Given the enormous popularity of GPS among civilian users, and the critical applications for the military, it is not surprising that a large body of literature has arisen about the origins of this remarkable technology. The curators of the new Time and Navigation exhibition discuss this history, and we have illustrated it with a few select artifacts, such as the engineering model of the Navy’s NTS-2 satellite, one of the key demonstrators of the technology that led to the deployment of the GPS constellation. GPS is the result of a collaboration among many contributors. Rather than try to sort out the various claims, I would like to take a closer look at one component of the system, namely the method by which signals from the constellation of satellites are encoded and transmitted to receivers on Earth. The technique is known as “spread spectrum,” and it is widely employed not only by GPS but also by WiFi and Bluetooth wireless connections to computers, by cordless telephones, and by most cell phones in the United States. In fact, spread spectrum and its cousins have created a revolution in communications, which may in a few years relegate traditional narrowband radio to our sister museum, the National Museum of American History.
Most histories of GPS credit the US Air Force, correctly, with suggesting this coding scheme for GPS. They also credit the genesis of the idea to the Austrian actress Hedy Lamarr, dubbed by Hollywood producer Louis B. Mayer (of MGM Studios) as “the most beautiful woman in the world” after she emigrated to the U.S. in 1937. I will pass on commenting on her contribution to the cinema, but it is worth exploring just what she did regarding secret coding, and how that relates to GPS. In 1940, Lamarr met the American avant-garde composer George Antheil, who just returned to America from Paris, where he had created a sensation pushing the boundaries of classical music. Among his more outrageous compositions was one in which he placed a number of player-pianos on the stage, each producing “canned” sounds. For us the relevance of this story is that Lamarr later emigrated to the US and became a fiercely patriotic champion of the Allied cause after America’s entry into World War II in 1941. While living in Austria she had been married to an industrialist named Fritz Mandl, and over dinner conversations he had with his colleagues, she became acquainted with some of the advanced weapons the Nazis would later employ to such great effect. Among them were radio-controlled glide bombs—the predecessors to the “smart bombs” so much in the news today. Inspired by Antheil’s compositions, she came up with the notion of using perforated paper tape (not as wide as a player-piano roll), to rapidly switch the frequency of the transmitter on a ship that launched the torpedo. An identical tape on the torpedo would switch, or “hop,” the frequency of the receiver, to match the transmitter. Think of a car radio, with which you can rapidly select a radio station by pushing a button, and not twist the tuning dial. The technique depended on the precise synchronization of the two tapes, but for a torpedo that only had to work for a short period of time. And of course it depended on the enemy’s not knowing the sequence of frequency hopping—the sequence had to appear random, although it was not. Lamar applied for and was granted a patent for a “secret communication system” in 1942, but the Navy did not use her invention. Decades later it was rediscovered and became the basis for secure communications. The paper tapes are now replaced by digital computer circuits, which generate sequences of “pseudo-random numbers” (PRN) that hop the frequencies of the transmitter and receiver in synch. Because the technique requires a wider band of frequencies than a normal radio transmission (think again of the car radio), it is called “spread spectrum,” as it spreads the signal across a wider band.
Now back to GPS. GPS uses spread spectrum. It requires a larger bandwidth than a narrowband radio would require to transmit its signals. The satellites transmit primarily on two frequencies, 1575.42 MHz and 1227.60 MHz, but if you tune an ordinary radio scanner to those frequencies, you hear nothing but background noise. The frequency does not hop, however. The signals are multiplied by a pseudo-random sequence, which is transmitted and recovered by the receivers using the same sequence. There are two main pseudo-random codes, a short one used in civilian receivers, and a longer one used by the military. Like the system proposed by Lamarr, this also spreads the signal out; it also trades bandwidth for power, which is why a normal receiver hears nothing—the signal is well below the noise threshold. The adoption of this technique gives GPS a number of advantages: the receivers do not need a dish or otherwise large antenna to pick up the signals, and the different codes allow for both civilian and military use of the same system. The low power also means that the signals cannot be received indoors or under dense tree cover, a drawback that future generations of GPS satellites may address. So if this technique did not come from Hedy Lamarr, where did it come from? Of that we know less. But there are hints that it may have come from another system developed during World War II. If true, that story is every bit as mysterious and intriguing as Lamarr’s. Much of the initial research apparently was done by the cryptographic community and remains classified. But there are some tantalizing hints. A few years ago, The U.S. National Security Agency (NSA) published a pamphlet describing a method of scrambling speech, which was used during World War II by Winston Churchill and Franklin D. Roosevelt, among others. The NSA claims this “SIGSALY” system was “The Start of the Digital Revolution.” For an agency known for its reticence, this publication represented a major revelation. The invention introduced pseudo-random noise, which was recorded mechanically on a phonograph disk, and superimposed this on the speech channels. At the other end an identical disk, synchronized to start at the exact same moment, subtracted the noise. The disks were destroyed after each use. The system was installed in several locations by 1943, including one in the Pentagon and one at Churchhill’s command post under the Admiralty Building near Number 10 Downing Street.
The NSA pamphlet shows a system of two phonographs, each of which contained one of the platters. For some of us, seeing this photograph immediately evokes another phenomenon—the use of twin turntables by hip-hop musicians to sample and otherwise electronically alter sounds on the dance floor. Is there a connection? Possibly—the methods of speech scrambling, developed in the 1930s and 1940s primarily by Bell Telephone Laboratories, have been cited as direct ancestors to current pop music. So the next time you use your car GPS receiver to tell you how to get to a restaurant, think of Hedy Lamarr, or better yet, Grandmaster Flash.