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predictable and unknown and unpredictable jamming, and each of the alternative strategies identified so far, and their pros and cons.

INTRODUCTION

To properly address the need for robust radionavigation, it is prudent to first agree upon what is meant by the term robust. After exploring a number of sources, the most appropriate definition found, one that applies to processes, organizations, or systems and best promotes the theme of this discussion, is the ability to withstand or overcome adverse conditions. This then leads us to define robust radionavigation as the provision of PNT services that are strong, sturdy, and able to withstand or overcome adverse conditions.

For radionavigation, the term adverse conditions implies situations where the accuracy, availability, integrity, or continuity of the data or information carried by a radionavigation signal is impacted so as to produce unacceptable, unsafe, or unsecure results. This occurs in the presence of interference.

Interference comes in a number of different varieties. It can be intentional or unintentional. Many, if not most, instances of radionavigation interference have been from sources that were totally unaware that they were causing a problem. Interference can be predictable or unpredictable. Some radio interference is actually planned and mitigations can be put in place to minimize, if not eliminate adverse effects. Interference can be both manmade and environmental. Recently much discussion has occurred in regards to our solar cycle and how increased sun-spot activity has the potential for significant impacts to GNSS-provided services. Interference can be crude or sophisticated (sometimes referred to as jamming or spoofing), the latter being much more problematic. While losing radionavigation services is never pleasant, not knowing that the services have been lost and relying on instrumentation that is faulty can be much worse. Interference can be either widespread, affecting hundreds of square miles and thousands of feet of airspace, or localized, affecting only specific operators and operations. Finally, interference can be continuous or random. While a constant-on jammer causes problems, locating one that randomly “pops up” and stays on for short periods of time can be much more problematic, because it promotes uncertainties in users—the “should I or shouldn’t I” problem. In the case of safety and security operations, the answer is inevitably “I should not,” making the intermittent interferer as effective, but more deceptive than the constant interference source.

Still, when assessing whether a condition is adverse, one must do so in light of the radionavigation system being employed—both on the transmitter and receiver ends. What is adverse for one may not be adverse for another, and that is a basis for determining an appropriate alternative PNT strategy that ensures safety and security and minimizes the impact to the economy. Some PNT systems rely on extremely low-power signals while others employ high-power transmissions. Some rely on line-of-sight signals, while others employ ground waves. Some have



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