6. Direct Sequencing

For modern GPS transmissions, the method is quite a bit more streamlined. Instead of using a code to alter the channel, GPS transmits data on a 1575.42Mhz carrier. It uses a C/A code running at 1Mhz to modulate the carrier. And using the magic of computer technology, essentially the data stream from the satellite is XOR'd into the C/A pattern and modulated. The navigation data stream is generated at 50Hz and the "channels" are changing at 1Mhz. The result is the signal is spread out over a 1Mhz band in a very smooth fashion.

The next series of examples depict a hypothetical navigation message that says "HELLO." Additionally, I'll be depicting things in ASCII to enhance readability. Actual GPS satellites transmit using a binary PSK that phase modulates the data at the bit level. Needless to say, very fascinating but outside the scope of this article to detail. So, use your imagination.

Table 2. Data Stream XOR C/A Code Stream = Transmitted Result

As seen in the above table, the slower hypothetical "hello" navigation message is combined into a much faster C/A code. By taking the random looking output and modulating it with a carrier frequency, the resulting signal is spread out over 1Mhz. This process digitally replicates the piano roll method and is known today as direct sequencing. Information is buried into the snowy hiss of what appears to be static, much the same way digital watermarks are smeared onto a photograph or a track on a CD. They can not be found, unless you know where to look. So basically, if you have the identical C/A code, then you can pull the wheat out of the chaff.

GPS Correlators Explained

Synchronize Your Clocks!

Thanks to Albert Einstein's General Theory of Relativity, it's known that a very fast ticking clock can be affected several different ways while it is in orbit. And since each GPS satellite is essentially an atomic clock in orbit, the entire constellation is corrected relativistically. Compared to our clocks on the ground, GPS clocks are sped up by the difference in gravity and slowed down by the difference in speed. Amazing enough, Einstein's theory pretty much predicted the actual relativistic effect almost down to a tee - the ticking of an orbiting clock is thrown off by 446.47 parts in 10^12. Imagine a 2Ghz Intel Core 2 Duo CPU placed in orbit: you'd discover the real speed is off by about 1 clock cycle. In order for GPS to function, the satellites must be synchronized. The more errors that are eliminated, the more "together" the transmissions are. We'll see why this is important later.