Assimilator

Project Members: Jahshan Bhatti, Daniel Shepard, Ken Pesyna, Todd Humphreys

Outside Collaborators: Brent Ledvina, Coherent Navigation

Former Project Members: Zach Tschirhart


Summary: What will GNSS receivers look like five years from now? The answer, of course, depends on the application. Mass-market receivers used in applications that do not require precision positioning and timing (e.g., hand-held units for hikers) will likely remain simple single-frequency L1-C/A-based GPS devices. On the other hand, a growing segment of military and civilian GNSS users will demand greater accuracy and reliability from their receivers than can be offered by single-frequency GPS. They will want their GNSS devices to be multi-frequency to combat ranging errors due to ionospheric delay, and multi-system to improve satellite availability and robustness against signal interference.

Major commercial GNSS receiver manufacturers already have product roadmaps in place that anticipate these demands. Manufacturers realize that they will be at a competitive disadvantage relative to their peers if they only offer a subset of available GNSS signals to sophisticated users. "Why should I have to choose between signals?'' their customers will complain, "I'd like all of them!''

Then there is the issue of GNSS security. There was a time, perhaps 20 years ago or more, when computer users were largely unconcerned with the security of their personal computers. That time has passed. As any victim of a computer virus knows, firewalls, anti-virus software, and protocols for secure data transfer are no longer optional, but required. Likewise, the deepening dependence of the civil infrastructure on GNSS—especially for timing synchronization—and the potential for financial gain or high-profile mischief make civil GNSS jamming and spoofing a gathering threat. Since the publication of the U.S. Department of Transportation's Volpe Report on GPS dependence nearly a decade ago, GNSS security researchers have repeatedly warned that civil GPS is not yet secure, and that users trust its signals at their peril. As Professor David Last commented at a recent conference on GNSS security, ``Navigation is no longer about how to measure where you are accurately. That's easy. Now it's how to do so reliably, safely, robustly.''

Secure positioning, navigation, and timing (PNT) will require use of all available means: inertial navigation systems, stable frequency sources, multiple antennas, cryptographic authentication, and all radio frequency signals from which PNT information can be extracted—including non-GNSS signals and signals never intended to be used for PNT.

In short, PNT devices in critical applications five years from now will likely be remarkably capable and secure devices that adhere to an all-signals-in-view, all-available-means philosophy.

Meanwhile, however, the overwhelming majority of GNSS receivers—even those in critical applications—are simple L1 C/A-based devices that fail when signals are blocked or jammed, complaining ``Need clear view of sky.'' What is more, no commercially-available civil GNSS receiver, as far as the authors are aware, incorporates even rudimentary defenses against spoofing. Are these receivers to be considered obsolete? Perhaps. And perhaps the prudent course of action is to replace them with secure and reliable modern devices.

A decision to replace existing receivers, however, cannot be made lightly. The hundreds of thousands of deployed GNSS receivers across the globe today represent an enormous investment in equipment and training. Moreover, in many cases the GNSS receiver is only an embedded subcomponent of a larger PNT-reliant system. It may be inconvenient, unsafe, or expensive to replace these embedded devices with modern counterparts. Nonetheless, the vulnerability of existing receivers, embedded and otherwise, to signal obstruction, jamming, and spoofing, and their inability to make use of modernized GNSS signals and other signals of opportunity, leaves much to be desired.

As an alternative to replacement of existing equipment, we propose augmentation. A technique has been developed for upgrading existing GNSS user equipment to address their shortcomings without requiring hardware or software modifications to the equipment. The technique re-purposes the portable civil GPS spoofer described here to generate ``friendly'' spoofing signals whose implied navigation solution is derived from a fusion of GPS and other observables. The technique is embodied in a device, called the GPS Assimilator, whose output is injected directly into the radio frequency (RF) input of existing GPS equipment to immediately robustify the equipment against GPS outages and interference.


Related Publications:

The GPS Assimilator: a Method for Upgrading Existing GPS User Equipment to Improve Accuracy, Robustness, and Resistance to Spoofing

GNSS Assimilator: A method for upgrading existing GNSS user equipment to improve accuracy, robustness, and resistance to spoofing