Design and Implementation of a Scanning Ground Based Microwave Radiometer

We have built a low-cost microwave radiometer operating at Ka band (18~22GHz) at the ESSIC/CISESS Remote Sensing Lab for the purpose of student education and training. It can also be used as a ground observation instrument for in-situ experiments to support the JPSS program. The instrument can be operated under two different detection modes: total power and Dicke-type detection. All key components, such as the low noise amplifier, Low Noise Downconverting Block (LNB), band pass filter, antenna feed are purchased from the domestic commercial market. The components are then tested and assembled in our Lab. The cumulative cost to produce the instrument is under five thousand dollars.

The instrument is built based on the self-designed digital back-end processing module. A microcontroller is used to generate a PWM signal to control the Dicke switch, the signals then are processed with AC coupling, AC/DC amplification, demodulation, and finally being digitized through ADC processing available to the ATMEL ATMEGA 328p microcontroller through the on-board ADC register. The RF module of the instrument is very sensitive to the variation of temperature, and a temperature control system consisting of a temperature sensor and cooling fan is needed to keep the instrument in the required ambient temperature range. Another important function of the back-end processing module is data collection and transfer.

Recently, we have consolidated our onboard electronics into two PCBs and have added a mechanical scanning system to automate data collection over an elevation sweep. To accomplish this, we leverage a stepper motor controlling a polished aluminum reflector disk set at 45 degrees relative to the axis of the motor spindle. In addition to this, we have also added support for wireless control and communication with the instrument, this is done by means of LoRA. Two modes of control are used for the reflector system, controlled angle (holding) and repetitive sweeping. Testing in the lab has concluded that the reflector adds an acceptable amount of signal instability/degradation, and that the electromagnetic interference caused by the motor’s operations is also at an acceptable level.

Field testing on-site at ESSIC has demonstrated the working capability and sensitivity of the instrument, at a base gain of 250 counts, we see a ~14 count variation over an elevation sweep from -60 to 60 degrees. Further development seeks to improve sensitivity and signal stability. Furthermore, we plan to leverage a Digilent Arty A7 FPGA board in place of our current ATMEL-based signal processing unit to improve ADC resolution from 10 to 12 bit, and to add additional polarizations for more thorough data acquisition. We are currently in the development phase of a flight model of the instrument which will be flown as a drone or small aircraft payload, which we hope to accomplish this coming year.