WHAT IS THE 1st THING

THAT COMES TO MIND

WHEN YOU THINK OF

!!GPS

GPSYOU TURN ON YOUR PHONE TURN ON GOOGLE MAPS

GOOGLE MAPS NAVIGATE YOU

Global Positioning System

CONTENTS

1. Introduction2. Trilateration• 2-D Trilateration• 3-D Trilateration3. Signal & Its Contents4. GPS Calculation• Dilemma Of Atomic Clocks5. Differential GPS6. Cold Starts & Warm Starts7. Latest Advancements

INTRODUCTION• The Global Positioning System is 

a constellation of 27 Earth - orbiting satellites (24 in operation and 3 extras in case one fails).

 • The U.S. military developed and 

implemented this satellite network as a military navigation system,but soon opened it up to everybody else.

• Each of these solar-powered satellites make 2 complete rotations every day.

• The orbits are arranged so that at any time, anywhere on Earth, there are at least 4 satellites "visible" in the sky.

THE BASICS

• The GPS RECIEVER gets a signal from each GPS SATELLITE.

• The GPS receiver with that signal knows : Exact Position of the satellites in the sky Exact Time they sent their signals. 

• By subtracting the time the signal was transmitted (t1)   from the time it

was received (t2) ; ( t2 - t1 ) , the GPS can tell how far it is from each satellite.

• So with the travel time of the GPS signals from 3 satellites and their exact position in the sky, the GPS receiver can determine your position in 3 dimensions - [East / West], [North / South] and [Altitude ]

• The whole process of determining the position is called TRILATERATION

TRI-LATERATION

2-D 3-D

LETS FIRST UNDERSTAND IT IN

2D

IN 3-D IT CAN BE SLIGHTLY

TRICKY

IMAGINE YOU ARE SOMEWHERE IN

INDIA TOTALLY LOST

FOR SOME REASON YOU HAVE NO CLUE

WHERE YOUR ARE

FRIEND : SUSHANT

2-d Trilateration • You find a friendly local

[ SUSHANT ] and ask, Where am I ? He says :-"You are 24 km

from Sector 47 Noida."

• This is a NICE, HARD FACT but it is not useful by itself.

Why ?

• You could be anywhere on a Circle around Sector 47 Noida that has a radius of 24 km, like this:

FRIEND : AMJAD

2-d Trilateration

• You ask another friendly local [ AMJAD ] where you are ?

He says, "You are 9.3 km from Zeta 1, Greater noida "

• If you combine this information with the Sector 47 Noida , information, you have 2 circles that intersect.

• You now know that you must be at one of these 2 intersection points, if you are 24 km from Sec 47 and 9.3 km from Zeta 1.

FRIEND : HIMANSHU

2-d Trilateration• If a third person [ HIMANSHU ] tells you that you are 9.2 km from Delta 1 ,Greater

Noida, you can eliminate one of the possibilities, because the 3rd circle will only intersect with one of these points.

• VOILLA !! You now know exactly where you are :- SHARDA UNIVERSITY

3-D TRI-LATERATION• If you know you are 10 km from satellite

A in the sky, you could be anywhere on the surface of a huge, imaginary sphere with a 10-km radius.

• If you also know you are 15 km from satellite B, you can overlap the first sphere with another, larger sphere. The spheres intersect in a perfect circle.

• If you know the distance to a satellite C , you get a third sphere, which intersects with this circle at two points.

• The Earth itself can act as a fourth sphere & only one of the two possible points will actually be on the surface of the planet, so you can eliminate the one in space.

GPS SIGNAL - Satellite To Reciever

THE SIGNAL CONSIST'S OF..?

• GPS satellites transmit 2 radio signals. ( L1 & L2 )

• All satellites broadcast at : 1575.42 MHz ( Civilian / L1 signal) 1227.60 MHz ( Military / L2 signal)

• The signals travels by line of sight, meaning they will pass through clouds, glass, plastic etc but will not travel through solid objects such as buildings and mountains.

• Each GPS satellite continuously broadcasts a navigation message at 50 bits / second at an approx frequency of about 1600 Mhz.

• Each transmission lasts for about 30 seconds and it carries 1500 bits of encrypted data.

• This small amount of data is encoded with a high-rate pseudo-random (PRN) sequence that is different for each satellite.

• GPS receivers know the PRN codes for each satellite and so can not only decode the signal but distinguish between different satellites.

• The GPS signal contains 3 different bits of information a) PSEUDO RANDOM CODEb) ALMANAC DATAc) EPHEMERIS DATA

1. The Pseudo Random Code is simply an I. D. code that identifies which satellite is transmitting information.

ALMANAC DATA[ Long Term ]

DEFINITION :It is that data which describes the orbital courses of satellites.

KEY POINT : EVERY SATELLITE will broadcast almanac data for EVERY SATELLITE.

FUNCTION : Your GPS receiver uses this data to determine which satellites it expects to see in the local sky.It then determines which satellites it should track.

CRUX : The Almanac data is not precise and can be valid for MANY MONTHS.

EPHEMERIS DATA[ Short Term ]

DEFINITION : It is the data which tells the GPS receiver where each GPS satellite should be at any time throughout the day.

KEY POINT : Each satellite will broadcast its OWN ephemeris data showing the orbital information for that satellite only. Because ephemeris data is very precise , its validity is much shorter.

CRUX :The data is considered valid for up to 4 HOURS but different manufacturers consider it valid for different periods with

some treating it as invalid after only 2 HOURS.

GPS - CALCULATIONS

1) At a particular time (let's say midnight ), the satellite begins transmitting a long, digital pattern called a pseudo-random code.

2) The receiver begins running the same digital pattern also exactly at midnight.

3) When the satellite's signal reaches the receiver, its transmission of the pattern will lag a bit behind the receiver's playing of the pattern.

4) Length Of The Delay = Signal's Travel Time.

5) The receiver multiplies [ signal travel time * speed of light ] to determine the distance the signal has travelled.

6) Assuming the signal traveled in astraight line, this is the distance from receiver to satellite.

SO WHERE IN ALL THIS DOES THE

DILEMA OF ATOMIC CLOCKS

COMES IN

DILEMMA OF ATOMIC CLOCKS

• In order to carry out this calculation, the receiver and satellite both need clocks that can be synchronized down to nanoseconds. [ 1 s = 10 9 nanosecond] or 1 billionth of a second.

• 1/100th of a second error could introduce an error of 2994 km's.

• To make a satellite positioning system using only synchronized clocks, you need atomic clocks not only on all the satellites,but also in the receiver itself.

• But atomic clocks cost somewhere between 30 lacks to 60 lacks, which makes them a just a bit too expensive for everyday consumer use..

• SO NOW WHAT ..??

THE SOLUTION Every satellite contains an

expensive atomic clock, but the receiver itself uses a ordinary clock, which it constantly resets.

If the GPS receiver uses the signal from a 4th satellite it can solve an equation that lets it determine the exact time, without needing an atomic clock.

The reciever then has the same time value that all the atomic clocks in all of the satellites have. The GPS receiver gets atomic clock accuracy " For Free "

DESPITE THE PRESENCE OF

ATOMIC CLOCKS

WHAT COULD POSSIBLY GO WRONG

NOW !

??

HERE's WHAT CAN GO WRONG !!

• For one thing, this method assumes the radio signals will make their way through the atmosphere at a consistent speed for i.e. [ At the Speed Of Light -> 3 x 10 8 ms-1 ]

• In fact, the Earth's atmosphere slows the electromagnetic energy down somewhat, particularly as it goes through the ionosphere and troposphere.

• It's difficult to accurately factor this delay into the distance calculations.

• Problems can also occur when radio signals bounce off large objects, such as skyscrapers, giving the receiver a false impression that a satellite is farther away than it actually is.

DIFFERENTIAL GPSComes To Rescue

Differential GPS (DGPS) helps correct these errors.The basic idea is to gauge GPS inaccuracy at a stationary receiver station with a known location.

Since the DGPS hardware at the station already knows its own position, it can easily calculate its receiver's inaccuracy.

The station then broadcasts a radio signal to all DGPS-equipped receivers in the area, providing signal correction information for that area.

In general, access to this correction information makes DGPS receivers much more accurate than ordinary receivers.

COLD & WARM STARTS

COLD START : Almanac data is current but Ephemeris is not or has expired.

WARM START : Both Almanac and Ephemeris data is current.

• To compute a P-V-T ( Position / Velocity / Time ) solution the receiver will look for satellites based on where it 'thinks' it is roughly located and the almanac is current.

• It will lock onto the nearest satellites and begin downloading ephemeris data. Once data from three satellites has been received an accurate positional fix is calculated.

• If you are moving whilst trying to obtain a fix this process may take much longer than it would if you were stationary. Your receiver must complete reception of ephemeris data without error.

• Should any one packet not be received completely without error then it must start over again. Clearly doing this whilst moving leads to much higher error rates and longer fix times.

• If you are attempting a lock having re-located more than a couple of 100 miles since your last fix then the ephemeris data will in most cases no longer be valid.

• In this case the receiver will initiate a factory start and begin downloading both almanac and ephemeris data. This will extend the initial time to lock considerably.

WHAT HAPPENS WHEN ..?

LATEST UPDATE [ in gps technology ]

A cellphone GPS unit these days get a fix almost immediately.

They use ASSISTED GPS (A-GPS) as a way of improving the time to first fix, even allowing a fix in conditions where the GPS might not otherwise be able to function.

• The A-GPS device will use a data connection (e.g. 3G) to contact an assistance server. The server can supply Almanac and Ephemeris data so the GPS doesn't have to wait to receive them from the satellites.

• The server can also send an approximate location derived from cellphone towers, allowing an immediate fix.

• Once you are out of cellphone and WiFi coverage, a cellphone GPS unit has to rely on the satellites to provide the Ephemeris & Almanac date so, like a standard recreational GPS, it takes 1 - 2 mins to get a fix from a cold start.

THANK YOU

QUESTIONS ??

MADE BY

ELECTRICAL & ELECTRONICSSHARDA UNIVERSITY

AKSHIT GUPTA