Multiple independent timing sources provide redundancy.
China and Russia have back-up GPS systems. The USA has no GPS backup, although politicians have been talking about the need for 20 years. ATSC 3.0 stations have the hardware in place to provide nationwide backup. The US Department of Transportation is responsible for GPS. If The Office of Transportation Policy (TRA/OTP) develops and coordinates an ATSC 3.0 GPS backup policy, work and funding can begin. With planning and coordination, ATSC 3.0 broadcasters are positioned to be the future GPS backup in the USA.
If GPS stops, the USA stops. Virtually everything digital requires a GPS time stamp, from credit card transactions and power generation to radio and TV transmitter exciters. Specifically at risk to GPS failure is critical infrastructure such as the energy and power grid, communications, financial transactions, transportation including aviation, maritime, rail, surface, and pipeline, first responders, chemical, dams, information technology, critical defense manufacturing, healthcare and public health, food, and agriculture. It’s a crucial national issue that can go hypercritical in the blink of an eye. US TV broadcasters are in the best possible position to quickly become the GPS backup plan.
Patrick Diamond, Ph. D. is with Diamond Consulting in Ayer MA, and is a member of NASA Space Based Position Navigation & Timing Advisory Board. In his BEIT session “Delivering Traceable Reference Time Via ATSC 3.0-based Broadcast Positioning System (BPS)”, he outlined how BPS-enabled ATSC 3.0 TV stations can evolve into a self-synchronizing mesh network for position and timing signals at a minimal cost.
The Preamble Timestamp
Dr. Diamond’s presentation was preceded by a paper presented by Mark Corl with Triveni Digital, titled “BPS ATSC 3.0 Broadcast Emission Time Stabilization System Proof-of-Concept.” Corl’s presentation described a proof-of-concept system, developed by multiple partner companies under the direction of NAB Pilot and Tariq Mondal at NAB, that provides a closed loop time stabilization system.
The preamble of an ATSC 3.0 frame carries the bootstrap emission timestamp. If this timestamp is accurate, a receiver can synchronize its clock using the ATSC 3.0 signal. Because the ATSC 3.0 signal can carry data, the location of the transmitting antenna can be sent with the signal as data. Armed with the precise bootstrap emission time and the location of the transmitting antenna, a receiver at a known location can maintain a very accurate clock.
A receiver can calculate its own location if it receives emission timestamps from three or more ATSC 3.0 transmitting antennas with known locations. A receiver can also improve its past time and location estimates if the past timestamping errors are reported in the data sent over the ATSC 3.0 broadcast.
Location spoofing resiliency increases when an ATSC 3.0 station listens to the signals from neighboring ATSC 3.0 stations and reports the neighbors’ signal measurements in the data pipe.
BPS is a system and method of estimating time and position at a receiver using a small pipe in the ATSC 3.0 broadcast signal stream. One of the features of BPS is its independence and potential for stand-alone use, and it can continue to operate when GPS/GNSS, internet, and cellular connectivity are unavailable.
BPS can estimate both position and timing at the receiver, however Dr. Dimond’s paper focused on the timing aspect. He said reliable timing information can be derived from a single BPS-enabled ATSC 3.0 signal if the receiver location is known.
To transmit accurate timing information, a TV station needs an accurate and trusted timing reference. BPS-enabled TV stations have the flexibility to invoke one or more timing sources to form a robust reference time, such as an ensemble of available clocks, cross-checked time sources, and the like. BPS-enabled TV stations receive signals from neighboring towers and under the timing service concept, transmit those neighbor stations’ identity, timing, and location and affects a self-synchronizing network. Dr. Diamond’s paper discussed methods and algorithms in detail for synchronizing BPS-enabled ATSC 3.0 TV towers.
In a self-synchronizing BPS network, a few stations used as master timing references result in all stations providing accurate time.
Positioning, Navigation and Timing (PNT) delivered by GPS is a National Security Concern. PNT signals and other data from GPS satellites allow infrastructure capabilities to function reliably. Without PNT, the US economy would halt.
Other nations are aware of this weakness and have implemented augmentations within their national boundaries to mitigate this exact same weakness. The US needs to do the same. The PNT problem has been recognized as critical since at least 2010, and so far, nothing has been done beyond “studying the problem.” Dr. Diamond’s paper described a viable infrastructure-ready solution to this national security weakness: namely, a Broadcast Position System (BPS) based on ATSC 3.0.
Critical infrastructure (CI) environments have special needs for this clock signal. The precise clock is used to synchronize critical operations and functions in each CI. For example, mobile wireless network 4G and 5G base stations need precision time synchronization for Time Division Duplex (TDD) systems. The requirement for time synchronization in 3GPP radios is a 1 PPS signal, accurate to within 1.1 µsec across the entire network. The 1 PPS signal needs to be traceable to Coordinated Universal Time (UTC).
Rules in Equity Trading Systems aka ‘stock exchanges’ require time stamps to correlate to UTC within 1.0 µsec. Each trade’s entire dataset must be stored for regulatory review for up to 7 years.
Machines called synchro-phasors are used to manage and regulate energy being added to the overall grid from all sources, including conventional power plants, wind farms, and solar panel arrays. It is critical to add new energy in a way that assures it is compatible with the existing energy flow. The synchro-phasor ensures the new power is added within one degree of top dead center of the 60 Hz energy carrier. For new energy to be successfully integrated, all synchro phasors need to be time correlated with each other within 1.0 µsec and correlated to UTC.
This critical level of control requires all ATSC 3.0 clocks to be correlated to UTC. Overall, 200 ns accuracy satisfies requirements for all CI industries, if traceable to UTC.
Station Time Sources
A trusted BPS-enabled timing source uses at least three independent time sources. Stations can then operationally leverage a range of robust timekeeping strategies, such as clock ensemble algorithms, redundancy cross checking schemes, or orthogonal source comparison. The BPS transmitter timekeeping system (TKS) can be used to improve accuracy (UTC offset error), resilience by mitigating faults or threats, and other internal performance needs in the broadcasting sector.
In the U.S., the most reliable timing sources would be UTC obtained from the National Institute of Standards and Technology (NIST) or the U.S. Naval Observatory (USNO) and securely delivered to the TV station via optical fiber, satellite link or microwave link. GPS can also provide reference time, as well as Cesium and rubidium clocks located at the station. Additionally, eLORAN signals can be used if available. Significantly, signals from neighboring BPS-enabled ATSC 3.0 stations can be used as reference timing sources.
+20 dB above threshold BPS coverage for general users in the continental US is shown in orange. At or slightly above the threshold is shown in green.
The BPS Network
Tall towers and high power may have line of sight to other tall towers located hundreds of miles away. With directional receiving antennas, one BPS-enabled station may be able to detect signals emitted from many neighboring towers. If those ATSC 3.0 signals are also BPS-enabled, a TV station can use the neighboring BPS signals as timing references.
BPS can satisfy the Critical Infrastructure timing needs if GPS becomes unavailable. If all TV stations participate, single-tower coverage of BPS signals spans all of CONUS and miles of coastal and border areas. BPS towers can form a self-synchronizing network with a few master stations. A BPS tower synchronization algorithm is proposed to provide the best accuracy to all users throughout the network.
The BEIT papers of Mark Corl et al. and Dr. Diamond et al. both contain significant engineering details and algorithms beyond what space allows. The complete papers are available at 2023 BEITC Proceedings - PILOT (nabpilot.org).
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