SubscribeThe development of lightweight, solid-state microwave systems by Microwave Associates Communications Equipment Division (M/A) in the early in the 1960’s marked the beginning of live airborne video for the public safety and military sectors.
Microwave Associates changed their corporate name to M/A-COM in 1978, and in 1986, M/A-COM sold the equipment division to the founders of Microwave Radio Communications.
Some of the first Microwave Associates portables provided live video from NASA helicopters that recovered US Astronauts after the splashdown of Mercury and Apollo space flights. The New York City Police Department installed Microwave Associates equipment in their first airborne systems in the late 1960's, with receive antennas located on the 81st floor of the Empire State building. The US Marines deployed ruggedised M/A transmitters in 1969 on drone helicopters, for airborne reconnaissance during the Vietnam conflict. In the mid-1970's, TV news organizations seized on the opportunity, and began using airborne coverage as a competitive weapon in the ratings war.
The usefulness of live video from a flying camera mount has proven itself in thousands of situations. To their credit, the news organizations pushed hard for improvements in technology that also improved the state of the art in public safety applications.
As the FCC mandated conversion to digital television continues to move forward, a number of new developments have become available that lend themselves well to improvements in features and performance for airborne systems of all types.
Microwave Transmission
With some special exceptions, the nature of microwave transmission limits its use to line of site communications; with signals behaving very much like light beams. They can be focused, reflected, partly obscured, blocked, and bent.
Solid objects like buildings, trees, and hills can partially or completely block a microwave signal if the object is directly in the path between the transmitter and the receiver. If an object with a reflective surface is close to, but not in the path, the signal may be reflected or bounced off the object. Signal reflections can be useful or bothersome, depending on the circumstances.
In figure 1, you can see a direct path from the helicopter to the receive site, and several indirect paths from the signal bouncing off nearly buildings. The receive site may be located on a tall building, or a tower as shown, or it may be a suitcase portable receiver like the MRC Tactical Receiver Case, also known as the TRC. The combination of direct and indirect signals is commonly called multi-path.
Prior to the development of digital transmission technology, multi-path had been a constant problem, as it causes severe distortion of the direct signal, or in extreme cases, cancels it completely. The issue stems from the fact that analog transmission uses a single frequency carrier wave for all of the information that must be transmitted, and multipath signals usually affect one specific frequency at a time.
The development of COFDM, which is a digital multi-carrier transmission technology, has minimized the effects of multi-path by spreading the information out over a range of closely spaced carrier frequencies. The graphic in Figure 2 contains a simplified view of analog transmission, as having only one path (or water pipe in this case) from end to end. If that one pipe breaks, the flow is disrupted.
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