Project for On-Board Autonomy - Vegetation (PROBA-V)
Mission summary
PROBA-V was launched on 6 May 2013 and was designed to bridge the gap in space-borne vegetation measurements between SPOT-VGT (March 1998 – May 2014) and Sentinel-3. The PROBA-V mission objective was to ensure continuity with the SPOT-VGT mission’s heritage. Throughout the mission, PROBA-V’s geometric and radiometric accuracy were maintained well within the requirements. PROBA-V flies at an altitude of 820 km in a sun-synchronous orbit, with a local overpass time at launch of 10:45 h. Because the satellite has no onboard propellant, the initial overpass time of 10:45 Local Time (LT) has gradually drifted to 08:31 h as of September 2021 (see Figure 1).
Figure 1: PROBA-V Local Overpass Time evolution, 7 May 2013 - 16 September 2021.
Instrument characteristics
The VEGETATION instrument has a Field Of View of 102o, resulting in a swath width of 2,295 km. This swath width ensures a daily near-global coverage (90%), whereas full global coverage is obtained every 2 days. The central camera observes at 100 m nominal resolution, which covers a swath of about 517 km, thereby ensuring global coverage every 5 days.
PROBA-V observes in four spectral bands: BLUE (centered at 0.463 µm), RED (0.655 µm), NIR (0.837 µm), and SWIR (1.603 µm). Observations are taken at resolutions between 100 and 180 m at nadir and up to 350 m and 660 m at the swath edges for the VNIR and SWIR channels, respectively (Francois et al., 2014). Final PROBA-V data products are disseminated at 100 m, 300 m, and 1 km resolution. The instrument and spectral characteristics will be explained in more detail below. The flight and payload characteristics are summarized in Table 1.
Table 1: PROBA-V payload and flight characteristics.
| Characteristic | Value |
|---|---|
| Altitude [km] | 819 – 827 |
| Local overpass time at launch [h] | 10:45 |
| Inclination [o] | 98.7 |
| Daily coverage [%] | 90 (daily) |
| 100 (2 days) | |
| Ground resolution [m] | 96.9 VNIR |
| 193.8 SWIR |
PROBA-V’s optical design consists of three cameras. Each camera has two focal planes, one for the short wave infrared (SWIR) and one for the visible and near-infrared (VNIR) bands. The VNIR detector consists of four lines of 5,200 pixels. Three spectral bands were implemented, comparable with SPOT-VGT: BLUE, RED, and NIR. The SWIR detector is a linear array composed of three staggered detectors of 1,024 pixels. Each used detector line is labelled as a strip. Each camera therefore has 6 strips. The instrument plane layout is shown in Figure 2.
Figure 2: PROBA-V instrument configuration.
Table 2 lists the radiometric characteristics of the PROBA-V spectral bands.
Table 2: PROBA-V spectral, radiometric, and geometric characteristics. Lref refers to the Top-Of-Atmosphere (TOA) irradiance in the respective spectral band. FWHM = Full Width at Half Maximum, SNR = Signal to Noise Ratio.
| Band name | Centre wavelength [µm] | Spectral range @FWHM [µm] | SNR @L<sub>ref [W m-2sr-1 µm-1] |
|---|---|---|---|
| BLUE | 0.464 | 0.440 – 0.487 | 177 @111 |
| RED | 0.655 | 0.614 – 0.696 | 598 @110 |
| NIR | 0.837 | 0.772 – 0.902 | 574 @106 |
| SWIR | 1.603 | 1.570 – 1.635 | 720 @20 |
Figure 3 shows the PROBA-V spectral response functions in comparison with its predecessor SPOT-VGT.
Figure 3: Spectral response functions for PROBA-V (solid lines), SPOT-VGT1 (dashed lines), and SPOT-VGT2 (dotted lines), for the BLUE, RED, NIR, and SWIR channels. US Geological Survey (USGS) 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.