESORBITATION MANEUVERS

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SPOT 1 Earth observation satellite de-orbiting

• 9. END OF LIFE OPERATIONS
• PRELIMINARY OPERATIONS
• Wed. November 26-th, 2003 inhibition of
  surveillances.
• END OF LIFE SCENARIO
• A last thrust of 2400s, with maximum visibility
  on TTC stations. (HBK, AUS, KRN, and Svalbard
  see diagram on the next slide)
• On Aussaguel TTC station (satellite in night
  mode)
  disconnection of the 3 on-board battery.
• On Kiruna TTC station (just after the thrust
  end) command to turn off the
  telemetry transmitter.
• With this scenario, we expected use all the
  residual propellant...

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SPOT 1 Earth observation satellite de-orbiting
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SPOT 1 Earth observation satellite de-orbiting

• END OF LIFE OPERATIONS (Friday November 28, 2003)
• HBK 20h29 - 20h41 all was nominal.
• AUS 20h50 - 21h01 disconnection of the 3
  onboard battery. gt beginning of yaw
  steering anomaly, sign of the first helium bubble
  in the propellant.
• 210029 drop of pressure on the two sensor,
  sign of dry running.
• 21h04 (on KRN station) command to turn off the
  telemetry transmitter gt end of SPOT 1 telemetry
  reception at 210405 (UTC). Great
  emotion in the SPOT Control Centre at this time
• Later in the evening, the OCC announced a final
  orbit with a perigee altitude of 581 km, and 801
  km for the apogee.
  (confirmed by
  the NORAD two-lines on Monday December 1-st, 2003)

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SPOT 1 Earth observation satellite de-orbiting
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SPOT 1 Earth observation satellite de-orbiting

• 10. CONCLUSION
• Faced with the orbital debris problem, CNES has
  defined rules for its future projects (see
   security requirements - space debris 
  document).
• Unfortunately, these requirements can t be
  directly applied to SPOT 1 satellite which was
  developed anteriorly.
• However, the CNES, as far as possible, will try
  to apply its official recommendations and its
  international positions.
• Particularly, it will respect the rule which
  limit to 25 years the orbital lifetime after the
  end of the operational mission.
• SPOT 1 is now on an 581x801 km eliptical orbit,
  and will return in 16 year.
• 1 16613U 86019A 03335.39851850 .00013189
  00000-0 23421-2 0 9541
• 2 16613 98.6694 47.2406 0154400 175.7235
  184.5210 14.59793079606825 (NORAD TLE - 01/12/03)
  

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SPOT 1 Earth observation satellite de-orbiting

• The SPOT constellation, before SPOT 1 de-orbiting
  (artist view).
• Thanks to
• The Operations and Flight Dynamics teams, from
  CNES Toulouse.
• Special thanks to
• Fernand ALBY, CNES Space Debris activity
  responsible.
• Céline BONNEAU and Michel HORBLIN, from
  EADS-ASTRIUM Toulouse.
• Brigitte BEHAL and Eric AUBEL, my family, and to
  all my friends.