Over time, GPS dependencies have become deeply embedded in much of the nation’s critical infrastructure, as shown in Figure 1 — from emergency services and transportation systems to critical manufacturing and logistics operations. For the past 20 years, however, efforts to protect these assets with a true backup system have stalled, despite the establishment of the U.S. Space-Based Positioning, Navigation and Timing (PNT) Policy in December 2004.
With the recent Notice of Inquiry from the U.S. Federal Communications Commission (FCC), an updated list of technological options is now on the table. However, most would require building new infrastructure or rely on quantum-based technologies that are still years away from being practical or available.
U.S. GPS Efforts Separating
Since its inception in 1977, GPS has drawn from a single technology to serve civil and military sectors. Now, with space — particularly satellites — becoming physically contested in wartime scenarios, the military is embarking on its own approach. This includes pairing GPS with military- grade receivers to improve service and protection for the global GPS layer. And two new layers are being developed as part of a multi-layer approach, deemed the “regional” GPS layer (i.e., per country) and the “local” GPS layer (i.e., per metro).
Yet, with this new system — although supporting modular, open-systems integration — the Department of Defense (DOD) is now distancing itself from other future endeavors, including supporting civil critical infrastructure. The future DOD PNT system will not follow the same path to civil/military use as was taken by GPS. The PNT capabilities employed by the DOD as such will be increasingly classified. The civil effort has not only been left to fend for itself, but it also has been tragically fragmented across many federal departments and agencies. We can only hope the recent FCC focus will help to solidify the civil GPS efforts.
Doors Open for New Solutions
The new orientation of the civil approach opens the door to significant focus on local and regional GPS services. Specifically, a new approach is based on data from the Earth’s “RF geospatial layer,” where geospatial is “relating to or denoting data that is associated with a particular location.” This layer’s data is about available RF signals, which can be used to derive the location of a particular end device anywhere in the blanket of signals. Devices using this new approach will be unencumbered by the intricacies and costs of satellite technology or having to be joint solutions required to meet military standards.
This also opens the door to the power of solutions available through consortia, which can tap into an order of magnitude more benefits through hearty partnerships. All of which also leads to the much-needed speed-to-market.
The Biggest Advantage
In the U.S., more than 110,000 towers transmit a variety of RF signals available to derive PNT. These towers provide a wide range of three-tower geometries needed for PNT calculations and enable strong resiliency (as an adversary cannot disable them all).
Two systems, in particular, are worthy of close consideration. The broadcast industry’s proposed Broadcast Positioning System (BPS) uses ATSC 3.0 infrastructure along with the existing MerlinTPS adaptive RF signal system. Both these systems take advantage of existing RF infrastructure prevalent in most developed and developing countries.
Don’t Fall Into the eLoran Trap
eLoran has been suggested by some as a viable alternative used for deriving PNT. However, this technology has notable shortcomings. The portion of the RF band it uses has several limitations. For example, eLoran is based on a 100 kHz signal, a low-frequency band that is highly susceptible to atmospheric noise.
Although some propose the use of existing AM towers for the eLoran signal, most are ~300 ft, of which eLoran tends to operate with 1600 ft towers. Attempts to operate eLoran using these shorter towers will make for reduced efficiency. Another misconception is about the proposed use of existing AM tower guidewires for transmission. At these wavelengths, that would restrict the towers to be 900 miles apart, having an impact on maintenance.
eLoran would require building new infrastructure for U.S. deployment, including 12 new towers and transmitters. The number of installations requiring significant maintenance and this low number can be taken out in physical warfare.
The eLoran system requires tight synchronization of the signals between each of its towers and the national epoch, requiring additional infrastructure with its attendant maintenance. eLoran supporting position accuracy is rated at 10 m to 20 m CEP, which is not within the FCC requirement of less than 3 m CEP.
Timing accuracy is +/- 50 ns, which meets today’s precision needs, although it is quickly becoming inadequate as needs in the precision timing market continue to increase.
Finally, the eLoran service is transmitted on one known frequency and in a published format, making it more vulnerable to jamming.
GPS RF Systems Pack a Punch
Given the issues associated with eLoran, other technologies must be considered. One such technology is available today and provided by a commercial company, MerlinTPS, which can transfer market-available, precise timing down to +/- 10ns. Such as precise timing provided by another commercial entity, Hoptroff, for example. Both companies currently provide the necessary components of a viable terrestrial GPS.
As a consortium, MerlinTPS/Hoptroff could deliver precise timing wirelessly to broadcast TV towers for BPS, while eliminating the need for signal conditioning and additional synchronization equipment at each tower, or any other related infrastructure.
MerlinTPS combined with BPS could provide all GPS services for primary and backup (not just timing). MerlinTPS can also fill in services for BPS edge cases having poor geometries. These services include portable and mobile devices. MerlinTPS is also able to handle both the enterprise and civil approaches similarly.
New open doors create freedom to quickly address the urgent national security need for reliable, alternative PNT. The consortium approach, adding commercially available technology to the broadcast infrastructure, allows for collaborative development while preserving individual market opportunities, making it an attractive proposition for all participants.
