GPS BASICS

What is GPS? The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.

GPS Global Positioning System Network of 24 satellites (with spares) Developed by Department of Defense Operational 24 hours/day Available worldwide Land, sea and air Works in all weather conditions It is FREE!! GPS stands for Global Positioning System. It is a network of 24 satellites (with spares) placed into orbit by the Department of Defense (DOD). It works anywhere in the world, 24 hours a day, in all weather conditions, and on land, air or sea. GPS is based on the coordinate system. The best part is that it is FREE to use!

Satellite Constellation

Navigation Tools of the Past Sextant Chronometer How did we navigate before GPS? Early sailors and travelers used the sun, stars and landmarks to navigate. They used sextants to make precise observation of the sun, stars, and planets to chart their course. In the 18th century clockmaker John Harrison invented the chronometer. It allowed navigators to know what time it was in two places at once, which helped to determine longitude. In the 1930s radio beacons were used to navigate. Their accuracy level was several miles. The space race in the 1950s and 1960s was the beginning of the GPS navigation system. In 1957 the Russians launched Sputnik, which became the world’s first artificial satellite. The radio waves emitting from Sputnik allowed researchers to determine Sputnik’s location. This caused them to think they could determine a location on earth from a satellite in space.

Beginnings TRANSIT developed by Navy in 1960s Used to track nuclear submarines Air Force created own system Used to locate vehicles on land or in air The Navy developed TRANSIT in the 1960s to track their fleet of nuclear submarines. It consisted of three satellites. The problem with TRANSIT was it had low accuracy and was not available 24 hours a day. This technology was used until 1996 and was replaced by GPS. During this time the Air Force also created a system to accurately locate vehicles on land or in the air.

NAVSTAR Combined Navy and Air Force systems in 1973 NAVigation Satellite Timing And Ranging (NAVSTAR) First used in combat during Operation Desert Storm – 1991 Full Operational Capability – April 27, 1995 In 1973, these two combined and formed NAVSTAR. NAVigation Satellite Timing And Ranging (NAVSTAR) is the official DOD name for GPS. The first GPS satellite was launched in 1978. It was first used in combat during Operation Desert Storm in 1991. The GPS system reached full operational capability on April 27, 1995.

GPS System Space Segment – Satellite Constellation Control Segment– Monitoring Stations and Ground Antennas User Segment – Receivers A GPS system consists of three segments – Space, Control and User. The Space segment consists of the satellite constellation. The Control segment consists of the master control station located at Schriever Air Force Base in Colorado. There are also four stations that monitor the satellites and three ground antennas. The User segment consists of the receivers and antennas receiving the signal on Earth.

Map of Monitoring stations

GPS Satellite GPS satellite

Satellite Information Weighs approximately 2,000 lbs Travels 7,000 mph 17 feet across with solar panels extended Last 10 years Orbit 12,500 miles above Earth Circle the Earth twice daily GPS satellites weigh approximately 2,000 lbs (1 Ton), travel 7,000 mph, last ten years and are 17 feet across when solar panels are extended. They are powered by solar energy but do have backup batteries for emergencies. Satellites are orbiting 12,500 miles above the Earth. Each satellite circles the Earth twice daily. The number of satellites in orbit changes, the following website gives information concerning the status of each satellite: http://tycho.usno.navy.mil/gpscurr.html.

In the next lesson we will learn: How GPS works Coordinates – Latitude and Longitude Creating and Finding Waypoints using a GPS receiver

How Does GPS Work?

How it works Satellite circles the Earth and transmits signal Signal contains time it was sent and its location All satellites send their signal at the same time Difference in time to reach receivers is used to determine location As each GPS satellite circles the earth, it transmits a radio signal called a pseudo random code. Each signal is encoded with information used to determine a receiver’s location. The signal transmission includes the time the signal was sent and the satellite’s location in space. Receivers on earth receive this signal. All the satellites in the constellation send their information at the same time. However, they arrive at different times due to the distance the signals travel.

Formula: Distance = Speed x Time Signal travels at the speed of light – 186,282.3976 miles per second Speed and Time are known, use to calculate Distance The signals travel at the speed of light which is 186,282.3976 miles per second (mps). The formula Distance = Speed x Time is used to determine the distance between a satellite and a location on earth. This formula is used because the both the speed and time are known. For example, it takes a signal 1 second to reach a receiver. So, we know that the receiver is 186,282.3976 miles from the satellite. However, the actual calculation is slightly more complicated than this.

Example of formula calculation

Trilateration Process of measuring the distance from at least three satellites Three satellites calculate 2D position (Latitude and Longitude) Four or more satellites calculate 3D position (Latitude, Longitude, and Altitude) GPS uses a process called trilateration to determine a precise location. Trilateration is the process of measuring the distance from at least three satellites to determine a location on earth. Three satellites calculate a 2D position while four or more satellites calculate a 3D position. 2D positions consist of latitude and longitude measurements. 3D position contains both latitude and longitude in addition to altitude measurements. The differences between the two is that a 2D position can track movement, while a 3D position can calculate speed, bearing, trip distance, distance to destination, sunrise/sunset, etc.

Trilateration You have no idea where you are. You ask someone and they say you are 125 miles from Wichita, KS. Someone else says you are 250 miles from Springfield, MO. Another person tells you that you are 65 miles from Oklahoma City, OK. Where are you? Stillwater, OK

One satellite can locate a receiver’s position somewhere on a sphere Two satellites can locate a receiver’s position to a circle representing the intersection of two spheres . One satellite can determine a receiver’s position somewhere on a sphere. Two satellites narrow the position down to a circle where the two spheres intersect. Three satellites locate the position to one of two points at the intersection of all three spheres. The second point is automatically ruled out because it is located somewhere in space. Three satellites can locate a receiver’s position to one of two points represented by the intersection of three spheres.

Latitude Latitude lines run horizontal Equator is 0° North and South Poles are 90° GPS is based on the coordinate system using latitude and longitude lines. Latitude lines run horizontal with the equator being 0°. The North and South poles are each 90°. They are noted as north or south of the equator.

Lines of Latitude Latitude lines

Longitude Also known as meridians, run vertical The Prime Meridian in Greenwich, England is 0° Lines range from 0° to 180° International Date Line is 180° Longitude lines, also known as meridians run vertical. Longitude lines run from 0° to 180°. The Prime Meridian located in Greenwich, England is 0° and the International Date Line is 180°. They are noted as east or west of the Prime Meridian. Sometimes a negative is used to designate south or west instead of S or W. On a GPS unit this negative may show up as a zero in front of the coordinates.

Lines of Longitude

Instantaneous positioning with GPS Accuracy of +/- 10 m (30 ft) error (horizontal) +/- 15 m (45 ft) error (vertical) Your location is: 37o 23.323’ N 122o 02.162’ W Since each satellite maintains time very accurately (~1 nanosecond = 30 cm), it can tag its signal with the time of transmission. This information, together with the location of the satellite, also transmitted to the receiver, is sufficient to allow the receiver to calculate the range to each satellite provided that the receiver also knows also knows the time. Equipping receivers with atomic clocks would be very expensive, so the receiver tags the signal with the time of its internal clock to create a “pseudorange” (range plus an unknown clock correction). The software in the receiver than uses the pseudoranges from four satellites to solve for the four unknowns: three coordinates and the receiver clock correction.

Uses of GPS Locating Tracking Navigating Mapping Timing GPS is used in locating, tracking, navigating, mapping, and timing. Locating is determining a basic location of someone or something. Tracking is the process of monitoring something or someone as it moves. Navigating is determining where you are going or getting from one location to another. Mapping includes surveying and mapping locations around the world. Timing involves frequency, precise time measurements, and time intervals.

Using GPS What are uses you can think of? Boating Fishing Hunting Camping Hiking Biking Rafting

Using GPS continued Scouting from land or air, Horseback riding, Hot air ballooning, Aviation, Snowmobiling, Skiing, Search and Rescue, Four wheeling, Emergency Vehicle Tracking, Highway Driving Geocaching, Surveying, Military, Precision Agriculture keeps track of cargo containers as they are shuffled around Singapore’s busy port. Geologists can measure the subtle shifts in the earths crust...movements of just a few millimeters location of trucks as they transfer goods throughout the us....also keeps track of scheduled stops and time behind the wheel. guides passenger cars along unfamiliar roads guides pleasure boats between points..... jet plane to determine its exact position guides a cruse missile to its target meteorologists measuring delays in gps signals caused by atmosphere to aid in forecasting weather. farmers use it to help spread fertilizer on their fields so it can be distributed equally experimental systems are leading blind people about Locating pets and even people (kids)

Using GPS continued Survey disaster areas, Map movement of environmental disasters (Oil spills, wild fires, floods, etc) Mapping fallout shelters, sidewalks, streets, trees, trails, etc. Measuring the growth or decline of mountains

How does GPS improve these activities? Makes the location more precise User Friendly Mapping is easier Universal system Fun! Ask the students if they can think of other benefits of GPS.

What can cause inaccurate measurements? Multipath ‘echoes’ Mountains, trees, towers, and buildings are just a few examples of possible obstructions. Remeber, the GPS signal is an RF signal that can easily be blocked. <B> If obstructions are present, it’s important that you note the azimuth and elevation above the horizon of these structures<B> You can then account for these in your planning. Also, some structures that are obstructions in one direction can also be sources of multipath in another direction.<B> Beware of other sources of multipath.<B> What can cause inaccurate measurements? Natural & Man-made barriers or reflectors can distort measurements If we were outside right now, what would cause potential multipath errors?