Global positioning system (gps)

UNIT V
Global Positioning System (GPS)
Global Positioning System or GPS, is a technology that can give
your accurate position anywhere on earth (latitude/longitude).
It can be defined as “a satellite-based navigation system which records and displays
location of an object on earth (in terms of coordinates, i.e. latitude and longitude) as
well as height of a place above mean sea level.
A GPS device receives signals from satellites and these signal codes are processed &
converted into values showing position, time and velocity (what we see on screen of
GPS device). The instrument to which we generally call „GPS‟ is basically a GPS
receiver and it is a small part of a large system. A complete Global Positioning
System consists of following three segments;
1. space segment,
2. control segment and
3. user segment.

Space segment
Space segment consists of dedicated satellites for Global Positioning System and
are referred as space vehicles (SVs). There are twenty four satellites in a nominal
GPS constellation. Out of these twenty four satellites three are spare satellites
which start operating if some of the functional twenty one satellites have some
operational problem.
These satellites remain in six orbital planes (four in each) and are positioned such
that five to eight of these SVs visible from any place on the earth at a given point
of time.

Control Segment
The control segment consists of tracking stations located around all over world.
There are master control station and monitoring stations in control segment.

User segment
User segment of GPS is what we uses i.e. GPS receiver. It also includes the GPS
user community. A GPS receiver consists of a screen for displaying information,
buttons to operate it and antenna to receive signals from satellites. The antenna
may be in-built in receiver instrument or it may be externally attached to it.
Signals from minimum four satellites are required for a GPS receiver to compute
location (x, y & z) and time (t). GPS receivers are essential part of navigation
system of air crafts and ships.

Function of Space Segment
•

The Space Segment is designed to
consist of satellites orbiting the earth at
approximately 20200 km every 12
hours.

•

Each GPS satellite has several very
accurate atomic clocks on board. The
clocks operate at a fundamental
frequency of 10.23MHz. This is used
to generate the signals that are
broadcast from the satellite.

•

The satellites broadcast two carrier
waves constantly.
L1 carrier has two codes
modulated upon it.
• The C/A Code or
Coarse/Acquisition Code is
modulated, has a length of
one millisecond ; its chipping
rate is 1.023MHz with
corresponding wavelength of
300 mts.

P-code or Precision Code. Has the frequency of 10.23MHz.this refers to
the sequence of 10.23 million binary digits or chips per second.
Frequency also referred as the chipping rate of p-code.
P-code is extremely long and repeats only after 266 days. Portion of
seven days each are assigned to the various satellites. As a
consequence, all satellite can transmit on the same frequency and can be
identified by their unique one week segment . This technique also called
as the Code Division Multiple Access (CDMA)

L2 carrier

: has just one code modulated upon it. The L2 P-code.

Broad cast messages : for precise geodetic applications, third type of
signal transmitted from the GPS satellite is broad message sent at a rather
slow rate of 50 bits per sec and repeated every 30 sec. chip sequence of pcode and C/A code are separately combined with the stream of messages
bit by binary addition i.e. same value for code gives 0 and different value
gives 1,

Function of Control Segment
• The Control Segment consists of
one master control station, 5
monitor stations and 4 ground
antennas distributed amongst 5
locations roughly on the earth's
equator.
• The Control Segment tracks the
GPS satellites, updates their
orbiting position and calibrates
and synchronizes their clocks.

• A further important function is to
determine the orbit of each
satellite and predict it‟s path for
the following 24 hours. This
information is uploaded to each
satellite and subsequently
broadcast from it. This enables
the GPS receiver to know where
each satellite can be expected to
be found.

Function of Control Segment
…..cont
The most important tasks of the control segment are:

• Observing the movement of the satellites and computing orbital data
(ephemeris)
• Monitoring the satellite clock sand predicting their behavior
• Synchronizing on board satellite time
• Relaying precise orbital data received from satellites in communication
• Relaying the approximate orbital data of all satellites (almanac)
• Relaying further information, including satellite health, clock errors etc.

User Segment
The User Segment comprises of anyone using a GPS
receiver to receive the GPS signal and determine their
position and/or time. Typical applications within the user
segment are land navigation for hikers, vehicle location,
surveying, marine navigation, aerial navigation, machine
control etc.

Global Navigation Satellite Systems (GNSS)
Global Navigation Satellite Systems (GNSS) is the standard generic term for satellite
navigation systems (Sat NAV) that provide autonomous geo-spatial positioning with global
coverage. GNSS allows small electronic receivers to determine their location (longitude,
latitude, and altitude) to within a few meters using time signals transmitted along a lineof-sight by radio from satellites. Receivers calculate the precise time as well as position,

which can be used as a reference for any purpose.
Till date we have following GNSS launched by different country;
1. NAVSTAR Global Positioning System: by United States
2. GLONASS by Russian

Planning to launched
1. European Union's Galileo positioning; scheduled to be operational in 2014
2. Beidou navigation system; by The People's Republic of China by 2015-2017.

NAVSTAR GPS
Navigation Satellite Timing and Ranging Global Positioning System
It is a satellite based radio navigation system providing precise three dimensional position,
navigation and time information to suitably equipped users everywhere on a continuous

basis.
GPS has been under development in the USA since 1973. It is primary a military system with
limited asses to civilian users. GPS satellite network is operated by the U.S. Air Force to
provide highly accurate navigation information to military forces around the world. The
network is also being used by a growing number of commercial products
The NAVSTAR Global Positioning System is managed by the NAVSTAR GPS Joint Program
Office at the Space and Missile Systems Center, Los Angeles Air Force Base, California.
Navstar continues to perform as the world‟s premier positioning and navigation systems.
Endeavors such as mapping, aerial refueling, geodetic surveying, and search and
rescue operations have all benefited greatly from GPS‟s accuracy. .

GLONASS
"GLObal NAvigation Satellite System" is a radio-based satellite navigation system,
developed by the former Soviet Union and now operated for the Russian government by
the Russian Space Forces. It is an alternative and complementary to the United States'
Global Positioning System (GPS), the Chinese Compass navigation system, and the
planned Galileo positioning system of the European Union (EU).
GLONASS was developed to provide real-time position and velocity determination,
initially for use by the Soviet military for navigation and ballistic missile targeting. It was the

Soviet Union's second generation satellite navigation system, improving on the TSIKLON
system which required one to two hours of signal processing to calculate a location with high
accuracy. By contrast, once a GLONASS receiver is tracking the satellite signals, a position
fix is available instantly. It is stated that at peak efficiency the system's standard positioning
and timing service provide horizontal positioning accuracy within 57–70 meters, vertical

positioning within 70 meters, velocity vector measuring within 15 cm/s, and time transfer
within 1 µs (all within 99.7% probability).

European Union's Galileo positioning
Galileo is a global navigation satellite system (GNSS) currently being built by the
European Union (EU) and European Space Agency (ESA). This project is an
alternative and complementary to the U.S. Global Positioning System (GPS) and
the Russian GLONASS.

Beidou Navigation System
It is a project by China to develop an independent satellite navigation system.
The current Beidou-1 system (made up of 4 satellites) is experimental and has
limited coverage and application. However, China has planned to develop a
global satellite navigation system consisting of 35 satellites (known as Compass

or Beidou-2).

Positioning Methods Using GPS
There are several different methods for obtaining a position using GPS. The
method used depends on the accuracy required by the user and the type of GPS
receiver available. Broadly speaking, the techniques can be broken down into
three basic classes:
– Autonomous Navigation (or Point Positioning):
• Position of a stationary or moving point is determined with respect to a
well defined coordinate usually by three coordinate values by using a
single GPS receiver, by observation to four or more satellite.

– Differential Phase positioning (Relative Positioning):
• The coordinate of an unknown point is determined with respect to a
known point. In other words relative positioning aims at determination of
the vector between two points by observation to four or more satellites by
two receives placed at two points simultaneously.
– Differentially corrected positioning:
• It is a system in which differences between observed and computed
coordinates or ranges known as differential correction at a particular
known point called reference station are transmitted to users (GPS
receives called rovers at other points) to improve the accuracy of the
user‟s receiver position

Triangulation
GPS receivers calculate the position of objects in two dimensional or three
dimensional space using a mathematical process called trilaterlation or
Triangulation.
Trilateration can be either two dimensional or three dimensional.

Two dimensional
Trilateration

Locate a place which is
1). 300 KM from City A
2). 200 KM from City B
3). 400 KM from City C

Two dimensional
Trilateration
Effect of Errors

D2+e2
D+e1
Foghorn 1

Foghorn 2

D3+e3

Foghorn 3

Estimated
Location Area
of Ship

Three dimensional
Trilateration

Dilution of Precision
The accuracy with which a position can be determined using GPS in navigation mode
depends, on the one hand, on the accuracy of the individual pseudo-range measurements,
and on the other, on the geometrical configuration of the satellites used. This is expressed

in a scalar quantity, which in navigation literature is termed DOP (Dilution of Precision).

There are several DOP designations in current use:

• GDOP: Geometrical DOP (position in 3-Dspace, incl. time deviation in the solution)
• PDOP: Positional DOP (position in 3-Dspace)
• HDOP: Horizontal DOP (position on a plane)
• VDOP: Vertical DOP (height only)

Basic of GPS Signaling Process

Transit time
The signals transmitted by the satellites take approx. 67 milliseconds to reach a receiver.
As the signals travel at the speed of light, their transit time depends on the distance
between the satellites and the user.
Four different signals are generated in the receiver having the same structure as those

received from the 4 satellites. By synchronising the signals generated in the receiver with
those from the satellites, the four satellite signal time shifts Δt are measured as a timing
mark as shown in figure. The measured time shifts Δt of all 4 satellite Signals are used to
determine signal transit time.

GPS Signals/ Satellite Signals
The signals from a GPS satellite are fundamentally driven by an atomic clocks.
The fundamental frequency is 10.23 Mhz.

Satellite Distance measured from the multiplication of time difference and
speed of light it is called as pseudorange.
Dis tan ce (or pseudorange)
Where, C is the speed of light.

T TS

C

Function of Satellite signals
The following information (navigation message ) is transmitted by the satellite at a rate of 50
bits per second;

• Satellite time and synchronisation signals
• Precise orbital data (ephemeris)
• Time correction information to determine the exact satellite time
• Approximate orbital data for all satellites (almanac)
• Correction signals to calculate signal transit time
• Data on the ionosphere
• Information on satellite health
The time required to transmit all this information is 12.5 minutes. By using the navigation
message the receiver is able to determine the transmission time of each satellite signal and
the exact position of the satellite at the time of transmission.

Each of the 28 satellites transmits a unique signature assigned to it. This signature consists
of an apparent random sequence (Pseudo Random Noise Code, PRN) of 1023 zero
sandones.

Information is encoded in the form of binary bits on the carrier signals by a
process known as phase modulation.

The binary digits 0 and 1 are actually represented by multiplying the electrical
signals by either +1 or -1.

There are three types of code on the carrier signals:
1. The C/A code (i.e. course acquisition code)
2. The P code (i.e. precise code)
3. The Navigation Message

C/A code
It is a unique Gold code on each satellite, which is a pseudorandom sequence of bits with a
repeating sequence length of 1023. C/A bit transitions occur at 1.023 Mhz. Since, the
fundamental frequency in the satellite is 10.23 Mhz, so this represents one transition every
10 cycles. At this rate of bit transitions, the full sequence of 1023 bits is transmitted in 1 ms.
Therefore, the sequence repeats 1000 times per second. The chip length (distance between
bit transitions) is 293 m. Therefore, the sequence repeats every 300 km.

P code
The P code is generated from a combination of two different registers (i.e. linear feed back
register), in such a way that it repeats every 266.4 days. Each 7 day section is assigned a
“PRN code.” Satellites are often identified by their PRN number; the user should beware that
any given satellite can have its PRN code changed. Therefore, PRN codes should not be
used in place of Satellite Vehicle Numbers (SVN) when talking about particular satellites.
There are 38 possible PRN codes; given that there are 24 nominal satellites, some PRN
codes are left unused. The PRN sequence is reset at Saturday midnight, defining the start of
“GPS week.”

The carrier can transmit the P code at 10.23 Mbps, with a chip length of 29.3
meters. Again, the basic information is the satellite clock time or transmission, which is
identical to the C/A information, except that it has ten times the resolution.

The Navigation Message
The Navigation Message can be found on the L1 channel, being transmitted at a
very slow rate of 50 bps. It is a 1500 bit sequence, and therefore takes 30
seconds to transmit. The Navigation Message includes information on the
Broadcast Ephemeris (satellite orbital parameters), satellite clock corrections,
almanac data (a crude ephemeris for all satellites), ionosphere information, and
satellite health status.

Satellite Signal Generation (Block Diagram)

Differential GPS (DGPS)
A horizontal accuracy of approx. 20m is probably not sufficient for every situation. In order to
determine the movement of concrete dams down to the nearest millimetre, a greater degree
of accuracy is required.
In GPS it can be with a reference receiver which will always be used in addition to the user
receiver. This is located at an accurately measured reference point (i.e. the co-ordinates are
known). By continually comparing the user receiver with the reference receiver, many errors
can be eliminated. This is because a difference in measurement arises, which is known as
Differential GPS (DGPS).

Principles:
• DGPS based on the measurement of signal transit time (achievable accuracy approx. 1m)
• DGPS based on the phase measurement of the carrier signal (achievable accuracy
approx. 1cm)
Depending upon the purpose of the accuracy, we have different DGPS;
• Local area DGPS
• Regional area DGPS
• Wide area DGPS

GPS Surveying Techniques
 Static survey
 Rapid static survey
 Stop-and-go survey

 Continuous kinematics survey
 Real-time kinematic (RTK) survey

GPS Static Survey
Static GPS surveying typically uses a network or multiple baseline approach for positioning.
It may consist of multiple receivers, multiple baselines, multiple observational redundancies
and multiple sessions. A least squares adjustment of the observations is required. This
method provides the highest accuracy achievable and requires the longest observation
times – from less than an hour to five hours or longer.
A variation of the static survey is the fast-static method (also called rapid-static by some
manufacturers of GPS equipment). This will allow shorter occupation times (i.e., 8 to 20+
minutes) than static positioning and may use a radial baseline technique, network
technique, or a combination of the two. Baseline lengths may not exceed 10 kilometers for
L1 only receivers and 20 kilometers for L1/L2 receivers
Typically, the occupation time is a minimum of 8 minutes for baseline up to 20 km and a
minimum of 12 minutes for baselines up to 30 km. Fast Static requires a least squares
adjustment or other multiple baseline statistical analysis capable of producing a weighted
mean average of the observations. More than one base station will be used to provide
redundancy for each vector.

Static survey
 stable platforms or pillars
 Long distances (10 km to thousands of kilometres)
 Long occupation time (hours to days)
 Control surveys
 Simultaneous recording at several stations
 Observation rates varying from 5 to 30 seconds
 Reducing multipathing effects
 Post-processing required

Rapid static survey
 shorter distances (up to 10 km)

Reference
receiver 2

 shorter occupation time (10 minutes)

1

 densification of control networks
2

 Observation rates varying from
3

a second to a few seconds
 Post-processing required
 2 reference receivers required

Reference
receiver 1

4

Stop-and-go survey
 distances less than 1 km

 1 minute occupation time
 observation rates of seconds
 initialisation required
 repeat initialisation when less
than 4 satellites are being tracked

initialisation

Reference receiver

Continuous kinematic survey
 initialisation required
 non-stop occupation

 observation rates of 1 second

Real-time kinematic (RTK) Survey

antenna

radio

antenna
radio

receiver

receiver

Initialisation Methods
 Static survey
static survey between any two points (usually short baseline) is performed with

sufficient measurements. Specific details are in equipment documentation.
 Known baseline
survey is performed between any two
points whose coordinates are
previously determined. Usually one
epoch is sufficient. Only ambiguities

are estimated with constraining the
position vector.
A

B

 Antenna swap
Step 1: Reference & rover receivers are located over well defined marks,
collecting simultaneous observations for a period of 1 minute (A)
Step 2: Reference & rover receivers are swapped without changing the
tripods, collecting observations for a period of 1 minute (B)
Step 3: Reference & rover receivers are swapped again to return back to
their original locations, for a period of 1 minute (C)
In general, the first two steps are sufficient to resolve the integer ambiguities.
However, the third step is recommended for a further check.

Reference
A

Rover

Rover

Reference
B

Reference

Rover
C

 On the fly
 the first three methods require the receivers to be stationary
 there are restrictions in some applications, such as aerial
photogrammetry where camera positions are determined with GPS. It is
not possible to stop the aircraft to perform the above initialisation
techniques.
 The “on the fly” method resolves the integer ambiguities while the
receiver is moving.
 5 satellites with good geometry are required, 6 or more are
preferred.
 Dual frequency receivers are required.
 Ambiguity resolution in 5 minutes, 2 minutes with 6 or 7 satellites.
 Specific details given in the equipment documentation.

GPS Surveying Techniques
1. Autonomous
2. Static
3. Fast Static
4. Post Processed Kinematic
5. Real Time Kinematic (RTK)
6. Continuous Kinematic
7. Airborne GPS
8. Networked RTK (VRS)

9. Code based data collection

Energy Transfer Mechanisms
Conduction: molecule to
molecule within a
substance

Convection (and advection):
mass movement of a fluid

Radiation: absorption of
electromagnetic waves

Image Interpretation Keys
Image interpretation key is simply a reference material designed to permit
rapid and accurate identification of objects or features represented on aerial
images. A key usually consists of two parts.

a). Collection of annotated or captioned images or stereo-grams
b). Graphic and/or world description
These materials are organized in a systematic manner that permits retrieval
of desired images by, e.g. data, season, region, subject

Elements of photo interpretation: are following eight elements;
1. shape (depends on the object outline) 2. size (relative to one an other)
3. tone (brightness-hue, color)
4. texture (smooth or coarse)
5. shadow (helps to determine height)
6. site (location helps recognition)
7. association (features that are normally found near object)
8. pattern

Image Interpretation Strategies
Image interpretation strategy can be defined as a disciplined procedure that
enables the interpreter to relate geographic patterns on the ground to their
appearance on the image.
We have following five categories of image interpretation strategies given
below;
1. Field Observations – Identification in the field by observation, photography,
GPS, etc.
2. Direct Recognition – Direct recognition intuitiveness Inference –
3. Interpretation by Inference - based on knowledge and possible surrogates
or proxy
4. Probabilistic Interpretation – Utilizing addition data in the form of overlays
to reveal relationships
5. Photomorphic Regions – Areas of relatively uniform tone and texture

Global positioning system (gps)

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