Expert Advice: Test-Based Civil Receiver Certification

December 1, 2011  - By 0 Comments
Logan Scott

Logan Scott

By Logan Scott

Disaster-preparedness plans recognize the individual’s role in his or her own survival. When storms approach, have water, food, and basic survival gear on hand. It takes time for help to arrive.

The civil GPS industry faces an oncoming storm of interference, and the receiver is the first line of defense. As we integrate GPS into all facets of our lives and infrastructure, we become more subject to disruptions, both unintentional and intentional. Newark International Airport now sees several jamming events per day. In Taiwan, one airport experiences an average of 117 events per day!

How can civil PNT infrastructure be made more resilient?

Faced with jamming, spoofing, and cyber attacks, receivers must take basic precautionary measures. They must recognize jamming and spoofing attacks to avoid generating hazardously misleading outputs. Situational awareness is key. Accurate and specific alarms must be generated so users can take action and authorities can be notified. Regular threat-signature updates can improve situational awareness, much like antivirus updates on a computer. Fire alarms don’t put out fires but they do save lives and improve response time.

Twenty years ago, computers rarely had firewall or antivirus protection. As GPS becomes more deeply integrated into communications-enabled systems, its utility increases exponentially but so does its vulnerability to cyber attack. When you update your GPS software or your maps, how do you know they have not been compromised? How do you know your receiver is authentic?

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Figure 1. There are demonstrated, well known attacks that can cause receivers to output misleading information without warning. Many of these attacks can be detected using simple methods. Some receivers incorporate detection and countermeasures techniques. Many don’t. Receiver certification provides GPS buyers with a starting point for selecting GPS receivers. Certified receivers can accurately report on interference so it can be located and stopped.

The U.S. Navy recently discovered counterfeit routers in several of their installations. Well-developed computer security methods such as the Trusted Platform Module found in more than 300 million computers can help secure GPS receivers without impeding innovation.

The government can also play a role in improving receivers by providing an authenticatable civil signal structure. Well-documented Public Key Infrastructure methods such as digital signing and occasional, short-spread spectrum security-code bursts can be added to the new L1C signal. Receivers voluntarily using these signal features can establish signal provenance with extremely high confidence.

The public, unclassified keys needed to process these features could be sold and used as a revenue source for the GPS system. Receivers that choose not to use these features can ignore them without adverse impact other than weaker security. The large numbers of in-theater military users who rely on civil signals would also stand to benefit.

Finally, I would note that situationally aware receivers can provide specific and detailed reports about what they see. Interference-monitoring systems such as Patriot Watch will need detailed reports to sort and associate the multitude of reports they receive into a coherent picture of what is actually happening. To provide adequate geographic coverage, interference monitoring systems will need to accept reports from diverse receiver types on an opportunistic basis. In short, they will have to rely on crowdsourcing as a major operational input.

As Brad Parkinson noted during my presentation of this material to the November 9 meeting of the National PNT Executive Committee Advisory Board (“Receiver Certification: Making the GNSS Environment Hostile to Jammers and Spoofers,” at www.pnt.gov/advisory/2011/11/), in the early days of electricity, a lot of houses burned down because of electrical problems. Underwriters Laboratories helped immensely by testing electrical equipment to make sure it was reasonably safe, and consumers looked for the UL label. A voluntary, basic receiver certification process similar to Underwriters Laboratories should be pursued to provide the user community with a basis for selecting receivers.


Logan Scott has more than 32 years of military and civil GPS systems engineering experience. At Texas Instruments, he pioneered approaches for building high-performance, jamming-resistant digital receivers. While at Omnipoint, a cellular carrier, he developed cross-system interference mitigation strategies. He holds 33 U.S. patents.

GPS World staff

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