A miniature, battery-powered, semiautonomous environmental monitoring instrument contains advanced meteorological sensors, a Global Positioning System (GPS) receiver for determining its position, radio-communication circuitry, and a controller that performs measurement, control, and data-communication interface functions. The instrument could serve as a high-accuracy radiosonde, though its intended use is in providing reference measurements for calibration and comparison of ordinary radiosondes.
The primary functions of the instrument are to periodically measure humidity, pressure, temperature, and position, and to report the measurements to a base station. The base station records the data stream as received, displays key parameters on a screen to enable monitoring of status, and provides an uplink (radio-communication) interface for sending commands and operational parameters needed for instrument functions.
The meteorological sensors in the instrument are a commercially available micromachined pressure sensor, three commercially available fast-response thermistors, and a surface-acoustic-wave (SAW) hygrometer.
The SAW hygrometer was developed at NASA's Jet Propulsion Laboratory and was described in "Fast, High-Sensitivity Dew-Point/Frost-Point Hygrometer" (NPO-20006), NASA Tech Briefs, Vol. 21, No. 5 (May 1997), page 10a. A surface-acoustic-wave (SAW) device serves as the resonant frequency-selecting element of a radio-frequency (RF) oscillator. The frequency of oscillation is sensitive to condensation on the surface of the SAW device. The SAW device is mounted on a thermoelectric cooler, which is used to effect feedback control of the temperature of the SAW device to maintain equilibrium between condensation and evaporation. The feedback-controlled temperature is near or at the dew or frost point and constitutes a measure of humidity.
Three custom circuit boards incorporated into the instrument drive the SAW device at its resonance frequency, provide signal conditioning, and implement digital control and communication.
In addition to the three custom circuit boards, the instrument contains a GPS receiver and an RF modem.
The base station consists of a laptop computer equipped with an RF modem. In addition to serving as a command and control interface as described above, the base station provides a display of real-time data and a moving map with a superimposed mark that represents the position of the instrument.
This work was done by Michael E. Hoenk, Robert Watson, and Greg Cardell of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.
This Brief includes a Technical Support Package (TSP).
Integrated Enviromental Monitoring Instrument
(reference NPO-20582) is currently available for download from the TSP library.
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