SPOT

Overview

SPOT - the Satellite Earth Observation System was designed in France, and developed with the participation of Sweden and Belgium. The SPOT satellites capture panchromatic and multispectral imagery in resolutions ranging from 2.5m to 20m. The satellites are fitted with two independent imaging instruments, containing detector arrays that operate using a "Push Broom" technique. This results in high geometric accuracy across the full 60km wide swath. Each instrument is also fitted with a steerable mirror that allows it to image areas up to 27 degrees east or west off the vertical, which increases the revisit capability and provides stereo imagery for digital elevation modelling. Latest in the series of these satellites is the SPOT 5 satellite.

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Sensor Information

SPOT 5 carries two HRG instruments (High Resolution Geometric), derived from the HRVIR instruments on SPOT 4, offer higher resolution up to 2.5 meters in panchromatic mode. HRS imaging instrument (High Resolution Stereoscopic) able to acquire stereopair images almost simultaneously for optimized production of digital elevation models.

SPOT 5 platform
Total mass	3,000 kg
Solar array (end of life)	2,400 W
Dimensions	3.1 x 3.1 x 5.7 m
Lifetime	5 years
Launch vehicle	Ariane 4
Recording capacity	90-Gbit solid-state memory, 550 images
Onboard image processing	Up to 5 images, 2 downlinked in real time, 3 stored onboard using a 2.6 compression ratio
Image telemetry link (8 GHz)	2 x 50 Mbps
Orbit determination	DORIS in real time, 5 m rms
Location accuracy	50 m

HRG instruments in twin-vertical viewing configuration

SPOT HRG Technical data
Mass	356 kg
Maximum power (depending on mode)	344 W
Dimensions	2.65 x 1.42 x 0.96 m
Oblique viewing angle	± 27°
Focal length	1.082 m
Field of view	± 2°
Performance	Panchromatic	B1 B2 B3	SWIR
Spectral range (panchromatic band)	0.49-0.69 µm	B1 0.50-0.59 µm B2 0.61-0.68 µm B3 0.79-0.89 µm	1.58-1.75 µm
Detectors per line	12,000	6,000	3,000
Number of lines	2, offset	3, registered	1
Detector pitch	6.5 µm	13 µm	26 µm
Integration time per line	0.752 ms	1.504 ms	3.008 ms
Ground sample distance	5 x 5 m single image 3.5 x 3.5 dual image	10 x 10 m	20 x 20 m
Signal-to-noise ratio	170	240	230
Modulation transfer function	> 0.2	> 0.3	> 0.2

Products at a resolution of 2.5 metres are generated using a unique sampling concept dubbed Supermode, patented by CNES. Supermode uses sophisticated processing techniques to yield a 2.5-metre image from two 5-metre images. These two images are acquired simultaneously by a sensor with two rows of 12,000 CCD detectors offset by one-half pixel across the satellite ground track and by 3.5 pixels along track.

Stereopair acquisition by the HRS instrument.

SPOT HRS Technical data
Mass	90 kg
Power	128 W
Dimensions	1 x 1.3 x 0.4 m
Field of view	± 4°
Focal length	0.580 m
Detectors per line	12,000
Detector pitch	6.5 μm
Integration time per line	0.752 ms
Forward/aft viewing angle relative to vertical	± 20°
Performance
Spectral range (panchromatic band)	0.49 μm - 0.69 μm
Ground sample distance Across track Along track	10 m 5 m
Modulation transfer function	> 0.25
Signal-to-noise ratio	> 120

The HRS imaging instrument points forward and aft of the satellite, giving it the ability to acquire stereopair images almost simultaneously in a single pass. HRS does not have an oblique viewing capability. It observes a swath of 120 kilometers centered on the satellite ground track, with a repeat cycle of 26 days. Resolution is 10 meters in panchromatic mode and elevation accuracy will be better than 15 meters.

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Product Types

SPOT data are offered in mainly two product types:

SPOT Scene
SPOT View

SPOT Scenes are standard products which undergo basic preprocessing. They are available at several pre-processing levels (1A/1B, 2A). SPOT Views supply up-to-date information in the form of extremely reliable product underlays which can be used directly in a geographic information system (GIS) or map-making software.

SPOT Scene Products

Level 1A

Level 1A imagery is corrected by normalizing CCD response to compensate for radiometric variations due to detector sensitivity. No geometric corrections are performed. Level 1A preprocessing thus leaves data in almost raw form.

Level 1B

Level 1B applies the same radiometric corrections as level 1A. Geometric corrections compensate for systematic effects, including panoramic distortion, the Earth's rotation and curvature, and variations in the satellite's orbital altitude.

Level 2A

Level 2A scenes are rectified to match a standard map projection (UTM WGS 84), without using ground control points. Level 2A is the entry-level map product. A global DEM with a post spacing of one kilometre is used. Geometric corrections use a resampling model that compensates for systematic distortion effects and performs transformations needed to project the image in a standard map projection (UTM WGS 84). This model is based on known viewing parameters (satellite ephemeris data and attitude, etc.) and does not use external measurements. Other map projections or mean rectification elevations are available on request.
Size
Depending on the surface of the area to be studied:

complete scenes: 60 km long x 60 to 80 km wide, according to the viewing angle.
extracts from subscenes
(2.5 m and 5 m in black and white and 10 m in colour):

1/2 scene : ~ 42 km x 42 km
1/4 scene : ~ 30 km x 30 km
1/8 scene : ~ 21 km x 21 km

SPOT View Products

Level 2B

Level 2B imagery is georeferenced, that is, scenes are framed in a given map projection and tied to ground control points (GCPs, obtained from a map or topographic surveys) for even better location accuracy.

Level 3

Level 3 imagery is georeferenced like level 2B. Level 3 products, also called orthoimages, are pre-processed using a digital elevation model (DEM) to correct residual parallax errors due to relief. Geometric corrections consist in "orthorectifying” imagery using a resampling model that compensates for systematic distortion effects and performs transformations needed to project the image in a specified map projection (Lambert conformal, UTM, oblique equatorial, polar stereographic, polyconic, etc.). Corrections are based on a model of the satellite’s flight dynamics and on GCPs and a DEM.

Levels of preprocessing

Size
Depending on the surface of the area to be studied:

complete scenes: 60 km long x 60 to 80 km wide, according to the viewing angle.
standard mapsheets centred on an area of interest (digital mosaic of several Spot scenes)

30’ x30' : ~ 54 km x 54 km at the equator
15’ x15' : ~ 27 km x 27 km at the equator
7’ 30" x 7’30" : ~ 13 km x 13 km at the equator

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Preprocessing levels and location accuracy

The location accuracy is given at one sigma (1 σ) for all levels. It is evaluated on the basis of a statistic calculated from a large number of scenes acquired from September 2003, across the globe. The location accuracy indicated for levels 1A, 1B and 2A applies to flat terrain.

SPOT Elevation Products

Besides 2D imagery products, SPOT also provides 3D products which contain elevation information as well. The range of SPOT 3D products includes:

SPOT DEM
SPOT DEM Precision
the Reference3D database

HRS validated coverage map and available Reference3D tiles - 21 May 2008

Reference3D, SPOT DEM Precision and SPOT DEM products are available off the shelf and deliverable rapidly for areas in blue (Reference3D tiles).

SPOT DEM

A SPOT DEM is a digital elevation model produced by automatic correlation of stereopairs acquired by the HRS instrument on SPOT 5. Unlike a digital terrain model (DTM), which describes the distribution of terrain or “bare earth” heights, whereas a DEM includes “first surface” elevations including vegetation and man-made structures.

Sampling

A SPOT DEM is a gridded file with step sizes of 1 second of arc (30 m on the equator, E/W dimensions vary according to the latitude).
Resampling to 20 m available.

Absolute horizontal accuracy without ground control points

The absolute horizontal accuracy is the positional error in the DEM with respect to the ground truth. For a SPOT DEM, this depends on the dimensions of the area of interest or on the availability of Reference3D (French defense database) data for this area.

Absolute vertical accuracy without ground control points

The absolute vertical accuracy is the elevation error in the DEM with respect to the ground truth. It depends mainly on the slope of the terrain.

SPOT DEM Precision

SPOT DEM Precision is the only DTED level 2 product of its kind on the market. It contains two layers of information:

An HRS DEM
Quality and Traceability metadata

Sampling

SPOT DEM Precision is a gridded file with step sizes of 1 second of arc (30 m on the equator, E/W dimensions vary according to the latitude).
Resampling to 20 m available.

Precision

SPOT DEM Precision offers unmatched performance and coverage of more than 15 million km² (50 million km² by 2009).

Reference 3D

Developed in partnership by Spot Image and the French survey and mapping agency IGN, Reference3D is a geocoded database containing three layers of information:

a DTED level 2 HRS DEM
GPS-compatible HRS Orthoimage
quality and traceability metadata

Reference3D is the keystone of any application that calls for very-high-accuracy elevation data.
Reference3D is unrivalled in the world for accuracy, coverage, with more than 15 million km² already covered (50 million km² by 2009).

DEM

These files contain a uniform grid of terrain elevation values of an area of interest and are obtained through automatic correlation of SPOT HRS stereopairs.

Sampling step: 1 second of arc (~ 30 m at the equator, varying according to latitude)
Absolute elevation accuracy: 10 m @ 90% for a slope < 20°
Absolute planimetric accuracy: 15 m @ 90%

HRS orthoimages

These images are orthorectified from the DEM and have a high degree of geometric quality and location accuracy

Absolute planimetric accuracy: 16 m @ 90%
Sampling step: 1/6 second of arc (~ 5 m at the equator - varying according to latitude)

Quality and traceability data

These are quality indicators for data sources and provide references and footprints for DEMs and orthoimages,
They describe the processing done for producing DEMs (masks),
They give accuracy estimates.

Size

Reference3D comes in geographic tiles of 1°x 1° covering emerged land areas and aligned along parallels and meridians.

The surface area of a tile, equivalent to 111 km x 111 km at the equator, decreases as the latitude increases.

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An unlimited source of ground control points

Reference3D orthoimages provide ground control points accurate enough to ensure GPS compatibility: 16 m CE90 or 10 m RMS.
They can be used to orthorectify imagery from most Earth observation satellites—e.g. SPOT, FORMOSAT-2, KOMPSAT-2, QuickBird, IKONOS, Envisat—when positional data are not available.

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