Skoltech researchers Alessandro Golkar and Ksenia Osipova, and former Massachusetts Institute of Technology (MIT) student Giuseppe Cataldo (now working at NASA’s Goddard Space Flight Center) have developed, as part of a Skoltech-collaboration MIT, a model to help engineers create and select the most promising conceptual designs for satellite radar systems. By optimizing the design of these rapidly evolving instruments, the model promotes their faster and more cost-effective introduction, leading to better maps and monitoring for storms, floods and landslides. The to study was released in Acta Astronautica.
Satellite imagery of the Earth is used to monitor agricultural land use, ocean ice cover, coastal changes and hostile weather events. These observations are made in different bands of the electromagnetic spectrum, including radio waves. Unlike optical or infrared imagers, radars observe targets regardless of their lighting, bypass clouds, and generally perform well in all weather conditions.
However, in order to provide the same resolution as a shorter wavelength instrument, the radar must be physically larger, which makes it difficult to install on a satellite. One solution is to use synthetic aperture radars. SARs achieve high resolution by artificially increasing their aperture, or antenna “size”. Mounted on a satellite, an SAR emits a radar pulse and travels a certain distance before the pulse returns and is picked up at a different location. The distance traveled is then factored into the virtual antenna size, as if it were much larger, resulting in better picture quality with a relatively small antenna.
Despite this trick of inflating the aperture, SARs have always been flown on large and expensive satellites, as the radars were still quite large and consumed a lot of power. This has changed with the advent of smaller and lighter SARs. These are in the early stages of development but are evolving rapidly, already taking over tasks such as oil spill detection and monitoring.
As the number of smaller and smaller satellites in orbit increases, SAR engineers are wondering which of them are possible supports for miniaturizing radars. This is particularly relevant because recent research suggests that dozens of so-called micro- or nanosatellite-based SARs working together could vastly outperform large conventional SAR missions, if cost-effectiveness is factored into the equation.
With the expansion of the range of options, it becomes increasingly difficult to balance the performance characteristics of the radar against the other parameters of a SAR launch mission. Some of the variables involved are the available orbits, radar and satellite models – along with their physical dimensions and a host of characteristics, such as data rate and power consumption. This complexity requires a computational approach to support the design of future SAR-based Earth observation missions.
To address this problem, a recent study led by Skoltech presents a mathematical model for creating optimal SAR conceptual designs. The model optimizes SAR characteristics with a method called commercial space exploration. This term, which is a combination of ‘compromise’ and ‘play space’, implies that the model will help early stage designers analyze the many tradeoffs involved in the process, quickly evaluating many design alternatives and identifying the optimal solutions to pursue. .
The article demonstrates the utility of the model by examining radar instruments on a wide variety of small satellite platforms: 1,265 feasible radar designs are reduced to less than 44 optimal designs for different radio frequencies. The researchers conclude that small satellites are a feasible platform for 8-12 GHz and 4-8 GHz high-frequency radars, but not for the 1-2 GHz band. The conditions for making the latter type of SAR feasible are discussed, as well as the feasibility limits and technical constraints regarding the requirements associated with instruments and spacecraft. The pulse repetition frequency appears to be the main limiting constraint in the SAR commercial space. In other words, this characteristic is the most powerful factor – before power consumption, antenna size, data rate, etc. – to reduce radar configurations to a limited set of feasible designs.
In a separate analysis, the team considers radars for the very small 3U CubeSat platform, identifying 44 optimal designs out of around 13,000 feasible candidates. The study explores the operational constraints required for the development of these innovative miniaturized radars. The authors conclude that SARs for CubeSats are instrumentally feasible and propose that their designs now be considered at the mission level – as well as the implications for spacecraft design.
The model presented in the study applies to radar systems mounted on a single satellite. However, it could be extended in the future to take into account the means of combining SAR satellites into constellations.