Sunday, October 31, 2004

A Question on GPS

Recently there was a MAIL at Chennai .NET User Group - CNUG on implementation of GPS on a web application… somehow I had this thought that this question generalizes and overshadows many internals of GPS , which would leave the basics behind the scenes… and many people would implement this without understanding the intricacies of the GPS…

Here is a small writeup i wrote on GPS in simple terms…in reply to that mail...

Have you ever wondered how this whole concept of GPS works synchronously?? Implementing something without understanding the crux would be fine sometimes, but absolutely lacks fun and adventurism. Thought I shall share something I know to clarify those generalization. If the basics does not interest you, very well go to the last part of the mail for the direct answers….

Okay the answer to your question is… The answer is YES and NO :-)
Let me justify my answer, but first some basics on GPS, I shall try and keep this as simple as possible. Global Positioning System(GPS) is a worldwide navigation system built around 24 satellites and millions of receivers. Each satellite has a very exact atomic clock to keep time accurate so that the deviation is less than 3 nanoseconds. Fine, but how do they help in location services ? All these satellites have an exact model of their orbit around the earth, and constantly broadcasts it’s time-stamped position to the ground stations. Now by measuring the differences in timestamp and satellite positions, a receiver on earth can determine its position using triangulation.The killer application of the GPS is, to know a position in real time. But, there are also a few other elements related to basic navigation, such as converting latitude/longitude information to easting/northing the coordinates used by many maps, or calculating distance/direction between known positions.

GIS – geographic information systems, complements the GPS very much. GIS system typically contains information about objects(e.g. roads,hospitals etc) and other characteristics in a database. This database is used to plot maps or make queries such as “how many hospitals are located in a particular area”.

But, how do we normal humans get information using these GPS and GIS ? the answer is the GPS Devices usually connected to a computer/PPC using serial ports/USB/Bluetooth, and as always we have issues with the proprietary protocols like SiRF and Garmin, but much like our TCP/IP which acts as a standard, we have NMEA 0183 (National Marine Electrinocs Association) you should have guessed who invented this protocol and why.

Surprisingly, NMEA is a very simple ASCII based protocol. The GPS Sends NMEA data continuously over the serial port. There are 3 types of NMEA Sentences – GGA, RMC and GLL, these sentences start with a “$” and provide details like UTC,Latitude,Hemisphere,Longitude,direction et al as comma separated values and ends with a “*” followed by the checksum. Some GPS receivers send all the three sentences, but not a mandatory one. Latitude determines the position in the south to north direction(0 @ equator, -90d @ south pole, +90d @ northpole), likewise longitude determines west to east direction.

How how do we exactly pin point a location on earth…i.e. “datum” in GPS Terms, for this you need to refresh your science and mathematics part of ur brains you studied at school, i.e. earth is an ellipsoid (sphere that is slightly pressed together at the top and bottom) the parameters that define ellipsoid is major axis, minor axis and displacement from center of the earth – this is datum. The datum used in GPS is called “WGS84”, therefore a position described in lat/long defines a point on a datum and thus we are able to pinpoint and locate a particular point on earth. You can call that type of data as “GeoCoded”. This datum is just projected on the map projections. These datas are manipulated using simple Cartesian coordinate systems and mapped against something called as UTM – Universal Transverse Mercator

Okie okie…. Now I’m answering your question… after all these information have been fetched, calculating the distance between two points is as simple as using your Pythagorean theorem and simple trigonometry.

Ah… should you go thro’ all these troubles, just to get a distance between 2 places… if you are adventurous the answer is yes, otherwise no :)

Now, the real part of the answer…. You can very well use the location services provided by the location servers of many vendors or use third party API’s that work on geocodes.
Such as those provided by
1. Microsoft MAP Point SDK – if your applicaton is extensible on implementation of location services, this is the best bet, but you might need to signup for an map point account with Microsoft. This will enable you to access the web services exposed to get more location based information. If you are a MSDN subscriber you might get it free. Just browse MSDN for more details.
2. Use third party components, some of my recommendations would be http://www.franson.biz/
3. Signup with some API Providers like Advantage API from MapQuest.

Ah…. A Long post !!! but fine :-) basics are pretty important….

references Franson.biz...

posted by Logu Krishnan : 10:21 AM


I would like to know if there is any GPS service provider in Chennai, if I were to have a GPS device or it does not require for a provider to be located here ?

It would be great if you could post more on getting GPS devices work in Chennai.

To my knowledge there are no GPS Providers in Chennai. But Infact I'm getting in touch with my friend specializing in remote-sensing and GIS @ Anna University. I should be able to give you a possible reply in couple of days. Please leave your email address over here so that i shall be able to inform you.

If you are using GPS you need 2 things
1. GPS reciever...there is no restriction of using device as your position is recognised using satellites..you are free to collect your locations and store it GPS device itself.

2. A Map with full information i.e. database attached to it with lat-longs of all features on Map.

Features means Roads, Railways, Buildings, Small streets, lakes, playgrounds etc all are features of map.

In India, we don't have that database which has full information. We have maps with information upto road level only and due to various policies of india government most of map data are not sold without proper justifications.

As if now there are no service providers for maps with navigation facilities in india but there are news that very soon you would see that into indian automobiles.

You need to wait for some more time to see GIS business applications in India.

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