GPS
The Global Positioning System (GPS) is a space-based global navigation satellite system (GNSS) that provides
location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line
of sight to four or more GPS satellites. It is maintained by the United States government and is freely accessible
by anyone with a GPS receiver.
Typically GPS is able to provide position information to within a few metres, allowing accurate
positioning to be made. It is also possible to extract timing information that enables frequencies and time
to be very accurately maintained. Frequency stability performance figures of systems using GPS timing
are far in better than crystal or many other accurate frequency sources.
The performance and ease of use of GPS has meant that it is now an integral part of everyday life, with
many portable or car-based "satnav" systems being used, as well as many mobile phones incorporating
them to enable them to provide location information superimposed on the maps from the phone or
satnav.
GNSS stands for GLOBAL NAVIGATION SATELLITE SYSTEM. Any group of satellite
which is providing global coverage for navigation solution is termed as GNSS. Currently
GPS, GLONASS are only operational GNSS in which GPS is widely used. GNSS provides PNT
solution( POSITION , NAVIGATION, TIME) to any user which has proper navigation
receiver. GPS is US satellite systems operated by Department Of Defense( DoD).
What is GPS?
The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24
satellites and their ground stations .
GPS satellites constellation
The basic concept behind GPS is that signals are transmitted from the satellites in space and
these are received by the receivers on or near to the surface of the earth. Using timing it is
possible to determine the distance from each satellite and thereby using a process of
triangulation and a knowledge of the satellite positions the position on Earth can be
determined.
All satellites send timing information, so the receiver knows when the message was sent. As
radio signals travel at the speed of light they take a very short but finite time to travel the
distance from the satellite to the receiver. The satellites also transmit information about their
positions. In this way the receiver is able to calculate the distance from the satellite to the
receiver. To obtain a full fix of latitude, longitude and altitude, four or more satellites are
required, and when the receiver is in the clear, more than four satellites are in view all the time.
A fix of just latitude and longitude can be obtained from three satellites.
The fully operational GPS satellite system consists of a constellation of 24/ (28) operational
satellites with a few more in orbit as spares in case of the failure of one. The GPS satellites are in
one of six orbits. These are in planes that are inclined at approximately 55° to the equatorial
plane and there are four satellites in each orbit. This arrangement provides the earth user with a
view of between five and eight satellites at any time from any point on the Earth.
Using economic ground based receivers GPS is able to provide position information to within a number of
metres. The economic costs have also meant that it is now fitted to many motor vehicles, while separate
GPS receivers can be bought for a few hundred pounds or dollars. As a result it is widely used by private
individuals, as well as many commercial and professional users. In fact the primary use for GPS is as a
military navigation system. The fact that it is used so widely is a by-product of its success.
However, even a very small clock error multiplied by the very large speed of light — the
speed at which satellite signals propagate — results in a large positional error.
Therefore receivers use four o r more satellites to solve for the receiver's location and time.
WHAT ARE THE COMPONENTS OF GPS?
The system consists of three segments
•Space Segment
•Control Segment
•User Segment
SPACE SEGMENT
This segment consists of 24 (30+ given on net) orbiting satellites in 6 different orbit
paths at about 20000 km altitude.Each Satellite has an orbital period of about 12
hrs.At any time at least four satellites are above the user’s horizon at minimum
elevation of 9.5 deg.
Following data is transmitted by each satellite over two frequencies
•Its Identity
•Its Position.
•Extremely accurate Time of sending the message which is provided by the atomic
clocks fitted on each of the satellites.
CONTROL SEGMENT
The control segment is composed of:
•A master control station (MCS),
•An alternate master control station,
•Four dedicated ground antennas, and
•Six dedicated monitor stations.
The satellites are controlled by keeping them on their correct orbital
paths and updating them regularly with information so that data
transmitted by them are accurate.
The Control Segment is the sole responsibility of the Department of
Defense (USA).
USER SEGMENT
The user segment consists of the antenna
and the receiver which is fitted on board a
ship.
There are multiplier supplier of these
equipment, who pay license fees to USA
for use of positional information
broadcast by the GPS satellites.
How position fixing is done by GPS?
GPS uses the ranging principle to fix the ship. The GPS concept is based on time
and the known position of specialized satellites. The satellites carry very stable
atomic clocks that are synchronized to each other and to ground clocks. Any drift
from true time maintained on the ground is corrected daily. Likewise, the satellite
locations are known with great precision. GPS receivers have clocks as well;
however, they are not synchronized with true time, and are less stable. GPS
satellites continuously transmit their current time and position. A GPS receiver
monitors multiple satellites and solves equations to determine the precise
position of the receiver and its deviation from true time. The time difference
between data transmitted from the satellite and its reception at the receiver is
measured (which is in nano-seconds) and integrated with speed of light (3 X 10
meters per second). i.e. d= s x ∆t At a minimum, four satellites must be in view of
the receiver for it to compute four unknown quantities (three position coordinates
and clock deviation from satellite time).
GPS receivers
A large number of GPS receivers are available today. They make widespread use of digital signalling
processing techniques. The transmissions from the satellites use spread spectrum technology, and the signal
processors correlate the signals received to recover the data. As the signals are very weak it takes some time
after the receiver is turned on to gain the first fix. This Time To First Fix (TTFF) is of importance, and in early
receivers it could be as long as twelve minutes, although modern receivers use many more correlators are able
to shorten this considerably.
When using a GPS receiver the receiver must be in the open. Buildings, or any structure will mask the signals
and it may mean that few satellites can be seen. Thus the receivers will not operate inside buildings, and urban
areas may often cause problems.
CONTROL PANEL
MODE – SELECTS NAV1, NAV2, NAV3 OR
PLOTTER SCREEN
MENU – RECALLS THE MENU
CLEAR – CLEAR NUMBERS ALSO STOP ALARM
ENT – ENTERS A NUMERICAL / OPTIONAL
PARAMETER
EVT – STORES EVENTS POSITION
CTRS – CHANGES DISPLAY CONTRAST 8 LEVELS
SELECTION - SELECTS PARAMETERS
MOB – ACTIVATES MOB FUNCTION
PWR DIM – DIMMER
OFF – POWER OFF
NUMERIC – ENTERS NUMERIC VALUES
CURSOR SHIFT – SHIFTS CURSOR
CONTROL PANEL
SCROLL / SHIFT
MENU ENTER
DISPLAY MOB
WPT GO TO
MARK PLOT
ZOOM IN ZOOM OUT
CENTER CURSOR
TONE CLEAR
POWER
RECEIVER
CONTROL PANEL
NAV-1 NAV-2
• POSITION (USE SEL TO
• SPEED
CHANGE SCREEN)
• LATITIUDE / LONGITUDE
• SPEED
• COURSE
• COURSE
• ALTITUDE
• DTG / COURSE TO STEER
• SPEED MADE GOOD
• X-TRACK ERROR
• COURSE MADE GOOD
• WAY POINT – DTG / COURSE
• RESET / WPT / RTE / ANCW
TTG / ETA
CONTROL PANEL
PLOT - SCREEN (4)
NAV-3
TRACK OF VESSEL
LATITUDE / LONGITUDE
SPEED
ROAD MAP – X-TRACK ERROR
COURSE
SPEED
LATITUDE /
COURSE
LONGITUDE
TTG / ETA FOR NEXT WAY POINT
DTG/ TTG – NEXT W.P.
CONTROL PANEL
STORING PRESENT POSITION
PRESENT POSITION CAN BE STORED IF REQUIRED BY NAVIGATOR.
MAN OVER BOARD
NEED POSN REF. AT LATER STAGE
ROUTE / WAY POINT SET UP
PASSAGE PLAN ROUTE NUMBER CAN BE GIVEN AND STORED.
AS GPS HAS AN INPUT TO RADAR , W.P APPEARS AS A CIRCLE ON RADAR.
SIMILARLY WAY POINTS (W.P) CAN BE GIVEN AND STORED IN A PARTICULAR ROUTE.
CONTROL PANEL
ANCHOR POSITION
ANCHOR POSITION CAN BE MARKED AS SOON
ANCHOR IS DROPPED.
TURNING CIRCLE CAN BE MADE AND ALARM SET.
Some GPS receivers may use additional clues or assumptions (such as
reusing the last known altitude, dead reckoning, inertial navigation
, or including information from the vehicle computer) to give a less
accurate (degraded) position when fewer than four satellites are
visible.
Alarm will sound in above cases.
• SIGNAL STATUS FROM GPS SATELLITES CAN BE MONITORED.
• LAT. / LONG ARE CALCULATED BASED ON WGS-84 WITH GPS SYSTEM .
• ANTENNA HEIGHT CAN BE SET
• A LOW SATELLITE CAN GIVE WRONG READING A LIMIT CAN BE SET
(e.g.10 DEG) – CALLED MASKING SATELLITE
GPS ERRORS
• Error Correction
• Types of error (slows signal)
• Charged particles in ionosphere
• Water vapor in troposphere
• Ground obstructions
(1) IF THE CONFIGURATION OF THE AVAILABLE SATELLITES IS NOT SUITABLE, ACCURACY OF THE
FIX IS AFFECTED.
(2) IONOSPHERIC AND TROPOSPHERIC DELAYS OF SIGNALS DUE TO REFRACTION. THESE DELAYS
CAN BE REASONABLY PREDICTED & ARE FED INTO THE SOFTWARE OF RECEIVER.
(3) SATELLITE CLOCK ERROR
(4)USER CLOCK ERROR
(5)DEVIATION OF SATELLITES FROM THEIR PREDICTED ORBITS
(6)RECEIVER ERROR: CAUSED DUE TO INTERNAL NOISE,COMPUTATION ERROR ETC.
GPS – Errors – Multipath error
• If satellite signal arriving at
ship antenna directly and
having been reflected by some
other object.
• This will cause distortion in
signal.
• Locating the antenna at a
suitable place will minimize
this error.
GPS – Errors – Deviation of satellite from the predicted path
The satellites are monitored and their
path predicted by ground based segment.
Between 2 consecutive monitoring there
may be minor drift from their predicted
path result in small position inaccuracy.
ALARMS
GPS FIX ALARM
WARNS OF POOR GPS POSITIONING DATA
ANCHOR WATCH ALARM
IN CASE SHIP DRIFTS OUT OF TURNING CIRCLE
PROXIMITY ALARM
WHEN NEARING WAY POINT
(Cross-Track-Error) XTE ALARM
WHEN VESSEL MOVES AWAY OF CROSS TRACK LIMIT SET.
COURSE DEVATION ANGLE
IN CASE VESSEL MOVES OUT OF COURSE DEVIATION LIMIT SET.
DGPS :
Differential Global Positioning
System (DGPS) is an enhancement
to Global Positioning System that
uses a network of fixed, ground-
based reference stations to
broadcast the difference between
the positions indicated by the
satellite systems and the known
fixed positions. These stations
broadcast the difference between
the measured satellite pseudo
ranges and actual (internally
computed) pseudo ranges, and
receiver stations may correct their
pseudo ranges by the same amount.
The correction signal is typically
broadcast over UHF radio modem.
DGPS techniques are called the
Ground Based Augmentation System
To overcome the disadvantages of
accuracy denial.
Systems are totally automatic.
Improve the accuracy of the GPS fix
only in areas where the system
operates, typically about 200 nm.
Additional equipment must be
installed.
position updates of 0.5 seconds
reported position-fix accuracy of
about 1 to 3 metres rms
Still dependent upon the supplier of
the GPS signals.
Why we need Differential GPS?
That improved accuracy has a profound effect on the
importance of GPS as a resource. With it, GPS becomes more
than just a system for navigating boats and planes around
the world.
This advanced version or the enhancement to Global
positioning System or the GPS is DGPS i.e. Differential
Global positioning System or DGPS. DGPS provides a
better and improved location accuracy than GPS from a
nominal GPS accuracy of 15 meters to that in the best
implementation of about 10 cm. It increases the accuracy
of the locations or the coordinates derived from the GPS
receivers.
For Example:
Canadian Coast Guard (CCG) uses DGPS for correcting
most of the errors caused in GPS signals by taking the
reference stations and then transmitting these corrected
errors within CCG or USCG coverage area who is having
DGPS receiver.
GPS, GLONASS, BDS, QZSS, GNSS, IRNSS: What are they?
If you own a smartphone, then the chances are that you have encountered or used its
satellite navigation features, which most of us commonly refer to as GPS. But the GPS is
just one of the satellite navigation systems that smartphones use, and if you’re
wondering what they are and want to make sense of it, then read on to find out.
GPS (United States of America)
The Global Positioning System or GPS, originally Navstar GPS, is the most popular
satellite navigation system for military and civilian use, and as the name
suggests, operates globally. It is owned by the United States government and
operated by the U.S. Air Force Space Command (AFSPC). Its first launch was in
February 1978, so the system has been around for quite a while. Don’t worry
though, as the Air Force launches new satellites to replace aging ones when
needed. As of October 18, 2018, there were a total of 31 operational satellites in
the GPS constellation.
A-GPS
The Assisted GPS or Augmented
GPS (A-GPS or aGPS) is different
from the other systems on the
list, as it works in tandem with a
GPS. Compared to standalone or
self-ruling GPS devices that
depend solely on information
given by satellites, an A-GPS
equipped device like the
smartphone uses cell tower data
to enhance its quality and
precision. This allows location-
based services to quickly pinpoint
your location when there’s poor
satellite signal like indoors.
GLONASS (Russia)
While the U.S. has GPS, Russia has
the Globalnaya navigatsionnaya
sputnikovaya sistema or “Global
Navigation Satellite System” or
GLONASS. Like the GPS, the
GLONASS is used for military and
civilian purposes. The development of
GLONASS began in the Soviet Union in
1976 and launched numerous rockets
since October 12, 1982. It consists of
24 satellites in orbit (plus three spares)
for full global coverage.
BDS (China)
Next, we have China and their
BeiDou Navigation Satellite
System (BDS) which first launched
on October 30, 2000. It started
with three satellites with Beidou-1
and served mostly users in China
and neighboring regions. China
decommissioned Beidou-1 at the
end of 2012 and moved on to
Beidou-2 with ten satellites in
orbit. In 2015, China launched the
Beidou-3 with 23 satellites in orbit
with the intention to provide global
coverage in 2020 with a
constellation of 35 satellites.
QZSS (Japan)
The Quasi-Zenith Satellite System
(QZSS) is a project of the Japanese
government and operated by the Japan
Aerospace Exploration Agency (JAXA),
with an aim to be a satellite-based
augmentation system (SBAS) for the
U.S. GPS to be receivable in the Asia-
Oceania regions, with a focus on Japan.
The first Quasi-Zenith Satellite (QZS-1)
launched on September 11, 2010. In
2017, the Japanese government
launched three more on separate
occasions, the QZS-2, QZS-3, and QZS-
4, bringing the total to four. Japan
intends to bring the total number up to
seven after 2018.
GNSS (European Union)
The European Union (EU) wanted to
have their own independent high-
precision positioning system, so
they do not have to rely on the U.S.
GPS Russian GLONASS, or Chinese
BeiDou. So in 2011, they launched
Galileo which is a global navigation
satellite system (GNSS), operated
by the European GNSS Agency
(GSA) and the European Space
Agency (ESA). As of July 2018, it has
a total of 26 satellites in orbit with
global coverage and used for
civilian and commercial purposes.
The complete 30-satellite Galileo
system (24 operational and 6 active
spares) is expected by 2020.
IRNSS (India)
India has the Indian Regional Navigation Satellite
System (IRNSS), with an operational name of
NavIC or Navigation Indian Constellation. It is
operated by the Indian Space Research
Organisation (ISRO) and mostly covers India for
military and commercial purposes. The system
was developed partly because access to foreign
government-controlled global navigation satellite
systems is not guaranteed in times of conflict or
war. The Indian government approved the project
in May 2006 and launched its first satellite on
July 1, 2013. It currently has a total of seven
satellites in orbit.
As you might have noticed, these satellite
navigation/positioning systems vary based on the
country that operates them. Since smartphones
are used globally, it is only logical that these
devices are compatible with these systems, so
location-based apps like Google Maps and Waze
work properly. Not all smartphones support all
