By Dr. Gary Deel
Faculty Director, School of Business, American Public University
To make global positioning systems (GPS) work, designers had to overcome the challenge of time dilation.
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The phenomenon of time dilation – first theorized by Albert Einstein – affects the rate at which time passes for objects moving at different speeds. What does this have to do with GPS? The answer relates to how orbits work.
It is easy to think of an orbiting satellite as just an object floating in space. In reality, satellites in orbit are actually in freefall. The only reason they do not fall back to Earth is that they are moving horizontally at such tremendous speeds that the Earth’s surface curves away at the same rate of speed at which the satellites are traveling.
The horizontal speed necessary to achieve a stable orbit varies depending on the altitude and eccentricity of the orbit. But as an example, satellites in low-Earth circular orbits generally travel at around 17,500 mph in order to maintain their orbits. Now that speed is nowhere near the speed of light, but it is enough to cause the effects of time dilation to become apparent in a very small way. Even small changes can make for big differences over time, and for GPS clocks that depend upon extreme precision and synchronization, that can be a real problem.
Time Dilation Proposition Was Purely Theoretical until the First Satellites Were Launched
Einstein’s time dilation proposition was actually purely theoretical until mankind launched the first satellites in the late 1950s and early 1960s. Previously, physicists had never had the opportunity to test the hypothesis because there were no objects with clocks on them moving fast enough to observe the effects of time dilation.
However, after launching the first few spacecraft, it became clear that Einstein’s theory was more than just a formula on a blackboard. For every day that passes on Earth, satellites in orbit lose approximately seven microseconds due to time dilation. Again, this may sound like a miniscule amount, but it aggregates over time and eventually the compounding effect can throw off the alignment between ground clocks and satellite clocks. Because GPS relies on this precise synchronization to deliver accurate location data, time dilation threatened to render the entire GPS network so inaccurate as to be utterly useless.
An Algorithm in Timekeeping Software Adds Back the Lost Seven Microseconds
So how did engineers solve the problem? The answer is quite simple. An algorithm was written into the timekeeping software onboard orbiting satellites that manually adds back the seven microseconds lost each day. The effects of time dilation are thereby canceled out and the clocks in space and on the ground are able to keep accurate, reliable synchronized time with one another.
Once the GPS network is able to determine precisely where a device is located on the Earth’s surface, high-resolution imaging and mapping are then married to the process to tell us where we are relative to our local surroundings. If GPS can pinpoint the latitude and longitude of a device, and then cross-reference that location data against a known database of high-resolution maps that show roadways, buildings, and other major infrastructure, the network can then deduce where the user is with respect to these reference points.
Incidentally, the speed and direction of the device on the ground also help the GPS network to assess its location. For example, if a device is at a location that seems to coincide with the route of a major highway, and the device is also traveling south parallel to that highway at 65 mph, the system can infer that the device is probably in an automobile traveling southbound on the highway.
With respect to roadways, points of confusion still frequently arise when GPS capabilities are not precise enough to differentiate between roads that are next to or on top of one another. For example, highway exit ramps often run closely parallel to the highways themselves before diverging. For the GPS network, it is often difficult to discern whether a GPS-equipped automobile has in fact left the highway until the vehicle travels far enough and changes direction.
Similarly, often roads and bridges are built on top of one another, and sometimes run parallel. The distances between the upper level and the lower level of a bi-level freeway or bridge might only be a few meters. In these cases, it can be difficult for GPS, even with lots of satellites and the most sophisticated maps available, to determine which road a vehicle is actually on.
Modern GPS Interfaces Include Options So Users Can Designate How They Intend to Get to Their Destination
Issues with navigation become even more complex when the mode of transport being utilized is not obvious. Is a user traveling by automobile? Or on foot? If on foot, is the individual intending to take advantage of public transport options such as buses and above- or below-ground rail? Answers to these questions are critical as they necessarily alter the types of navigation information to be provided.
Fortunately, modern GPS interfaces include options so that users can designate how they intend to make their way to their destination. If they are driving, modern GPS can provide directions as well as real-time traffic alerts and delays, which themselves are a product of historical data and real-time monitoring of vehicles with GPS. If traffic is a consistent problem on certain roads at certain times of day, GPS can prioritize the monitoring of these high-congestion areas and offer route alternatives and updates in real-time.
For pedestrians, the GPS network can provide directions by way of sidewalks, parks, alleys, pedestrian bridges, and other routes that motor vehicles are unable to take. For people traveling by public transit, GPS can direct them to bus or railway stations and even offer real-time departure schedules so users know when the next train or bus will be leaving.
These types of advanced GPS networks can also be linked to user calendars, so that phones and other smart devices can recommend departure times in order to reach certain destinations for designated appointments. The GPS-enabled device can advise you when you will need to leave in order to arrive on time, based on the known route and predicted traffic and weather conditions. All of these conveniences rely on sophisticated remote sensing data.
GPS offers a host of enormous benefits in the 21st century that make traveling easy compared to navigation before the space age. However, it is far from a simple process, so the next time you use your smartphone GPS to guide you somewhere, remember to give thanks to the scientists and engineers who make it all possible.
About the Author
Dr. Gary Deel is a Faculty Director with the School of Business at American Public University. He holds a JD in Law and a Ph.D. in Hospitality/Business Management. He teaches human resources and employment law classes for American Military University, the University of Central Florida, Colorado State University and others.