UK SG4
CP(02)07
Inter Agency Space Debris Coordination Committee
Recommendation on Disposal of Satellites from the
Geostationary Region at End of Life
Background
During the 15 th meeting of the Inter Agency Debris Committee (IADC), held in
Houston in 1997, the Working Group responsible for addressing debris mitigation issues reviewed the International Telecommunications Union (ITU) recommendation on geostationary orbit (GEO) protection. In order to assess the ITU recommendation, studies were presented by individual agencies relating to the prediction of long term orbital stability in the geostationary region. As a result of these discussions, the Working Group reached a consensus on an alternative proposal, defining both an empirical formula and operational procedures that should be applied to GEO satellites at end of operational life (EOL). This proposal was presented to the Steering Group of the IADC and was subsequently endorsed. This paper summarises the rationale and approach adopted by the IADC when addressing the GEO EOL issue and presents a series of qualitative and quantitative recommendations.
Rationale
The GEO region is a unique resource that offers significant benefits to operators from the standpoint of station-keeping requirements, ground visibility and coverage, and a relatively benign orbital environment. It must therefore be protected to ensure future exploitation in a cost-effective and sustainable manner. Given the current limitations (primarily specific impulse) of space propulsion systems, it is impractical to retrieve satellites from GEO altitudes or to return them to Earth at the end of their operational life. To avoid an accumulation of redundant platforms in GEO, and the associated increase in population density and potential collision risk that this would lead to, satellites should be manoeuvred out of the GEO region at the end of their operational life. In order to ensure that these redundant satellites do not present a collision hazard to satellites being injected into the GEO region, they should be manoeuvred to higher, rather than lower, altitudes. The target disposal altitude should be sufficiently high that, under the influence of perturbing forces, the satellite cannot interfere with existing operational satellites in the GEO region. A protected region must therefore be established which defines the nominal orbital regime within which operational satellites will reside.
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Re-orbiting Requirement
The basic requirement is that following disposal into a higher altitude orbit, the spacecraft, which will be under the influence of perturbing forces, must not migrate back into the region of space to be protected:

H -

> h (1) minimum altitude threshold for re-orbiting
 altitude perturbation

H h manoeuvre corridor station-keeping zone geostationary altitude, 36000 km h

H
 minimum altitude increase in GEO plane of re-orbited spacecraft maximum descent of re-orbited spacecraft due to perturbations minimum altitude of space above GEO which is to be protected
GEO Protected Region
It was agreed that h , the region to be protected above GEO, should be 200 km based on the following:
1) The classical station keeping window of +/- 0.1 degrees in the
North/South and East/West directions defines the on-station regime in relation to the nominal geostationary altitude, and corresponds to: a.

75 km perpendicular to the orbital plane b.

75 km tangential to the orbit c.

37.5 km in the radial direction (or altitude)
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2) Above this altitude an operational corridor exists for effecting changes in the longitude of a spacecraft, extending from 37.5 km to 200 km above the nominal geostationary altitude.
Perturbations to a satellite in a supersynchronous orbit
The motion of a satellite injected into a super-synchronous altitude orbit will be perturbed in a periodic manner due to the influence of:
 the gravitational influence of the asphericity of the Earth
 the gravitational attraction of the Sun and Moon
 the radiation pressure of the Sun
The overall orbital perturbation

can be represented empirically by two components. The combined influence of the periodic gravitational perturbations should not exceed 35 km for any satellite, or:
 grav
< 35 km (2)
The maximum extent of the solar radiation pressure (SRP) perturbation will depend upon the individual characteristics of a satellite and is given (in km) by:

SRP
< 1000 Cr A/M (3) where:

=
 grav
+

SRP
C r
is the reflectivity coefficient of the satellite at beginning of life and will vary between between 1 and 2 depending upon its surface characteristics
A is the aspect area of the satellite exposed to the Sun (m 2 )
M is the dry mass of the satellite (kg)
A/M will typically take a value between 0.01 and 0.1 depending upon the characteristics of the satellite.
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Combining equations (1), (2), and (3) gives the minimum reorbit altitude requirement to ensure that the satellite, following end of life disposal, does not return within 200 km of the geostationary altitude:

H > 235 + 1000 Cr A/M (4)
Fuel Budget and Margin
Spacecraft operators are encouraged to monitor the use of on-board propellant to ensure adequate fuel is available to achieve the required manoeuvre at end of life. In addition it is recommended that a fuel margin be added to the budget to effect a change in orbit which exceeds the minimum threshold in order to reduce the effect of orbital determination inaccuracies, possible execution errors, and the consequences of spacecraft deterioration and anomalous fragmentation following injection into the disposal orbit.
It is recommended that a multiple manoeuvre (minimum of 3 burns) strategy be followed to raise the orbit perigee to the projected minimum altitude, thereby minimising the consequences of failure of the propulsion system due to either malfunction or inadequate fuel margin.
Once the minimum perigee altitude has been reached, the semi-major axis should be increased without decreasing perigee, using all remaining propellants and, if feasible, pressurants.
Once any remaining propellants and pressurants have been expended, all further stored energy sources on-board should be passivated (e.g. batteries, gyros) to avoid the possibility of explosive fragmentation.
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