Lecture 7:Global Positioning

System (GPS)

Radio Aids & Navigational Systems

Learning Outcomes

At the end of this lecture, the student should be able to: Explain the basic working principle of GPS Describe the advantages of GPS

Introduction

GPS is stands for Global Positioning System.

Official name of GPS is Navigational Satellite Timing And Ranging Global Positioning System (NAVSTAR GPS)

Introduction

What is GPS-Global Positioning System?

4 satellites/orbit. Total orbit = 6. Thus, GPS consists of 24 satellites.

Each satellite orbits the earth every 12 hours (2 complete rotations every day).

Thus, every point on the Earth will always be in radio contact with at least 4 satellites.

GPS Segment

Control Segment

Space Segment

User Segment

Three Segments of the GPS

Monitor Stations

GroundAntennas

Master Station

Three Segments of the GPSThree Segments of the GPS

What is the purpose of GPS The purpose of GPS is to show you

your exact position on the Earth anytime, in any weather, anywhere.

Who developed GPS?

First developed by the US DOD (United States Department of Defense)

Feasibility studies begun in 1960’s.Pentagon appropriates funding in 1973.

First satellite launched in 1978. System declared fully operational in April, 1995.

Open to the public, 2000. Currently controlled by the United States Air Force.

It costs about $750 million to manage and maintain the system per year

Who Can Use GPS

Military

Automobile

Aircraft Navigation

Marine

Individual

Who can use GPS?

Automobiles: show moving maps (highways, buildings)

Military: target detection. Aviation: for aircraft navigation Marine: Provides precise navigation

information to boaters.

Why do we need GPS?

One drawback of using radio waves generated on the ground (like NDB,VOR,DME or ILS) is that you have only two choices:

A system that is very accurate but doesn’t cover a wide area

A system that covers a wide area but is not very accurate

• We need a powerful system such GPS because GPS can provide accurate information and cover wide area.

GPS Frequency

GPS operates in the UHF band. There are two types of services available:

1. For Civilian/Research Use Standard Positioning System L1 (1575.42 MHz), L2 (1227.60 MHz), L5

(1176.45 MHz) – For Civilian L4 (1379.913 MHz) – For Research

2. For Military Use Precise Positioning System L3 (1381.05 MHz)

GPS Receiver

Handheld GPS Receivers

Casio GPS wristwatch

Components of the SystemSpace segment 24 satellite vehicles Six orbital planes

Inclined 55o with respect to equator

Orbits separated by 60o

20,200 km elevation above Earth

Orbital period of 11 hr 55 min

Five to eight satellites visible from any point on Earth

Block I Satellite Vehicle

GPS Satellite Vehicle

Three nickel-cadmium batteries

Two solar panels Battery charging Power generation 1136 watts

S band antenna—satellite control

12 element L band antenna—user communication

Block IIF satellite vehicle (fourth generation)

GPS Satellite Vehicle

Weight 2370 pounds

Height 16.25 feet

Width 38.025 feet including

wing span Design life—10 years

Block IIR satellite vehicle assembly at Lockheed Martin, Valley Forge, PA

How GPS works

GPS receivers tuned to the frequencies of GPS satellites.

Each GPS satellites then transmit signals to the GPS receivers . The signals, moving at the speed of light, c = 3 x 10⁸ m/s.

These signals indicates satellite’s location and the current time.

The GPS receiver measures the time taken for a signal to travel from satellite to receiver.

The distance to each GPS satellites can be calculated:Distance = Speed of Light x Time

Knowing the distance from at least 4 GPS satellites, the GPS receiver can calculate the position (latitude, longitude and altitude )

Triangulation

A GPS receiver's job is to locate four or more of these satellites, figure out the distance to each, and use this information to deduce its own location.

This operation is based on a simple

mathematical principle called triangulation or trilateration.

Triangulation in three-dimensional space can be a little tricky, so we'll start with an explanation of simple two-dimensional trilateration.

An Example of 2D Triangulation Imagine you are somewhere in the United States

and you are TOTALLY lost -- for whatever reason, you have absolutely no clue where you are.

You find a friendly local and ask, "Where am I?" He says, "You are 625 miles from Boise, Idaho."

This is a nice, hard fact, but it is not particularly useful by itself. You could be anywhere on a circle around Boise that has a radius of 625 miles

Where in the U.S. am I?

To pinpoint your location better, you ask somebody else where you are.

She says, "You are 690 miles from Minneapolis, Minnesota.“ If you combine this information with the Boise information, you have two circles that intersect.

Where in the U.S. am I? (Cont’d)

If a third person tells you that you are 615 miles from Tucson, Arizona, you can eliminate one of the possibilities, because the third circle will only intersect with one of these points. You now know exactly where you are…

Where in the U.S. am I? (Cont’d) You are in Denver, CO!

This same concept works in three-dimensional space, as well, but you're dealing with spheres instead of circles.

3D Triangulation

Fundamentally, three-dimensional trilateration is not much different from two-dimensional trilateration, but it's a little trickier to visualize.

Imagine the radii from the examples in the last section going off in all directions. So instead of a series of circles, you get a series of spheres.

GPS Triangulation

If you know you are 10 miles from satellite A in the sky, you could be anywhere on the surface of a huge, imaginary sphere with a 10-mile radius.

Earth

10 miles

GPS Triangulation (Cont’d) If you also know you are 15 miles from

satellite B, you can overlap the first sphere with another, larger sphere. The spheres intersect in a perfect circle.

10 miles15 miles

GPS Triangulation (Cont’d) The circle intersection implies that the

GPS receiver lies somewhere in a partial ring on the earth.

PossibleLocations of GPS Receiver

Perfect circle formed fromlocating two satellites

GPS Triangulation (Cont’d) If you know the distance to a third satellite,

you get a third sphere, which intersects with this circle at two points.

GPS Triangulation (Cont’d)

The Earth itself can act as a fourth sphere -- only one of the two possible points will actually be on the surface of the planet, so you can eliminate the one in space.

Receivers generally look to four or more satellites, however, to improve accuracy and provide precise altitude information.

GPS in AVIATION

GPS receiver in the aircraft’s cockpit

Can provide the pilot with navigational information in terms of longitude,

latitude and altitude.

What GPS can tell you?

GPS also can tell you What direction you are heading Speed (How fast you are going) Your altitude A map to help you arrive at a destination Distance (How far you have traveled) Time (How long you have been traveling) Estimated time of arrival (When you will

arrive)

ADVANTAGES OF GPSin AIRCRAFT NAVIGATION

GPS can reduce the aircraft delays problem,

GPS can reduce journey time and fuel consumption

GPS can maintain high levels of flight safety.

GPS can increase airway & landing capacity for aircraft.

GPS more economical

A GPS receiver in the cockpit provides the pilot with accurate position data and helps him keep the airplane on course.

Using GPS, aircraft can fly the most direct routes

between airports

Advantages of using GPS in Aircraft Navigation

GPS can reduce the aircraft delays problem GPS can provide accurate & continuous

data during all-weather condition. This can avoid flight delays.

GPS can reduce fuel consumption Civil aircraft typically fly from one

waypoint to another. With GPS, an aircraft's computers can be

programmed to fly a direct route to a destination.

This can save fuel and time

Advantages of using GPS in Aircraft Navigation

GPS can maintain high levels of flight safety. Improved situational awareness (quick

alert about emergency) GPS also can simplify and improve the

method of guiding planes to a safe landing, especially in poor weather.

GPS can increase airway & landing capacity for aircraft. GPS system more flexible. Many aircrafts

can depend on GPS system at one time.

Advantages of using GPS in Aircraft Navigation

GPS more economical Maintain economies from reduced

maintenance and operation of ground-based systems (such as VOR/DME/NDB station)

GPS NAVIGATION SYSTEM There are two advanced GPS

NAVIGATION System which are

1.LAAS - Local Area Augmentation System.

2.WAAS - Wide Area Augmentation System

Local Area Augmentation System (LAAS)

“LAAS is a precision approach and landing system that relies on the Global Positioning System (GPS) to broadcast highly accurate information to aircraft on the final phases of a flight.

LAAS support precision approaches and landing capability to aircraft operating within a 20- to 30-mile radius of the airport.

LAAS approaches will be designed to avoid obstacles, restricted airspace, noise-sensitive areas, or congested airspace.”

Local Area Augmentation System (LAAS)

One LAAS covers multiple runway ends

Contributing technology for high precision terminal area navigation services.

Quick data transferring among LAAS, Aircraft and GPS satellites

Benefits of LAAS

Replaces ILS systems that are expensive to maintain

Increases efficiency of arrival and departure operations and improves usage of runway capacity

Supports fuel efficiency. Improves access to airports

during extremely low visibility operations

Wide Area Augmentation Wide Area Augmentation System (WAAS)System (WAAS)

38 Reference Stations

3 Master Stations

4 Ground Earth Stations

2 Geostationary Satellite Links

2 Operational Control Centers

WAAS Benefits

Serves all classes of aircraft during flight operations in all weather conditions at all locations.

Provides precise navigation and landing guidance to pilots at all airports, including thousands that have no ground-based navigation aids

Overcomes obstacles to ground-based systems, such as mountainous terrain

WAAS Benefits

Reduces operating and maintenance costs associated with ground-based navigation aids

Makes more airspace usable to pilots, provides more direct en-route paths, and provides new precision approach services to runway ends

Through international cooperation provides a global navigation system for all users

GPS Limitations

The GPS must “see” the satellites, so it does not work well in dense forests, inside caves, underwater, or inside buildings.

Signal Interference: Sometimes the signals interfered before they reach the receivers.

Changing atmospheric conditions change the speed of the GPS signals as they pass through the Earth's atmosphere

Obstruction

Earth’s Atmosphere

Solid Structures

Metal Electro-magnetic Fields

Signal Interference