One of the last unexplored frequency ranges in radio astronomy is the frequency band below 30 MHz. New interesting astronomical science drivers for very low frequency radio astronomy have emerged, ranging from studies of the astronomical dark ages, the epoch of reionization, exoplanets, to ultra-high energy cosmic rays. However, astronomical observations with Earth-bound radio telescopes at very low frequencies are hampered by the ionospheric plasma, which scatters impinging celestial radio waves. This effect is larger at lower frequencies. Below about 5 MHz at night or about 10 MHz during daytime, the ionosphere is even opaque for radio waves. Although the ionosphere is transparent at frequencies above roughly 10 MHz, Earth-bound radio astronomy is affected by the short-term phase fluctuations of the received celestial radio waves. Advanced calibration techniques are needed and are currently being developed for the LOFAR telescope array. Still, it will be very difficult to calibrate the array at low frequencies. Both effects will lead to a loss in sensitivity and spatial resolution. An additional factor which limits the sensitivity of Earth-bound radio telescopes at frequencies in the band below 30 MHz, is the world-wide occurrence of very strong transmitter signals (RFI).
Because of the above reasons, the frequency band below approximately 30 MHz is one of the last unexplored frequency ranges in astronomy. A radio telescope in space would not be hampered by the Earth’s ionosphere. Therefore several initiatives have been started recently to explore this unexplored frequency band. In 2009 an ESA-project, Distributed Aperture Array for Radio Astronomy in Space (DARIS) was carried out to investigate the possibilities for a low-frequency radio telescope in space using small satellites. The overall conclusion was that using today’s technology it should be possible to make an array in space. The OLFAR project, funded by STW, is developing new technology to realize a swarm of satellites in space using nano-satellites.
The team in the Netherlands currently working on this project are ASTRON, the Universities of Twente, Delft, Groningen and Nijmegen, Dutch Space, ESA, ISIS, Axiom IC, AEMICS, SystematIC, National Semiconductors.
Several subsystems and ideas are researched at the moment. In this project , funded by the NanoNext program, the antenna system for the reception of astronomical signals will be studied.
The aim of this research is to develop design methods for robust antenna systems suitable for the harsh conditions in space. The project will have a system level approach which means that the antenna elements, the LNAs, matching circuits, and the associated calibration systems all will need to be studied.
Topics to be covered:
- Development of antenna system concepts that can be used for a space-based low-frequency radio telescope. The small size of the satellites limits the antenna design in size as well. Several antenna designs should be further researched, like using active dipoles, helix antennas, or tripoles (and magnetic loops). The overall antenna pattern is important to characterize, since it will impact the reception of the astronomical signals. Implementation aspects will impact the design of the antenna – eg. what are the conditions for the stability of the antennas – and what are possible materials to implement such …
- Low power integrated LNA design. The power budget on nano-satellites is very tight, so very low power subsystems must be designed.
- Modeling the low-frequency electromagnetic characteristics of the antennas. A very accurate model is needed for characterizing the antenna. This is important for the calibration routines.
- Development of calibration routines. The antenna systems is used in an antenna array of several satellites (up to thousands of satellites).
- Alternative designs.
- Demonstration system.