SIGINT: The Mission CubeSats are Made For

The collection of radio frequency (RF) signals by means of interferometry is an area that shows great promise for small satellite applications and is a shared interest of business and the scientific and military community. SIGnals INTelligence or SIGINT is one of the oldest missions for satellites, especially for its subfield, ELectronic INTelligence (ELINT), the analysis and localization of RF-signals. Unfortunately, the accuracy that customers demand from such systems in order to merit their costs is often incongruent with detection techniques that rely on single nanosatellites (such as Angle of Arrival methods). Accuracy is strongly related to aperture size; rigid antennas are therefore limited to the available surface area of small satellites. Typical accuracies that can be expected of AOA-techniques range from 0.1° – 1°1. Factoring in orbital altitude, this results in geolocation accuracies of 10 km or more for RF-sources close to the satellite’s nadir, increasing rapidly with distance from nadir for missions in LEO. Using a single CubeSat solution with rigid antenna systems limits the type of RF-emitters that can be geolocated with high accuracy (\u3c 0.1°) to X-band (or shorter wavelengths). Deployable structures and small satellites that do not adhere to the CubeSat standard offer a limited solution as there is limited volume available for deployment mechanisms. One of the key benefits of using CubeSats is their lower unit and launch cost. This enables technical solutions that depend on distributing the desired functionality over many satellites, instead of investing in highly sophisticated single satellite payloads. This approach has in the past been studied for space-based interferometers like Orbiting Low Frequency Antennas for Radio Astronomy (OLFAR) enabling far larger diameter “apertures” than could be fitted on a single satellite while at the same time simplifying the development and deployment2. The same technologies that enable these scientific missions are at the heart of satellite formations for the purpose of identifying and geolocating RF-emitters on the Earth’s surface, such as inter-satellite datalinks, station-keeping systems and precise avionics. The overlap is not limited to these enabling technologies but also extends to system level characteristics. One of the big obstacles for CubeSat missions beyond LEO is their reliability. CubeSat missions beyond LEO face two hurdles that amplify each other, on the one hand the radiation environment becomes significantly more hostile, complicating the use of COTS components and on the other hand the cost of replenishment increases drastically with distance from Earth. Missions such as OLFAR thus require a step change in the reliability of the subsystems in order for them to be affordable and cost effective. At the same time these same reliability improvements would further decrease the cost of ownership of LEO spectrum monitoring (or SIGINT) constellations

Communication, Localization and Synchronization of Spacecraft for Swarm Missions

Swarm missions are based on the use of several spacecraft working together to pursue a specific task for a specific mission. To allow these elements to work together, it is necessary for them to be able to communicate with each other and to synchronize themselves within the swarm. Moreover, the mission may likely require knowing the relative or absolute positions of the spacecraft in the swarm. In order to collect simultaneous measurements allowing computing localization and synchronization in the swarm, a full duplex CDMA communication method is studied by CNES. An Inter Satellite Link (ISL) transmitter prototype is currently under development and first performance evaluation is conducted. CNES is also working on measurement signal processing. Based on signal exchange between satellites, one can estimate jointly the distance and clock offset between a pair of satellites. In parallel, CNES is developing a swarm simulator implying both dynamics and functional behavior of each spacecraft in the swarm. First, this simulator will be software only but its architecture will allow integration of hardware equipment in a future version. This simulator will be used for the validation of the services provided by the link at a system level

Space-based Aperture Array For Ultra-Long Wavelength Radio Astronomy

The past decade has seen the rise of various radio astronomy arrays, particularly for low-frequency observations below 100MHz. These developments have been primarily driven by interesting and fundamental scientific questions, such as studying the dark ages and epoch of re-ionization, by detecting the highly red-shifted 21cm line emission. However, Earth-based radio astronomy below frequencies of 30MHz is severely restricted due to man-made interference, ionospheric distortion and almost complete non-transparency of the ionosphere below 10MHz. Therefore, this narrow spectral band remains possibly the last unexplored frequency range in radio astronomy. A straightforward solution to study the universe at these frequencies is to deploy a space-based antenna array far away from Earths' ionosphere. Various studies in the past were principally limited by technology and computing resources, however current processing and communication trends indicate otherwise. We briefly present the achievable science cases, and discuss the system design for selected scenarios, such as extra-galactic surveys. An extensive discussion is presented on various sub-systems of the potential satellite array, such as radio astronomical antenna design, the on-board signal processing, communication architectures and joint space-time estimation of the satellite network. In light of a scalable array and to avert single point of failure, we propose both centralized and distributed solutions for the ULW space-based array. We highlight the benefits of various deployment locations and summarize the technological challenges for future space-based radio arrays.Comment: Submitte