FLOATS (Fiber-optic Laser Operated AtmosphericTemperature Sensor)

FLOATS is the acronym for Fiber-optic Laser Operated Atmospheric Temperature Sensor, a distributed temperature sensing system designed to provide high-resolution atmospheric temperature profiles up to 2 km beneath constant-altitude balloons. It used a fiber-optic cable to measure temperature based on Raman scattering, achieving a vertical resolution of 3 m and a minimum sampling period of 20 seconds. The system was specifically developed for the tropical tropopause layer (TTL) and aimed for deployment aboard super-pressure balloons as part of the Stratéole 2 campaign. FLOATS was developed by a team from the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado at Boulder, in collaboration with researchers from NOAA's Chemical Sciences Laboratory and NorthWest Research Associates.

In the image at left we can see an scheme of the instrument (click to enlarge). It comprised three major components: a distributed temperature sensing unit (DTS), a mechanical reeling system, and an end-of-fiber unit (EFU). The DTS included a commercial optical bench modified for high-altitude conditions, control electronics, and a reinforced fiber-optic cable. The optical bench used a 785 nm laser to generate Raman scattering signals, from which temperature was determined through calibration methods. Reference temperature sections, insulated within the gondola, provided baseline measurements. The reeling system controlled the deployment and retraction of up to 2000 m of fiber-optic cable, using a carbon-fiber-reinforced nylon spool, servomotors, and a braking mechanism. The EFU, a lightweight sub-gondola, contained sensors for GPS positioning, temperature, and pressure measurements at the cable's endpoint, ensuring data accuracy at the lower limit of the profiling range.

The system was built to withstand harsh environmental conditions, operate autonomously, and meet stringent weight and power requirements for long-duration flights in the TTL. The fiber-optic cable itself was coated with a liquid crystal polymer for durability and UV resistance, with a breaking strain sufficient to meet regulatory safety standards. The optical and mechanical components were optimized to minimize losses due to bends and junctions in the fiber, ensuring accurate signal transmission over the entire deployed length. Calibration methods accounted for environmental effects and system-specific optical losses, using a four-reference section optimization process to dynamically adjust temperature retrieval parameters during each flight.

Balloon launched on: 1/22/2021
Launch site: Wyoming Balloon Launch Facility, Laramie, Wyoming, US  
Balloon launched by: Wyoming University
Balloon manufacturer/size/composition: Zero Pressure Balloon Raven - 283 m3
Flight identification number: WY933
End of flight (L for landing time, W for last contact, otherwise termination time): 1/22/2021
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 4 h

This was a validation mission of the system using a zero-pressure balloon. It was launched in January 22, 2021 from Laramie, Wyoming and reached a float altitude of approximately 19 km. The flight tested FLOATS's optical, mechanical, and calibration systems under stratospheric conditions. The instrument deployed a 1170 m fiber-optic cable, retrieving temperature profiles during ascent and at float altitude. The retrieved temperatures showed strong agreement with onboard radiosonde measurements, with root-mean-square errors below 0.4°C for reference sections and less than 1°C compared to the radiosonde during ascent. At float altitude, the deployed cable exhibited minimal curvature, validating the positional accuracy of temperature measurements.

The FLOATS system demonstrated its ability to detect temperature and wind perturbations, providing evidence of gravity wave activity. Spectral analysis indicated the presence of small-scale waves, highlighting the system's potential for investigating atmospheric dynamics in the upper troposphere and lower stratosphere. The validation flight confirmed FLOATS's viability as a robust tool for long-duration atmospheric temperature profiling.

• A fiber-optic distributed temperature sensor for continuous in situ profiling up to 2 km beneath constant-altitude scientific balloons Atmospheric Measurement Techniques, Volume 16, Issue 3, 2023, pp.791-807