Every DB counts - improved detection sensitivity of phased array radars thanks to cryotechnology
With the new, exciting research topic »cryo-cooled receiver systems for phased array antennas« Fraunhofer FHR is leading the way to further improve the sensitivity and, with this, the capabilities of radars. The technical implementation for the lowering of the system noise temperature has great potential for the development of trendsetting space surveillance radars.
After the successful completion of a first research project on behalf of the DLR Space Administration, the work in the field of cryotechnology was intensified in 2018. In the process, technologies were designed to lower the noise temperature of radar receivers. The optimization of the signal-to-noise ratio (SNR) is essential for the detection and cataloging of tiny objects in low Earth orbit (LEO), thus helping to improve the current awareness of the space situation.
Development of a Suitable Measuring Environment
To significantly increase the sensitivity, the receiver has to be cooled down to temperatures as low as -270°C (~4°K). To determine the improvement, the noise temperature has to be measured in a receiving system, and this places special demands on the measuring environment. Thus, evacuated stainless steel vessels (dewar) have been developed that allow for the cooling down to very low temperatures through the vacuum and the thermal shielding of the experimental platforms from the environment.
Currently, a large measuring dewar (height 840 mm, diameter 620 mm) is used for a variety of examinations. The performance characteristics of the used cryo-cooler including all add-on components were measured and a detailed thermal load diagram was created. This is used to determine the necessary number of cryo-coolers for a future scalable phased array system.
At the moment, an additional smaller vacuum vessel (25 x 25 x 10 cm3) is being set up and optimized. Because of the lower mass and the smaller anechoic chamber, this dewar is more flexible in its use, thus offering a platform for smaller and faster low temperature experiments. To reach a temperature of < 20 K on the experimental platform, a two-stage cryo-cooler is used. In the first stage (77 K), the thermal load irradiating from the outside is carried off. In the second stage, the experimental platform is at < 20 K. The gilding of the radiation shields in the first stage provides for an additional reduction of the thermal load, as the thermal radiation is reflected.
Measuring Technology to Determine the Noise Temperature – Heated Load
The correct determination of the noise temperature at < 20 K requires fully developed high frequency measuring technology, as the material and HF properties change significantly at very low temperatures. Furthermore, the heat input of the 290 K ambient temperature environment (e.g. by HF cables) to the < 20 K experimental platform has to be kept as low as possible. To achieve this, the necessary noise source is brought directly into the environment with T < 20 K. For an optimized measurement of the noise temperature, a so-called »heated load« is developed. This heated load consists of a 50 ohm HF terminating resistor that can be set to different defined temperatures by means of a heating resistor. This method allows for a very precise determination of the noise temperature. The challenge here is that, on one hand, the terminating resistor has to be thermally decoupled from the system so that the component is not also heated up but kept at a temperature in a thermally stable manner. On the other hand, the resistor has to be connected in a thermally sufficient way in order to quickly reach the desired low temperatures.
The First Prototype of a Scalable Individual Receive Channel
To implement a flexible cryogenic phased array radar, the cryo-cooled receive channel is designed for scalability. First, a mechanic support structure is necessary for the first amplifiers. This structure also acts as a radiation shield for the first and the second stage. In the process, the structure has to resist the mechanical stress caused by the cooling while also maintaining the different temperature levels thermally decoupled from each other. A first successful prototype has already been developed and submitted to a variety of tests.
Cryo-suitable HF Windows
To design a future cryo-suitable phased array system so that it is vacuum-tight and can be penetrated by electromagnetic radiation, a suitable radome material has to be found. For the first examination of these HF windows, a special plastic cap in the form of a torispherical head was designed for the large dewar. This HF window makes it possible to build a cryo-cooled receiver where the antenna is cooled as well. First vacuum and stress tests show that the torispherical head offers the necessary stability. The analysis of alternative materials and radome forms as well as HF examinations are still in progress.
The knowledge gained from this flows into the development of enhanced performance cryogenic phased array radar systems and has the potential to decisively help other research fields to progress as well.