Signal GPS improvements

Gps Control Segment Map
Gps Control Segment Map
GPS receivers make it all transparent to the user, but the GPS civilian signal setup will actually improve as new frequencies become operational. Above left, the system’s global monitoring and control network.

The incredible accuracy and reliability of the Global Positioning System, which has been in use for more than two decades, has given it the quality of a force of nature — always there, doing its thing, like the seasons or the tides. In reality, of course, a sizeable group of U.S. Air Force personnel and civilian contractors work behind the scenes not only to operate the system but to improve it. Let’s take a dive into a somewhat complicated aspect of GPS that most users never consider: the steady improvements to GPS civilian navigation signals. 

The original Global Positioning System that was declared operational in 1995 had satellites that broadcasted three signals: the course acquisition or C/A code, the P code and the encrypted Y code. The P and Y codes are intended for use by military receivers. Ocean voyagers with civilian receivers use the C/A code. The C/A code is broadcast at 1,575.42 MHz, which is dubbed the L1 frequency and the P(Y) code uses 1,227.60 MHz, which is called the L2. (The P(Y) code is also broadcast on the L1 — I did say this is a bit complicated!) 

Even though it was the first and least accurate of the civilian signals, the original C/A code still provides users with roughly 30-meter (98-foot) accuracy anywhere on the globe and does it continuously. This was a huge improvement over the less accurate and more intermittent fixes from the previous satellite system called Transit. With Transit — depending on your latitude —you might have to wait 90 minutes between fixes. 

As revolutionary and impressive as this was both for navigation and for timing applications, the GPS brain trust decided that the system could be improved upon. This has led to the development of three new civilian signals: L2C, L5 and L1C. 

The L2C signal is broadcast on the L2 frequency along with the P(Y) code. (How these signals are modulated to carry different streams of data is really getting into the weeds and is way beyond even the scope of this piece!)

L2C was devised to improve accuracy of navigation, to provide an easier to track civilian signal than the L1 C/A code, and to be a redundant signal in case any localized interference masked the L1 signal. The easier to track part of the L2C improvement is via something called “dataless acquisition.” This involves the L2C signal including something called a pilot carrier broadcast along with the data signal. The pilot carrier makes it easier for a GPS receiver to pick up the L2C signal and then start decoding the data component.

Another improvement introduced in the L2C signal is something called forward error correction, a technique that allows a receiver to determine if the data being sent is correct. 

One of the errors introduced into satellite radionavigation signals is due to the fact that the signal travels through the electrically-charged ionosphere. In the original version of GPS when it only broadcast the one C/A civilian signal, there was no way for ordinary marine receivers to correct for ionospheric delay. With the advent of the L2C it is now possible for units to correct for ionospheric effects by comparing the C/A and L2C signals and factor out the ionospheric delay. According to Air Force Captain Jonathan Teer, Space Systems Command lead engineer for Position, Navigation and Timing Signals Management, this is one of the L2C’s major benefits. 

“The main advantage of the L2C signal is that it enables directly correcting the ionospheric delay when used in conjunction with the L1 C/A signal,” Teer wrote in an email. “L2C enables faster signal acquisition, enhanced reliability, and greater operating range, making it easier to receive under trees and even indoors.”

Though not yet officially operational, the L2C is broadcast by 23 satellites and should be useable from all satellites by 2023.

The improvements don’t stop there, however. The next change was to introduce a new signal, the L5, which is broadcast at 1,176.45 MHz. This is a protected frequency in the aeronautical services band for safety of life applications such as aircraft precision approach guidance. The signal has an improved signal structure for enhanced performance, has a higher transmitted power than L1/L2 signal and a wider bandwidth and provides redundancy should the other signals be masked by interference. “The L5 signal was designed primarily to meet the aviation safety of life community’s need for a second signal at a different frequency,” Teer wrote, “but any receiver designed to receive L5, not just aviation users, can use the L5 signal.”

The L5 frequency is currently broadcast by 16 satellites and is also “pre-operational.” The entire constellation should be L5-capable by 2027.

That would seem to cover all the bases, yet there is another signal due to be added called the L1C. This can be thought of as the “new and improved” C/A signal. It will be broadcast on the L1 frequency like the present C/A code and will be backward compatible with the C/A code but will incorporate some of the upgrades seen in the newer signals. It will have a pilot carrier for dataless acquisition and easier tracking, higher broadcast power and, perhaps most interesting, will allow GPS receivers to work more effectively with other satnav systems. “The L1C signal was developed to be the next generation high-performance civil signal in the L1-band,” Teer wrote, “to provide better compatibility and interoperability with other radionavigation satellite service systems, including the European Galileo and Japanese QZSS.”

The L1C signal is currently broadcast by four satellites and should be available from all GPS satellites by the late 2020s as newer satellites are launched and older ones retired from service.

By Story and photos by Craig Smith