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Radiometer Aids Fire Detection, Fig. 8Measured results are shown in Fig. 8, where the measured output voltage mean values are plotted versus test input PRF. By interpolating the measured data, a linear (as expected) relation in between the input power and the output DC voltage has then been extrapolated. The obtained radiometer transfer function is given in Eq. 1:

VDC(mV) = γPRF(mW) + V0 = 1.13 × 1011 × PRF(mW) + 556.6    (1)  

Given the radiometer calibration data obtained, it is possible to determine a linear function relating the antenna temperature TA and the measured receiver output, VOUT, when the radiometer input is connected to the antenna to observe the surrounding scene. In particular, VOUT is proportional to TA through the Boltzmann constant, the receiver bandwidth, and the calibration factor, γ of Eq. 1. From Eq. 1, one can conclude that, for the realized prototype, a 1 K increase in the antenna temperature corresponds to a 3.9-mV increase in the radiometer output signal.

Based on the same measurement setup, the radiometric resolution (standard deviation associated with the estimated antenna temperature) has also been measured by first calculating the standard deviation of VOUT and then by considering the linear relation between the measured output signal and the input antenna noise temperature (TA, as in Eq. 1). With the current approach, a 1.2 K radiometric resolution has been measured for 300 K antenna noise temperature. With digital filtering strategies, the obtained radiometric resolution can be further reduced: by applying a mobile average filter (with filter order N = 50) the radiometer sensitivity has been decreased down to 0.3 K.  The ADC sampling frequency is 50 Hz; a 50-order filter corresponds to a 1-s integration time.

Radiometer Aids Fire Detection, Fig. 9  

The radiometer was then tested in an open-space environment, using the measurement setup of Fig. 9. A 20-dBi Ku-band horn antenna was connected to the radiometer input. The antenna was placed at roughly a 1-m height and pointed toward the fire spot, with fire area of 0.09 m2 (30-cm-side square). The horizontal distance between the fire and the instrument was 3 m. The experimental results are shown in Fig. 10. Raw data were acquired with the ADC and then filtered with a 50-order mobile average digital filter (both raw data and filtered acquisitions are plotted in Fig. 10).

Radiometer Aids Fire Detection, Fig. 10

A maximum increment in radiometer output voltage of 80 mV was observed when the fire was at maximum (or 20 K in terms of radiometric contrast based on Eq. 1). This value is compatible with the predicted radiometric contrast for the current measurement setup,1 confirming the feasibility of the proposed approach for fire-detection applications. Fire extinguishing was recorded during a subsequent acquisition. As Fig. 10 shows, as the fire goes out, the output signal (starting from a maximum value) slowly decreases to the value corresponding to the absence of fire. Recorded local increments correspond to temporary increase in the fire area due to the presence of wind.

In summary, the Ku-band radiometer prototype exhibits radiometric resolution of 0.3 K for a 300-K antenna temperature, comparable with the performance of similar devices.1 Improved resolution may also be possible with increased integration time. Preliminary experimental results have indicated the usefulness of the instrument as a fire sensor: a fire spot of 0.09 m2 at a 3-m distance has been detected with a 20-dBi horn antenna, with corresponding increment in the radiometer output signal of 80 mV.2-5

The radiometer can be improved and additional features need to be implemented in the sensor. First of all, the dimensions of the device may be reduced. For the RF circuitry, rather than using commercial components (isolator, switch, or RF amplifier), in-house developed devices may be considered: this would allow for miniaturization as well as a cost reduction. Moreover, temperature monitoring still needs to be implemented in the instrument.

Finally, the absolute accuracy of the instrument as temperature sensor has not been evaluated. For fire-detection applications, however, this parameter is of minor importance as far as the increase in the antenna noise temperature rather than its absolute value should be properly measured. In short, these experimental results confirm the feasibility of the proposed design approach and represent a first step toward the realization of low-cost, high-performance remote temperature sensor suitable for surveillance and industrial applications.

G. Bianchi, R&D Engineer

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