A Guide to Space, Earth Observation and Land Management

EO and remote sensing can be confused. In short, EO encompasses all ways of collecting information about the Earth, including both remote sensing and on-the-ground methods. Remote sensing refers to gathering data from a distance, usually with satellites or aircraft.

Spatial resolution refers to the level of detail in an image, determined by the distance between pixels on the ground (Ground Sample Distance, or GSD). A smaller GSD means finer detail. 

• Low-resolution images have a GSD of over 300 metres, sufficient for broader applications like weather forecasting.
• High-resolution images have a GSD of below 5 metres - common for public satellite data. 
• Commercial satellites and UAV photogrammetry (such as drones) can have a very high-resolution GSD range between 1 to 2.5 cm.

Higher-resolution images are useful for detailed tasks like precision agriculture, but they are more expensive and generate larger files, making them better suited for smaller areas.

Revisit time refers to how long it takes for a satellite to observe the same area again. This time varies depending on the satellite's orbit and agility. Most EO satellites are in Sun-synchronous orbits, which help maintain consistent lighting for images. Revisit time is shorter near the poles and longer at the equator. To reduce waiting times, multiple satellites or agile satellites that can adjust their view are used. 

For example, Sentinel-2 consists of two identical satellites Sentinel-2A and Sentinel-2B. The satellites are on opposite sides of the Earth (180° to each other), in a sun-synchronous orbit. Each individual satellite has a revisit time of 10 days, therefore the overall revisit time of Sentinel-2 is 5 days.

Spectral bands refer to different ranges of light or energy that sensors on remote sensors can detect. These bands capture energy from various parts of the electromagnetic spectrum, like visible light (what we see), infrared, or ultraviolet light. Multi-spectral imaging means using sensors that can detect several different spectral bands at once. For example, a multi-spectral satellite might capture images in red, green, blue, and infrared bands. Each of these bands gives us different information about Earth's surface, like the health of vegetation, water quality, or soil conditions.