CURRENT STATUS OF RESEARCH AND DEVELOPMENT OF SCANNING ARRAY FOR HYPERMULTISPECTRAL RADIOWAVE IMAGING (SAMRAI) MOUNTED ON SATELLITE

T. Maeda¹, Y. Kobayashi¹, N. Tat Trung¹, Y. Takei¹, T. Yano¹, N. Tomii¹

¹Japan Aerospace Exploration Agency

Microwave remote sensing is a technology that reveals the characteristics of materials by measuring the microwave energy emitted by them. A microwave radiometer (MWR) is an instrument that measures such microwave energy. Japan Aerospace Exploration Agency (JAXA) has been operating the AMSR series (AMSR, AMSR-E, AMSR2, AMSR3) of satellite-borne MWRs for more than 20 years. We define these here as conventional MWRs. Through the operation of the AMSR series, the following problems with conventional MWR were revealed.

1. Observation frequencies and bandwidths are limited, and when artificial microwave power interferes with weak natural microwave power, it becomes impossible to separate and distinguish the two. This is known as the so-called radio frequency interference (RFI) problem.

2. Observing the Earth’s surface from a satellite with a practical spatial resolution requires sharpening the beam width of the receiving antenna using a large reflector. In addition, continuous scanning of the Earth’s surface requires rotating the reflector. After all, carrying a conventional MWR requires a large satellite, increasing manufacturing costs.

3. User needs for improved spatial resolution are high, which requires a larger rotating reflector. As the size of the reflector increases, it becomes technically difficult to rotate it continuously. In order to solve the problem 1), we studied and developed ultra-wideband hyperspectral MWR (DSuRAD) using an ultra-high-speed AD converter and onboard signal processing such as FFT (doi: 10.1109/LGRS.2020.2990707).

On the other hand, a phased array antenna (PAA) is known as an effective means for solving the problems 2) and 3). However, a conventional PAA is typically limited to a single observable frequency due to the receive bandwidth of the antenna elements. Our another study shows that true time delay (TTD) ultra-wideband interferometric MWR can be achieved by applying DSuRAD technique to PAA (doi: 10.1109/LGRS.2020.3023697).

These preliminary studies will make it possible to realize a new MWR that is completely different from conventional ones. We have named this new MWR SAMRAI (Scanning Array of hyper-Multispectral RAdiowave Imaging), and are conducting research and development with funding from the Japan Science and Technology Agency (JST).

SAMRAI is equipped with a PAA consisting of 14 ultra-wideband antenna elements capable of receiving frequencies from 1 to 40 GHz. The received ultra-wideband horizontally and vertically polarized microwave signals are each sampled at a rate equivalent to 80 GSPS (samples per second). By delaying the received signal of each element and performing spectrum/correlation processing using FFT, the microwave spectrum at 27 MHz intervals is measured in the target direction (i.e. observation point). Due to the effect of PAA, the footprint of each observation point is small.

In this way, SAMRAI has more than 3000 observation frequency channels for both horizontal and vertical polarization at each observation point with a small footprint. The observation frequencies of SAMRAI are continuous while those of the conventional MWR were discrete. Therefore, SAMRAI can identify the observation frequency channel contaminated by RFI, and by excluding it, it becomes possible to retrieve geophysical values without being affected by RFI.

SAMRAI was first developed for helicopter installation, and observations and performance evaluations were carried out from 2022 to 2023. Currently, improvements are being made based on the results of helicopter observations, and SAMRAI is being developed for installation on a 200 kg-class satellite. This satellite will be launched by the first quarter of 2028, with the aim of demonstrating it in orbit. In this presentation, we introduce the various updates applied to the hardware of the satellite-mounted SAMRAI to identify and separate RFI.