To L2C or Not to L2C? That Is the Operational Question.

July 22, 2015  - By
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Half of the GPS constellation now transmits the new civil signal, L2C. In a matter of weeks, that number will crest into the majority of the constellation when IIF-10 is set active and operational to users. By the end of the year or early 2016, look for 18 usable satellites transmitting L2C. That could be considered a nominal initial operating capability (IOC), though it is unlikely to be declared as such by the Air Force. We can anticipate a full operating capability (FOC) within five years. Many high-precision GPS receivers currently embody L2C signal processing capability.

As Oscar Colombo, research scientist at NASA, noted in a recent CANSPACE contributed note, “This seems like a moment to start seriously thinking about using L2C as much as possible.”

This month’s newsletter presents an amalgam, a panel discussion in virtual print, on several aspects and viewpoints stimulated by his posting,

Some readers may want to peruse this U.S. government bulletin for a general description of L2C; others who feel sufficiently informed may skip directly two paragraphs down to “Three issues might be in the way of that being a practical proposition.”

“L2C is the second civilian GPS signal, designed specifically to meet commercial needs. Its name refers to the radio frequency used by the signal (1227 MHz, or L2) and the fact that it is for civilian use. There are also two military signals at the L2 frequency. When combined with L1 C/A in a dual-frequency receiver, L2C enables ionospheric correction, a technique that boosts accuracy. Civilians with dual-frequency GPS receivers enjoy the same accuracy as the military (or better). For professional users with existing dual-frequency operations, L2C delivers faster signal acquisition, enhanced reliability, and greater operating range. L2C broadcasts at a higher effective power than the legacy L1 C/A signal, making it easier to receive under trees and even indoors. The Commerce Department estimates L2C could generate $5.8 billion in economic productivity benefits through the year 2030. The first GPS IIR(M) satellite featuring L2C launched in 2005. Every GPS satellite fielded since then has included an L2C transmitter.”

Oscar Colombo’s CANSPACE note continues:

“Three issues might be in the way of that [using L2C as much as possible] being a practical proposition, and I would appreciate comments on some or all of them:

“(1) The fact that the new L2C navigation code (CNAV) is being transmitted, but flagged as pre-operational by the USAF, indicating that this organization is not yet ready to guarantee its fitness for use.

“(2) The quarter-wave phase difference with the heritage signal L2.
This one is important to know when fixing the ambiguities of differential observations (double differences and first order differences between satellites) combining L2C data from IIR-M and IIF satellites with those with only L2 (IIR and IIA). Some high-end commercial receivers correct for this phase difference, some don’t.  The latest RTCM document I’ve seen that touched on this issue came out in 2013 (RTCM Standard 10403.2, Paragraph 3.1.8, Table 3.1-5), and listed the choices , at that time, by nine leading manufacturers on this matter. The list does not include all of present-day manufacturers of high-end receivers, a list that changes over time.

“(3) There is no proper place for L2C in files in the widely used Rinex 2.11 format.
In principle, this can be taken care of by using data files in the Rinex 3 format. However, the use of Rinex 3, that has some major departures from 2.11, is not universal yet.

“Does anyone  know of an up-to-date, reliable and comprehensive list of receiver manufacturers showing those that correct and those that do not correct for the quarter-wave phase shift?”

GPS World contributing editors Eric Gakstatter (Geospatial Solutions) and Don Jewell (Defense) had a private conversation about the above, which I now make public.

Don Jewell: “I can address this from a policy and operational perspective but you [Eric] have a better feel for the users perspective.

“With two more successful IIF launches there will then be 18 L2C SVs broadcasting that signal, and that is considered by the government to be nominal IOC, an initial operating capability. Regardless of where you are on the Earth, shy of 60 deg N and 60 deg S, you should always have at least one or more L2C SVs in view.

“We are probably looking at 2023 (8 more years) before the L2 carrier phase is in jeopardy of shifting without notice. If indeed that ever happens.

“So from an operational and providers (HQ AFSPC, 50SW and 2SOPS) perspective, certainly the L2C signal should be useful and reliable. Just not normally guaranteed until FOC or full operational capability is declared, usually with 24 SVs broadcasting L2C. With no premature losses that will be halfway through the GPS III launch schedule ~ 2019-20.

“Schedules are dynamic and always subject to change of course.”

[Editor’s note: The L2 carrier isn’t going to go away or shifting to another frequency. What might go away is the P(Y) modulation on the L2 carrier if the DoD considers the P(Y) signals redundant once the M-code is fully embraced. If the P(Y) signal on L2 is no longer transmitted, then civil receivers currently using the P(Y) signal to obtain L2 carrier-phase measurements will no longer be able to do this.]

Eric Gakstatter replied, “I’ve heard that some manufacturers say they are taking advantage of L2C when there are IIRM or IIFs in view and maybe some of the receivers I’m using are doing so. I’ve not paid attention to it.

“It could be helpful in areas where users are trying to work in difficult environments such as near and under tree canopy.

“In the case of RTK, I would think the reference station would have to broadcast L2C data.”

A CANSPACE reader provided the following useful reference, which although it dates from 2012, still contains much immutable data: “The most recent view on the situation I have with L2C can be found here.”

This links to the presentation slides from an American Geophysical Union 2012 Fall Meeting paper, “The Effects of L2C Signal Tracking on High-Precision Carrier Phase GPS Positioning: Implication for the Next Generation of GNSS Systems,” by Frederick Blume, Henry Beglund, and Lou Estey of UNAVCO, a non-profit university-governed consortium, facilitates geoscience research and education using geodesy.

Oscar Colombo and other CANSPACE subscribers have contributed several further notes t the L2C discussion string.  To read them, it’s possible to access the archives here.
Or you can more simply and elegantly subscribe to CANSPACE; see instructions here.

Last month, Richard Langley and Oliver Montenbruck jointly communicated the following interesting aspect of the U.S. Federal Radionavigation Plan to CANSPACE readers:

“in the new version of the FRP is a new phrasing of the earlier statement on guaranteed availability of the P(Y) signal only up to 2020:

“The [U.S. Government (USG)] commits to maintaining the existing GPS L1 C/A, L1 P(Y), L2C, and L2 P(Y) signal characteristics that enable codeless and semi-codeless GPS access until at least two years after there are 24 operational satellites broadcasting L5. Barring a national security requirement, the USG does not intend to change these signal characteristics before then. Twenty-four satellites broadcasting the L5 signal is estimated to occur in 2024. This will allow for the orderly and systematic transition of users of semi-codeless and codeless receiving equipment to the use of equipment using modernized civil-coded signals. Note that it is expected that 24 operational satellites broadcasting L2C will be available by 2018, enabling transition to that signal at this earlier date. Civilian users of GPS are encouraged to start their planning for transition now.”

Finally, Richard Langley notes that “I have a student looking into which Precise Point Positioning engines can currently process L2C observables, but his report is not yet available. Also, we are looking into adding an optional L2C processing capability to the University of New Brunswick PPP software, GAPS (GPS Analysis and Positioning Software), but that’s a month or so away.”

As a postscript, there is a trending discussion at the LinkedIn group, GNSS R&D, Using C\A acquisition products to acquire L2C long code.

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Alan Cameron is the former editor-at-large of GPS World magazine.

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