The SPOT (Satellites Pour l’Observation de la Terre or Earth-observing Satellites) remote-sensing programme was set up in 1978 by France in partnership with Belgium and Sweden.

The SPOT satellites constellation offers acquisition and revisit capacity allowing to acquire imagery from anywhere in the world, every day. Each SPOT payload comprises two identical high resolution optical imaging instruments, which can operate simultaneously or individually in either panchromatic (P mode: a single wide band in the visible part of the spectrum) or multispectral mode (XS mode: the green, red, and infrared bands of the electromagnetic spectrum). The orientation of each instrument’s strip-selection mirror can be remotely steered by the ground stations, offering an oblique viewing capability up to angles of +/- 27 degrees from the satellite’s vertical axis. In this way, the temporal resolution is shortened from 26 to 4-5 days for the temperate zones. On SPOT 5 , the HRS (High-Resolution Stereoscopic) imaging instrument also allows simultaneous acquisition of stereopairs.

Since the deorbitation of SPOT 2 in 2009, after almost 20 years of service, satellites SPOT 4 and 5 together ensure the provision of high-resolution SPOT images and of VEGETATION global images.

The continuity of the SPOT program is planned with the development of the Pleiades system, as well as Spot 6 and 7, which should offer 2-meter resolution images in a 60-km by 60-km swath. SPOT 6 and 7 are scheduled to launch in 2012 and 2014 respectively.

As for the VEGETATION Continuity Program, it will be provided by the small Belgian satellite Proba V (launch planned for 2011-2012) and by the Sentinel-3 satellite developed in the framework of GMES (launch planned in 2013).

SPOT 1, 2 & 3
The first three satellites were identical and their payloads consisted of two identical HRV (Visible High-Resolution) optical instruments, data recorders (on magnetic tapes), and a system for transmitting the images to the ground-based receiving stations (downlink).

HRV sensors

SPOT 4
On Spot 4, the two identical optical instruments are HRVIR (Visible & Infrared High-Resolution) sensors. Spot 4 has also onboard the first VEGETATION instrument, developed for observation at global level.

altitude: 822 km
inclination: 98.7 degrees
orbit: sun-synchronous polar
period of revolution: 101 minutes
swath width: 60 x 60 to 80 km
repeat cycle: 26 days
satellite: SPOT 4 (24/03/1998 – still operational)

HRVIR sensors
The HRVIR sensors are very similar to the HRV sensors of the previous generation (same spatial resolution and possibility of orienting the mirrors). However, they differ by:
• the presence of an additional spectral band in the middle-infrared band;
• the panchromatic (0.51-0.73 µm) band’s being replaced by the B2 (0.61-0.68 µm) band, which can function equally well in ‘10m’ and ‘20m’ mode; and
• onboard superimposition of all of the spectral bands.

VEGETATION Sensor
The VEGETATION programme is co-financed by the European Union, Belgium, France, Italy, and Sweden and being conducted under the supervision of the CNES (National Centre for Space Studies, France). The aim of the VEGETATION instrument is to provide accurate measurements of the main characteristics of the Earth’s plant cover. Practically daily global coverage and a resolution of 1 km make this sensor an ideal tool for observing long-term regional and global environmental changes.
VEGETATION works independently from the HRVIRs. It includes a wide-angle radiometric ‘camera’ operating in four spectral bands (blue, red, near-infrared, and middle-infrared). Given its 2,250km swaths, this instrument is thus able to cover almost all of the Earth’s dry land in just one day.

Several products are available, including daily and ten-day synthesis products (at full resolution as well as 4km and 8km reduced resolutions) for the geographical areas that the user has defined as well as for complete global coverage. The VGT images are processed, archived and distributed by the Belgian research institute VITO. VITO is also responsible for distribution in Belgium. Archive data older than 3 months are available for free on the website http://free.vgt.vito.be.

SPOT 5
The main payload consists of high resolution imaging instruments delivering the following product improvements compared to Spot 4:
- the HRS (High-Resolution Stereoscopic) imaging instrument dedicated to taking simultaneous stereopairs of a swath 120 km across and 600 km long;
- a ground resolution of 5 and 2.5 metres in panchromatic mode;
- a resolution in multispectral mode of 10 m in all 3 spectral bands in the visible and near infrared ranges.

The spectral band in the short wave infrared band (essential for VEGETATION data) is maintained at a resolution of 20 m due to limitations imposed by the geometry of the CCD sensors used in this band,

The Spot 5 spectral bands are the same as those for Spot 4 (see below). The panchromatic band does, however, return to the values used for Spot 1-2-3. As requested by many users, this ensures continuity of the spectral bands established since Spot 1.

altitude: 822 km
inclination: 98.7 degrees
orbit: sun-synchronous polar
orbit period: 101 minutes
swath width: 60 x 60 to 80 km
repeat cycle: 26 days
satellite: SPOT 5 (04/05/2002 – still operational)

HRG sensors (High Resolution Geometric) sensors
Two HRG instruments are capable of generating data at 4 resolution levels with the same 60 km swath.

HRS sensors
The ability to acquire stereopair images quasi-simultaneously (90 sec) is a considerable advantage for the quality of digital elevation model (DEM) production. The resemblance between the two images is indeed maximum.

Characteristics
- spectral band: panchromatic
- resolution: 10 m, along the track sampling: 5 m
- imaging swath (centred on the satellite track): 120 km
- maximum scene length: 600 km
- viewing angle of the telescopes:+ and - 20°

VEGETATION 2 Sensor
The VEGETATION sensor remains unchanged in comparison to the one installed onboard SPOT 4 and ensures the continuity of global data delivery

Source: http://eoedu.belspo.be