Table 1. Major capabilities of Envisat
• Sun-synchronous orbit: 800 km, 10 a.m.
descending node, 35-day repeat cycle
• Stellar yaw steering, for accurate pointing and
Doppler compensation of SAR
• 1900 Watts, 2500 kg for instruments
• Data recovery at up to 100 Mbps direct via
X-band or via Artemis
• On-board storage in solid-state recorders for
regional and global missions
• S-band command and control; 2 kbps uplink,
4 kbps downlink
rbulletin 106 — june 2001
26
The Envisat Satellite and Its Integration
P.A. Dubock, F. Spoto
Envisat Programme, ESA Directorate of Earth and Environment Monitoring from
Space, ESTEC, Noordwijk, The Netherlands
J. Simpson, D. Spencer
Astrium Ltd., Bristol, UK
E. Schutte & H. Sontag
Astrium GmbH, Friedrichshafen, Germany
Introduction
Envisat is the largest and most complex freeflying
satellite ever built in Europe. It will carry a
comprehensive series of instruments designed
to observe a whole series of interrelated
phenomena that characterise the behaviour of
the Earth’s environment as a system. The
satellite, together with its related ground
systems, will continue and extend the data
services provided by the Agency’s earlier ERS-
1 and ERS-2 satellites. In particular, Envisat
should substantially increase our knowledge of
the factors determining our environment. It will
make a significant contribution to environmental
studies, notably in the areas of atmospheric
chemistry and ocean studies, including marine
biology.
The observations made by Envisat will
eventually be continued and extended by a
series of new, smaller satellite programmes
being initiated within the Agency’s Earth
Observation Envelope and Earth Watch
Programmes.
Background
The Columbus Programme approved at the
ESA Ministerial Council Meeting in The Hague
in 1987 included the development of a multimission
Polar Platform as part of the
International Space Station. Following a series
of studies and iterations with potential users, an
implementation re-using the equipment and
architecture of the Spot-4 spacecraft bus
design, although with a significantly enlarged
structure, was decided upon. The main
development phase (Phase-C/D) for the Polar
Platform programme was awarded to British
Aerospace in Bristol (UK) – later to become
Matra Marconi and now Astrium Ltd. – in late
1990.
Meanwhile, in the Earth Observation area, the
Agency was considering, as ERS-1 grew closer
to launch, how to continue and extend the
services offered. In 1988, these elements were
drawn together in an ESA proposal to its
Member States for an overall ‘Strategy for Earth
Observation’. These considerations led to the
adoption of the POEM-1 programme, using the
Polar Platform, at the Ministerial Council
Meeting in Munich in November 1991. There
continued to be an evolution in the payload
complement for POEM-1. This culminated in a
splitting of the payload into separate Envisat
and MetOp satellites, which was finally agreed
at the next Ministerial Council in Granada in
November 1992. A Phase-C/D contract for the
procurement and support of the Envisat
payload (the so-called ‘Mission Prime Contract’)
was awarded to Dornier Satellitensystem, now
Astrium GmbH, in July 1992.
For the early years of this new millennium, Envisat is ESA’s major
contribution to the study of the Earth as a system. Carrying ten
sophisticated instruments – both optical and radar – it is the largest
and most complex satellite ever built in Europe. It has been designed
and tested over a period of more than 10 years. Much of the
integration and test programme was conducted on site at ESTEC, in
Noordwijk (NL). It will be the first satellite launched into a polar orbit
by Ariane-5.
This article summarises the design and engineering of Envisat, and
explains the model philosophy and test approaches used. The launch
campaign plans are also briefly described.
The satellite makes use of the multi-mission
capability of the Polar Platform originated in
the Agency’s Columbus Programme. This
development also forms the basis for MetOp.
The Polar Platform in turn has drawn heavily on
the equipment and technologies developed
within the framework of the French Spot
programme. Almost all of the instruments on
the satellite have been specifically developed
for Envisat, with one or two having a strong
design heritage from ERS.
the envisat satellite and its integration
27
As a result of the programmatic origin of the
satellite, there remain two large contracts for its
implementation:
– the Polar Platform Prime Contract (Astrium
Ltd.), and
– the Mission Prime Contract (Astrium GmbH).
These two large contracts, interfacing with
each other at some of the technically most
critical on-board locations, caused a number of
problems during the development programme.
The satellite integration programme has,
however, largely been carried out at ESTEC
following the closure of Astrium’s Bristol site. As
a result, many of the technical personnel have
been collocated (with ESA) at ESTEC. This, and
the grouping of both contractors within the
Astrium company, has ensured a much
smoother technical path for the programme in
its final phase.
The organisation of the Agency’s project teams
initially also reflected the programmatic split,
with separate project divisions for Polar
Platform and payload. More recently the project
teams hav
More recently the project
teams have been merged within a single
division. This too has simplified the technical
conduct of the programme.
Major capabilities
The satellite is designed for a Sun-synchronous
polar orbit (Table 1). The planned operating
altitude is 800 km, although a range of altitudes
can be selected allowing variations in the
repeat cycle of the ground track. The local time
at the equator for the descending node has
been selected as 10.00 a.m., which optimises
illumination conditions for part of the optical
payload.
The selected orbit has a repeat cycle of 35
days and an orbital period of 100.6 min. Its
inclination is 98.54 deg, which implies small,
uncovered areas at the poles for instruments
with limited swath widths. The on-board
systems allow the ground track to be
maintained within 1 km and the local hour to
within 5 min. One of the on-board instruments
(DORIS), when used in conjunction with a
dedicated set of ground stations,
provides realtime
knowledge of position to within 50 cm,
and a precision altitude restitution to within 5
cm.
In nominal operations, the satellite is pointed
using star trackers in a ‘stellar yaw-steering
mode’. In this mode, the satellite is yawed to
compensate for the apparent motion of the
Earth across-track beneath the satellite. This
compensation simplifies the processing of
Doppler signals from the synthetic-aperture
radar. When using the star trackers, random
pointing errors will be less than 0.0085 deg,
and the stability over all periods of up to
170 s better than 0.015 deg. Attitude
estimation will be better than 0.04 deg. This
pointing performance allows both adequate
geographical location for data measured on the
Earth’s surface, and a vertical resolution when
viewing the atmosphere at the limb of better
than 3 km.
The satellite provides an average of 1900 W for
instrument operations through sunlit and
eclipse portions of the orbit. This enables all
instruments except MERIS and ASAR to be
operated continuously throughout the entire
orbit. MERIS, the Medium-Resolution Imaging
Spectrometer, requires sunlight to operate.
ASAR, the Advanced Synthetic-Aperture
Radar, produces such enormous quantities of
data in its high-resolution mode that its
operation is limited to 30 min per orbit.
The SM includes eight batteries, and the solar
array. This is a flat-pack array designed for the
Polar Platform by Fokker Space (NL), based on
their standard design elements. Once deployed,
the array is rotated to point continuously towards
the Sun using a solar-array drive mechanism,
which is attached to the base of the central cone.
The propulsion module on top of the cone
contains four tanks, which hold 300 kg of
hydrazine.
A single central computer containing both
command and control and AOCS functions
performs on-board data management. It
controls the SM equipment via a standard onboard
data-handling bus. The central computer
also communicates with the central computer
of the PLM via the same bus.
The data-handling capabilities of the spacecraft
have also been sized to support the global and
regional missions. All instruments except
MERIS and ASAR operate continuously,
together producing 4.6 Mbps. There are
separate additional regional missions for
MERIS (up to 25 Mbps) and ASAR (up to 100
Mbps). On-board storage in redundant solidstate
recorders allows the recording and
dumping of all data from the global and regional
missions. Data can be downlinked directly
when overflying a suitable ground station, such
as Kiruna in Sweden, via a fixed X-band
antenna, or when within visibility of Artemis, via
a steerable Ka-band antenna.
Command and control of the satellite and
payload is via an S-band transponder. The
satellite will normally be operated by uplinking a
24-hour command timeline. Housekeeping
data is available in real time when over a
ground station, but is also included in the global
mission data stream.
The satellite is designed for launch only by
Ariane-5. It has a total launch mass of 8100 kg,
of which 2150 kg are instruments. The physical
size of the spacecraft requires the Ariane-5
long fairing, for which Envisat is the first
customer.
Major components
The satellite is made up of two major subassemblies,
the Service Module (SM) and the
Payload Module (PLM), with a simple structural,
electrical and avionics interface between them.
All instruments are physically located on the
PLM, to which the instruments have a largely
standardised interface. The modularity of the
design has made it possible to conduct by far
the majority of the integration work on the SM,
PLM and instruments in parallel.
Service Module
The SM provides the standard satellite support
functions, and was subcontracted to Astrium
SAS. It is based on the design of the
Spot Mk-
II service module, but with a number of
important new developments, particularly in the
structure and solar-array areas (Fig. 1).
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