Global Positioning System

Number C-02-11

U.S. Department
of Transportation
Federal Transit


400 Seventh St. S.W.
Washington, D.C. 20590

Dear Colleague:

In recent years, there has been an increasing reliance by transportation systems on Global Positioning System (GPS). In March 2002, the Department of Transportation formally accepted recommendations made in the Vulnerability Assessment of the Transportation Infrastructure Relying on the Global Positioning System (Report) dated August 29, 2001. The Report was written by the Volpe National Transportation Systems Center in response to a Presidential Decision Directive. The Report is an assessment of whether appropriate policies, plans, and activities are either in place or underway to mitigate the vulnerabilities of the GPS.

The Federal Transit Administration (FTA) encourages each transit agency that employs or plans to use GPS systems to conduct its own GPS risk assessment. Transit Intelligent Transportation System (ITS) applications of GPS typically include Automatic Vehicle Location (AVL) for operations management, including locating vehicles during emergencies and mechanical failures. However, widespread integration of other transit ITS technologies such as automatic passenger counters, automatic traveler information systems, navigation and route guidance systems, and others are increasingly dependent on GPS signals. Most AVL systems feed location of paratransit fleets and provide real-time transit information for travelers at kiosks, in stations, or on the internet.

In the event of a severe GPS outage, many ITS systems will be rendered inoperable. In order to mitigate these vulnerabilities, FTA encourages transit agencies to deploy backup systems or procedures. Transit agencies also are encouraged to train personnel to recognize non-standard GPS performance and become familiar with use of back-up systems and procedures. At a minimum, transit agencies can revert back to systems and operating procedures used for locating vehicles prior to the use of GPS. In emergency situations where a transit vehicle needs to be located, an operator can send a priority request to talk to the dispatcher and then describe the vehicle’s location via the voice radio.

In light of renewed emphasis on reinforcing our nations security systems, I believe a self-assessment of GPS vulnerability is timely. Your attention to providing for policies and backup plans in the event of a GPS outage will ensure the vitality of the transportation infrastructure. For further information, the Volpe report, Vulnerability Assessment of the Transportation Infrastructure Relying on the Global Positioning System, is available through the Coast Guard Navigation Center website at I thank you in advance for heightening your awareness of this issue.


Jennifer L. Dorn