Earth Observation Satellites
Satellites come in all shapes and sizes, they are made for different applications, measure different parts of the electromagnetic spectrum, have different orbits, they are typically located around an altitude of 700km in the Lower Earth Orbit (LEO).
Open Satellites
There are a number of satellites that provide free data. The table below lists some of them.
| Mission / Sensor | Resolution | Swath | Revisit | Bands |
|---|---|---|---|---|
| Sentinel-2 | 10 m | 290 km | 2-3 days | 12 |
| Landsat-8 | 30 m | 183 km | 16 days | 11 |
| MODIS | 250 m | 2330 km | 1-2 days | 36 |
| Sentinel-1 | up to 5m | up to 400km | 6 days | C-band dual pole |
Types of Sensors
Optical
Optical sensors measure the visible and infrared part of the electromagnetic spectrum. They rely on sun lighting up the surface of the Earth and measure the light reflected back. In optical images, Earth's surface is often blocked by clouds.
Synthetic Aperture Radar (SAR)
Synthetic aperture radar emits a microwave pulse and measures its reflection, SAR can image the surface of the Earth in day or night and with and without the presence of clouds. SAR images are hard to interpret and suffer from speckle noise.