GPS is Ready for its Close-up
Though they existed only in the realm of science fiction until just a few years ago, devices using GPS technology have become common. We use GPS in our cars, our phones and our computers. We use it to keep our kids safe, prevent loss, increase productivity, even to keep time.
Hollywood has fueled a lot of misconceptions about GPS, and global positioning in general. There are no trackers as small as a pill, and they can’t be implanted under someone’s skin. The size issue doesn’t stem from the positioning system or the receivers themselves. It’s all about the power source. Though receivers can be as tiny as a fingernail, and most are smaller than a quarter, the battery tech has not been able to keep up. So a GPS receiver like those in the movies might be able to be manufactured, but it wouldn’t actually work. Yet.
What is GPS?
GPS stands for Global Positioning System. The system is made up of at least 24 satellites orbiting the Earth. The satellites communicate with specialized receivers on the ground, providing the exact position of the receivers. As of this writing, there are 31 operational satellites in the GPS constellation. Put in place by the US military starting in 1989, the GPS satellite constellation transmits a signal for its own use and a separate signal that anyone with the technological wherewithal is free to access. This has allowed manufacturers to integrate the technology into their products.
GPS satellites are constantly transmitting a signal toward the Earth, which includes their exact position and the precise time as measured by an atomic clock. Receivers pick up these transmissions, calculate how long it took the signal to reach them, and measure that against their own internal clock. By picking up a signal from at least 3 satellites, the device can then figure out exactly where it is using a process called trilateration: “If satellites are here, here, and here, I must be here.” The only information that is actually transmitted by a GPS satellite is its trajectory, along with those of all the other satellites in use, and the exact time of the transmission. The receiver then uses this information to calculate its position in 3-dimensional space as a set of coordinates. We’ll talk more later about why this is important.
Planar orbits are planned and maintained so that most areas around the globe are constantly in view of at least 4 satellites. The more satellites in view of a receiver, the more precise it is at detecting location. Under ideal conditions, a receiver’s position can be calculated to within a few feet, if not a few inches. The accuracy of a GPS receiver can vary based on multiple factors beyond coverage, like sensitivity, sources of interference, and the kind of satellites in view.
The Satellite Blocks, Current and Future
Currently, there are four types of functioning satellites in the GPS constellation, known as Blocks, with a fifth on
There were 10 GPS Block IIA satellites still in use as of August, 2011. Two of these have been in service for over 20 years. They were launched between November of 1990 and November of 1997, with an expected lifespan of 7½ years. Though they’re aging fast, Block IIA satellites have done impressive work, performing longer than anyone could have expected.
To replace the graying Block IIA satellites, the Block IIR satellite was developed by Lockheed Martin and began service in 1997 with the last launched in 2004. The 12 orbiting IIRs are the core of today’s Global Positioning System.
GPS Block IIR(M) satellites began launching in 2005. These improved versions of Block IIR added new jam-resistance for military signals, in addition to being the first to broadcast on L2C, a second civilian signal. L2C is designated for use in commercial applications, improving on accuracy for dual-frequency receivers. L2C also broadcasts at a higher power, allowing for better signal penetration in areas with heavy vegetation cover, and even inside buildings.
The final satellites in the second generation of GPS are known as Block IIF. They first began service in 2010 and the second was launched in 2011. Ten more are planned to launch and replace failing Block IIA units. The IIF series has a longer expected lifespan, 12 years, and is capable of broadcasting on the L5 frequency, the third frequency intended for civilian use. Though it has only gone through preliminary testing, it is expected to begin broadcasting in 2012. L5 is intended for “safety-of-life transportation” applications, such as commercial airlines. In conjunction with the first two civilian GPS frequencies, through a technique known as “trilaning,” L5 is expected to provide accuracy under a meter without any sort of augmentation.
Under development is GPS Block III. The US government has committed to purchase two satellites in this series, with the option to purchase an additional 10. The first is slated to launch in 2014, and will add a fourth civilian GPS signal, L1C. L1C will allow the GPS network to interact with satellite navigation systems maintained by other governments (Russia, Europe and possibly China), with the promise of improved service and accuracy worldwide.