Why Time May Not Be on Your Side

The ATIS Synchronization Committee (SYNC) is at the forefront of the industry in doing work to examine GPS vulnerability and methods of GPS backup for time and frequency synchronization.   The group has recently published a technical report which discusses this in detail, GPS Vulnerability (ATIS-0900005).

GPS (Global Positioning System) has become so ubiquitous in everyday life that most people take it for granted, although it is used in cars, planes, computers, smartphones, and wearables, to name a few common examples. While GPS is associated primarily with navigation, location tracking and mapping, it is also critical to providing precise time, time intervals, and frequency information critical to many of today’s technologies.

Why Is Time Important?

Each GPS satellite contains multiple atomic clocks that provide precise location and time information to any receiver capable of receiving and decoding the timing signals from satellites in the GPS constellation.  This enables users to determine the time to within 100 billionths of a second.

Precise time information is essential to the operation and technology evolution of several critical infrastructure segments including telecommunication networks, electrical power grids, and financial networks – all of which rely on precision timing for synchronization and operational efficiency.

Mobile networks use GPS time to synchronize base stations, allowing mobile handsets to share limited radio spectrum more efficiently and to synchronize call handoffs.  GPS also plays a key role in enabling geolocation of Enhanced 911 (E911) emergency calls originating from wireless handsets.

Financial markets leverage GPS timing to timestamp business transactions, providing a consistent and accurate way to maintain records and ensure their traceability.

Utility companies use GPS time to enable efficient power transmission and distribution, for demand pricing where billing accuracy is paramount, as well as to provide status measurements at frequent points to determine where or if a problem exists in the grid.

Other market segments with critical time-dependent functions include rail transportation, trucking, and other transit operations; shipping and maritime; agriculture; air traffic control and seismic monitoring.

GPS Interference/Vulnerabilities

Despite it being central to so many functions, GPS reliability and security has always been limited by a relatively low-powered signal. The weaker the signal, the more sensitive it is to disruption. This leaves it open to interference from several sources including radio emissions in nearby bands, intentional or unintentional jamming, as well as naturally occurring space weather.

Signal jammers are one of the most common sources of interference. While illegal to market, sell, or use in the United States, they are readily available from many sources on the Internet.  Jammers operate by transmitting radio signals that overpower, jam, or otherwise interfere with authorized communications.  GPS jammers have been shown to block navigation signals used by ships, aircraft, and ground transportation. Further, they could have a significant negative impact on systems that depend on GPS for position, navigation, and timing, such as first responder systems.

With the continuing growth and demand for wireless services, the need for additional spectrum has been of paramount importance to network operators.  As such, there is concern that the repurposing of various radio frequencies, including the satellite communications bands next to GPS, could interfere with GPS receivers that do not have the necessary filters to reject adjacent-band transmissions.

GPS in Telecommunications Networks

Within telecommunications systems, GPS facilitates the precise synchronization of networks operated by different network providers.  In addition, this synchronization is necessary for network operations and scalability.

Without precise timing, calls over mobile networks could be dropped or experience interference.  The networks themselves would experience packet loss and become spectrally inefficient.  Broadcast video would experience interruptions, while the overall quality of service – including location-based accuracy – would decline.

Further, any impact to the GPS signal impacts a large number of customers.  For example, a problem with a GPS receiver located at a wireless base station could impact all wireless handset users that connect to that base station.

As such, the need for secure high-performance timing sources is critical and becoming more so as newer technologies, such as LTE-A and 5G are deployed in the network.

Telecom networks support many different synchronization requirements, depending on the service.  Current wireless networks based on LTE require timing accuracies of + 1.5 ms to + 5 ms, demand carefully designed networks and a source Primary Reference Time Clock (PRTC) delivering +100 ns (plus or minus 100 nanoseconds).  Driven by ever more stringent timing requirements such as those for 5G, a new Enhanced Primary Reference Time Clock (ePRTC) has been developed with the capability of delivering +30 ns accuracy.

Mitigation Options

Within the ATIS report on GPS Vulnerability are proposals for how to mitigate GPS vulnerabilities on critical infrastructure timing receivers:

These include the following:

  1. Navigational Message Authentication on modernized GPS civil signals
  2. Atomic clock time holdover
  3. Sync over fiber networks
  4. eLoran (Enhanced Long Range Navigation)
  5. WWVB (a time signal radio station operated by the National Institute of Standards and Technology).
  6. Terrestrial beacon networks
  7. Communication satellite timing
  8. Differential time transfer

With the exception of Navigational Message Authentication (NMA), which offers a possible mitigation strategy against a spoofing attack, all of the other proposed mitigation solutions are methods for transporting time and phase synchronization that provide an independent or semi-independent timing mechanism to the receiver and are immune from localized sources of GPS degradation such as multipath and improper GPS antenna installations.

With precision time playing such a critical role in our telecommunications infrastructure, the need to make GPS as secure as possible becomes particularly important. As more and more services depend on these networks to be as reliable as possible, GPS has become an increasingly integral part of our economy.  This makes mitigation of GPS vulnerabilities of paramount importance and provides strong motivation for an alternative timing dissemination system available at a national scale.