The inter-satellite link (ISL) in swarm and constellation missions is a key enabler in the autonomy of the mission. OLFAR (Orbiting Low Frequency Array for Radio astronomy) is one such mission where 10 to 50+ nanosatellites are placed in the Lunar orbit and perform astronomical observations from the far-side of the moon. Each of the nanosatellite in the swarm would carry a receiver that performs observations between 0.3 - 30 MHz, which are the least explored frequency bands in radio astronomy, thus attracting a large scientific interest.
Observations in this frequency bands from Earth are highly challenging as the ionosphere is opaque to these frequency bands. Furthermore, RFI (Radio Frequency Interferences) generated on Earth makes it highly challenging to perform astronomical observations below 30MHz band. The impediments faced by Earth-based or near-Earth-based radio astronomy for these frequency bands is the motivation to perform measurements from the far-side of the moon.
The purpose of using a swarm of nanosatellites to perform low frequency observations is to enable the realization of long observation baselines and additionally, the effective aperture of observation increases with the number of satellites. For the swarm of nanosatellites to operate as a single aperture, it is very important to cross-correlate the information collected by each satellite and this is where the ISL becomes very crucial. Apart from exchanging data collected by the payload, other information such as attitude and timing information needs to be exchanged.
This work derived mission level requirements which would be used to define a suitable communication architecture for space-based radio astronomy missions such as OLFAR. The approach chosen for communication system for such a swarm mission will comprise of two types of ISL: High data-rate directional link that will be used to exchange payload date and low data-rate omni-directional link that will be used to exchange attitude, timing information and be used for localization, positioning and ranging of the nanosatellites in the swarm. This work will present link budgets to show the feasibility of the proposed communication architecture and derive the specs to further design the transceivers.

The frequency range below 30 MHz remains one of the last unexplored frequency ranges in radio astronomy However, Earth-based observations at these wavelengths are severely impeded, due to man-made radio frequency interference (RFI) and atmospheric opacity. To overcome this impediment, various space-based radio astronomy studies have been proposed in the past decade, notably the OLFAR (Orbiting low Frequency Antennas for Radio Astronomy) study, which proposed a satellite swarm for ultra-long wavelength observation. To realize this mission, various technological challenges of a satellite swarm are currently being addressed, particularly antenna design, navigation, communication, distributed processing, and overall system and mission design. Secondly, the RFI levels at various altitudes from Earth is currently unknown, which is a hindrance in general for radio astronomy. To this end, we propose the use of high-altitude ballooning experiments to validate OLFAR sub-systems in pseudo-representative conditions. Furthermore, these ballooning experiments will measure the RFI in the ultra-long wavelength spectrum at various altitudes from Earth. Our project is termed LOBE (Low-frequency observations using high-altitude Balloon Experiments), and in this paper, we present an overview of the science objectives, payload, and the technological and programmatic challenges of the LOBE project.