Explains about the GPS system overview.

1. G. Nagesh Kumar
Sr. Assistant Professor
GPREC, KURNOOL,
A.P.
 GLOBAL POSITIONING SYSTEM- AN
INTRODUCTION

2. Where on earth am I ?

3.  GPS is the shortened form of NAVSTAR
 NAVSTAR - acronym for NAVigation
System with Time And Ranging Global
Positioning System.
3

4.  Standard generic term for satellite navigation
systems that provide autonomous geo-spatial
positioning with global coverage.
 Using a GNSS system the following values can
accurately be determined anywhere on the
globe
 Exact position (longitude, latitude and altitude
co-ordinates) accurate to within 20 m to
approximately 1 mm.
 Exact time (Universal Time Coordinated, UTC)
accurate to within 60ns to approximately 0. 5ns 4

5.  Current global navigation systems
 GPS
 GLONASS - RUSSIA
 Proposed global navigation systems
 COMPASS - CHINA
 DORIS - FRANCE
 Galileo – EUROPE
 IRNSS – Indian Regional Navigation Satellite
System
 QZSS - JAPAN
5

6.  GPS-United States' Global Positioning System
(GPS), which as of 2009 is the only fully functional,
fully available global navigation satellite system.
 32 medium Earth orbit satellites in six different
orbital planes, with the exact number of satellites
varying as older satellites are retired and replaced.
 Operational since 1978 and globally available since
1994, GPS is currently the world's most utilized
satellite navigation system.
6

7.  Relatively high positioning accuracies
 Capability of determining velocity and time
 Signal availability to users
 No user charges and low cost hardware
 All–weather system
 Three dimensions
7

8.  Space Segment – All Operational satellites
 Control Segment – All ground stations involved
in the monitoring of the system : Master Control
Stations, Monitor Stations & Ground Control
Stations
 User Segment – All civilian and military users
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9. 9

10.  Consists of upto 32 operational satellites orbiting the
earth
 They orbit at a height of 20,180 km above the Earth’s
surface and are inclined at 55° to the equator.
 Any one satellite completes its orbit in around 12
hours
 Due to the rotation of the Earth, a satellite will be
its initial starting position above the earth’s
surface after approx. 24 hours(23 hours 56 minutes)
10

11. GPS satellites orbiting the earth in 6 orbital planes
11

12.  To receive and store data transmitted from
control segment stations
 To maintain accurate time by means of several
on-board atomic clocks
 To transmit information and radio signals to
users on two L- band frequencies
 To maintain a stable platform and orbit for the
L- band transmitters
12

13.  Satellites transmit signals in 2 frequencies – L1
& L2
 L1 CARRIER
19 cm wavelength
1227 MHz frequency
Navigation
Carries Coarse / Acquisition Code (C/A Code )
and Precise Code ( P- Code)
C/A Code – Rougher Positioning
13

14.  L2 carrier
24 cm wavelength
1775 MHz
P Code – encrypted- unusable by civilians with
one receiver but is usable when performing
differential corrections between 2 or more
receivers; used by military
Navigation Message – called Ephemeris- tells
where satellites are located w.r.t WGS-84.
14

15.  Continuous stream of data transmitted at 50 bits
per second. Each satellite relays the following in
formation to Earth:
 System time and clock correction values
 Its own highly accurate orbital data (ephemeris)
 Approximate orbital data for all other satellites
(almanac)
 System health, etc.
 The navigation message is needed to calculate
the current position of the satellites and to
determine signal travel times.
15

16.  Data stream -
modulated to the HF carrier wave of each
individual satellite.
 Data is transmitted in logically grouped units
known as frames or pages.
 Each frame is 1500 bits long and takes 30 seconds
to transmit.
 The frames are divided into 5 subframes. Each
subframe is 300 bits long and takes 6 seconds to
transmit.
16

17.  In order to transmit a complete almanac,
25 different frames are required.
 Transmission time for the entire almanac is
therefore 12.5 minutes.
 Unless equipped with GPS enhancement , a
GPS receiver must have collected the complete
almanac at least once in order to calculate its
initial position.
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18. Functions of the Control Segment
 Observing the movement of the satellites and
computing orbital data (ephemeris)
 Monitoring the satellite clocks and predicting
their behavior
 Synchronizing onboard satellite time
 Relaying precise orbital data received from
satellites
18

19.  Relaying the approximate orbital data of all
satellites (almanac)
 Relaying further information, including satellite
health, clock errors etc.
 Oversees the artificial distortion of signals (SA,
Selective Availability), in order to degrade the
system’s positional accuracy for civil use.
19

20. (1) Schriever Air Force Base
(2) Hawaii
(3) Cape Canaveral
(4) Ascension Island
(5) Deigo Garcia
(6) Kwajalein.
20

21.  GPS receivers used to receive the GPS signal
for determination of position and time.
 Consists of an antenna and preamplifier, radio
signal microprocessor, control and display
device, data recording unit and power supply
 Decodes the timing signals from the ‘visible’
satellites and having calculated their distances,
computes its own latitude, longitude, elevation
and time.
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23.  Working Mode Accuracy
 Autonomous 15-100 metres
 Differential GPS 0.5- 5m
 Real Time Kinematic Float 20 cm – 1m
 Real Time Kinematic Fixed 1cm – 5 cm
23

24.  The basic split of receivers is based on the
number of satellites the receiver can track at a
time.
 Each tracked satellite requires a channel.
 Receivers usually have between one and
twelve channels.
24

25.  Since four satellites are required for an accurate
position, any receiver with less than four
channels must necessarily be a sequencing
receiver.
 A sequencing receiver tracks one satellite,
drops that one and goes to the next in
sequence, and so on until at least four satellites
have been tracked.
 The whole process then begins again.
25

26.  Single-channel receivers are the cheapest and
smallest.
 Two-channel receivers process the signal from
one satellite while tracking the next satellite.
 Receivers with four or more channels are
continuous receivers. Each channel tracks one
satellite. No gaps or delays in tracking occur.
26

27.  Six channels are better than four, since another
satellite or two is a benefit, but eight channels
is a only a small step better than six.
 Five or six satellites within easy "viewing"
distance are common.
 The seventh and eighth satellites are further
afield and require a larger antenna to capture.
27

28.  Obstructions low on the horizon may block the
farthest satellites and negate the advantage of
eight or more channels.
 Ten- and 12-channel receivers are usually
reserved for benchmark locations and other
activities requiring similar accuracy
28

29.  One calculates position by establishing the
distance relative to reference satellites with a
known position.
 The distance is calculated from the travel time
of radio waves transmitted from the satellites.
29

30.  Satellites with a known position transmit a
regular time signal.
 • Based on the measured travel time of the
radio waves (electromagnetic signals travel
through space at the speed of light c =
3,00,000km/s) the position of the receiver is
calculated.
30

31. The distance D is calculated by multiplying the travel time by ΔT by
velocity of light c.
D= ΔT × c
31

32. 32

33. Satellite Navigation Systems use satellites as time signal transmitters.
Contact to at least four satellites is necessary in order to determine the three
desired coordinates (Longitude, Latitude, Altitude) as well as the exact time.
33

34.  Satellite Navigation Systems employ satellites
orbiting high above the Earth and distributed in
such a way that from any point on the ground
there is line of sight contact to at least 4 satellites.
 Each one of these satellites is equipped with
onboard atomic clocks.
34

35. Atomic clocks -
Most precise time measurement instruments known
, losing a maximum of one second every 30,000 to
1,000,000 years.
 GNSS satellites transmit their exact position and
onboard clock time to Earth.
 These signals are transmitted at the speed of light
(300,000 km/s) and therefore require approx.
67.3 ms to reach a position on the Earth’s surface
directly below the satellite.
 The signals require a further 3.33 µs for each
additional kilometer of travel.
35

36.  To establish position, all that is required is a
receiver and an accurate clock.
 By comparing the arrival time of the satellite
signal with the onboard clock time the moment
the signal was transmitted, it is possible to
determine the signal travel time
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37. 37

38. 38

39. The position is determined at the point where all three spheres
intersect
39

40.  Four satellites are the minimum to secure only one,
absolutely technically, trigonometrically
unambiguous location. Three work in practice as
the absurd location can be eliminated.
40

41.  Receiver and the atomic clocks onboard the
satellites are to be synchronized
 If the time measurement is accompanied by a
constant unknown error (∆t), in 3dimensional space,
there are four unknown variables viz.
longitude (X) , latitude (Y) , height (Z) &
time error (∆t)
These four variables require four equations, which
can be derived from four separate satellites.
41

42. Satellite Navigation systems are deliberately constructed in such a way that
from any point on Earth, at least 4 satellites are “visible”. Thus despite
inaccuracy on the part of the receiver clock and resulting time errors, a position
can be calculated to within an accuracy of approx. 5 – 10 m.
42

43.  The exact location of any one satellite at any
given time is not correct.
 Factors such as gravity, influence from the
earth, moon, sun, and even other stars move
the satellites a little.
 The solar wind can also push the GPS satellites
out of the intended orbits.
43

44.  Without absolutely accurate knowledge of the
satellite locations the receiver position fixing
will not be correct.
 Each satellite broadcasts its position in an
"ephemeris".
 A receiver can target a satellite, receive the
ephemeris, and determine its own position
relative to the satellite
44

45.  A receiver doesn't have to be programmed
with all the satellite location information
because it picks up locations as needed.
Receiving the ephemeris for a satellite takes
about 30 seconds.
 The clock in each satellite must be absolutely
correct for the correct distance from each
satellite to the receiver to be known. Even the
tiniest fraction of a second error in time will
throw off the distance calculation.
45

46.  Each satellite carries four atomic clocks, the
most accurate timing device known, but small
influences affect the accuracy of the clocks.
 So, neither satellite location nor distance is
exactly known, and the theoretical position by
triangulation has to be adjusted
46

47.  The clock in the GPS receiver must be exactly
in synchronous with the satellite clocks to
eliminate all error in distance.
 Since the satellite clocks don't agree with each
other, it is unrealistic to expect the GPS receiver
to be dead on.
 Fortunately, consistency is sufficient.
47

48.  Each satellite sends its time signal to the
receiver. The receiver notes when the signals
were sent and then calculates the distances.
 These distances are compared and any
adjustments necessary to correct for a fast or
slow receiver clock are calculated. The
adjustment, however, is not exact, and some
distance errors occur.
48

49.  1. Ionosphere and troposphere delays
 Satellite elevation.
 The density of the ionosphere is affected by the
sun.
 Water Vapour also affects the GPS signal.
49

50.  Satellite and Receiver clock errors
 Orbital errors
 Number of satellites visible
 Satellite geometry/shading
 Intentional degradation of the satellite signal
50

51.  Two levels of service
 Precise Positioning Service (PPS)
 Standard Positioning Service ( SPS)
 Precise Positioning Service
 Accurate positioning, velocity and timing
service
 Only to authorized users -primarily intended
for military purposes.
51

52.  U.S. military users, NATO military users and
other selected military and civilian users.
 PPS receivers can achieve 0.2 metres per second
3-D velocity accuracy, but this is somewhat
dependent on receiver design.
 Selective Availability (SA) and Anti-Spoofing
(A-S).
52

53.  To reduce GPS position, velocity, and time
accuracy to the unauthorized users.
 SA operates by introducing pseudorandom
errors into the satellite signals.
 Subjects Satellite Clocks to ‘ dithering’ that
alters its time slightly
 Ephemeris is broadcasted different from actual
53

54.  The A-S feature is activated on all satellites to
negate potential spoofing of the ranging
signals. The technique encrypts the P-code into
the Y-code. Users should note the C/A code is
not protected against spoofing.
 Encryption keys and techniques are provided
to PPS users which allow them to remove the
effects of SA and A-S and thereby attain the
maximum accuracy of GPS.
54

55.  The SPS is a less accurate positioning and
timing service which is available to all GPS
users.
 In peacetime, the level of SA is controlled to
provide 100 metre horizontal accuracy.
 The SPS is primarily intended for civilian
purposes, although it has potential peacetime
military use.
55

56.  Two levels of accuracy –
 "Mapping Accuracy" which usually implies
"corrected" accuracy within a few feet or less.
This is usually sufficient for most GIS work.
 "Surveying Accuracy" which is usually
advertised as having "sub-centimeter" accuracy.
These units are significantly more expensive.
56

57.  Measure of strength of satellite geometry and
related to spacing and position of satellites in
the sky
 Ratio of standard deviation of one coordinate
to the measurement accuracy
 Low DOP factor is good - satellites are in
optimal configuration for reliable GPS position
57

58.  Satellite Geometry
Errors introduced by satellite orientation are
referred to as Position Dilution of Precision
(PDOP)
PDOP is composed of two pieces:
HDOP or Horizontal Dilution of Precision &
VDOP or Vertical Dilution of Precision
58

59.  PDOP affects reported position accuracy as a multiplier
of the sum of all the other distance and location errors.
 If the sum of distance, timing, location, SA and other
errors is 15 feet and PDOP is 4, the total error is 4*15 =
60 feet.
 A continuous receiver receives continuously, and
constantly calculates and adjusts as more information
becomes available.
 So the maximum error possible is not the likely error,
and methods are available to remove most of the likely
error.
59

60.  HDOP is minimized when the position being
located is in the middle of the satellites being
tracked.
 With three satellites the best case is each satellite
equidistant from the position and at 120 degrees
from each other.
 Further, the satellites should be as low on the
horizon as possible so that the elevation of the
satellites above the position is zero.
 The worst HDOP occurs when all the visible
satellites are to one side of the position and high
above the horizon
60

61.  VDOP is minimized when one satellite is above
the position and one satellite is below the
position.
 This is impossible. The position can never be
"between" the satellites, so elevation is more
difficult to establish than position on the plane.
61

62.  VDOP is not a severe problem since elevation
is not usually as important as position on the
plane.
 The accuracy is more than sufficient for most
purposes.
 Most receivers allow a maximum HDOP and
VDOP to be selected.
 A rule of thumb is that vertical values are
about one-third as accurate as horizontal
values.
62

63.  This correction should be applied to all
reported locations when accuracy within 50
feet or less is required.
 Two receivers, one fixed and one roving.
63

64.  Fixed receiver is in a known
position, established either
by survey or by long term
measurements of GPS
signals.
 Fixed receiver compares its
reported position to the
position it knows it
occupies, and stores the
correction necessary to
rectify reported and known
position.
 The corrections are stored
by date and time since the
corrections are different for
every second.
64

65.  The roving receiver collects and processes
signals and stores the reported positions, doing
the best it can with the information available.
 This information is brought back to the office
and the corrections calculated by the fixed
receiver are applied to the rover’s positions.
65

66.  The corrected positions from the roving
receiver are easily accurate to within five feet.
Since corrections are applied after the fact, this
is called post-processing differential correction.
 Corrections from the fixed receiver are
applicable to an area within 200 or 300 miles of
the fixed receiver. Therefore, only one fixed
receiver is usually necessary within an
operating area.
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67. 67

68.  An immediate, hence real-time, correction
applied to the rover measurements.
 Fixed receiver calculates the correction
necessary for this moment, and then sends the
correction by radio to the rover. The rover
applies the correction to its current
measurements and adjusts position
accordingly.
 Real-time correction is not as accurate as post-
processing correction.
68

69.  Slight delay in the radio signal
 All the atmospheric difficulties the original
GPS signals must face also apply to the radioed
correction.
 Terrain features may bounce or block the
correction.
69

70.  Since the fixed station and rover must be in
radio contact, more fixed stations are required
for real-time correction than for post-
processing correction.
 Real-time correction adds to the cost of the GPS
unit and is rarely necessary when collecting
data for GIS.
 Post-processed differentially corrected data
will always be more accurate than real time
differentially corrected data.
70

71.  Height above a surface must be measured
relative to that surface.
 For example, most elevations are reported as
being some number of feet or meters above
mean sea level.
 Unfortunately, mean sea level is an average
that changes locally because of gravity,
spinning force, sun-moon-planet orientation,
and a dozen other things.
71

72.  Geoid - Surface along which the gravity
potential is equal everywhere and to which the
direction of gravity is perpendicular
 Ellipsoid
 GPS receivers allow to specify the ellipsoid or
the geoid for calculating elevation
 If relatively correct elevations are needed that
will be consistent over an area, use the
ellipsoid.
72

73.  Most commonly used geoid model for GPS
work is the 1984 World Geodetic System--
WGS84.
 Another common model is the 1927 North
American Datum, called NADCONUS27, for
North American Datum CONtinental US 1927.
This is the model used for most USGS maps.
 Match the GIS datum, or at least make sure that
a transformation to the needed datum is
available.
73

74.  Location - determining a basic position (Where am
I?")
 Navigation – getting from one location to another
(Where am I going?")
 Tracking - monitoring the movement of people
and things
 Mapping - creating maps of the world (Where is
everything else?")
 Timing – bringing precise timing to the world
("When will it all happen?")
74

75. 75

76.  In- Vehicle Equipment (IVE)
 Fleet Management Server (FMS)
 Internet based User Interface (UI)
IVE, fitted into the vehicle, calculates
its position on the basis of
information received from GPS
Satellites.
This information is sent in a SMS
message to the FMS, at configured
intervals.
FMS maintains all vehicle positions in
a centralized database. FMS also
maintains alarm details, trip details,
SMS numbers, etc.
76

77.  Global Positioning System (GPS) can give
position of a point anywhere on the globe with
high accuracy. GPS can measure base lines for
high accuracy, it controls points without any
line of sight requirement.
 No need for control points, long traverses or
resections
 GNSS/GPS determine the position
77

78.  Real Time Kinematic GPS determines the
position to centimeter accuracy within a few
seconds at ranges up to 100 km from a
reference station.
 With GPS Total station , it is possible to:
 to go in the shortest possible time;
 to fix the position with GPS and
 to survey with the total station.
78

79.  GPS and its Applications’, 2005, Notes of the Short
term Course organized by Indian Institute of
Technology Roorkee.
 A.P. Cracknell and L.W.B. Hayes, 2003,”Introduction
to Remote Sensing”, Taylor and Francis.
 C.P. Lo, Albert K.W. Yeung, 2007, “Concepts and
Techniques of Geographic Information Systems”,
Prentice Hall of India Private Limited.
79

80.  John R. Jensen, 2004, “Remote Sensing of the
Environment”, Pearson Education Series in
geographic Information Science.
 Satheesh Gopi., 2005, ‘Global Positioning
System – principles and Applications’, Tata
McGraw- Hill Companies, New Delhi.
 http://en.wikipedia.org/wiki/Global_Position
ing_System. accessed on 30-4-2009.
 http://en.wikipedia.org/wiki/Global_Navigat
ion_Satellite_System accessed on 30-4-2009.
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81. THANK YOU
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