Wednesday, April 17, 2024

Demystifying GPS

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A contemporary Yogi, philanthropist and self-proclaimed Guru, likened GPS to a Guru. He said – A GPS will guide you when you are in an unknown territory, just like Gurus would! However, here’s my take on it – GPS is not just about guiding you in an unknown terrain, but can do lot more than that! How’s that? Read on….

Firstly, let’s try to answer the question “What is GPS?” Also, what is GNSS, GloNaSS? Far too many acronyms? A GPS is Global positioning system, which much of us know. But, what many of us don’t know is – it is one of the GNSS out there. GNSS – Global Navigation Satellite system, is a bunch of satellites up there in the space, which helps to pinpoint your geographical location. This could be done using a receiver, which receive and decipher the signal and information sent by these satellites. The receiver has to recover signals from multiple satellites to be able to accurately compute the geographical location. More the satellites, it is more likely that the location would be accurate. Things would be a little clearer later. In addition to location information, GPS also gives you accurate frequency and time information.

While GPS is a global navigation satellite system managed by the Americans; Russians, European union and Chinese have their own version of GNSSs. While Russians have GloNaSS, the Europeans the Galileo, the Chinese BeiDou; India has its own indigenous IRNSS, which is in its final stages of making. All of these are similar to GPS and some of the receivers in the market have the capability to receiver more than one GNSS signals. Receivers supporting more than one GNSS stand a better chance of receiving more satellites and therefore at any point of time and at any location, they are more likely to generate accurate location information. However, let us focus on GPS to understand its operation and its use.

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GPS, operational since 1978 and globally available since 1994, is currently the most widely used global navigation satellite system. Although originally intended for military applications – for precise delivery of weapons to intended targets; its utility was extended for civilian use by diluting some of the information so that precision is restricted to couple of meters, to prevent wrong people from misusing it for wrong purpose. However, improving the precision is still possible through special techniques like differential GPS, etc. Incidentally, India has satellite based augmentation system called GAGAN (GPS Aided Geo Augmented Navigation) which helps to improve the accuracy of GNSS receiver by providing reference signals. GAGAN is more intended for aiding flights and without digressing, let us keep to GPS.

GPS consists of upto 32 satellites in medium earth orbit (~20,000 Kms) in 6 different orbital planes, with exact number of satellites in use at any point of time varying as older satellites are retired and replaced. By design, there are atleast 24 of them for proper positioning at any location on the earth. More satellites are deployed to improve reliability and availability of the system. The orbits are arranged so that at least six satellites are always within line of sight from almost anywhere on the Earth’s surface. These satellites send signals coded with information carrying time, frequency and data that could be used to decipher the location information.

The modulation technique used is Direct Sequence Spread Spectrum (DSSS) which allows multiple satellites to share the same frequency band. Code Division Multiple Access (CDMA) spread-spectrum technique ensures the low-bit rate message data is encoded with a high-rate pseudo-random (PRN) sequence (code) that is unique for each satellite. With the PRN codes for each of the satellite, the receiver will be able to receive data from each of the satellite in view, in a sequence. CDMA spread spectrum renders the GPS signals highly robust and immune to interferences.

GPS satellite constellation
GPS satellite constellation

The satellite send coded data actually on two separate signals – L1 (1.57542 GHz) and L2 (1.2276GHz). The L1 signal carries Coarse Acquisition code (C/A code) for civilian use, encrypted Precision (P(Y)) code for military purpose and the Navigation message. The L2 signal also carries the encrypted precision code for military purpose. The precision code could be decoded only if the receiver has the encryption key. P(Y) code when decrypted carries information required to improve the precision – for example removing the effects of the ionosphere.

GPS signals
GPS signals

Any receiver intended for civilian purpose will receive both L1 and L2 signals, but can decode and decipher only C/A data and therefore will not be able achieve high level of precision in location. If you recollect my earlier statement that precision (in location information) is diluted for civilian use and this is how it is done. A receiver intended for military use can receive L2 signal and decode the precision data by having the encryption key and therefore can be more precise. To clarify the difference between accuracy and precision – for a given set of satellites visible at a location at a specific time and given reasonable signal strengths of their signals, a civilian receiver will have a larger circle on the map to pinpoint the location on a map, while a military receiver will have a much smaller circle on the map to pinpoint the location on the map. The receiver could be anywhere within the circle. Therefore, a military receiver would be in a position to pin point the exact location. The illustration below explains the difference between accuracy and precision.

Accuracy vs. precision
Accuracy vs. precision

GPS follows a technique which is a variation of Triangulation called Trilateration, to compute the location. A GPS receiver determines its position on the surface of the earth by timing the signals from four satellites. The technique uses precise location of satellites in space, precise time information from each satellite and measuring distance using the travel time of the radio signals. [It is possible to compute location using three satellites also, but the receiver would have to use additional information from mobile network to aid the computation and/or it would have to compromise on the accuracy.]

Here’s how the GPS receiver figures out its location. It uses distances of the receiver from multiple satellites to compute the location on the earth, provided precise location of the satellites are known. The receiver uses accurate time information from the satellites to compute the time of flight of the signal and from that it can compute its distance from the satellite. Let us assume that the receiver is able to see 4 satellites. You may consider d1, d2, d3 and d4 as the distance travelled by the signals from the satellites. If you draw four spheres with centers being the location in the space of these four satellites, with d1 to d4 as their respective radius, then these four spheres will all intersect at one point on the surface of the earth, which is the location of the receiver.


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