Thursday, March 3. 2011
Space Blog caught up with Shaun Kenyon from SSTL’s Mission Concepts following a busy IAC 2010 for the eagerly awaited follow up to his interview.
Today we’re looking at Inter-Satellite Links (ISLs). Whereas most satellites communicate via one or more groundstations on Earth, there are a number of reasons why communication between satellites is attracting increasing interest from the space community – from reducing latency and sharing costs to powering robust and intelligent multi-satellite systems.
ISLs – the challenges
LEO-GEO ISLs
Mission Concepts is looking at how low-earth orbit satellites such as Earth observation satellites can use existing geostationary satellites to relay messages eg. Task a satellite.
The principle is simple – right now most satellites can only be tasked to acquire an image during a pass above a ground station. Once it has then acquired the image, this data can be transferred by high-speed downlink to this or another groundstation later in its orbit.
Increasing the amount of groundstations in use is another way to reduce the time it takes to task a satellite. However, groundstations can be expensive to operate, especially when the satellites are maintained in a polar orbit, with groundstations in remote areas such as the Arctic or remote groundstations such as the
Troll satellite station – as you might expect the logistics of getting supplies to the North Pole and providing high speed Internet are formidable!
By using just one geostationary satellite to relay commands, it is already possible to increase the window during which new commands can be issued to a satellite to nearly all the time. With two satellites this could be further improved with inter-satellite communications. Renting such basic capacity from a geostationary satellite could be quite a cost effective way to improve the window for tasking satellites – but most importantly it makes it possible to task a satellite very quickly.
Of course using a geostationary satellite to relay back information from a spacecraft in low earth orbit is nothing new – NASA and the USAF have
done this for decades, although a commercial version has not yet materialised.
“In the best case scenario, we could task a satellite and download the image within a 30 minute timeframe. This would be a huge advance for satellite imaging.” Commented Shaun Kenyon.
With the implementation of the joint European Commission/ESA Global Monitoring for Environment and Security (GMES) programme, it is estimated that the European space telecommunications infrastructure will need to transmit six terabytes of data every day from space to ground. This led to another interesting concept, the
European Data Relay Satellite (EDRS).
Bent pipe ISLs
For Earth observation bent-pipe ISLs are particularly interesting. The principle is that two satellites work together as a source and a relay. There are essentially two types of bent pipe ISLs.
1. A “scout” satellite spots a target then tasks a second satellite via a low-rate satellite link.
2. Faster downlink with X-band etc for data transfer.
IridiumNEXT is an interesting example that SSTL is watching closely. The satellite communications provider is using medium-scale data rate ISLs for satellite phone communications.
ISLs can also be used to coordinate formations of satellites. In particular, Mission Concepts is looking at the feasibility of using terrestrial Wi-Fi technology to coordinate a “swarm” of nanosatellites. Fast and low latency communication is possible because the satellites are relatively close together, and in the vacuum of space range is greatly improved.
