Application of DSP Techniques in Airborne Evaluation of Ground - Based Radio Navigational Aids

The digital band-pass filters used in the system and the DSP algorithm realizing the FM discriminator were designed using Matlab [3]. The filter coefficients and discriminator algorithm are incorporated in a program running on the DSP board. The DSP board samples the signal emitted by the ground-based radio navigation aid and processes the sampled data to obtain ILS and VOR information. The sampling is done at a 1.56 kHz rate in ILS mode and at a 39.9 kHz rate in VOR mode. The DSP board communicates the processing results to an Intel 80486 host processor via a dual port RAM (DPRAM) at a 30 Hz rate. Since the DSP chip is not optimized to perform division, the operations requiring division are done on the 80486 host processor. Results As a result of utilizing DSP techniques the cost of building and operating a flight inspection system was reduced. The DSP-based system reduces the cost of building the system in two ways: • the system can be built using off-the-shelf hardware instead of specialized hardware (like hybrid RC networks on a single substrate), and • the engineering costs are reduced because there is no need to design complex mechanisms to compensate for the measurement variations caused by the analog hardware. The cost of operating a DSP-based inspection system is also lower than the cost of operating a traditional system. There are two reasons for this: • due to the reduction of the number of necessary hardware components the equipment is more compact, and it can be flown using a lighter and therefore cheaper airplane, and • the time to perform the inspection is reduced because flight time is not spent on recalibrating the equipment. Field trials conducted during acceptance testing of three units delivered to the FAA indicated excellent correlation with results obtained using FAA fully-automatic systems. Operation of the system at many facilities has proven that time to complete inspections and cost of operation are minimized. Conclusion A DSP-based flight inspection system was built for commercial use. The application of the DSP approach resulted in the reduction of the analog hardware to a single A/D converter. High-order bandpass filters were implemented without the instabilities inherent in a high-Q hardware filter. The price of building and operating the system is significantly lower than in the case of systems using analog electronics.