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command the antenna to dwell for some time at the end of each step and average several short
duration measurements to filter out the always present scintillation “noise.”
7.4.19
Rain Fade
Rain Fade refers to the absorption of an RF signal by atmospheric rain, snow, or ice. It is most prevalent
at frequencies above 11 GHz. A heavy rain storm can reduce the satellite signal level received by a Ku-
band or Ka-band antenna system to essentially zero in a matter of seconds. Lesser storms, which are
much more common, can still have significant effects long before they arrive at the antenna’s location,
especially when the antenna is operating at low look (elevation) angles. Any significant signal changes
not related to the pointing of the antenna system can have a detrimental effect on the performance of
an auto-pointing antenna system.
Rain fades are usually short duration events, so the antenna typically returns to normal operation very
quickly.
7.4.20
Signal Sources
The AAQ is designed to interface with four different types of signal sources; RSL (broadband power
meter), Beacon Receiver, the AvL DvB receiver and a variety of Modems. Each of these devices falls
short of being the ideal signal measurement device for an auto-point antenna system. That ideal
device would have the following key characteristics:
1.
Reliable confirmation that the signal originates from the target satellite
2.
High sensitivity and dynamic range
3.
Near instantaneous response time
4.
Frequent (>~10 Hz) and accurate signal level estimates
5.
Affordable incremental system price
A device that excels in all of these areas would assure very fast and reliable satellite acquisition and
peaking while minimizing complexity in the AAQ software. Modems tend to do very well on
characteristics (1) and (5), but fall short on the others. The RSL scores well on items (3) – (5). The
best Beacon Receivers meets most of the above criteria, but they struggle with the important
characteristic (5). See appendix listing for Beacon receivers and other signal source options.
