WASHINGTON (AP) - Trucks. Trains. Cattle. Communications satellites. Bumblebees.
The common bond is the transponder, a bit of high-tech that's become a quiet necessity of today's world. The little-known device helps keep airliners from colliding, allows trucking companies to track their loads, helps farmers keep tabs on their cows' milk output, leads cops to stolen cars.
They let the military keep tabs on shipments of armaments and relay television signals to millions of dishes.
Some Americans carry a variation of the transponder to buy gasoline, an electronic key marketed by Mobil Corp. that allows customers to charge purchases by waving the "Speedpass" at the gas pump.
Indeed, a tiny transponder glued to a bumblebee allows researchers to track its movements to the nest.
At its most basic, a transponder is little more than an electronic receiver and transmitter designed first to recognize a particular signal, then reply.
"Essentially, it is used to identify or communicate with a vehicle or container or something that moves around," explained Rick Schuman, director of system applications for the Intelligent Transportation Society of America.
The use that probably affects most people is in communications satellites, where transponders receive and rebroadcast signals from Earth.
Growing numbers of Americans are installing small satellite dishes to receive the signals. In England they sprout from the sides of flats and homes across the country.
Get your signal by cable? That's merely a centralized satellite receiver. Even folks using only over-the-air television depend on satellites to get the network show to their local station.
The last time you traveled on a commercial airliner, the plane's transponder received radio signals from ground radar stations and other aircraft and replied with its altitude and an identifying code. That's how air traffic controllers can tell apart those moving dots on their radar screens.
And thanks to a system called the traffic alert and collision avoidance system, airliners avoid one another because transponders recognize when a plane is too close and warn the pilots to change course.
The government plans a system called "free flight" for airliners to travel with little ground control, using complex new transponders to track surrounding traffic and tell the pilot how to avoid a collision.
Tom Staggs, of AlliedSignal Inc., a leading transponder manufacturer for aviation, said the new devices could let pilots view radar signals seen by air traffic controllers and provide a print communications link - like the words that sometimes crawl across a television screen - to avoid misheard voices on the radio.
Transponders were recently in the news when the military lost track of a truck carrying four dummy missiles. Standard military practice is to place tracking equipment on trucks carrying munitions, weapons and other sensitive equipment, but that truck's broke down.
The truck was found when the driver called his office to say that he tried to charge dinner at a truck stop and was told his company credit card had been canceled. The company had done it in the hope he would call in.
Within three years, a tiny short-range transponder may become a lifesaver for police officers by preventing crooks from taking a cop's pistol and using it.
A .40-caliber semiautomatic pistol developed by Colt's Manufacturing Co. Inc. uses radio-frequency signals to block unauthorized people from using the firearm. A receiver inside the firearm's magazine picks up electronic signals from a transponder worn by the officer inside a wristband or ring.
Automobiles too can benefit from transponders: to find their way using the Global Positioning Satellite system, to lead police to stolen vehicles, to pay tolls on so-called intelligent highways.
Schuman estimates that 2 million commuters currently use short range transponders to pay daily tolls on highways in New York, California, Virginia, Texas, Florida, Louisiana and other areas.
On a more prosaic level, farmer Don Jensen of Stanley, N.Y., has placed transponders around the necks of his 800 dairy cows to enable weighing meters to measure how much milk each produces.
"We know quickly which cows are making us money and which ones aren't," Jensen said.
The result can be serious for the cow - automatic gates send some to the veterinarian, some to the breeder and some to the butcher.