Survey Perspectives – Late May 2008

May 19, 2008  - By
Image: GPS World

So, You’ve Been Hearing About L5

It seems in the world of high-precision GPS, there is always something new to consider. A new satellite launched … a new signal being broadcast … a new whiz-bang product introduced.

In fact, engineers are designing GNSS receivers to be “compatible” with signals that aren’t even being broadcast yet. Even more interesting is that salespeople are selling GNSS receivers that are “capable” of receiving these signals that aren’t being broadcast yet, and satellite navigation systems that don’t even exist yet, such as Galileo.

The third civil GPS signal, L5, has been in the news lately, so hearing about it has probably got your antenna up. First of all, let me clarify that a useful L5 signal is still years away – several years.

The L5 news you’ve been hearing about lately is a test of the L5 frequency from a modified Block IIR-M satellite built by Lockheed-Martin last year. For ~$6 million, Lockheed modified one of the Block IIR-M satellites to broadcast some test data on the L5 frequency (1176Mhz). The modified Block IIR-M satellite is scheduled to launch next month.

However, the L5 test data being broadcast won’t be usable by your GNSS receiver that is capable of receiving GPS L5. In fact, I’m almost certain that even if you purchased an L5-capable GNSS receiver today, a serious software update (if not hardware) will be required even when usable data is eventually broadcast on L5.

For a quick review, let’s summarize which satellite models are/will broadcast the various signals:

  • Block I/II/IIA (26 are broadcasting) – L1 C/A, L2 P/Y
  • Block IIR-M (six are broadcasting, two remain to be launched) – L1 C/A, L2 P/Y, L2C
  • Block IIF (12 are planned, first launch planned in 2009) – L1 C/A, L2 P/Y, L2C, L5
  • Block IIIA (eight or possibly 10, first launch planned for 2014) – L1 C/A, L2 P/Y, L2C, L5, L1C

The first Block IIF is scheduled for launch in 2009. At an aggressive launch rate of three per year, all twelve wouldn’t be in orbit until the end of 2012. Even then, we’ll only have twelve satellites broadcasting on L5. Users will derive some benefit from those twelve, but nothing like you would with a full constellation.

Since there are only twelve Block IIF satellites planned, having more than twelve satellites broadcasting L5 means we’ll have to wait for the Block III satellites to be launched. This could go well into the next decade as the U.S. Air Force awarded the contract to Block IIIA satellites to Lockheed just last week, with a first launch optimistically scheduled for 2014.

Besides launching satellites, there are many other L5 technical issues that need to be addressed, such as the control segment infrastructure (software and hardware). There are also significant non-technical issues, money being a big one. Launching satellites is an expensive endeavor; up to $100 million per launch is the number commonly referred to. This is one major reason why GPS satellite launch schedules are a moving target. As much as I’d like to see more satellites in orbit, I understand why the bean counters might want to push out GPS satellite launches for a year when there are 31 operational satellites today, the most ever in history. For that reason, I wouldn’t care to place a bet on where we’ll be with L5 four years from now anymore than I’d bet on where Garmin stock will be four years from now.

What’s so cool about L5 anyway?

Setting launch schedules, infrastructure development, and funding aside, L5 offers some really significant benefits to the high-precision user. First and most significant is that the days of worrying about ionospheric activity will be over. At 1176 Mhz, the L5 frequency separation from L1 (1575Mhz) is significant enough that the ability of your receiver to directly mitigate the ionospheric refraction will render the iono beast essentially harmless.

If you’re relatively new to GPS (in the last six years or so), you will experience the iono beast in the coming years before L5 is ready to help. The current solar cycle has just started and will continue for eleven years. It will peak around 2011 to 2012. The next solar cycle’s affect on GPS is worthy of a newsletter column by itself, and I will have one in the coming months.

Another benefit of L5 is that its broadcast strength is about four times stronger than L2C. A stronger signal combined with a superior code structure means that you’ll get more robust performance in tough GPS conditions. That’s great news for high-precision users working in marginal GPS conditions.

Breaking News on GPS modernization

On Friday of last week, the Office of Space Commercialization published a notice in the U.S Federal Register requesting public comments from within the United States and beyond on the U.S. government’s plan to phase out codeless and semi-codeless access to GPS by December 31, 2020.

This is an historic step. It will be the first time in history that GPS equipment will be rendered obsolete. Specifically, the units affected are high-precision, dual frequency GPS receivers that weren’t designed to use L2C or L5. For example, I have a set of dual-frequency RTK receivers that are about 12 years old – designed well before L2C and L5 specifications were finalized. They work great now, but would be affected by this change.

Granted, we’re talking about 2020, twelve years from now. But there are a great number of you (and me, to a certain extent) that subscribe to the “if it ain’t broke, don’t fix it” philosophy, and even if we did upgrade, we’d hang on to the old equipment as a back-up.

It’s important to recognize that the equipment will not stop working on December 31, 2020. The idea is that the DoD will no longer guarantee the availability of the Y-code on Block IIR-M, Block IIF and Block IIIA satellites that would used by codeless and semi-codeless receivers. Legacy Block II satellites would continue to broadcast Y-code until those satellites are decommissioned. But by 2020, not many legacy Block II satellites will still be operational. Legacy Block II satellites comprise 25 of the current 31 operational satellites today.

After I digest the Office of Space Commercialization’s notice a bit more and talk to a few people, I’ll publish my thoughts in this newsletter in the coming weeks.

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