SmallSats and CubeSats in Low-Earth-Orbit (LEO) have for many years been limited by the short ground station overpass time (around ten minutes per overpass). The use of LEO satellites for real-time applications have therefore been restricted.
Delayed data may be acceptable in some cases, but in others the delay has a negative impact on the use and relevance of transmitted data. The limited overpass time causes satellites to store and prioritize large amounts of data, until they can be transferred to a ground station and then onto the user. Both the issue of time-sensitivity and amounts of data can be alleviated to some extent by facilitating more frequent overpasses and thereby increasing the opportunity for satellites to ‘dump’ collected data. However, if higher connectivity were to be obtained by establishing more ground stations, this would be an overwhelming investment. So what other alternative do we have for improving connectivity?
What is IDRS?
In 2010 Inmarsat initiated the development of a variant of its Aeronautical Broadband Global Area Network (BGAN) system for providing real-time connectivity to LEO Satellites. The system was termed SB-SAT. As part of this effort, GateHouse developed and implemented the SB-SAT protocol stack. In early 2014, Addvalue took on the development of a satellite communication terminal and subsequently launched a 6-U cube satellite with the satellite communication terminal as the payload in December 2015 to start 14 months of in-orbit technical feasibility and proof of concept demonstration. The technical trial was a complete success and Addvalue is presently commercializing what is now termed the Inter-satellite Data Relay Service or IDRS for serving the New Space LEO satellite market.
Equipped with the IDRS terminal, a LEO satellite gains on-demand, low latency data connectivity at L-band frequencies via the Inmarsat I4 satellite constellation in Geostationary-Earth-Orbit (GEO). The Inmarsat I4 satellites relay data through Inmarsat ground stations to users or operators, making full use of Inmarsat’s mobile broadband system infrastructure.
The IDRS technology enables new use cases for LEO satellites missions by providing connectivity in all of Inmarsat’s I4 global coverage. Smaller and cheaper LEO programmes can leverage satellites that stay in frequent communications contact without the cost of building an extensive ground station infrastructure around the globe. By significantly increasing the connectivity, the use of LEO satellites will undeniably increase both in terms of volume and variety. Indeed, the game-changing nature of IDRS technology will greatly liberate the use of LEO satellites for many new and innovative applications and level the playing field for new players from developing countries around the world, especially in the Middle East and Africa because of their vast underserved demands in applications such as real-time IoT, wide-area environmental surveillance, maritime and aerospace monitoring.
How does IDRS work?
The IDRS service consists of an on-board satellite terminal based on the SB-SAT system solution. The satellite terminal provides IP data connectivity when the satellite is within coverage of one of the four Inmarsat I4 satellites.
The terminal uses either an omni, switched or directional antenna, and GPS receiver data to point to the visible I4 satellites. It then logs on and activates a data connection. Now the terminal is on-line and can provide Full-Duplex IP data connectivity at up to 492 kbps. The terminal seamlessly handles handovers between the 200 narrow spot beams of each Inmarsat I4 satellite. Since the relative velocity of a LEO satellite is much higher than the normal aeronautical use case for Inmarsat terminals, GateHouse developed and implemented an algorithm in the protocol stack to mitigate challenges caused by the velocity. The algorithm intelligently predicts when the handover is required and initiates negotiation with the ground station ahead of time such that the handover is done successfully before the signal of the previous spot beam is lost.
When the terminal enters coverage of a new GEO satellite, an antenna repointing is required (for a directional or switched antenna configuration). This again is a predictive process, but will take a few seconds while the antenna is repointed, signal re-acquired and terminal logs on to the new satellite and ground station.
While the existing Inmarsat Aeronautical BGAN terminal already has provisions for handling Doppler shift and precise burst timing, conditions are much more challenging in LEO orbit. The IDRS terminal has been built and extensively tested under worst-case conditions of Doppler shifts up to 50 kHz and burst clock timing shift rates of 54 microseconds per second. Under these conditions, the IDRS satellite terminal requires special predictive handling of Doppler compensation and burst link timing, compared to an aeronautical BGAN terminal.
How can IDRS create value?
In an earth imaging mission with only a limited number of ground links, a traditional LEO mission will have only ground connectivity for 10-15 minutes each time a ground station is passed. This may happen as rarely as only 3-4 times a day in the case of a mid-latitude ground station in locations such as the Middle East, North Africa and Southern Europe, or at best up to 14 passes a day in the case of a polar ground station. This limits the ability to react to new data from the satellite and best utilize the limited on-board storage capacity and downlink capacity. With real-time access to the satellite provided by the IDRS service, a relatively low volume but high value data can be uploaded and downloaded continuously or on-demand via the IDRS link. A mission operator can task the whole LEO constellation on-demand, thereby minimising the latency between order and delivery of images. The operator can, for example, download low resolution images in real-time, decide which data is the most interesting and command the satellite to take high-resolution imagery of an area of specific interest. This could be downloaded via the high-speed dedicated ground link when the ground station is passed. These use cases completely change the reaction time window and ability to best utilize the satellite’s operation in orbit and storage capacity, enabling entirely new concepts of operation that weren’t previously possible. Assuming that the LEO satellite can be operated via IDRS, only the operator will further benefit from availability of existing groundstations and fully approved global spectrum. This is similar to the benefit you get when you buy a cellphone and subscription, which provides access to connectivity and the spectrum.
Applications that will greatly benefit from IDRS real-time communications capabilities include Earth Observation missions with vastly improved response time to customers’ image requests, or surveillance missions such as real-time monitoring of ship movements and border control in political “hot spots” such as the Persian Gulf. The real-time delivery of data would be particularly advantageous in monitoring illegal ship movements and pirate activities such as near the Somalian coast, thereby enabling timely response to irregular or illegal behavoiur.This could be implemented using a combination of optical, SAR (for night vision) and AIS satellites with real-time tasking and quick-look capabilities. The IDRS real-time capabilities will also enable Satellite based ADS-B missions to deliver their airplanes tracking information to Air Traffic Controllers in real-time.