I glossed over one important detail. How can such minute antennas be used to communicate with satellites? The secret lies in the communication technology used. A (PRN) of length 1023 bits is generated by the satellite. The receiver generates them at the same time. By correlating the phase of the code with that generated by the receiver, we can know the time taken by the signal to travel. This "code phase" receiver is not fine grained and so we use "carrier phase" tracking to lock onto the carrier frequency, which is more accurate. Each satellite has a corresponding PRN sequence which is known to the receiver or transmitted by the earth stations to the receiver.

The reason behind using PRN sequences is that they are so resistant to noise and jamming since the signal can be recovered fully be taking more samples. If you understand CDMA, this is the perfect application for that technology. That is how we can get by with small antennas. And of course, our information content and bandwidth requirement is so little that noise doesn't affect us much.

Now we have corrected almost all of the errors that confronted us. But the errors cannot be eliminated completely. To improve the accuracy still further, we use something called a differential GPS. Here, we have a known fixed reference receiver and a rover receiver (our normal GPS receiver). The reference receiver will know the exact errors in calculation since its own position is already known. The rover receiver compares with the reference receiver and applies the error corrections, thus resulting in amazing accuracy.

As mentioned above, the more satellites we use, the more accurate is our measurement. There are receivers that use up to fifteen satellites to determine the location.

GPS as a technology that tells us our position accurately is highly effective. We can map every point on earth and above, within one meter precision using a tiny GPS receiver. The sources of error and the means used to correct them have also been described. The use of an additional fixed receiver to improve the accuracy results in much higher accuracy. This is what is used in differential GPS. GPS combined with maps can be used for navigating anywhere on earth. Searching and surveying are just few of the areas where GPS can be used.

Girish Venkatachalam is a senior software engineer at MindTree Consulting.
He can be contacted at girishv@mindtree.com