Today it is used mostly in rotary aircraft – helicopters. It was also used in commercial fixed-wing aircraft initially for several years, including secondary navigation support in some military planes, before being replaced by global positioning system (GPS) navigation. Doppler navigation systems transmit four frequency-modulated continuous-wave radar beams directed downward at about 45 degrees, fore and aft, at 8,800 MHz. These beams reflect or “echo” off the ground and are then read by the Doppler computer aboard the aircraft, measuring the differences in frequencies of returning signals. By analyzing frequency changes of the beams fore and aft, the system derives the speed and direction the aircraft has moved over the ground to determine the aircraft’s ground speed and location. If the Doppler signal is ever lost, the system automatically switches to dead reckoning – also known as “memory” operation – using measured airspeed corrected by the last determined wind vector.
The main advantage of Doppler navigation is its self-contained design. Because the Doppler system does not need external data for it to function, it is resistant to tracking or signal-jamming, which allows the system to operate well in areas that are “off the grid.” By comparison, GPS navigation requires satellite data, but GPS signals are unavailable in parts of the world, and they can be jammed or disrupted by hostile attacks or atmospheric conditions. Doppler navigation works independently, giving flight crews accurate, jam-resistant guidance and velocity data without interruption. That’s why Doppler navigation is widely used in military, rescue, and other helicopters that commonly operate in remote and less developed areas, especially in contested zones. The key downside of Doppler navigation is that it only works effectively at relatively low altitudes, because the higher the aircraft climbs, the further away it is from the topography that its radar signals must bounce off of. There are also limitations over some rough terrains and errors due to wave motion over water. However, due to the frequent need to operate in irregular, complex environments and land on uncertain topographies, the benefits of Doppler navigation to helicopter flight far outweigh other potential issues.
While Doppler navigation systems in general are highly accurate under the right circumstances, today’s more advanced Doppler radar navigation systems for helicopters, low flying unmanned aerial vehicles (UAVs), and other low altitude aircraft integrate them with inertial navigation systems (INS), global positioning systems (GPS), or combined INS / GPS systems for more effective, more compact avionics that leverage all of their respective strengths. For U.S. Department of Defense (USDoD) military aircraft in particular, it is essential that these systems also comply with Assured Position Navigation and Timing (A-PNT) and Resiliency and Software Assurance Measures (RSAM) regulations.
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