Cryogenic phased array radars for space surveillance
In the future, the utilization of cryogenic techniques should greatly enhance the sensitivity of phased array radars. Scientists at Fraunhofer FHR have taken on the great technical challenge of developing efficient cooling measures to reduce system noise temperature.
The utilization of radar technology for space surveillance to determine the position of debris particles and satellites in near-Earth space is growing in importance. Active phased array radar systems play a significant role in the area of radar surveillance. The electronically steerable beams facilitate the simultaneous surveillance of various cardinal points as well as high, inertia-free beam agility and therefore allow the fast and precise tracking of moving targets.
A high signal-to-noise ratio (SNR) is very important for the detection of space debris. This parameter depends on the intrinsic noise temperature of the individual receive channels. If this is too high, weak signals can no longer be distinguished from noise and will therefore remain undetected. For this reason, one of the next challenges in future phased array projects for space surveillance lies in the technical realization of efficient cooling measures for the individual receive channels. Particularly in the area of space debris detection, it is essential that the SNR be improved through the reduction of the system temperature while using a defined equivalent isotropic radiated power so that even the smallest particles can be detected in low Earth orbit (LEO).
In 2017, the aerospace management of the German Aerospace Center (DLR-RFM) commissioned the ARB department of Fraunhofer FHR to investigate cryogenic receivers for phased array antennas in general, with a particular focus on systems that are similar to the GESTRA system. This led to the creation of a new research area at Fraunhofer FHR.
Present technical situation
In radio astronomy, cryo-cooled receiver to reduce the system temperature in single-pixel detectors, such as the 100-meter parabolic reflector Effelsberg, has already been established for decades. Due to mechanical and high-frequency challenges, techniques for the reduction of receiver noise in phased array systems are still in the very early stages of development. The research now conducted at Fraunhofer FHR focuses on the design and optimization of various approaches for the cryo-cooling of phased array systems. A cost-benefit analysis is also carried out to shed light on economic aspects.
As the largest share of the system noise temperature is generated by the first amplifier (Low Noise Amplifier, LNA) in a receiver chain, the primary focus lies on the effective reduction of inherent noise by cooling the first amplifier stage.
Freezing improves detection sensitivity
When not cryo-cooled the first amplifier in the receiver chain is in an environment with room temperature (290 K, corresponds to 20°C). Research activities now focus on cooling the first amplifier stage to below 20 K (-253°C) with gaseous helium to achieve system temperatures below 50 K (-223°C). The radar equation shows an improvement in the signal-to-noise ratio of approx. 3 dB, with the result that objects with a radar cross section of half the size can be detected in orbit.
Experiments at up to -269°C
Some of the initial – internally funded – project activities involved the creation of a suitable infrastructure. Appropriate measurement chambers that allow the safe handling of inert gases (e.g. helium ) and liquid nitrogen were constructed.
Various tests will be conducted in these measuring facilities to investigate which low-noise amplifiers offer the best performance when cryo-cooled and how the measuring techniques for temperature monitoring can be optimized. The biggest challenge here lies in the mechanical development of vacuum-tight, stable and high frequency permeable experimental environments parallel to the achievement of an optimal thermal connection between the cryo-cooler and the first amplifier stage. In cooperation with one of the leading companies in the areas of cryogenic engineering, a dewar was developed specifically for this purpose. With the dewar, the challenging high frequency and material experiments can be carried out at temperatures of up to 4 K (-269°C).
On successful completion of the first phase, attention will turn to the realization of a highly sensitive, phased array-based space surveillance radar. The implementation of a cryo-cooled receiver system for radar applications would be a milestone in the improvement of receiver sensitivity. This important technology would also have the potential to take many other research areas a decisive step forward.