During the early development days of radar and electronic-warfare (EW) systems, the electromagnetic (EM) spectrum seemed almost infinite. Not only was so much frequency spectrum seemingly available, such as 2 to 18 GHz, there was also the sense that it "belonged" to the military because of their critical needs during wartime. But here in present time, bandwidth now appears something considerably less than in nite, with the US Federal Communications Commission (FCC) doing all they can to manage what is now clearly a limited resource, by means of such things as frequency allocations and frequency auctions.
Even military electronics systems planners must change their ways of thinking in terms of bandwidth, since what is available must be wisely used. As an example, several articles in this issue (see pp. 34 and 46) report on various methods for control stations to communicate with unmanned aerial vehicles (UAVs), either by satellite links or by line-of-sight (LOS) microwave links. Traditionally, these communications formats have been specified by the common data link (CDL) definitions as part of the frequencies and modulation formats for use between control units and UAVs. But even in tactical systems as sophisticated as UAVs, the bandwidth was not always used efficiently, often falling back on simple analog modulation schemes such as frequency modulation. While UAVs have been used effectively on the battlefield, their numbers have been relatively few at any one time, so that CDL channel congestion and interference have not been a problem. But in a situation where more than a handful of UAVs are needed over a small operating space, the limits of the CDL format may become painfully obvious.
Fortunately, the growing adoption of the tactical common data link (TCDL), a communication format designed specifically for UAVs and compatible with existing CDL systems, programmability and fl exibility are possible on the battlefield. Th e Ku-band system (with option for X-band) operates with uplink frequencies of 15.15 to 15.35 GHz and downlink frequencies from 14.40 to 14.83 GHz. By changing the frequency converter and the antenna, a UAV system can be quickly modified for X-band operation.
The TCDL format is difficult to jam and features low probability of detection. It can be encrypted for added security, and can support data rates to 45 Mb/s. An operator can even select waveforms and data rates during a fl ight under software commands from the control station. As mentioned in the story on low-noise ampliflers (LNAs) and power amplifiers for TCDL applications on p. 46, the entire TCDL modem assembly can be made with readily available components in a compact form factor that is light enough even for the smallest UAVs. With all the advances in imaging, motor, power, and other systems in UAVs, it was only fitting that the communication link should follow.