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GPS Modernization: L2C Signal
|Author||Topic: GPS Modernization: L2C Signal|
posted 09-25-2014 08:11 AM ET (US)
With the addition of GPS satellite IIF-7 to usable status in the GPS constellation, the total number of satellites sending the L2C signal has reached 14. The L2C signal is a component in GPS modernization, and is intended for use by civilians. The L2C signal will permit GPS receivers with dual signal capability to perform their own atmospheric delay correction, resulting in increased position accuracy.
The signals are identified as "L" in reference to the L-band microwave frequency designator standard of the Institute of Electrical and Electronics Engineers (IEEE). L-band is from 1,000 to 2,000-MHz and is known as the "long wave" microwave band.
The L1 C/A or coarse acquisition signal is transmitted at 1,575.42-MHz. The L2C signal is transmitted at 1,227.6-MHz, and is transmitted in a manner that should produce better received signals than the L1 C/A signal, even though the carrier power is not as high. This is anticipatd to improve reception in difficult environments, such as indoors or outdoors under cover of trees.
Present marine GPS receivers only utilize the L1 C/A (coarse acquisition) signal. If the GPS modernization program continues on schedule, by 2018 there should be 24 satellites in the GPS constellation transmitting the L2C signal. At that time it will be interesting to see what sort of progress will have been made in marine GPS receivers. Will they evolve into dual-frequency GPS receivers and make use of the L2C signal?
posted 10-30-2014 11:30 AM ET (US)
With the successful launch of GPS IIF-8 and its anticipated coming successful entry into operational status, the constellation of the Global Positioning System (GPS) will soon have 15 satellites that are sending the L2C signal. With 15-satellites there is a reasonable chance that at least four of them ought to be in view to a user, assuming their orbits are spread across the various slots and planes of the constellation. That suggests that obtaining a position fix with the L2C signal may soon become a reality.
Using the L2C signal will require a more advanced GPS receiver than the typical consumer grade or recreational boat grade receiver of today. There will be, no doubt, some lag in the use of L2C by the general public, but there doesn't seem to be any particularly big boundary to cross for widespread use of L2C to become fairly common in the future.
It is also interesting to recognize that the choice of L-band for radio navigation satellite systems like GPS was very much on purpose. A characteristic of L-band signals is the ability to penetrate through the ionosphere and troposphere. The ionosphere generally remains transparent for L-band frequencies. At lower frequencies the ionosphere can be reflective, and signals cannot pass through the ionosphere or would be attenuated. The troposphere is similarly transparent to L-band signals, while for higher frequencies the presence of water molecules or certain gases in the troposphere can cause radio signals to scatter or be attenuated. The frequencies in the L-band tend to be high enough to penetrate through the ionosphere and low enough not to be scattered or attenuated in the troposphere. The selection of L-band frequency was no accident in the design of GPS, making it an all-weather system. And, of course, it is in bad weather when you often most need to know your position.
posted 11-01-2014 01:36 PM ET (US)
When I wrote "there doesn't seem to be any particularly big boundary to cross for widespread use of L2C to become fairly common in the future," I was not aware of the existence of a rather big boundary: there seems to be a schism in thinking about the L2C signals and their deployment into actual use. Even though there are plenty of satellites in orbit now that have the capability to send the L2C signal, and, indeed are sending it, there seems to be no navigational message being sent on that signal. So the L2C signal is presently of no use to anyone, other than perhaps for testing the concept. What is the reason for this?
Not too long ago, noted GNSS expert Don Jewell wrote:
Source: GPS World, February 2103
Can the government of the USA really be so screwed up that one department can spend billions of dollars launching satellites that another department then says must not be used for twenty years? Perhaps not. A more recent article announced the beginning of transmission of navigation messages on the L2C signal:
Source: GPS World, April 2104: http://gpsworld.com/ dod-announces-start-of-civil-navigation-message-broadcasting/
posted 11-02-2014 09:01 AM ET (US)
The following notice was recently issued by the Air Force:
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