Hedy Lamarr's beauty and celebrity and perhaps her technical mind made her the ideal person to help promote Emerson radio and TV sets in the early 1950s.

Thanks to actress Hedy Lamarr, the early foundation for spread spectrum technology was laid out decades ago. The Navstar (GPS) constellation is the single largest spread spectrum machine in existence, broadcasting microwaves covering every square inch of the planet; a scary thought until you realize that through the magic of spread spectrum, the energy of the signal is smeared out so widely, it exists almost below the natural background static of the universe. Thanks to correlators, the direct side benefit of having to time-shift C/A codes to match orbiting transmitters is the calculation of the receiver's distance from various satellites. If you will grant me some poetic license, Hedy Lamarr's and George Antheil's synchronized clock piano-roll system for lockstepping the transmitter frequencies is actually an ancestral relative of the modern day correlator. So it's by no means serendipity to say we have Hedy Lamarr to thank for the convenience of modern day GPS.

Sidebar: Interesting Facts

Many people believe that their GPS computes their speed using the time it takes them to cover the distance between an earlier location and their current location. However, many receivers calculate velocity using Doppler shift. Just as a speeding truck blowing by you sounds distinctly different from a slower truck, the different satellites in their different orbits do the same trick. A speeding truck makes a distinctly different sound from a slow moving truck. The different satellites in their different orbits also produce Doppler shifts in their transmissions. Using complicated math, Doppler shift and the precisely known orbits of these satellites, your GPS receiver can tell you your speed more accurately than your car's speedometer.

Few people know that GPS satellites are equipped with NUDET sensors. These special gamma ray instruments come to life when a nuclear detonation occurs. When a nuke blows up, it generates a pulse of radiation moving at the speed of light. The radiation wavefront is detected by each GPS satellite in sequence. Using "reverse location lookup" the GPS satellites report the precise location of the detonation.

Most GPS devices today employ an Almanac. An Almanac carries basic long term information about the GPS constellation, informing the correlators what satellites to look for and what satellites not to bother with. Almanacs are downloaded from the GPS satellites and decrease the time it takes to search for satellites when you turn on a GPS device. Ephemeris data, downloaded similarly to an Almanac, contains more precise but short lived orbital data that if available can vastly reduce the fix acquisition time. This is why the first time you turn on a GPS, it takes a lot longer to get your position than if you simply turn it off after first use and back on again. Almanac and Ephemeris data are download the first time.

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