The Global Positioning System (GPS) is a satellite-based navigation system made up of at least 24 satellites. GPS works in any weather conditions, anywhere in the world, 24 hours a day, with no subscription fees or setup charges. The U.S. Department of Defense (USDOD) originally put the satellites into orbit for military use, but they were made available for civilian use in the 1980s.
How does GPS work?
Getting lost while driving is close to a lost art. With GPS in your car’s navigation system, in the portable navigation device on your dashboard, or in your smartphone, it’s easy to pull up a map and see where you are, or get directions to where you’re going. The hardest part may be speaking or keying in the data if your car has a clunky interface, or if it doesn’t let you enter an address while the car is moving, even if you’re driving through a sketchy neighborhood just outside the airport in a rental.
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 measure your running route, map a golf course, find a way home or adventure anywhere.
To calculate your 2-D position (latitude and longitude) and track movement, a GPS receiver must be locked on to the signal of at least 3 satellites. 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, but that depends on the time of day and where you are on the earth. Some devices can do all of that from your wrist.
Once your position has been determined, the GPS unit can calculate other information, such as:
- Trip distance
- Distance to destination
- Sunrise and sunset time
- And more
How accurate is my GPS receiver?
GPS started life as a military-first satellite system meant to increase the accuracy of aircraft, submarines, and their ordnance. Until 2000, the signal available to civilians was degraded to provide less accuracy. The unencrypted time signal sent to non-military users was randomly offset to provide no better than 100-meter accuracy. This was called selective availability. It meant that a car’s location might be off by one or two city blocks, and “take the next right turn” might come a block too soon or too late.
Some Garmin GPS receiver accuracy is improved with WAAS (Wide Area Augmentation System). This capability can improve accuracy to better than 3 meters, by providing corrections to the atmosphere. No additional equipment or fees are required to take advantage of WAAS satellites. Users can also get better accuracy with Differential GPS (DGPS), which corrects GPS distances to within an average of 1 to 3 meters. The U.S. Coast Guard operates the most common DGPS correction service, consisting of a network of towers that receive GPS signals and transmit a corrected signal by beacon transmitters. In order to get the corrected signal, users must have a differential beacon receiver and beacon antenna in addition to their GPS.
Today, the GPS receiver in a car is accurate to 10-15 meters. This is with satellite receiver modules costing only a few dollars. Standalone GPS receivers claim accuracy within 3 meters (10 feet) and higher cost units are good to a few centimeters. A $25,000 Trimble Total Station GPS system can be accurate to less than an inch. What does this mean? Variants of high-precision GPS systems can remotely control road construction equipment flattening and shaving of the earth to create a roadway.
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 buildings and mountains. However, a modern receiver is more sensitive and can usually track through houses.
A GPS signal contains 3 different types of information:
- Almanac datatells 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.
- Pseudorandom codeis 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
- Ephemerisdata is needed to determine a satellite‘s position and gives important information about the health of a satellite, current date and time.
GPS and the self-driving car:
Self-driving cars rely on optical sensors and 3D maps to understand precisely where they are and where the hazards are. GPS could play a larger role when it’s accurate to the inch, not the meter or 5 meters. That’s coming. At the least, the car needs to center itself in a 12-foot lane and not waver more than a foot off center.
A snowplow could still venture out in crappy weather and forge ahead at least at moderate speed with GPS either guiding the steering wheel, or warning the driver that the plow is off-course and drifting off the road. What’s not possible at 60 mph in the next five years might be workable at 30 mph.
With hyper-accurate GPS, it could act as an independent auditor of the car’s sensors — for instance, if it believed LDW was no longer performing perfectly. In rainy or mildly snowy weather where optical systems degrade, or the lane markings are partially obscured, the combination might help the car maintain lane-centering a bit longer. Eventually it would shut down.
The GPS Satellite System:
The 31 satellites that currently make up the GPS space segment are orbiting the earth about 12,000 miles above us. These satellites are constantly moving, making two complete orbits in less than 24 hours. They travel at speeds of roughly 7,000 miles an hour. Small rocket boosters keep each satellite flying on the correct path.
Here are some other interesting facts about the GPS satellites:
- Transmitter power is only 50 Watts or less.
- A GPS satelliteweighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.
- A full constellation of 24 satelliteswas achieved in 1994.
- The first GPS satellitewas launched in 1978.
- The official USDOD name for GPS is NAVSTAR
- GPS satellitesare powered by solar energy, but they have backup batteries onboard, in case of a solar eclipse.