SPOT 4
innovations,
Innovations at the SPOT 4
operations control centre,
Innovations at the
SPOT 4 programming centre,
Innovations
at the SPOT 4 archiving and preprocessing centre
Innovations at the
SPOT 4 image quality centre.
With the arrival of SPOT 4, the entire Spot system has been extensively
upgraded to offer higher performance, impoved operational services and better
all-round response to the needs of Spot Image customers.
The SPOT 4 satellite underwent a thorough design review
and upgrade compared with its predecessors. Among a wide variety of improvements and
refinements, some of the most important are:
- The payload includes a new instrument, Vegetation, which, as
its name indicates, is used for operational monitoring of vegetation cover; more
specifically, Vegetation will provide daily, global information on the continental
biosphere and crops. The two HRVIR instruments and Vegetation have been designed to complement
each other : B2, B3 and SWIR bands all register perfectly, allowing data to be
interpreted at several scales. It will thus be possible to take advantage of
Vegetation's high revisit capability (one global image per day, with a resolution
of 1 km) and of the high resolution of the HRVIR instruments (10 metres).
- The new satellite architecture integrates the lessons learnt from SPOT 1, 2 and 3
and offers increased reliability (a more modular design, improved
interface compatibility, increased redundancy, limitation of single-failure points, etc.).
- A new shortwave infrared band ( from 1.58 to 1.75 µm) enables early
observation of plant growth through analysis of vegetation water content. The main
SPOT 4 instrument is called HRVIR as opposed to HRV for its three predecessors.
- The two (HRVIR) imaging instruments are
entirely independent of each other: in fact, one of them can acquire
imagery while the other moves its strip- selection mirror in
order to focus on a specific area. This is an extremely useful innovation since it allows
easier acquisition of isolated targets ( areas equal to a standard SPOT scene,
i.e. 60 x 60 km), or less, which account for almost three-quarters of customer orders.
- The onboard recorders are based on an all-new design
(made in France by Enertec); their storage capacity has not only been doubled
(40 minutes compared with 22 for SPOT 1, 2 and 3), but this function has been
reinforced by incorporating a "solid-state memory" which can
hold 10 Gbits. The advantage is that access to imagery data is as easy as with a
CD, i.e. it is quick and direct, whereas with recorders, the data have to be read
in the sequence in which they were recorded: this means even quicker access to Earth
imagery when needed, for example following a natural disaster.
- Image data can still be sent as they have been up to now, to the score or so Direct Receiving Stations, but
also, using a laser link, to a relay satellite, in geostationary orbit:
this bold experiment using a PASTEL terminal is being conducted by the European Space
Agency (the " Silex " experiment).
- The HRVIR instruments are protected against polarization or blinding by
direct sunlight.
- The radiometric quality of the imagery now offers greater stability of
instrument responses, thus facilitating comparison or mosaicking of scenes of the same
place, taken at different times or under different conditions (generation of satellite image maps). The same is true for the accuracy in
determining scene coordinates, by optimizing use of DORIS.
These improvements are very important, since up to now, some SPOT scenes could not be
validated due to poor radiometric quality or to the geographic area not being entirely
covered.
- Passenger payloads include:
- PASTEC, which will be used to study the satellite's environment
during launch and in orbit (acquisition of thermal, radiation and micro-vibration data).
- The DIODE software will be tested for real-time
calculation of the satellite's position using data provided by DORIS ; this position will be relayed to the ground
as image telemetry auxiliary data.
- A radar transponder will be used to calibrate certain ground radars
(including those at the CNES space centre in French Guiana).
- PASTEL is a component of the SILEX experiment which will send
SPOT 4 imagery to the Artemis geostationary relay satellite.
- ESBT is a " spread-spectrum transponder " supplied by
the European Space Agency. The aim is to validate a direct 4 Kbit/sec link between
SPOT 4 and the control centre in Redu, Belgium, via the Artemis satellite.
- POAM III will follow-on from the American POAM II mission
(flown on SPOT 3), to continue taking measurements of the ozone layer and aerosols
above the Earth's poles. This instrument has been provided by the US navy's "Naval
Research Laboratory" .
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The SPOT 4 satellite operations control centre is called in French the "CMP
(Centre de Mise et Maintien à Poste SPOT 4)" ; it is the ground component which
tracks and controls the satellite throughout all orbital phases:
The SPOT 4 CMP is another member of the CMP family
which includes dedicated operational centres for SPOT 1 and SPOT 2.
These centres are at the CNES site in Toulouse. By sharing facilities a single team can
manage continous operation of all SPOT satellites. Naturally each CMP has its own
characteristics, depending on the satellite for which it is responsible, but all have the
same basic functions.
In fact, each CMP consists of two parts:
The CMP now has a third entity for managing and programming the Vegetation payload.
This is the Vegetation Programming Centre (CPV).
The major innovation of the CMPs is increased automation procedures at
CCS, CGS and CPV which are performed according to a pre-determined schedule based on
satellite rendez-vous with the ground segment (regular, daily passes over the TT&C stations). Each day a CMP can monitor and command a
satellite during five passes of about 10 minutes each. These are high points during each
day's operational routine, which involves preparing data for uploading to the satellite,
interpreting data received from the satellite or exchanging information with outside
bodies (the Programming Centre, the Archiving and
Preprocessing Centre, …)..
In general, human intervention mainly consists of seeing that these tasks are correctly
performed. Of course, in the event of difficult operations (changing the satellite's orbit,
uploading a new version of the onboard software, etc.) or satellite anomalies, the
operator can always take over very quickly.
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Upgrades at the Programming Centre, based on feedback from
programming of SPOT 1, 2 and 3, mean that Spot Image now has an efficient tool to
exploit SPOT system imaging capabilities to the full:
- Daily cloud cover forecasts for almost the whole world, supplied by
Météo-France, Toulouse, allows for a 30% reduction in the number of cloudy
scenes.
- The programming centre's new capabilities allow it:
- to compile, in anticipation of customer requests, uniform archive surveys
(i.e. large surveys using a given spectral mode).
- to acquire same-day stereopairs using two SPOT satellites. The orbital
configuration of SPOT 1 and 2, or SPOT 2 and 4, is such that stereopairs of
certain regions of interest can be acquired on the same day, the acquisition times for the
two scenes (with different viewing angles) differing by just 20 minutes or so.
- to use two or three SPOT satellites, thus significantly increasing the system revisit
capability and correspondingly reducing the time needed to acquire coverage of any region
of interest.
- to offer direct receiving
stations a new programming service, known as SIP, whereby the Spot Image programming
centre programs their service requests to give them the benefit of its optimization
resources.
- Thanks to its new scheduling capabilities and access to both global
climate data and daily cloud cover forecasts, the programming centres can also give
customers accurate estimates of the likelihood of successfully acquiring any given
programming request. More detailed and more frequent progress reporting allows
the centre to provide customers with a range of information, including:
- latest estimated likelihood of PR acquisition (this figure being updated every day
according to the number of "successful" scenes acquired the day before and the
impact of any new PRs to be taken into account),
- the number of successful scenes acquired in response to a given PR,
- the time remaining until the end of the PR survey period.
- Ground measurements of reflectance data for all SPOT scenes acquired since 1986 have
been analysed to adjust the sensor gains according to the type of landscape and the month
of the year. Indeed, one of the programming centre's key roles is to select the optimal
sensor gains, each day and for each scene to be acquired, to ensure that the dynamic range
of each is as large as possible.
- The orbital slots allocated to a direct receiving station can now be programmed so that
real-time image data are encrypted and that only the target station has the decryption
key. This ensures that each receiving station's programming requests remain strictly
confidential. This is important, since almost half of all the PRs received by Spot Image
come from direct receiving stations, many of which have overlapping "acquisition
circles".
- Software development is now based on the Ada programming language and Hierarchical
Object Oriented Design (HOOD), thus considerably improving the adaptability and
reliability of every program. This has also facilitated the introduction of changes and
improvements without giving rise to the type of functionality regression used over the
last three years with SPOT 1 and 2.
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Inventory and scene production capabilities for image telemetry received at Aussaguel
or Kiruna have been upgraded significantly with the creation of a new archiving and
preprocessing centre (CAP) at Spot Image in Toulouse and at Satellitbild in Kiruna. This
means that it is now possible to provide users with products in less than one day when the
scene is either available in the catalogue or has been acquired without clouds.
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Know-how acquired with the first SPOT satellites has made it possible to create an
operations centre which image quality experts can use for accurate, real-time performance
monitoring.
Moreover the QIS team has been able to use instrument calibrating data (dark images,
calibration lamp images, images of the Sun or images of ground calibrated sites) to
regularly update parameters for geometric and radiometric processing; these calibration
coefficients are sent to all image production centres to ensure imagery of consistently
high quality.
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page updated on the 00-06-06