Lucky Number Five (L5)

Lucky Number FiveSystem Design & Test Newsletter, February 2009Feb 24, 2009By: Alan CameronGPS WorldBy this time next month, we should have witnessed GPS Modernization Truly Rising. In the form of IIR-20(M), the GPS satellite carrying the first payload to transmit the L5 signal. 20(M), originally slated to launch in June 2008 but hampered by launch vehicle problems, is now purported to rise March 24 from Florida’s Cape Canaveral Air Force Station.Also by this time next month, I hope to have just sent off to press the April issue of GPS World, containing an in-depth technical article on aspects of the L5 signal, by authors at a company intimately connected to the space vehicle.Not to hold our collective breath too long, let’s recap what we know so far about L5.In the beginning, GPS modernization called for:implementing military (M) code on the L1 and L2 frequencies for the Department of Defense (DoD)providing a new L5 frequency in an aeronautical radio navigation service (ARNS) band with a signal structure designed to enhance aviation applicationsadding the C/A code to L2.Before we get too excited, let’s remember that a useful L5 signal is still years away — several years. The L5 signal going into orbit next month (we hope) is purely a placeholder, to secure the frequency with the International Telecommunications Union. Lockheed has modified one of the last Block IIR-M satellites to broadcast some test data on the L5 frequency (1176Mhz).However, the L5 test data being broadcast won't be usable by your GNSS receiver even it it is capable of receiving GPS L5. It’s next to certain that an L5-capable GNSS receiver purchased today would require a serious software update (if not hardware) by the time usable data eventually starts broadcasting on L5.Further life for L5, after March launch and subsequent commissioning of IIR(M)-20, won’t resume until the troubled Block IIF satellites begin lofting. 12 are planned, with the first optimistically slotted for sometime in 2009. The IIFs will broadcast L1 C/A, L2 P/Y, L2C — and L5. Even at an aggressive launch rate of three per year, all twelve wouldn't be in orbit until the end of 2012. 12 satellites broadcasting on L5 would bring users some benefit, but nothing like that envisioned from a full constellation.Hurry up and wait for the next installment, Block IIIA, maybe in 2014, when the first of eight or possibly 10 may start broadcasting L1 C/A, L2 P/Y, L2C, L5, and L1C. Full operational capability may not arrive until the next decade.Besides launching satellites, L5 faces a bevy of technical issues still to be fully addressed, such as the control segment infrastructure (software and hardware). Not forgetting non-technical issues such as money. Launching satellites is an expensive hobby.Why L5 Will RockThe oncoming new signal will bring significant benefits to the high-precision user. Its most important uses will come in aviation, however.L5 will bring to an end the era of worrying about ionospheric activity. At 1176 Mhz, the L5 frequency separation from L1 (1575 Mhz) is significant enough that high-grade, triple- or even dual-frequency L5-capable receivers will mitigate the ionospheric refraction down to a nearly negligible factor. Because ionospheric refraction error is inversely proportional to frequency squared, ionospheric error at L2 is 65 percent larger than at L1, and at L5 it's 79 percent larger.L5’s broadcast strength will be roughly four times that of 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.Through the Past DarklyWay back in September, 2001, Tom Stansell, an advisor to the magazine and various government and private entities, envisaged this future for, well, turns out it was last year. We now take you back in time, to look forward in time — wistfully. A prize to the reader who first e-mails me the accurate count of prophecies that did not come true in the following account:The Scene: 2008 The meeting started at 9:00 AM in a small conference room at Acme Industries. Fred, Acme's product development manager, had attended ION GPS-2008 the previous week, and he wanted an update on the GPS chipset alternatives for the 2009 product introductions. . . . At the ION conference, GPS chipset vendors had impressed Fred with the wide variety of options available, including single-frequency and multi-frequency chipsets for all three civil GPS signals at L1, L2, and L5. He wanted a better understanding of the new options and what they might mean for Acme's markets."Thirty satellites now transmit L1 C/A," Valerie began, "but as this slide shows, only 20 have the L2 civil signal, and only nine have the L5 signal. I think Al agrees we can't sell single-frequency L2 products until there are at least 24 satellites in good orbit positions. Until then, even with a better signal, we can't overcome the geometry advantage a 30-satellite constellation gives L1-only products.""A year from now, in late 2009," she continued, "we expect to see a good 24-satellite L2 constellation, so I'm starting to design for L2. But there's no guarantee. I don't know whether we should put all our chips on L2 - no pun intended - or delay another year until we're sure of the constellation, or offer two flavors of equipment and let our customers decide. We also don't know what our competitors will do, so our options seem to be either picking one signal and taking the market risk or spending the extra money to cover both options.""Twelve IIR satellites were modernized into IIR-Ms to speed-up the availability of the military M code on L1 and L2 and the civil code on L2. However, it wasn't feasible to put L5 on the modified satellites. That had to wait for the IIF series now being launched. So, until the twelve IIR-M satellites reach end of life and are replaced, L5 won't be on every satellite. Any delay is a shame, however, because L5 is a great signal."The avionics manufacturers had a similar but even more difficult problem. Common practice was for avionics to be supported for 20 years after installation on a commercial airplane. Can you imagine the dilemma of knowing that signals which had never been launched would fill the sky years before the avionics was replaced? There was disagreement in the FAA about whether to use the L2 civil signal or not, because it isn't in an ARNS (Aeronautical Radio Navigation Service) band, even though it would be available years earlier than L5 and, by increasing signal redundancy, would give substantial protection against GPS interference. Also, it wasn't clear whether or when WAAS or LAAS would support either or both of the new signals. One solution was a modular design, supporting future upgrades, including software upgrades, by adding or exchanging plug-in components.""What about L5, then?" Fred asked.Charley answered, "We're excited about L5 and, if possible, we want to include it as part of the overall transition to consumer chipsets. Unfortunately, we may have to do something sooner than consumer chips are available, because we wouldn't want our competitors to have it first. Like Val said, so far there aren't very many signals, but in the future our high-end products will use all three frequencies. This will speed up ambiguity resolution and extend baselines by permitting better ionospheric corrections over long distances. This is an important improvement, but it certainly has challenged the antenna designers to maintain low-multipath patterns with good sensitivity and tightly controlled phase center characteristics for all three frequencies!"Valerie resumed, “We prefer to use the L2 civil signal now for consumer products rather than stick with the tried-and-true L1. It's a better signal."That doesn't mean it's better for everything, so we won't abandon L1, and for many future applications we'll use L5. That's the beauty of having three rather different civil signals to choose from, because we can choose the best signal for each application."Relative to L1, the raw signal power on L2 is 2.3 dB weaker, but on L5 it's 3.7 dB stronger, for a 6 dB advantage of L5 over L2. We hope these differences will slowly disappear as more GPS III satellites with increased L1 and L2 power are launched. Both L2 and L5 use FEC, and the data rate on L2 is 25 bits per second versus 50 on L1 and L5. Signal tracking threshold on both L2 and L5 is improved because one of the codes has no data. L2 is better than L1 C/A but not as good as L5, simply because L5 has four times more power than L2."One obvious conclusion is that L5 will be a very attractive signal in a few years when the number of signals in space catches up. However, right now it appears that L2 is superior to L1 for many applications, it's available years earlier than L5, and it may be better than L5 for a lot of future applications, even after there are enough L5 signals."Back in 2004, in an article titled “Acquiring Sensitivity,” Philip Mattos wrote:Like the Galileo signal, new GPS signals at L2C and L5 have faster code-rates, wider bandwidths, longer codes, so for the same sensitivity, the receiver must do five to 10 times as much work on the more complex signals during acquisition.GPS L5 and Galileo E5A share a common frequency, common basic code length of 10,230, common chipping rate of 10.23 MHz. and the use of tiered codes. Tiered codes generate a very long code by multiplying a medium length fast code by a short slow Neumann-Hoffman (NH) code. Thus, a pre-acquisition can be done within one chip of the slow code, the results stored, and the results post-processed at all possible synchronizations of the known, short slow code.GPS L5 and Galileo let us in by creating the very long code from two codes, the second of which is very slow, running one chip for each epoch of the primary code. This allows the acquisition to be done in two stages, integrating first for only the period of the primary code, and extending this to the full composite code only when high sensitivity is required. Of course, for tracking purposes, after acquisition, the long code is used all the time.The secondary codes are very short, 10 and 20 chips for the L5 signals, 20 and 100 for the E5A signals.However the chipping rate of the L5/E5A signals means that, in the short term, they will not be targeted by low-cost receivers. In ten years time, when full constellations of both systems are available, the higher speed and power and lower cost of silicon will have overcome this, and this group of signals may well become the most used of all the GNSS signals.Finally, in December 2005, in a Directions essay that he titled “The End of the Beginning,” Per Enge wrote:The forthcoming diversity of signals (L1/L2/L5) will obviate the danger due to accidental radio frequency interference (RFI), do much to tame the ionosphere, and mitigate multipath. This year, GPS has launched the first of the Block IIRM satellites that includes the civil signal and code on the L2 frequency. Within a few years, GPS satellites will have three civil frequencies. The third signal, L5, will be the most effective of all, 5 or so dB more powerful than L1, with a chipping rate of 107 chips per second (or 10 Mcps) compared to the 1-Mcps codes used by L1 and L2.The single greatest challenge for GPS this upcoming year is to accelerate the delivery of L5. Like the current L1 and unlike L2, L5 will reside in an Aeronautical Radionavigation System (ARNS) band of the radio spectrum. Thus, it is much more useful to the international civil aviation community. With its high chipping rate, L5 will also provide more protection against RFI and multipath. All told, it has very high value to safety-of-life applications like IFR flight. The civil aviation community and the FAA are working hard to ensure that suitable avionics and integrity monitors are available, and we must support their efforts.Welcome to the future, ladies and gentlemen. It is now. Or soon will be. I hope I am not too soon in raising a glass to L5.

Geodetic / Global Positioning Systems

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Lucky Number Five (L5)

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