L2C — not just vanilla GPS anymore

April 24, 2006  - By
Image: GPS World

Welcome to the second edition of GPS World’s Survey & Construction e-newsletter. My name is Eric Gakstatter (egakstatter@questex.com). I’ve spent the past 16 years in the GPS survey/mapping industry using many brands of GPS equipment and software. My first ten years in GPS were spent as a product manager and the last six years as a GPS user and consultant. I’m a non-partisan advocate for the GPS user community.

This subject of this month’s column is L2C. It’s not just about vanilla GPS anymore. GPS modernization weighs heavily in future of satellite surveying. What does L2C bring to the table? When do the new features become useful enough to start making equipment purchasing decisions? While some of the answers may be obvious, others may surprise you.

First of all, I’ll preface this column by noting that L2C is only a small part of the Global Navigation Satellite System picture that includes L5, GPSIII,  Glonass (Russia’s satellite system) and Galileo (Europe’s satellite system). Discussing all of the GNSS components is too much for one column so I’ll be chipping away at all of them in the coming months.

Last September (’05) was a big step for GPS modernization when the first IIR-M satellite was launched…starting the next phase of GPS with a second civilian signal (L2C). Currently, there is one civilian signal (L1 C/A). L2 was designed for military use…although civilian manufacturers have been very resourceful in developing codeless techniques for utilizing L2; therefore making dual frequency receivers (L1/L2) very useful for the user community.

Basically, L2C can be viewed as an add-on feature to the existing L2 band. In practical terms, L2C will help in two areas:

  • It will allow for user receivers to more accurately correct for the error that is generated when the GPS signal passes through the Earth’s ionosphere. L2C provides manufacturers with a new code, enabling them to address the ionospheric delay in a more direct manner than the codeless techniques used by today’s dual frequency receivers. It will also open the door for non survey-grade GPS manufacturers to design survey-grade dual frequency receivers at a much lower R&D cost with fewer patent blocks.
  • Contrary to popular belief, the L2C signal is weaker (not stronger) than L1 C/A code. The idea that L2C will “punch through the trees” with a stronger signal is incorrect. What L2C does offer is a more robust code structure with improved error-correcting methods that will allow it to be used more effectively in marginal satellite signal conditions than what we experience today. Just how much it will help will only be known when the satellite constellation is in place and the receivers are developed to optimize it.

These two enhancements will result in more competition in the survey-grade GPS receiver marketplace because survey-grade dual frequency receivers will be easier to design. With increased competition, it’s reasonable to expect more competitive prices. With L2C, you can also expect GPS to perform better in weaker satellite conditions.

With the benefits of L2C to the survey/mapping market clearly established, when are we going to be able to use it? Well, it’s going to be awhile. The publicized year of 2010 is not realistic as this point. Educated speculation says that 2012 is more likely. Don’t forget that it’s not just a matter of tossing satellites into orbit. There are control and management systems on the ground that need to be developed, tested and rolled-out to make use of the new signal. After the hype generated last September when the first IIR-M satellite was launched, the delays in the follow-up IIR-M launches have been disappointing. For example, the launch scheduled for 1Q 2006 has been pushed out until September…a full year after the first IIR-M.

From the launch schedule, you can see it’s a bit early to start making equipment purchasing decisions based on L2C. Yes, I think that manufacturers will do their best to exploit a partial constellation of L2C satellites and perhaps there will be some innovative
developments in that area, but just note that by the time there is a minimum constellation of L2C satellites in orbit, there will be another two or three generations of receivers that will have been introduced to the market.

I’m at the American Congress on Surveying and Mapping annual conference this week in Orlando. Look for my report on conference news in next month’s issue.

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