Georges C. BRONDINO

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Space debris avoidance strategies
on SPOT/HELIOS Satellites

• Georges C. BRONDINO
• HELIOS SATELLITES MANAGER
• CNES Toulouse
• FRANCE

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HELIOS 1A artist view
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SPOT / HELIOS Satellite family

• HELIOS 1A was placed into orbit on July 7 1995,
  for a combined french, italian and spanish use
• SPOT-4 placed into orbit on March 24 1998, uses
  the same platform as HELIOS 1A, and carry also
  the secondary mission VEGETATION, the passengers
  DORIS from CNES, POAM from Naval Research
  Laboratory (US), and PASTEL from ESA.
• HELIOS 1B was placed into orbit on December 4
  1999, and is the sistership of HELIOS 1A
• SPOT 1 (2/22/86) and SPOT 2 (1/22/90) are of an
  older conception, and SPOT 5 (5/4/02) represent
  the first platform of a new family type which
  will be followed by HELIOS 2A and 2B

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Introduction to space debris hazard
Due to the extraordinary expansion of space
activities through the past years, the low earth
orbits are completely polluted by all kinds of
space debris. The risk of collision with these
space debris increase regularly for all low earth
orbit satellites, such as SPOT / HELIOS earth
observation spacecraft.
The CNES operational teams that are in charge
with SPOT / HELIOS spacecraft have taken into
account the necessary strategies to allow to
minimize the risk of degradation, or even of
destruction, of the spacecraft due to the
encounter with a passive space debris
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HELIOS SPOT Orbit characteristics

• HELIOS SPOT are 3 axis fine earth pointed
  satellites, with quasi polar sun synchronous
  orbits (day ascending 27 days cycle for HELIOS,
  and descending 26 days cycle for SPOT)

The SPOT are geocentric pointed using earth
sensor The HELIOS are pointed using stellar
sensors, with rall/pitch/yaw evolutions
capabilities
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HELIOS Manoeuvre characteristics (1)

• These manoeuvres are always in two separate
  thrusts (near the poles) separated by half an
  orbit, the second thrust being used to
  circularise the orbit again.
• During these phases the attitude remain 3 axis
  controlled through reaction wheels actuation and
  gyrometers measurements.
• The phasing margins are 10 Km for Helios and 5 Km
  for SPOT, and the local hour is 13h17 at
  ascending node for Helios with a 150 seconds
  tolerance, and 10h30 for SPOT with a 15 minutes
  tolerance.

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HELIOS Manoeuvre characteristics (2)
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Alert procedures
Using NORAD two-lines data, the COO assume day
the selection of the space debris that can
collide with a spacecraft within CNES
responsibility, and a calculation of collision
probability is made. If the nearby distance is
less than 1,5 Km, if the probability of collision
is greater than 10-3, the alert sequence is
initiated
The COO alerts the operational teams of SPOT and
HELIOS Control Centre, and also the MONGE and the
French radar network to obtain, if possible
within the operational delay constraints, a more
precise measurement of the debris
trajectory. http//www.netmarine.net/bat/divers/mo
nge/
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Manuvre decision

• Using Helios reference orbit, and the eventual
  radar data, a more precise prediction of the date
  and the relative distance of nearby crossing will
  be calculated
• A preliminary estimation of the avoidance
  manoeuvre will be made by the operational team of
  the CMP, and the post-manoeuvre orbital data will
  be introduced in the COO software to verify that
  no new collision risk with another debris may
  occur.
• The CMP satellite team verify the feasibility of
  the manoeuvre, taking into account all the
  operational constraints including the ones
  attached to the following manoeuvre needed to
  come back in the operational orbital window.

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Operations (1)

• The manoeuvre is calculated using, for example
  for Helios satellite, the following approached
  formula
• d -5,745.Da.Dt
• with d avoidance distance in Km
• Dt duration between end of the manoeuvre and
  collision date
• Da orbital altitude increment in Km
• In case of emergency concerning the date of
  cross-over, we can consider a special case
  leaning to avoid the debris between the two
  thrusts

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Operations (2)

• We have to compute the manoeuvre using the
  following parameters
• - instant of the crossing over,
• - safety distance wanted
• - direction of the manoeuvre with respect to
  the equator phasing
• "forward" Dalt0, if we are west side of the
  specification,
• "backward" Dagt0, if we are east side of the
  specification
• - date of the first thrust
• - the fall of pressure in the tanks
• (compatible with the relative pressure
  difference allowed between the
• two tanks, with respect to the mechanical
  constraints of the tanks
• switching valves)

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Chronology
The chronology of the operations can be
synthesised as shown here, to be able to give the
minimum delay needed to be able to react to a
collision risk.
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Return to operational mode

• As soon as the manoeuvre has been successfully
  realised, we have to do a manual orbit
  restitution to prepare the come back manoeuvre.
• To be able to proceed successfully to these
  return operations, we must verify the following
  points
• - the availability of ground stations to be
  able to actualise the orbit, and
• to load the commands with respect to the
  necessary chronology
• - the validity of the restitued orbit issued
  from the first manoeuvre,
• - the evolution of the pressure in the
  propellant tanks
• to obtain the right thrust, and to respect
  the maximum pressure
• difference allowed between the two.
• If necessary, we will have to commute the tanks
  before the next manoeuvre.

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Particular consequences

• Realising an avoidance manoeuvre in an emergency
  context can imply to take into account external
  considerations as
• - a risk for the satellite, due to the
  realisation of 2 or 3 altitude manoeuvre,
• - an adaptation of the mission for 2 to 4
  days depending on the manoeuvres,
• - a degradation of the orbit restitution for
  several days after,
• - a problem of effective availability of
  ground stations,
• - a very important consumption of ergols
• - the COO feed-back to verify that no new
  collision risk with another debris
• may occur after this new manoeuvre