On Upsets of Analog Portions of Digital Flight Control Systems Due to Electromagnetic Interference 912144
Recent concerns about the susceptibility of digital flight control systems (DFCS) to electromagnetic interference (EMI) have lead to testing a representative DFCS for upset from possible carrier signal modulations. Tests performed on the analog inputs of a DFCS have confirmed the following traits:
Waveforms demodulated prior to system input must have modulations lying in the passband of the DFCS to cause upset
All else equal, AC-excited sensors are more sensitive to EMI than DC-excited ones
All else equal, CW signals upset system easier than pulsed signals
Upsets are not extremely sensitive to modulation waveform shape for CW signals
Analog DFCS inputs are susceptible to average, verses peak, voltages for system upsets
Analog sensors requiring AC excitation are more sensitive modulations near, or at, the excitation frequency
Frame synchronous operation is desirable to counter EMI-induced analog faults
Averaging sensor redundancy management algorithms are less susceptible to EMI than middle-picking algorithms.
Actuator output averaging mitigates effects of EMI much better than the practice of dedicating a certain channel to drive a certain control surface
Sensor inputs requiring the highest gain are the most susceptible
The above implies that the analog sections of a modern DFCS will not be upset by radar or other high power pulsed radiofrequency (RF) sources. High power commercial broadcasting stations can be a concern if and only if the detected signal levels are large enough to trip sensor health monitors, and these levels can be determined by examining system specifications. If EMI caused upsets are deemed a great problem, modifying the redundancy management system to handle multiple-correlated failures will be required to avoid misdiagnosis. Future tests will examine the ability of the fault-tolerant digital sections of the DFCS to be upset by EMI modulations.