Development and Calibration of Pressure-Temperature-Humidity (PTH) Probes for Distributed Atmospheric Monitoring Using Unmanned Aircraft Systems

Abstract

Small unmanned aircraft systems (UAS) are increasingly being used for meteorology and atmospheric monitoring. The ease of deployment makes distributed sensing of parameters such as barometric pressure, temperature, and relative humidity in the lower atmospheric boundary layer feasible. However, constraints on payload size and weight, and to a lesser extent power, limit the types of sensors that can be deployed. The objective of this work was to develop a miniature pressure-temperature-humidity (PTH) probe for UAS integration. A set of eight PTH probes were fabricated and calibrated/validated using an environmental chamber. An automated routine was developed to facilitate calibration and validation from a large set of temperature and relative humidity setpoints. Linear regression was used to apply temperature and relative humidity calibrations. Barometric pressure was calibrated using a 1-point method consisting of an offset. The resulting PTH probes were less than 4 g in mass and consumed less than 1 mA when operated from a 5 VDC source. Measurements were transmitted as a formatted string in ASCII format at 1 Hz over a 3.3 V TTL UART. Prior to calibration, measurements between individual PTH probes were significantly different. After calibration, no significant differences in temperature measurements across all PTH probes were observed, and the level of significance between PTH probes was reduced. Actual differences between calibrated PTH probes were likely to be negligible for most UAS-based applications, regardless of significance. RMSE across all calibrated PTH probes for the pressure, temperature, and relative humidity was less than 31 Pa, 0.13 °C, and 0.8% RH, respectively. The resulting calibrated PTH probes will improve the ability to quantify small variations in ambient conditions during coordinated multi-UAS flights.

Keywords: barometric pressure; calibration; distributed atmospheric monitoring; embedded systems; relative humidity; temperature; unmanned aircraft systems.

Figure 1

Top and bottom isometric views of the PTH probe design with main components identified.

Figure 2

Block diagram of the PTH probe microcontroller firmware generalizing functionality. The main thread handled sensor measurements, data processing, and data transmission. An interrupt service routine handled serial data input from a host device for setting the serial number, retrieving existing calibration data, setting the calibration data to specific values, and resetting the calibration data to factory defaults.

Figure 3

Example formatted string transmitted from the PTH probe to the host at 1 Hz over 3.3 VDC TTL UART. The checksum was the bit-wise exclusive OR of all 8-bit ASCII characters between the ‘$’ and ‘*’ characters and displayed as a 2-digit hexadecimal number. The terminator comprised the non-printable carriage return and line feed characters.

Figure 4

Example formatted strings received by the PTH probe from the host over 3.3 VDC TTL UART. The PTH probe responded by echoing the string with relevant data and checksum inserted before the terminator.

Figure 5

Eight PTH Probes in an environmental chamber. The environmental chamber’s temperature and relative humidity sensors and the weather station’s barometric pressure sensor were used as reference instruments for calibration and validation. The weather station’s ultrasonic anemometer also confirmed that airflow exceeded 1.0 m/s to achieve manufacturer-specified sensor response times.

Figure 6

Block diagram of the automated calibration system generalizing functionality.

Figure 7

Dimensioned drawing of the PTH probe. Units are shown in [millimeters] and inches.

Figure 8

Automated calibration program graphical user interface developed in MATLAB App Designer environment.

Figure 9

Time response of environmental chamber settings for temperature and relative humidity during a calibration run, with additional points of interest for the start of equilibrium threshold and calibration data collection periods. Lines show the temperature and relative humidity profiles over the duration of the calibration. O’s show when the chamber reached equilibrium and calibration data collection began. X’s show when calibration data collected ended.

Figure 10

Regression model and corresponding confidence interval for barometric pressure calibration data from representative PTH probe (SN: 0x0005).

Figure 11

Regression model and corresponding confidence interval for temperature calibration data from representative PTH probe (SN: 0x0005).

Figure 12

Regression model and corresponding confidence interval for relative humidity calibration data from representative PTH probe (SN: 0x0005).

Figure 13

Time response of environmental chamber settings for temperature and relative humidity during validation run, with additional points of interest for the start of equilibrium threshold and calibration data collection periods. Lines show the temperature and relative humidity profiles over the duration of the validation. O’s show that when the chamber reached equilibrium and validation, data collection began. X’s show when validation data collected ended.

Figure 14

Regression model and corresponding confidence interval for barometric pressure validation data from representative PTH probe (SN: 0x0005).

Figure 15

Regression model and corresponding confidence interval for temperature validation data from representative PTH probe (SN: 0x0005).

Figure 16

Regression model and corresponding confidence interval for relative humidity validation data from representative PTH probe (SN: 0x0005).