This presentation touches almost all the relevant and important topics related to GPS.

1. WHAT IS THE
1st THING
THAT COMES TO
MIND
WHEN YOU THINK OF
!!GPS

2. GPS
YOU TURN ON YOUR PHONE TURN ON GOOGLE MAPS
GOOGLE MAPS NAVIGATE YOU

3. Global Positioning
System

4. CONTENTS
1. Introduction
2. Trilateration
• 2-D Trilateration
• 3-D Trilateration
3. Signal & Its Contents
4. GPS Calculation
• Dilemma Of Atomic Clocks
5. Differential GPS
6. Cold Starts & Warm Starts
7. Latest Advancements

5. 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.

6. 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

7. TRI-LATERATION
2-D 3-D
LETS FIRST
UNDERSTAND IT IN
2D
IN 3-D IT CAN BE
SLIGHTLY
TRICKY

8. IMAGINE YOU
ARE SOMEWHERE IN
INDIA
TOTALLY LOST
FOR SOME REASON
YOU HAVE
NO CLUE
WHERE YOUR ARE

9. FRIEND : SUSHANT

10. 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:

11. FRIEND : AMJAD

12. 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.

13. FRIEND : HIMANSHU

14. 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

15. 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.

17. GPS SIGNAL - Satellite To
Reciever

18. 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

19. • 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 CODE
b) ALMANAC DATA
c) EPHEMERIS DATA
1. The Pseudo Random Code is simply an I. D. code that identifies
which satellite is transmitting information.

20. 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.

21. 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.

22. 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.

23. SO WHERE IN ALL THIS
DOES THE
DILEMA OF ATOMIC
CLOCKS
COMES IN

24. 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

25. 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 "

26. DESPITE THE PRESENCE
OF
ATOMIC CLOCKS
WHAT COULD POSSIBLY
GO WRONG
NOW !
??

27. 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.

29. DIFFERENTIAL GPS
Comes 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.

32. 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.

33. • 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 ..?

34. 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.

35. • 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

36. THANK YOU
QUESTIONS ??
MADE BY
ELECTRICAL & ELECTRONICS
SHARDA UNIVERSITY
AKSHIT GUPTA