Sweeping Aperture Neutral Drifts Sensor

The neutral atmosphere at satellite and rocket altitudes is particularly difficult to investigate as neutral gases aren’t directly measurable. The last successful wind measurement was performed by the Winds and Temperature Sensor (WATS) on the Dynamics Explorer 2 mission launched in 1981. However, the instrument requires two deployable baffles with motors, two pressure gauges, as well as a magnetic field and mass spectrum analyzer to identify atmospheric gases. The instrument baffles were required to move slowly enough for the instrument to reach equilibrium before making the measurement. These requirements are not amenable for operation on CubeSats. The Sweeping Aperture Neutral Drifts Sensor (SANDS) was invented to enable neutral atmosphere measurements on both small and large spaceflight platforms.

WATS schematic. Movable baffle (top) blocks the flow of neutral gas into the sensor when the baffle, sensor, and neutral gas flow direction align.
WATS temperature analysis procedure. The currents recorded as the baffle is rotated display a local minimum when aligned in the wind direction. Deviations from a calculated zero-temperature curve are used to extract ambient temperatures.


SANDS improved upon WATS by removing the deployable baffles, reducing the required motors and pressure gauges down to one, removing the magnetic field requirement, and removing the requirement for equilibrium operation (faster measurements). The baffles in WATS are replaced by anti-baffles in SANDS, openings in a front facing barrier, that produce beams of neutral gas to be measured as the satellite moves at high speed. An aperture rotates underneath this front barrier, periodically sweeping through the neutral beams and admitting gas into the instrument. The gas pressure is measured and recorded over time. With two anti-baffles, required to measure winds in two directions, the SANDS aperture encounters 4 neutral gas beams per rotation. This configuration ensures that uncertainty in the aperture position over time has no impact upon output accuracy.

Instrument measurements are typically compared to an analytical description to extract scientific parameters. For SANDS, this would require a slower instrument operating rate. Instead, a coupled set of differential equations were created and scientific parameters are extracted through comparison to the numerical integration outputs. Lab testing is not able to fully recreate the neutral gas beam as encountered in orbit. Nevertheless, this testing demonstrates SANDS responds as generally expected at ultra high vacuum.

SANDS is currently under construction for deployment on the upcoming NASA Cousins rocket mission.

SANDS schematic. Fixed anti-baffle (top) admits a flow of neutral gas onto the top plate. The gas enters the sensor when the aperture in the top plate rotates through the neutral gas beam.
A single aperture rotation for SANDS configured to measure two wind directions, neutral composition, as well as neutral temperature. The simulated sweep, including instrument errors, produces an accurate characterization of the neutral atmosphere.
Lab tests reporting SANDS internal pressure resulting from a periodic injection of gas as the aperture rotates past a gas source.

U.S. Patents

Dr. Stoneback was the lead inventor for SANDS. U.S. Patent 11,187,827