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Session A7: Cooperative Space-Based Sources: Signals

Waveform Adaptation for Contested RF Environments
Kojo Zilevu, Charles Connors, Mike Tinston, Enrico Mattei, Afsayh Saquib, Expedition Technology, Inc.
Location: Ballroom C
Date/Time: Tuesday, Jun. 4, 5:05 p.m.

A Positioning, Navigation, and Timing system intended to work in the presence of strong interference must have a way to ensure the intended signal is received with sufficiently accurate estimates of time/range and that the required ephemeris information can be communicated from the system to the user. To achieve these goals, we have designed a flexible waveform that has attributes tailored to a specific jamming structure while maintaining the ability to adapt to a changing interference environment. We present the waveform definition, spectral situation awareness, and the control architecture that allows the reconfiguration of the signaling framework.
Our waveform adaptivity is two-fold; we first optimized the waveform offline with a receive matched filter to achieve a high degree of orthogonality to the presence of known interference types. The transmit and receive waveform pair is used to develop the signal detection and ranging solution as well as the communications signaling. While the jammer orthogonality is encoded offline, the ability to adapt to the actual interference environment is also built in using an Orthogonal Frequency Division Multiplex (OFDM) representation of the signals (both transmit and receive) and selecting the OFDM subcarriers that correspond to the least interfered frequencies in a contested environment. This adaptivity is an online process. Additionally, the forward error correction and signaling rate can be adjusted to ensure communications in high interference environments and maximize throughput in low-interference environments. To this end, a control architecture that enables the use of spectrum awareness (SA) is needed to respond in contested environments to the threat at hand.
The capability of waveform adaptation has facilitated the need for a control architecture that can subscribe to 3rd party situational awareness or generate awareness in the target area. In both scenarios, each transmit and receive system has a backchannel communication link to enable communication with the controller. The controller framework includes a message broker hosted on the cloud to provide secure communication between the nodes. The broker, hosted on an Amazon EC2 central server, forwards commands to adjust their subcarriers in real-time. When integrated with a 3rd party, we can receive their situation awareness messages via the central server and forward them to the deployed nodes. We have successfully integrated and demonstrated use of 3rd party situational awareness messages. Furthermore, we can generate optimized subcarriers for the scene at our receiver and dispatch them to the transmitters via the central server. The control architecture has been successfully demonstrated in various contested EW environments. In addition to the subcarrier adaptation, our control architecture allows us to adapt other transmit parameters such as the learned jammer orthogonality. It is important to note that with a single receiver, we can adjust to the specific threat scene at the receiver location. However, when we scale to multiple receivers, the RF environment will be the “average” environment for all targeted users. By using the backchannel system, our navigation system can reconfigure and reprogram its PNT waveforms in real time and achieve robust and resilient performance in challenging spectrum environments.
In this talk, we will discuss our system implementation with an emphasis on the backchannel control architecture that allows for frequency agile waveforms to adapt in contested environments and ensure our transmitters and receiver are in-sync, communicating and navigating. We have successfully demonstrated this capability in a diverse set of contested environments at GPS test events.
Approved for public release; distribution is unlimited. Public Affairs release approval #AFRL-2024-0348

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