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

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:

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

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

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

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.

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

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

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

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

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

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.

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.

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.

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 healthy Suc-

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:

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

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

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.

***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.