Inter-Satellite Links for an Earth-Observation Constellation (AlertSat)

Satellite constellations are becoming more common as the prices for satellite launches drops. These constellations have different purposes such as communications, navigation or Earth-observation. Earth-observation constellations can be used to detect natural disasters such as wildfires, floods and landslides. Many of these events occur in remote areas that are hard to reach. This means that detecting that they have occurred can take a long time and the delayed response can have serious consequences for the area. Forest wildfires for example spread quickly and a delayed response can lead to the fire growing out of control.

These events can be detected using satellite imagery from Earth-Observation satellites. One issue is that even if it’s detected, satellites orbit Earth approximately once every 90 minutes in low Earth orbit, which for fast spreading disasters might not be fast enough. Another issue is that downlinking large amounts of data in the form of images often requires large, powerful and expensive satellites. The processing on the ground also takes time. This means that by the time the event has been detected it can already be to late. Combining the satellite sensors with software that can detect the critical events in-orbit, eliminates the need to downlink all images and only send the needed information. Instead it can be processed on-board the satellites, enabling smaller, less expensive satellites.

When a satellite detect an event there is no certainty that it is above a ground station. The information can’t be sent to until it has line of sight of one. Having many ground stations spread across the world would increase cost and is limited by the availability of antennas. The information can instead be shared with the other satellites in the constellation via Inter-Satellite Links to reduce the latency of the information.

The AlertSat constellation from the Swedish company PandionAI aims to do this. With it’s twin concept constellation design it will be able to do autonomous tasking, change detection and decision control using small, specialized satellites. When for example a wildfire is detected an Alert will be generated and quickly be sent through the constellation to a satellite within line of sight of a ground station so that it reaches the end user who can respond.

This thesis aims to answer the question of how Inter-Satellite Links can be used to reduce latency for the AlertSat constellation. It will identify the parameters, limits and trade offs that the communication has. Inter-Satellite Links have limits due to the distance that the satellites in orbit have to each other. The satellites are also limited in size and power which needs to be quantified for us to be able to see how much data that can be sent, while the link can still be closed.