L5 Demo Payload

May 1, 2009 By: Willard Marquis, Michael McFadden, Tom Powell, Jim Irvine, Captain Brandon Erwin GPS World

From Concept to Capability in Less Than 12 Months

On April 10, 2009, a GPS IIR-M satellite transmitted a signal on the L5 (1176.45 MHz) frequency for the first time, securing the use of this critical frequency band for future GPS satellites. Delays had pushed the planned first L5-capable space vehicle launch until after the August 2009 deadline for GPS to "use or lose" its primary status in the L5 band.

Figure 1 Power spectrum plot for the L5 demonstration signal on April 10, confirming the L5 signal power and modulation

The need to meet the bring-into-use (BIU) date for L5 became the driving force behind the L5 Demonstration Payload on GPS IIR-20, a Block IIR-M spacecraft built by Lockheed Martin and launched on March 24. Although much smaller in scope than a major new satellite program, the L5 Demo represents a true success story for the space acquisition business: a story of programmatic foresight, effective risk management, technical innovation, and close cooperation between government and industry to achieve a goal of national importance.

Radio Spectrum: They Ain't Makin' It Anymore

Few compelling narratives begin with a discussion of spectrum management, but the story of the L5 Demo must begin here, in a critical yet often overlooked part of any satellite system.

The Navstar GPS, arguably one of the crowning achievements of the space age, has spurred a second wave of GNSS such as GLONASS, Galileo, Compass, QZSS, and IRNSS. These new systems, like GPS, must operate in a few internationally sanctioned bands of spectrum within what is known as the L-band. These bands were selected by GPS for their favorable space-based radionavigation properties. Indeed, these segments of L-band, L1 (1559-1610 MHz), L2 (1215–1240 MHz), and L5 (1164–1215 MHz), are considered the beachfront property of radio frequency spectrum. Like any buildable lot near the water, two truths apply to L-band spectrum: it is extremely valuable and "they ain't makin' it anymore."

Thus, spectrum allocated to radionavigation satellite services (RNSS) is subject to regulation to ensure peaceful coexistence of its users. The primary regulatory body for RNSS spectrum is the International Telecommunication Union Radiocommunication Sector (ITU-R), based in Geneva, Switzerland. Several governments, including the United States, have their own domestic spectrum regulatory agency in addition to complying with ITU-R regulations. Many other countries adopt ITU-R rules for their domestic regulations.

The GPS L1 and L5 frequencies lie in bands designated for aeronautical radionavigation services (ARNS). Aviation users depend on these bands for safety-of-life (SoL) services. Although ITU-R regulations protect all RNSS bands, L1 and L5 receive additional protection due to the SoL services they support. The Federal Aviation Administration counts on GPS signals in these protected ARNS bands to provide dual-frequency ionospheric corrections for future aviation users, with significant availability, integrity, and accuracy advantages.