New GPS Civil Signal Rollout Will Aid Spoofing Countermeasures

As reported by Aviation Week: The U.S. Air Force is set to start early implementation of the long-anticipated GPS Civil Navigation (CNAV) message at the end of this month, and will use the process to help develop new countermeasures against spoofing.

The GPS satellites will begin the early broadcast of more accurate navigation messages on the new civil L2C and L5 signals, mainly to aid development of compatible user equipment and CNAV operational procedures. However, according to the Air Force, an element of the pre-implementation phase will evaluate new ways to protect against the growing threat of spoofing, in which vehicles can be put off course by counterfeit signals. Spoofing is a more insidious threat than jamming because users are not aware that their navigation system is being misled.

The development of spoofing countermeasures is viewed as increasingly vital because of the “safety of life” applications at which the L5 signal is aimed. L5 is the third civilian GPS signal, and will be broadcast in a radio band reserved exclusively for aviation safety services. In the future, aircraft will use L5 in combination with L1 C/A to improve both accuracy and signal redundancy. L2C is the second civilian GPS signal, and when combined in a dual-frequency receiver with the legacy L1 C/A civil signal, enables ionospheric correction that will improve accuracy. The signal broadcasts at a higher effective power than L1 C/A, which will make it easier to receive in areas where reception can be poor, such as under trees or indoors.

Air Force Space and Missiles Systems Center (SMC) GPS chief engineer James Horejsi says “part of the reason to push out CNAV is we are evolving from a static code to a series of flexible codes. That allows you to put additional information out, and this month we will be starting to figure out if there is a way to take advantage of these messages to minimize spoofing on the civil side.” Speaking at the Space Tech Expo in Long Beach, Calif., Horejsi says: “How to do we begin evolving the GPS architecture to minimize the ability of someone to do this? There is no simple fix, but there are things we are doing to address [it]. We just have to get it out there and let the user come up with innovative ways of using augmented signals to prevent spoofing.”

Horejsi adds that “spoofing is not unknown or new, and from a military perspective that’s why we’re going to M-code (Military). But the problem is we've been doing it for years so that’s why there has been an agreement to pull the introduction of that forward by at least a year and a half.” M-code is the new highly secure, anti-jam signal designed for the GPS III constellation.

The planned pre-operational continuous broadcast is the next phase of CNAV development and follows initial testing done in June 2013. Although L2C and L5 signals have been transmitted by GPS satellites for several years, they have not included a navigation message. Air Force Space Command (AFSPC) acknowledged last year that implementation had been delayed because additional testing was required. Issues uncovered during the evaluation phase in 2013 have all now been addressed, AFSPC says. CNAV uploads are expected twice weekly, with current accuracy levels not due to be matched or exceeded until full implementation in December 2014. Seven GPS IIR-M satellites currently broadcast L2C, and four GPS IIF satellites broadcast L2C and L5.