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

This paper presents the design, fabrication, and successful high-altitude balloon testing of CubeSounder, a low SWaP-C 180 GHz radiometer utilizing passive waveguide filter banks for atmospheric water vapor sensing.

The Gist

Imagine you are trying to listen to a very faint whisper in a crowded, noisy room. That's essentially what weather satellites do when they try to "listen" to the atmosphere to predict the weather. They listen for specific radio signals (microwaves) emitted by water vapor in the air.

For decades, the best "ears" for this job have been huge, heavy, expensive, and power-hungry machines. They are like a professional recording studio setup: massive mixing boards, heavy cables, and cooling systems. While they work great, they are too big and costly to put on small, cheap satellites.

Enter CubeSounder. Think of CubeSounder not as a recording studio, but as a high-tech, pocket-sized walkie-talkie that can still hear that whisper clearly.

Here is the story of how they built it, how it works, and what happened when they took it for a ride on a giant balloon.

1. The Problem: The "Heavy" Weather Ear

Current weather satellites use complex technology called "mixers" to process signals. It's like trying to translate a foreign language by first converting it to Morse code, then to Braille, and then back to English. It works, but it requires a lot of heavy machinery, power, and a very stable local clock (like a super-precise metronome).

The team at Arizona State University wanted to build a "lightweight" version. They wanted something small enough to fit on a CubeSat (a tiny satellite the size of a loaf of bread) that could still hear the atmosphere's whispers.

2. The Solution: The "Prism" of Microwaves

Instead of the complex translation method (mixers), CubeSounder uses a clever trick called a filter bank.

• The Analogy: Imagine white light shining through a prism. The prism splits the light into a rainbow of colors (red, orange, yellow, etc.).
• The CubeSounder Version: Instead of light, they use invisible radio waves. They built a custom metal block (a waveguide filter bank) that acts like a prism for microwaves.
  • The signal enters the block.
  • Inside, the block splits the signal into different "channels" based on their frequency (like splitting the rainbow).
  • Each channel goes to its own tiny detector (a diode) that just measures how much "energy" is there.

This is much simpler. It doesn't need a super-precise clock or heavy cooling. It's just a metal block with holes cut in it, connected to some cheap, off-the-shelf electronics.

3. The Build: "Lego" for Space

The team didn't build this from scratch in a vacuum. They used Commercial Off-The-Shelf (COTS) parts.

• The Amplifier: They used a standard, store-bought amplifier (like a volume knob) to boost the faint signal.
• The Detectors: They used standard radio detectors.
• The Brain: They used an Arduino (the same kind of microcontroller used by hobbyists) to read the data.

The only "custom" part was the metal block with the filter channels. They used high-precision CNC machines to carve these channels into aluminum. It's like 3D printing, but with metal and extreme precision.

4. The Test Flight: Riding a Giant Balloon

Before putting this on a satellite, they had to test it in the real atmosphere. They couldn't just test it in a lab; they needed to be high up, above most of the weather, to see how it performed in space-like conditions.

They partnered with World View, a company that flies giant, high-altitude balloons.

• The Journey: They launched the CubeSounder payload from Arizona. The balloon carried it up to the stratosphere (about 32 km or 20 miles high)—higher than commercial airplanes fly.
• The Challenges:
  • The "Glitch": The balloon had its own power supply that was "noisy." It was like trying to listen to a whisper while someone next to you was tapping a pen on a table. This created "glitches" in the data.
  • The Fix: The team wrote computer software to act like a "noise-canceling headphone." It identified the regular tapping (the glitches) and removed them, leaving only the atmospheric data.
• The Result: They flew four times. The final flight in 2024 was a huge success. They collected one month of continuous data, proving the device works, is durable, and can measure water vapor in the atmosphere with high accuracy.

5. Why This Matters

The paper concludes that CubeSounder is a game-changer because:

• It's Cheap: It uses parts you can buy at an electronics store.
• It's Light: It weighs a fraction of current instruments.
• It's Efficient: It uses very little power.
• It Works: It achieved the same sensitivity as the giant, expensive instruments currently on satellites.

The Big Picture:
Think of the current weather satellites as a fleet of massive, expensive cruise ships. They are great, but if one breaks, it's a disaster. CubeSounder is like a fleet of hundreds of small, cheap, fast speedboats. If one breaks, you just launch another one. This technology could allow us to put weather sensors on thousands of small satellites, giving us a much clearer, more frequent, and more detailed picture of our planet's weather and climate.

In short: They took a complex, heavy, expensive science instrument, shrunk it down to the size of a shoebox, filled it with cheap parts, and proved it works by sending it on a balloon ride to the edge of space.

Technical Summary

1. Problem Statement

Microwave radiometry is the primary driver for global numerical weather forecasting, yet current state-of-the-art instruments (e.g., ATMS, AMSU) suffer from high Size, Weight, Power, and Cost (SWaP-C) and system complexity. These instruments rely on heterodyne mixers and frequency-stabilized local oscillators, making them expensive to replace as they reach end-of-life. Conversely, promising alternatives from astronomy (e.g., SuperSpec, DESHIMA) often require sub-Kelvin cooling, rendering them unsuitable for low-SWaP-C atmospheric applications. There is a critical need for a scalable, low-cost, and uncooled spectrometer capable of water vapor radiometry in the millimeter-wave spectrum.

2. Methodology and Instrument Architecture

The authors developed CubeSounder, a dual-band (60 GHz/V-band and 180 GHz/G-band) direct-detection spectroradiometer. The system architecture diverges from traditional heterodyne designs by utilizing a passive waveguide filter bank for channelization.

• Signal Chain:

  1. Input: Broadband signal enters via a pyramidal feed horn.
  2. Amplification: The signal is amplified by commercial millimeter-wave Low Noise Amplifiers (LNAs).
  3. Channelization: The signal passes through a custom-designed millimeter-wave waveguide filter bank. This passive component consists of resonant cavities coupled to a main waveguide. Each cavity is tuned to a specific frequency channel, directing it to a unique output port.
  4. Detection: Each output port terminates in a millimeter-wave diode power detector.
  5. Readout: Signals are amplified by instrumentation amplifiers, digitized by an 18-bit ADC, and stored on an SD card. No high-speed ADCs or FPGAs are required.

• Filter Bank Design & Fabrication:

  • Simulation: The team utilized a batched simulation workflow using CST Studio for 3D Electromagnetic (E&M) modeling and Python scripts (with scikit-rf) for S-matrix cascading. This allowed for rapid iteration of filter spacing and dimensions to optimize overall passband performance.
  • Fabrication: The filter banks were machined as "split-blocks" (two halves milled to half-depth and mated). The G-band (180 GHz) prototype was machined in aluminum with 5-micron tolerance, while the V-band (60 GHz) prototype used 25-micron tolerance.
  • Components: The system relies heavily on Commercial Off-The-Shelf (COTS) components, including Radiometer Physics and Eravant LNAs, Pacific Millimeter Products (PMP) detectors, and Arduino-based readout electronics.

• Flight Testing Strategy:
  The technology was validated through a series of four high-altitude balloon flights using the NASA Flight Opportunities program and World View Stratollite platforms. The payloads included Faraday cages and metal mesh windows to mitigate Radio Frequency Interference (RFI) while allowing millimeter-wave transmission.

3. Key Contributions

• Novel Filter Bank Technology: Successful design, simulation, and fabrication of custom millimeter-wave waveguide filter banks that enable direct detection without cryogenic cooling.
• Low SWaP-C Architecture: Demonstration of a spectrometer system that achieves high sensitivity using only COTS components, significantly reducing mass, power, and cost compared to heterodyne systems.
• Flight Heritage: Successful integration and operation of the CubeSounder payload on stratospheric balloons, raising the Technology Readiness Level (TRL) from development to TRL 6.
• Data Processing Pipeline: Development of a robust data pipeline capable of handling real-world flight challenges, including RFI mitigation ("de-glitching") and Dicke-switching demodulation using a mechanical chopper.

4. Results

• Laboratory Performance:
  • G-band (180 GHz): Achieved a Noise Equivalent Temperature (NET) of ~200 mK√s per channel. This is within an order of magnitude of the theoretical radiometer limit (39 mK√s), with performance dominated by detector noise. The optical efficiency was measured at ~20%, and total gain at ~36 dB.
  • V-band (60 GHz): Initial lab results showed a peak channel performance of ~400 mK√s.
• Flight Performance:
  • Flights 1 & 2 (2021-2022): Demonstrated successful integration and operation of the G-band prototype, collecting data over four days despite early termination of Flight 1.
  • Flight 4 (August-September 2024): The most successful flight, yielding one month of high-quality data. The payload operated at altitudes up to 32,576 meters over the Rocky Mountain region.
  • Data Quality: The team successfully mitigated significant RFI (caused by an unrelated power supply on the balloon) using a brute-force time-based filtering method. After "de-glitching" and demodulation relative to the chopper wheel, the system produced meaningful brightness temperature measurements.
  • Calibration: A two-point linear calibration (using 293 K room temperature and 77 K liquid nitrogen loads) was successfully applied to convert digitized voltages to absolute brightness temperatures.

5. Significance

• Scalability for Weather Forecasting: CubeSounder proves that high-performance microwave sounding can be achieved with low SWaP-C, making it feasible to deploy these sensors on small satellite constellations (CubeSats) to increase temporal resolution in weather forecasting.
• Cost Reduction: By utilizing COTS components and replicable custom parts, the system offers a pathway to replace aging, expensive satellite instruments at a fraction of the cost.
• Pathfinder for Space: The successful TRL 6 demonstration on high-altitude balloons validates the technology for future spaceflight missions, potentially revolutionizing how atmospheric water vapor is monitored globally.
• Future Work: The authors plan to conduct full analysis of the flight data, compare results with ERA-5 reanalysis datasets, and propose the technology for future space-based development grants.