National Aeronautics and Space Administration

Glenn Research Center

System Description

The Guam Atmospheric Propagation Monitor was developed after the Goldstone and White Sands Site Test Interferometers. Since these systems were capable of measuring signal attenuation and phase differences at the two spatially separate locations, it was relatively straightforward to implement the well-characterized system for this site, as well.

As with the other systems, the Guam Atmospheric Propagation Monitor monitors an unmodulated beacon signal. This particular signal is a 20.7 GHz beacon broadcast from geostationary satellite UHF Follow-On (UFO8). The unmodulated beacon signals at 20.7 GHz are received, amplified and down-converted to 70 MHz in the RF feed box enclosure. The 70 MHz signals are further down-converted to 455 kHz (double conversion) in the IF box enclosure. These 455 kHz signals are sent to the indoor facilities for analog-to-digital conversion (3.64 MHz sampling rate, with 524,288 samples) and further signal processing. The post-detection bandwidth can be electronically adjusted between 7Hz to 222Hz to improve SNR if required.

The system block diagram for the two-element interferometer is depicted in Figure 1.

System block diagram of the Guam Atmospheric Monitor

Figure 1. System block diagram of the Guam Atmospheric Monitor. Select image to view larger.

In software, the in-phase (I) and quadrature (Q) components are determined to derive the signal amplitude and phase. Further, the Guam Atmospheric Propagation Monitor is outfitted with a 20.7 GHz radiometer to monitor the sky brightness temperature (gaseous absorption, noise) contribution to the communications system.