This week marks the thirtieth anniversary of the launch of the first SPOT satellite, making it one of the longest satellite time series datasets.
The French Space Agency (CNES) established the series of Satellites Pour lâ€™Observation de la Terre, known as SPOT. SPOT-1 was launched on the 22nd February 1986 and was fitted with revolutionary steerable mirrors, meaning the satellite could look in multiple directions enabling it to observe the same point on the Earth every five days.
Since SPOT-1 there have been six subsequent satellite launches, with a strong thread of sensor consistency throughout the series, meaning it is much easy to compare imagery over time. Currently, there are three active satellites in the series – SPOT-5, SPOT-6 and SPOT-7.
Looking at SPOTâ€™s history:
- SPOT-1, SPOT-2 and SPOT-3 all had identical imaging sensors, namely a panchromatic band with 10 m spatial resolution, and multispectral bands in green, red and near infrared (NIR) with 20 m resolution.
- SPOT-4 had identical multispectral bands to its predecessors, but it also added a middle IR (MIR) band. The panchromatic band operated at slightly different wavelengths, but with the same resolution. In addition, SPOT-4 also carried the first Vegetation instrument with blue, red, NIR and MIR bands at a 1 km resolution that effectively gave daily global coverage.
- SPOT-5, launched in 2002, offered a step change in spatial resolution. The multispectral green, red and NIR bands had an improved resolution of 10 m. The panchromatic band was at 5m, and returned to its original wavelengths. The vegetation sensor was identical to that flown on SPOT-4.
- The latest incarnations of the series, SPOT-6 and SPOT-7, launched in 2011 and 2014 respectively, operate as a tandem constellation and again offer an improvement in resolution. The panchromatic band is down to 1.5m, and the multispectral green, red and NIR bands are down to 6m. They are expected to provide data through to 2024.
A time-series continuity of the Vegetation Sensor has been provided by the Belgian Proba-V satellite that launched in 2013, and will be carried on into the future by the OLCI sensor on the recently launched Sentinel-3A mission.
Unlike the oldest time series data, Landsat, SPOT is still categorised as a commercial dataset and its imagery has to be purchased.
The US Geological Survey does have a contract to provide historical data from SPOT-4 and SPOT-5 for North America for some US government staff, and ESA provide limited datasets for approved projects. There was an announcement by the French Government in 2014 that SPOT satellite data, over 5 years old, would be free of charge for non-commercial users â€“ although weâ€™ve struggled to find it!
SPOTâ€™s applications have included exploring for gas, oil and minerals including routing pipelines; mapping the planet including forestry, topographical maps and urban planning; agriculture data to combat drought and support farmers decision making; emergency rapid response information for disaster relief and urban planning. In addition, from SPOT-5 onwards DEMâ€™s can be created using photogrammetry techniques, because of the instruments produce stereoscopic images allowing minute changes on the Earth to measured; and the global coverage of Vegetation sensor has also contributed towards climate change research.
SPOT-1 provided imagery ten days after the Chernobyl disaster, and also picked up photosynthesis in the area in 1988 using its NIR sensor, and was recently used to look at camps for Syrian Refuges in recent times.
The SPOT series of satellites have made a huge contribution the development of remote sensing time series datasets, and thatâ€™s worth celebrating.