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GPS Modernization

The Modernized L2 Civil Signal

September 1, 2001 By: Wai Cheung, Tom Stansell, Richard D. Fontana GPS World

Leaping Forward in the 21st Century



 

A funny thing happened on the road to GPS modernization: a signal suddenly changed.

After years of preparation, 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 applications
  • adding the C/A code to L2.

 

Implementation was underway when the System Program Director for the GPS Joint Program Office (JPO) asked whether it was wise simply to replicate the 20th-century C/A code in a 21st-century "modernized" GPS.

Responding to this challenge, a truly modern L2 civil (L2C) signal was designed in a remarkably short time to meet a much wider range of applications. The first launch of a Block IIR-M satellite in 2003 will carry the new signal, as will all subsequent GPS satellites.

As a result, civil GPS product designers eventually will have at least three rather different types of GPS signals to choose from. It also would be desirable for GPS III to add a modern civil signal to L1, further increasing the number of design choices. Depending on the application, designers will be able to select signals based on power, center frequency, code clock rate, signal bandwidth, code length, correlation properties, threshold performance, interference protection, and so on.

Figure 1 Introduction of new signals (authors' estimate, as funding not finalized)
Figure 1 Introduction of new signals (authors' estimate, as funding not finalized)

As well as describing the technical characteristics of the L2C signal, this article investigates how it will be used, what difference it will make, and how it will affect both users and manufacturers. To explore these issues, we invite you to eavesdrop on a meeting held on September 16, 2008.

 

The Scene: 2008

The meeting started at 9:00 a.m. 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. He had invited only three other people: Charley, who headed Acme's dual-frequency and high precision GPS product developments, Valerie, who headed GPS-based consumer product developments, and Albert, from marketing.

 

Under Fred's direction, Acme offered a wide array of GPS and non-GPS products for both the professional and consumer markets. Years ago Acme had recognized how important GPS was for many applications, so it acquired a few small companies with expertise in designing and applying positioning technology. By 2008, Acme had become a major supplier of GPS-based equipment for high precision, OEM, and consumer applications, although it had not entered the aviation or military markets.

Figure 2 New signal availability
Figure 2 New signal availability

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 knew Charley and Valerie were on top of these trends, so he wanted a better understanding of the new options and what they might mean for Acme's markets.

Fred asked Valerie to explain why she used only L1 C/A chipsets for consumer applications, while Charley had been using dual-frequency chipsets for years.

Valerie said "show slide one," and her palmtop transmitted it to the conference room projector (see Figure 1).

 

Consumer Applications

"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." Al made a note and nodded agreement.

 

"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."

"Why are there so many more L2 than L5 signals?" Fred asked.

Figure 3 L2 signal options in IIF satellites
Figure 3 L2 signal options in IIF satellites

Valerie showed slide two (see Figure 2) and explained, "Before the first IIR-M 'modernized' satellite was launched in 2003, GPS provided only three navigation signals, right from the first Block I in 1978 through the last unmodified IIR. Of the three signals, only the L1 C/A was designated for civil use.

"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."

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