The GPS system uses satellites to provide location and time information anywhere on Earth. It became fully operational in 1994 with 24 satellites. Modernization efforts now aim to implement new GPS III satellites and control systems. GPS calculates position by precisely timing signals from at least 4 satellites. It determines distance to each satellite and uses trilateration to find the receiver's location. Various augmentation systems can further improve GPS accuracy. Key sources of GPS error include effects of the ionosphere, satellite orbit and clock errors, multipath interference, and atmospheric conditions.

1. GPS (GLOBAL POSITIONING SYSTEM)
BACKGROUND INFORMATION:
The Global Positioning System (GPS) is a space-based satellite navigation system 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 to anyone with a GPS receiver.
The GPS program provides critical capabilities to military, civil and commercial users around
the world. In addition, GPS is the backbone for modernizing the global air traffic system.
Fig.1 The GPS project was started in 1973 to overcome the limitations of
previous navigation systems, integrating ideas from several
predecessors, including a number of classified engineering design
studies from the 1960s. GPS was created and realized by the U.S.
Department of Defense (USDOD) and was originally run with
24 satellites. It became fully operational in 1994.
Advances in technology and new demands on the existing system have now led to efforts to
modernize the GPS system and implement the next generation of GPS III satellites and Next
Generation Operational Control System (OCX). Announcements from the Vice President and
the White House in 1998 initiated these changes. In 2000, U.S. Congress authorized the
modernization effort, referred to as GPS III.
Fig.2 GPS III Satellite
In addition to GPS, other systems are in use or under development. The Russian GLObal
NAvigation Satellite System (GLONASS) was in use by only the Russian military, until it was
made fully available to civilians in 2007. There are also the planned European Union Galileo

2. positioning system, Chinese Compass navigation system, and Indian Regional Navigational
Satellite System.
Fig.3 GLONASS Fig.4 China: Compass Navigation System
Fig.5 IRNSS Fig.6 Galileo Positioning System
BASIC CONCEPT OF GPS:

3. A GPS receiver calculates its position by precisely timing the signals sent by
GPS satellites high above the Earth. Each satellite continually transmits messages that include
 the time the message was transmitted
 satellite position at time of message transmission
Fig.7 “THE GLOBAL POSITIONING SYSTEM” Measurement of code-phase arrival time from at
least 4 satellites are used to estimate four quantities: Position in 3 dimensions (X,Y,Z) and GPS
time.
The receiver uses the messages it receives to determine the transit time of each message and
computes the distance to each satellite. These distances along with the satellites' locations
are used with the possible aid of trilateration, depending on which algorithm is used, to
compute the position of the receiver. This position is then displayed, perhaps with a moving
map display or latitude and longitude; elevation information may be included. Many GPS
units show derived information such as direction and speed, calculated from position
changes.
Three satellites might seem enough to solve for position since space has three
dimensions and a position near the Earth's surface can be assumed. However, even a very
small clock error multiplied by the very large speed of light — the speed at which satellite

4. signals propagate — results in a large positional error. Therefore receivers use four or more
satellites to solve for both the receiver's location and time. The very accurately computed
time is effectively hidden by most GPS applications, which use only the location. A few
specialized GPS applications do however use the time; these include time transfer, traffic
signal timing, and synchronization of cell phone base stations.
Although four satellites are required for normal operation, fewer apply in special cases.
If one variable is already known, a receiver can determine its position using only three
satellites. For example, a ship or aircraft may have known elevation. 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.
GPS augmentation refers to techniques used to improve the accuracy of positioning
information provided by the Global Positioning System, a network of satellites used for
navigation.
Augmentation methods of improving accuracy rely on external information being integrated
into the calculation process. There are many such systems in place and they are generally
named or described based on how the GPS sensor receives the information. Some systems
transmit additional information about sources of error (such as clock drift, ephemeris, or
ionospheric delay), others provide direct measurements of how much the signal was off in the
past, while a third group provide additional navigational or vehicle information to be
integrated in the calculation process.
Fig.8 Local Area Augmentation System (LAAS) is a ground-based augmentation system (GBAS) that the U.S.
Federal Aviation Administration (FAA) is in the process of developing. If and when LAAS is completed, it will

5. provide very accurate GPS-based guidance at selected commercial airports. These signals will allow airliners to
land in very poor weather conditions, even including hands-off “auto-land” capability for newer aircraft.
Examples of augmentation systems include the Wide Area Augmentation System, Differential
GPS, Inertial Navigation Systems and Assisted GPS.
Other satellite navigation systems in use or various states of development include:
 GLONASS – Russia's GLObal NAvigation Satellite System (Globalnaya Navigatsionnaya
Sputnikovaya Sistema). Fully operational worldwide.
 Galileo – a global system being developed by the European Union and other
partner countries, planned to be operational by 2014
 Beidou – People's Republic of China's regional system, currently limited to Asia and
the West Pacific
 COMPASS – People's Republic of China's global system, planned to be operational
by 2020
 IRNSS – India's regional navigation system, planned to be operational by 2012,
covering India and Northern Indian Ocean
 QZSS – Japanese regional system covering Asia and Oceania
GLOBAL POSITIONING:
Navigation Skills
1. Entering Waypoints
Plotting a route with waypoints is easy. Simply press the MARK button (or, on some units,
press and hold the ENTER button). If you're marking a waypoint where you stand, you
can often do this with the single press of a button. You can also add multiple levels of
detail: a name (e.g.,"trailhead" or "waterfall"), the coordinates, the elevation and even
a short note. This is particularly helpful if you're marking waypoints for the trail ahead,
perhaps before you leave home. NOTE: Whenever starting a hike, add a waypoint
where you've parked your car.
2. Following Waypoints
With waypoints in place, your GPS receiver can guide you from point to point. Use the
FIND or GOTO button to identify a particular waypoint target. Then switch to the

6. Compass screen where the GPS receiver will give you a bearing and estimate the
distance and time of travel.
Fig.9 Personal Location Finder Fig.10 The ASTRO 320 GPS
3. Keeping a Track
If you take a spontaneous side trip from base camp or in any way venture into
unknown territory, one of your GPS receiver's most useful features, "tracking," comes into
play. When you enable the TRACK RECORDING feature, the GPS unit will automatically
set track points as you go, essentially laying a breadcrumb trail to show where you've
been.
You can adjust track points to be laid at specified intervals of time or distance. The
shorter the distance between track points, the more accurate the path back. For
example, track points set every 100 yards allow a greater risk of you wandering off
course versus track points set every 10 feet. The intervals you select should depend on
the presence of a marked trail, the terrain, the weather and other conditions that you
find. In addition to this essential guiding feature, tracking also allows you to record time
and distance traveled.

7. Making the Most of GPS
Sensors
This term refers to your unit's barometric altimeter and magnetic compass.
 Barometric altimeter:
All GPS units provide elevation as part of the information
gleaned from the satellites. But the advantage of also
having a barometric altimeter is that it operates
independently of this signal. So if the satellite signal
becomes too weak to be reliable, the barometric
altimeter can still give you an accurate elevation. And
since it measures air pressure, it gives you an idea of
approaching weather changes by displaying a chart of
barometric trends.
Fig.11 GPS Barometric Altimeter
 Magnetic compass:
The magnetic compass works in a similar manner to
your traditional capsule compass. Since you're still
carrying the latter compass (and a hard-copy map),
the magnetic compass is somewhat redundant. So if
you need to conserve the GPS battery life by only
using essential functions, you could turn off the
magnetic compass and just use your capsule

8. compass. This will not affect the navigational functions of the GPS receiver, which rely on
satellite signals.
Fig.12 WINTEC GPS Compass
Memory Cards
In addition to having preloaded maps, many GPS units allow you to download more maps
using CD-ROM software (available separately). Some GPS receivers give you even greater
flexibility by using removable micro SD memory cards. These cards are available preloaded,
or you can download maps from your computer to a blank card. If your GPS unit uses
memory cards, it's easy to organize your maps for maximum efficiency and ease. For
example, you could have one memory card for topographical maps, one for streets and
roads, and another for marine charts.
Fig.12 Memory card (GPS) Fig.13 Slot for inserting memory card in GPS

9. A BULLETED-MAP OF GPS:
Basic concept of GPS
1. Position calculation introduction
2. Correcting a GPS receiver's clock
Structure
1. Space segment
2. Control segment
3. User segment
Applications
1. Civilian
2. Restrictions on civilian use
3. Military
Communication
1. Message format
2. Satellite frequencies
3. Demodulation and decoding
Navigation equations
1. Bancroft's method
2. Trilateration

10. 3. Multidimensional Newton-Raphson calculations
4. Additional methods for more than four satellites
Error sources and analysis
Accuracy enhancement and surveying
1. Augmentation
2. Precise monitoring
3. Timekeeping
4. Carrier phase tracking (surveying)
RECIEVERS:
A GPS receiver does not replace a map and compass or the knowledge of how to use them.
Your GPS unit does augment and enhance your navigational abilities with technology. But
you should still always carry a detailed map of the area and a compass.
Fig.13 Block diagram of a simple GPS receiver

11. Fig.14 GPS receiver (With great GPS units out there that are less than $99)
These are common to virtually any GPS receiver intended for hiking:
 Give a location: A GPS unit accurately triangulates your position by receiving data
transmissions from multiple orbiting satellites. Your location is given in coordinates:
latitude and longitude or Universal Transverse Mercators (UTMs).
Fig.15 Universal Transverse Mercator (UTM) coordinates system
 Point-to-point navigation: A location or destination is called a "waypoint." For
example, you can establish a starting waypoint at a trailhead by using the location
function. If you have the coordinates for the campsite you're headed for (taken from
a map, resource book, website, mapping software program or other source), a GPS

12. can give you a straight-line, point-to-point bearing and distance to your destination.
Since trails rarely follow a straight line, the GPS' bearing will change as you go. The
indicated distance to travel will also decrease as you approach your goal.
Fig.16 GPS can compile various navigation statistics
 "Route" navigation: By combining multiple waypoints on a trail, you can move point-
to-point with intermediate bearing and distance guides. Once you reach the first
predetermined waypoint, the GPS receiver can automatically point you to the next
one or you can manually do this.
Fig.17 GPS receiver, route navigation
 Keep a "track": One of the most useful functions of a GPS unit is its ability to lay a
virtual "breadcrumb trail" of where you've been, called a track. This differs from a
"route," which details where you're going. You can configure a GPS to automatically
drop "track points" over intervals of either time or distance. To retrace your steps,
simply follow the GPS bearings back through the sequence of track points.

13. Fig.18 GPS identifying trail landmarks as waypoints
SATELLITES:
Acquiring Satellites
To provide reliable navigational information, including your position, a GPS receiver needs to
receive good signals from at least four satellites. To "acquire" satellites, turn on your GPS and
go to the Satellite screen:
 This will display the current configuration of the satellites and the strength of the
signals. It may take several minutes for the GPS unit to lock in to the satellites, so be
patient.
 If you see only a few satellites and weak signals, then don't rely on the GPS'
directions. Use your map and compass.
 A clear view of the sky gives you the best opportunity for an optimal satellite lock.
Tree canopy, canyons and tall buildings that obscure the view overhead or of the
horizon can impede reception. So look for a clearing or a high point where you can
get a stronger signal.
 You can sometimes acquire satellites faster if you turn it off, then power back on.
 Be sure the batteries are fully charged.

14. Reading Coordinates
To simplify map navigation, a system of coordinates is used. Coordinates divide the map into
a grid and identify a particular location by listing its relative position north/south and
east/west. To choose a coordinate system, simply go to the Preferences screen. The most
common coordinate systems used in GPS navigation are:
 DMS (Degrees/Minutes/Seconds): This is the standard way of listing latitude and
longitude:
Example: N47° 37' 12" W122° 19' 45".
In this example, N47° 37' 12" indicates that the north/south position is 47 degrees,
37 minutes and 12 seconds north of the equator; while W122° 19' 45" places the
east/west position at 122 degrees, 19 minutes and 45 seconds west of the Prime
Meridian (at Greenwich, England).
 DDM (Degree Decimal Minutes): A decimal version of DMS, DDM is used by
geocachers and a growing number of other GPS enthusiasts. In this format,
coordinates look like this:
Example: N47° 37.216' W122° 19.75'.
The north/south and east/west position remains unchanged. The difference is
that the seconds part of the location is converted to a decimal by dividing the
seconds by 60.
 UTM (Universal Transverse Mercator): This military-derived grid system is not tied to
latitude and longitude. It divides the map into a square grid with the grid lines all
1,000 meters apart. Most topo maps have UTM grid lines printed on them. The system
is metric-based and requires no conversion of minutes and seconds.
Example: 10T 0550368 5274319.
Here, "10T" identifies the map zone, "0550368" is the east/west or "easting"
number, while "5274319" is the north/south or "northing" number.

15. Your GPS receiver can automatically display whichever of these coordinate systems
you select. It can also convert coordinates from one system to another. This is helpful if
you're given coordinates for a location in one system (e.g., UTM), but want to actually
navigate in another (e.g., DDM).
Block
Launch
Period
Satellite launches
Currently in orbit
and healthySuc-
cess
Fail-
ure
In prep-
aration
Plan-
ned
I 1978–1985 10 1 0 0 0
II 1989–1990 9 0 0 0 0
IIA 1990–1997 19 0 0 0 10
IIR 1997–2004 12 1 0 0 12
IIR-M 2005–2009 8 0 0 0 7
IIF 2010– 2 0 10 0 2
IIIA 2014– 0 0 0 12 0
IIIB Theoretical 0 0 0 8 0
IIIC Theoretical 0 0 0 16 0
TOTAL 60 2 10 36 31
***(Last update: 24 May 2010)
ERRORS:

16. GPS error analysis which is found in Error analysis for the Global Positioning System is an
important aspect for determining what errors and their magnitude are to be expected. GPS
errors are affected by geometric dilution of precision and depend on signal arrival time errors,
numerical errors, atmospherics effects, ephemeris errors, multipath errors and other effects.
Variability in solar radiation pressure has an indirect effect on GPS accuracy due to its effect
on ephemeris errors.
The errors of the GPS system are summarized in the following table. The individual values are
no constant values, but are subject to variances. All numbers are approximative values.
Ionospheric effects ± 5 meters
Shifts in the satellite orbits ± 2.5 meter
Clock errors of the satellites' clocks ± 2 meter
Multipath effect ± 1 meter
Tropospheric effects ± 0.5 meter
Calculation- und rounding errors ± 1 meter
First let's get one thing straight, All GPS positions are not 100% accurate and thus must have
some error in them. Now we do have a couple of options open to cut down these errors
depending on our needs. They involve using different GPS receivers and different methods of
getting our positions. For example, surveyors may need very high accuracy, the kind needed
to measure the size of a quarter. On the other hand, to locate your house on a map, a far less
accurate position will do nicely.
Before we turn our attention to just how accurate our GPS positions are, we should have a
quick look at some of the errors that affect the positions we get from the GPS. Each of the
following errors has an impact on the accuracy of our GPS positions.
Orbital error The positions of the satellites obtained from the signal information are really a
prediction of where the satellite should be at a given moment, and can differ slightly from the
actual position. While steps are taken to predict the best positions (or orbits), they can't be
predicted perfectly all the time.
Clock errors The satellites and receivers both need very good clocks to do their job. The
smallest error can throw off the "range measurement" from the receiver to the satellite by

17. many 10's, 100's or even 1000's of meters. For example a 10 nanosecond (0.00000001 sec)
error would cause a 3-metre error in the range.
Ionospheric and Tropospheric Delay This occurs when the signals from the satellite are
delayed in their journey to the receiver by traveling through an area of charged particles,
called the ionosphere, above the earth and through our atmosphere.
Multipath errors The GPS signal may bounce off a nearby object. Imagine measuring the
length of your living room by stretching a tape from one end to the other, but over the top of
the sofa. You wouldn't get a very accurate measurement would you? Well, a range
measurement to a satellite by way of a nearby stop sign would, for example, certainly throw
off our GPS position.
GPS APPLICATIONS:
Many civilian applications use one or more of GPS's three basic components: absolute
location, relative movement, and time transfer.
 Clock synchronization: The accuracy of GPS time signals (±10 ns) is second only to
the atomic clocks upon which they are based.
 Cellular telephony: Clock synchronization enables time transfer, which is critical for
synchronizing its spreading codes with other base stations to facilitate inter-cell
handoff and support hybrid GPS/cellular position detection for mobile emergency
calls and other applications. The first handsets with integrated GPS launched in the
late 1990s. The U.S. Federal Communications Commission (FCC) mandated the
feature in either the handset or in the towers (for use in triangulation) in 2002 so
emergency services could locate 911 callers. Third-party software developers later
gained access to GPS APIs from Nextel upon launch, followed by Sprint in 2006,
and Verizon soon thereafter.
 Disaster relief/emergency services: Depend upon GPS for location and timing capabilities.
 Geofencing: Vehicle tracking systems, person tracking systems, and pet
tracking systems use GPS to locate a vehicle, person, or pet. These devices are
attached to the vehicle, person, or the pet collar. The application provides
continuous tracking and mobile or Internet updates should the target leave a
designated area.
 Geotagging: Applying location coordinates to digital objects such as photographs
and other documents for purposes such as creating map overlays.
 GPS Aircraft Tracking

18.  GPS tours: Location determines what content to display; for instance, information
about an approaching point of interest.
 Map-making: Both civilian and military cartographers use GPS extensively.
 Navigation: Navigators value digitally precise velocity and orientation
measurements.
 Phasor measurements: GPS enables highly accurate times tamping of power system
measurements, making it possible to compute phasors.
 Robotics: Self-navigating, autonomous robots using a GPS sensors, which calculate
latitude, longitude, time, speed, and heading.
 Recreation: For example, geocaching, geodashing, GPS drawing and way marking.
 Surveying: Surveyors use absolute locations to make maps and determine property
boundaries.
 Tectonics: GPS enables direct fault motion measurement in earthquakes.
 Telematics: GPS technology integrated with computers and mobile
communications technology in automotive navigation systems
 Fleet Tracking: The use of GPS technology to identify, locate and maintain contact
reports with one or more fleet vehicles in real-time.
As of 2009, military applications of GPS include:
 Navigation: GPS allows soldiers to find objectives, even in the dark or in unfamiliar
territory, and to coordinate troop and supply movement. In the United States armed
forces, commanders use the Commanders Digital Assistant and lower ranks use
the Soldier Digital Assistant.
 Target tracking: Various military weapons systems use GPS to track potential ground
and air targets before flagging them as hostile.[citation needed] These weapon
systems pass target coordinates toprecision-guided munitions to allow them to
engage targets accurately. Military aircraft, particularly in air-to-ground roles, use
GPS to find targets (for example, gun camera video from AH-1 Cobras in Iraq show
GPS co-ordinates that can be viewed with specialized software).
 Missile and projectile guidance: GPS allows accurate targeting of various military
weapons including ICBMs, cruise missiles and precision-guided
munitions. Artillery projectiles. Embedded GPS receivers able to withstand
accelerations of 12,000 gor about 118 km/s2 have been developed for use in 155
millimetres (6.1 in) howitzers.
 Search and Rescue: Downed pilots can be located faster if their position is known.
 Reconnaissance: Patrol movement can be managed more closely.
 GPS satellites carry a set of nuclear detonation detectors consisting of an optical
sensor (Y-sensor), an X-ray sensor, a dosimeter, and an electromagnetic pulse (EMP)
sensor (W-sensor), that form a major portion of the United States Nuclear Detonation
Detection System.

19. ***The U.S. Government controls the export of some civilian receivers. All GPS receivers
capable of functioning above 18 kilometers (11 mi) altitude and 515 meters per second
(1,001 kn) are classified as munitions (weapons) for which State Department export
licenses are required. These limits attempt to prevent use of a receiver in a ballistic
missile. They would not prevent use in a cruise missile because their altitudes and
speeds are similar to those of ordinary aircraft.