Satellite Pour l’Observation de la Terre – Végétation (SPOT-VGT) Mission and Instrument Overview
The VEGETATION Programme was jointly developed by France, the European Commission, Belgium, Italy, and Sweden. The satellite components of the programme (SPOT-4 and SPOT-5) were launched on 24 March 1998 and 4 May 2002, respectively. The VGT instruments on-board these satellites performed measurements from 21 April 1998 until 31 May 2014 and were tailored to monitor land surface variables with a near-daily global coverage at a medium spatial resolution of 1 km.
The VGT mission goals were to provide global observations for surface parameter mapping, agricultural, pastoral and forest production, as well as for terrestrial biosphere mechanism monitoring and modelling. SPOT-VGT data have been used for many applications, such as: * Biophysical parameter retrieval, such as LAI, fAPAR, and surface albedo * Land cover classification * Agricultural monitoring and food security * Burnt area detection * Forest monitoring * Environmental monitoring
Instrument characteristics
The VEGETATION instrument, hereafter referred to as VGT, with additions ‘1’ and ‘2’ for the instruments on-board SPOT-4 and SPOT-5, respectively, comprises 4 cameras that observed reflected solar radiation in the following bands: B0 (BLUE, centre wavelength 0.46 µm), B2 (RED, centre wavelength 0.68 µm), B3 (NIR, centre wavelength 0.85 µm), and SWIR (centre wavelength 1.66 µm). The cameras covered a field of view (FOV) of 101°, thereby covering a swath of 2,200 km. The nominal pixel resolution was 1.15 km at nadir, increasing to ~1.7 km at the swath edges. Each spectral band had its own optics with the associated detector unit in the focal plane. Each detector unit contained: * an interference filter for spectral selection * a 1728-detector linear array * a low-noise amplifier and CCD follower unit * a SWIR dedicated, closed loop active thermal control concept using a nearby Detection Interface Unit (DIU).
See Table 1 for more details on the instrument’s main payload and flight characteristics.
Table 1: SPOT-VGT payload and flight characteristics.
| Characteristic | Value |
|---|---|
| Altitude | 832 km |
| Local overpass time at launch | 10:30 h |
| Inclination | 98.7o |
| Coverage | 90 % (daily, 100 % per 2 days) |
| Instantaneous Geometric Field of View (IGFOV) | 1.15 km |
Figure 1 shows the spectral response functions of the four VGT1 and VGT2 bands and also includes typical vegetation and bare soil spectra. Full Width at Half Maximum (FWHM) spectral ranges and centre wavelengths are summarized in Table 2. The VGT1 response is slightly broader than VGT2 in B0 and B2 at the longer wavelengths, while the VGT1 NIR channel has slightly more response at the shorter wavelengths. The VGT1 SWIR channel is more or less entirely shifted to longer wavelengths, see Table 2 and Table 3 for more details on the spectral ranges, radiometric resolution and radiometric accuracy.
Figure 1: SPOT-VGT spectral response functions for channel B0 (blue), B2 (red), B3 (green), and SWIR (black) for VGT1 (solid line) and VGT2 (dashed line). Typical vegetation spectra for grass (solid dark green line), maple leaf (dashed dark green line), and bare soil (sandy loam, dotted brown line) are plotted for reference.
Table 2: Spectral ranges (Full Width at Half Maximum, FWHM) and centre wavelengths (in parentheses) for VGT1 and VGT2.
| Spectral band | VGT1 [µm] | VGT2 [µm] | Surface reflectance range [-] |
|---|---|---|---|
| BLUE (B0) | 0.437 - 0.480 (0.459) | 0.439 - 0.476 (0.458) | 0.0 – 0.5 |
| RED (B2) | 0.615 - 0.700 (0.658) | 0.616 - 0.690 (0.653) | 0.0 – 0.5 |
| NIR (B3) | 0.773 - 0.894 (0.834) | 0.783 - 0.892 (0.838) | 0.0 – 0.7 |
| SWIR (MIR) | 1.603 - 1.695 (1.649) | 1.584 - 1.685 (1.635) | 0.0 – 0.6 |
Table 3: Radiometric resolution (NEΔR), intra-image consistency, and radiometric calibration accuracy for all spectral channels.
| Parameter | Value |
|---|---|
| Radiometric resolution | 8 bits |
| Noise Equivalent Delta Radiance (NEΔR) | |
| BLUE | NEΔR = 0.003 for [0, 0.50] |
| RED | NEΔR = 0.001 for [0, 0.10] |
| NEΔR = 0.005(R-0.10)+0.10 for [0.10, 0.50] | |
| NIR, SWIR | NEΔR = 0.003 for [0, 0.50] |
| Intra-image consistency | 0.005 for any reflectance value |
| Calibration accuracy | Inter-band and multi-temporal: <3% |
| absolute: <5% |