TABLE OF CONTENTS As an example of realization of the design considerations in Chapter 5, the noise-injection receivers employed in the first generation of the DTU airborne multifrequency radiometer system will be reviewed. A more detailed description can be found in [1, 2]. The system comprises three receivers at 5, 17, and 34 GHz. The major electrical characteristics are shown in Table 6.1.
| Frequency | 5 GHz | 17 GHz | 34 GHz |
| Bandwidth | 250 MHz | 500 MHz | 500 MHz |
| Noise figure | 4.5 dB | 5 dB | 5 dB |
| Sensitivity ( ? = 8 ms) | 1.15K | 0.95K | 0.95K |
| Sensitivity ( ? = 64 ms) | 0.41K | 0.34K | 0.34K |
| Integration times | 4, 8, 16, 32, 64 ms | ||
| Input range | 0 313K |
The design of the 17-GHz radiometer is very close to that given in Chapter 5 (NIR mode only). The two other radiometers are of equivalent design. The only major differences between the radiometers (apart from the frequencies, of course) is that the 34-GHz radiometer features two antenna inputs and an associated switch, while the 5-GHz radiometer has half the bandwidth of the others and employs a microwave preamplifier before the mixer to enhance performance.
One of the basic assumptions in the noise-injection radiometer concept is the stabilization of the front-end microwave components to a temperature equal to the radiometric reference temperature. Hence, a temperature-stabilized enclosure is designed for these components. The directional coupler, Dicke switch, reference load, microwave filter, and the isolator have to go inside the enclosure but...
