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  GPS Modernization: L2C Signal

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Author Topic:   GPS Modernization: L2C Signal
jimh posted 09-25-2014 08:11 AM ET (US)   Profile for jimh   Send Email to jimh  
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?

jimh posted 10-30-2014 11:30 AM ET (US)     Profile for jimh  Send Email to jimh     
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.

jimh posted 11-01-2014 01:36 PM ET (US)     Profile for jimh  Send Email to jimh     
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:

quote:
...rumors abound that a certain government agency objects to the immediate activation of L2C and L5. This agency--and it is not hard to ferret out exactly which one--wants extensive testing to ensure there will be no complications with the Wide Area Augmentation System (WAAS) broadcasts. As an alternative, it proposes waiting until all the L5 satellites are on orbit before activating navigation messages on L2C or L5.

That’s 24 L5 [space vehicles] before the L2C and L5 PNT signals are activated with full navigation messages. At the current GPS rate of one launch per year, that means we will see the L2C ship come in sometime in 2034, or as late as 2037. Given the average life span of GPS [space vehicles] today, by 2037 every single L2C, L5, and M-code capable satellite on orbit today will have been disposed of without ever having broadcast any of the new signals for which they were designed.


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:

quote:
The Department of Defense announced that U.S. Air Force Space Command will begin broadcasting Civil Navigation (CNAV) messages on all operational GPS satellites capable of transmitting the L2C and L5 signals. L2C and L5 are the first of several new civil capabilities being added to GPS as part of the GPS modernization program announced in 1999. The L2C signal is designed to meet commercial needs and L5 meets safety-of-life transportation requirements.

Source: GPS World, April 2104: http://gpsworld.com/ dod-announces-start-of-civil-navigation-message-broadcasting/

jimh posted 11-02-2014 09:01 AM ET (US)     Profile for jimh  Send Email to jimh     
The following notice was recently issued by the Air Force:

quote:

General message to all GPS users:

The purpose of this notification is to inform users of an upcoming event related to the GPS satellite constellation. Air Force Space Command will be implementing CNAV messages on the GPS L2Cc and L5 signals beginning [Julian day-118 or April 28, 2014 at 1430GMT] with updates from the control segment approximately twice per week. The message populated signal content will include broadcast message types (MT) 10, 11, 12, 30, and 33. There are no planned GPS satellite outages or degradations for this activity. L2C and L5 CNAV messages should be transparent to GPS receivers that do not process L2C or L5 cnav messages. These populated signals are intended to facilitate user familiarization and development of compatible user equipment.

Note: until further notice, the L2C and L5 signals are considered pre-operational. A pre-operational signal means the availability and other characteristics of the broadcast signal may not comply with all requirements of the relevant interface specifications and should be employed at the users? Own risk. Therefore these signals should not be used for safety-of-life or other critical purposes. Any military or civil users who encounter user equipment problems following message population of the L2C and L5 signals should contact the applicable [points of contact] identified below as soon as possible. Aviation users should file reports consistent with FAA-approved procedures.

Update: The purpose of this notification is to inform users that the CNAV messages on the GPS L2C and L5 signals beginning on approximately [Julian day-118 or April 28, 2014 at 1430GMT], will include broadcast message types (MT) 10, 11, 30, and 33, but will exclude broadcast MT 12.

Additionally, the CNAV message health bits will indicate that the L1 and L2 signals are healthy, but that the L5 signal is unhealthy. The CNAV message health bits may function independently from the legacy message health bits and should not be considered reliable indicators of signal quality or performance during pre-operational use.

Reminder: until further notice, the L2C and L5 signals are considered pre-operational. A pre-operational signal means the availability and other characteristics of the broadcast signal may not comply with all requirements of the relevant interface specifications and should be employed at the users? Own risk. Therefore these signals should not be used for safety-of-life or other critical purposes. Any military or civil users who encounter user equipment problems following message population of the L2C and L5 signals should contact the applicable pocs identified below as soon as possible. Aviation users should file reports consistent with FAA-approved procedures.



Cf.: http://www.navcen.uscg.gov/?Do=constellationStatus

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