[Paper Review] CubeSounder: Low SWaP-C 180 GHz Radiometer for Atmospheric Sensing Tested on High Altitude Balloons

Kyle D. Massingill, Tyler M. Karasinski+15 more|arXiv (Cornell University)|Feb 26, 2026

Precipitation Measurement and Analysis0 citations

TL;DR

CubeSounder presents a low SWaP-C, millimeter-wave radiometer using passive waveguide filter banks for 60/180 GHz water vapor sensing, demonstrated on multiple high-altitude balloon flights achieving TRL 6.

ABSTRACT

Microwave sounding is the leading driver of global numerical weather forecasting, but is limited by the scalability of such instruments. With modern machining and commercial microwave components, it is now possible to design low size, weight, power, and cost (SWaP-C) microwave spectrometers while maintaining wide bandwidth performance. Here we report on the status of CubeSounder, a spectrometer tailored for water vapor radiometry that utilizes passive wave guide filter banks. After developing a prototype and high altitude balloon payload, we demonstrated CubeSounder on commercial stratospheric balloon flights. We report on our design process, especially the simulation and fabrication of the custom millimeter-wave filter banks. We also report the initial results of the data collected from the balloon flights.

Motivation & Objective

• Develop a scalable, low SWaP-C millimeter-wave spectrometer tailored for water vapor radiometry.
• Replace traditional heterodyne architectures with a passive waveguide filter bank to reduce complexity and power.
• Demonstrate prototype performance and flight readiness on high-altitude balloon missions.
• Evaluate end-to-end lab and in-flight performance to approach state-of-the-art sensitivity with lower mass and power.

Proposed method

• Amplify scene signals with broadband LNAs and channelize using a custom millimeter-wave filter bank.
• Directly detect each channel with diodes and read out using a low-cost, off-the-shelf electronics stack.
• Simulate filter-bank performance via end-to-end S-matrix cascades and CST Studio EM simulations with Python-based optimization.
• Construct split-block aluminum filter banks for G-band and V-band channels and integrate with commercial detectors (PMP) and LNAs.
• Demonstrate radiometer performance via lab measurements of S-matrix, optical efficiency, and channel noise, followed by in-flight data from NASA Flight Opportunities balloon campaigns.
• Use a two-point LN2/calibration scheme and chopper-based demodulation to convert voltages to brightness temperatures in flight.

Figure 1: Drawing of spectrometer system. Broadband signal enters the feed horn, is then amplified by LNAs and finally divided by filter-bank. The filter-bank is a passive component of directly milled metal based on a novel design.

Experimental results

Research questions

• RQ1 Can a low SWaP-C millimeter-wave spectrometer achieve competitive brightness temperature sensitivity in the 60/180 GHz bands using a passive waveguide filter bank?
• RQ2 What are the design, fabrication, and readout challenges for a scalable multi-channel 60/180 GHz radiometer intended for balloon and space platforms?
• RQ3 How does in-flight performance compare with lab measurements for CubeSounder, and what are the dominant noise and RFI factors?
• RQ4 Is CubeSounder’s architecture suitable for future small-satellite demonstrations given its mass, power, and data handling requirements.

Key findings

• The G-band channel achieves a noise-equivalent temperature near 200 mK√s per channel, within an order of magnitude of the radiometer-quantum limit for that band.
• The optical efficiency of the G-band filter bank is ~20%, with total spectrometer gain around 36 dB in lab tests.
• The V-band prototype shows initial in-lab sensitivity around 400 mK√s, with ongoing flight testing to address readout and gain tuning issues.
• Four high-altitude balloon flights demonstrated progression from single-band to dual-band operation and maturity to TRL 6, with one month of in-flight data in the final flight.
• The instrument benefits from commercial components and a custom filter-bank architecture to achieve low SWaP-C while maintaining broad bandwidth performance.
• In-flight data processing incorporates glitch detection and de-glitching, chopper demodulation, and linear LN2-based calibration to derive brightness temperatures.

Figure 2: An example of the waveguide filter-bank concept. Broadband light propagates through the waveguide and different frequency bands are selected off of the main waveguide by the five channels. Center frequency of each channel is defined by length of resonant cavity.