Since ancient times, humans have used the skies to find their way. Constellations in the night sky were used by sailors to figure out where they were and where they were going.

Today, all we need is a GPS receiver to be able to pinpoint our exact location anywhere in the world, utilized by our constantly used navigation systems. Nevertheless, we still need objects in the sky to figure out where we are and how to get to other places. Enter the satellites.

 

Pictured below: GPS Block IIR(M) Satellite
Rain

What is GPS?

Currently, there are 31 satellites in the GPS space segment orbiting about 26.000 kilometers above the earth: Navstar, a network of U.S. satellites that provide the global positioning system (GPS) services. 24 are in active use, with the rest meant as backup in case one fails. These satellites travel constantly, completing two orbits within 24 hours and travel at speeds of roughly 11.000 kilometers an hour. Each satellite is kept on course by small rocket boosters. The first generation of Navstar satellites began launching in 1978. The latest block of full GPS III satellites, called IIIA, started launching in 2018. 

The Global Positioning System (GPS) was developed by the U.S. Department of Defense. The GPS uses the transmission of microwave signals from the network of satellites to pinpoint a receiver’s location, as well as its speed and direction of travel. Initially restricted to the military, U.S. President Reagan authorized the civilian use of GPS in 1983 following the shooting down of a Korean airliner that strayed into Russian airspace. But a policy known as Selective Availability prohibited the use of high-quality signals by civilians, and only the military could access the best signal. Bill Clinton changed this policy on May 1, 2000, by instructing to turn off Selective Availability so that civilians could have the same accuracy that military personnel had. Since then the GPS system became one of the most widely used navigation aids worldwide very quickly. GPS devices today are quite compact and extremely accurate compared to their early predecessors, finding you within 5-1 meters or sometimes mere centimeters. Thanks to these advances, quite everyone now relies on GPS navigation systems to avoid getting lost. GPS, as a system, is made up of three parts: satellites, ground stations, and receivers.

Routing Options

How does it work?

 

GPS satellites circle the Earth twice a day in a precise orbit. Each satellite transmits a unique signal and orbital parameters that allow GPS devices to decode and compute the precise location of the satellite. The ground stations use radar to make sure they are actually where we think they are. GPS satellites carry atomic clocks that provide extremely accurate time. The time information is placed in the codes broadcast by the satellite so that a receiver can continuously determine the time the signal was broadcast. GPS receivers use this information and trilateration to calculate a user’s exact location. Essentially, the GPS receiver measures the distance to each satellite by the amount of time it takes to receive a transmitted signal. With distance measurements from a few more satellites, the receiver can determine a user’s position and display it electronically to find a way home or adventure anywhere.

 

To calculate your position (latitude and longitude) and track movement, a GPS receiver must be locked on to the signal of at least 3 satellites. The receiver then uses trilateration to determine your exact location on earth. Basically, it draws a sphere around each of the three satellites it locates. These three spheres intersect at two points – one in space, and one on the ground. The point on the ground where the three spheres intersect is your location.

 

With 4 or more satellites in view, the receiver can determine your 3-D position (latitude, longitude and altitude). Generally, a GPS receiver will track 8 or more satellites, depending on the time of day and where you are on the earth. Once your position has been determined, the GPS unit can calculate other information, such as speed, trip distance, distance to destination, sunrise and sunset time and more. GPS works in any weather conditions, anywhere in the world, 24 hours a day, with no subscription fees or setup charges. The ancient sailors of history would be astonished by the speed and ease of pinpointing your location today.

 

Pictured below: GPS III Satellite
Routing Options

What’s the signal?

GPS satellites transmit at least 2 low-power radio signals. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects, such as thick buildings and mountains. However, modern receivers are more sensitive and can usually track through houses.

A GPS signal contains 3 different types of information:

  • Pseudorandom code is an I.D. code that identifies which satellite is transmitting information. You can see which satellites you are getting signals from on your device’s satellite page.
  • Ephemeris data is needed to determine a satellite’s position and gives important information about the health of a satellite, current date and time.
  • Almanac data tells the GPS receiver where each GPS satellite should be at any time throughout the day and shows the orbital information for that satellite and every other satellite in the system.

What’s the signal?

GPS satellites transmit at least 2 low-power radio signals. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects, such as thick buildings and mountains. However, modern receivers are more sensitive and can usually track through houses.

A GPS signal contains 3 different types of information:

  • Pseudorandom code is an I.D. code that identifies which satellite is transmitting information. You can see which satellites you are getting signals from on your device’s satellite page.
  • Ephemeris data is needed to determine a satellite’s position and gives important information about the health of a satellite, current date and time.
  • Almanac data tells the GPS receiver where each GPS satellite should be at any time throughout the day and shows the orbital information for that satellite and every other satellite in the system.

Together in your navigation system

Combining the use of signals from the satellites with interactive onboard maps, GPS navigation systems can plot routes of travel to a given destination based on a number of variables. Some GPS navigation systems are interconnected with sources of traffic information, enabling them to automatically account for construction and congestion when determining the best route. If a driver misses a turn, GPS navigation systems can quickly correct the error with updated routing. Some navigation systems, like our BAREWAYS navigation system, not only just include traffic, but also weather and public warnings.

However, GPS isn’t foolproof. GPS receivers use a combination of signals from a network of satellites and ground stations to figure out where you are and where you’d like to go. It is really only as good as the satellite network and its map data. Without a clear and strong signal, your device can’t accurately establish your location. Tall buildings, dense foliage, mountains and even reflective objects can cause errors.

The United States, which developed and operates GPS, and Russia, which developed a similar system known as GLONASS, have offered free use of their respective systems to the international community. The International Civil Aviation Organization (ICAO), as well as other international user groups, have accepted GPS and GLONASS as the core for an international civil satellite navigation capability known as the Global Navigation Satellite System (GNSS).

 

Picture Credits:
2, 3, 4: U.S. Government photo, GPS.gov Multimedia Library.