The eye in the sky
While investigating radio waves in the 1880s, Heinrich Hertz set the stage for radar (radio detection and ranging). Most militaries in World War II used ground and ship-based radar to study weather, detect aircraft, and guide planes to their targets. One of radar's biggest drawbacks, however, is that it is line-of-sight, so it could not be used on targets over the horizon. (At sea level, the horizon is about 10 miles away.)
To get around this limitation, engineers developed HF radio (short wave) that peeked over the horizon by bouncing off the ionosphere. Another solution to the line-of-sight problem is to mount the radar antenna as high as possible. This approach, and the trend toward ever smaller electronic and electrical devices, eventually led to airborne systems, including today's E-2C Hawkeye and the E-8C J-Stars.
"Strangle my parrot?"
Prior to World War II, British engineers invented an aviation transponder, the Parrot, that would respond to properly coded interrogation pulses with a coded pulse of its own. It was designed to let ground operators discern one aircraft's radar return from another's. Returning Parrot pulses from a plane were used to plot that aircraft's range and bearing on a radar scope and contained coded information such as mission status, fuel remaining, or indicated an emergency.
During the war, the Parrot proved useful in discriminating between friendly and enemy aircraft. It was used to detect Germans fighters tailing British bombers back to their airfields and attacking just as they were landing and most vulnerable.
IFF returns, however, are more powerful than reflected radar returns, and can overwhelm them on a radarscope, often hiding radar returns from other aircraft. So ground operators would tell pilots to "Strangle your Parrot," which meant to switch it off or to standby. Some older traffic controllers still use the British terminology.
Britain's wartime version, a 10 X 10 X 10-in. box, originally contained a thermite bomb triggered by a destruct switch. The bomb was to be activated in case of imminent crash or capture to prevent Axis forces from discovering how it worked or deciphering the code. Unfortunately, the destruct switch was next to the "on" switch and more than one pilot inadvertently blew up his Parrot. Engineers installed an inertial switch that would self-trigger in case of crash and safety-wired the destruct switch.
In the U.S., the system is called IFF (identification friend or foe) or secondary radar and is still in widespread use. The technology has expanded to include several different IFF modes. Mode C, for example, is tied to an aircraft's altimeter and relays the plane's altitude to the IFF operator. Airliners, squawk or transmit code 7500 if they are hijacked, 7600 if they lose radios, and 7700 in case of other emergencies. The system is also carried on ships, manned and unmanned ground units, and civilian aircraft. There are also shipboard and airborne IFF interrogators such as the E-2C Hawkeye, AWACS, and most tactical navy ships.
Finding your way through the night
One of the first heavy users of aviation services was the Post Office which used it to move mail, often at night. To ensure mail got through, in 1921 the Post Office bought an 80-mile string of rotating beacons erected by the Army between Dayton and Columbus, Ohio. Pilots making the run between the two cities were always in sight of the next beacon. By the end of 1923, a similar line of lights stretched from Chicago to Cheyenne, and soon it went coast to coast. It let mail cross the continent in 29 hr eastbound, and 34 hr westbound, the difference accounted for by prevailing west winds. This was two to three days shorter than it took by train.
By 1932, technology had gone from rotating lights to radio beacons. The simplest just transmitted a three-letter identifier in Morse Code. Pilots would tune their radio directional finder, also called a radio compass, to the right frequency, and a needle on the display would point to the beacon. This let them triangulate their position or determine their heading.
Eventually this led to various radio-based navigation systems, including Hiran (high-ranging system), VOR (VHF omnidirectional radio range), Tacans (tactical air navigation), Vortac, Loran-A, which was developed and used in World War II, and Loran-C (long-range aid to navigation), and Omega (optimized method for estimating guidance accuracy). The next step was satellite systems initially used solely by the military. These systems include Transit, the first operational system in 1962, Secor (sequential collation of range), a U.S. Army network of 13 small (34 to 45-lb satellites), and finally NAVSTAR or GPS (global-positioning system). There were also several Soviet satellite systems, the most current being GLONASS.
Planes without pilots?
Aircraft today have automatic pilots, landing systems, inertial navigation and GPS, and they've long since been able to fly radio controlled without pilots or aircrew. Advances in avionics and NASA's self-learning neural networks, will bring pilotless planes even closer. NASA's neural nets, for example, identify and store stability and control characteristics of the testbed aircraft. The idea is that if the plane suffers inflight damage or a malfunction, it will recall those characteristics and be able to restore stable flight. The biggest hurdle to pilotless planes, especially those carrying passengers, will probably be public acceptance.