Doppler velocity sensor

A Doppler velocity sensor (DVS) is a specialized Doppler radar that uses the Doppler effect to measure the three orthogonal velocity components referenced to the aircraft. When aircraft true heading, pitch and roll are provided by other aircraft systems, it can function as a navigation sensor to perform stand-alone dead reckoning navigation calculations as a Doppler Navigation Set (DNS).

Doppler navigation systems are independent of surrounding conditions, perform with high accuracy over land and sea anywhere in the world, and are independent of ground-based aids and space-based satellite navigation systems.

Operational principles

To measure an aircraft three-dimensional velocity, a Doppler radar antenna is caused to radiate a minimum of three non-coplanar microwave electromagnetic beams toward the earth's surface.[1] Some of the energy is backscattered to the radar by the earth surface. With knowledge of the beam angles, three or more beam-Doppler frequencies are combined to generate the components of aircraft velocity.[2]

DVS transmission is performed at a center frequency of 13.325 GHz in the internationally authorized Ku band of 13.25 to 13.4 GHz.[3]

Uses

DVS are used on helicopters for navigation, hovering, sonar dropping, target handover for weapon delivery and search and rescue. Because the Doppler radar measures velocity relative to surface, sea current and tidal effects create biases. However, for sonobuoys dropping and over water search and rescue, velocity of the aircraft relative to water movement is expected.[4][5]

These radars were formally approved under the FAA TSO-65a[6] until 2013, and are designed in accordance with the Radio Technical Commission for Aeronautics (RTCA) DO-158 standard titled Minimum Performance Standards − Airborne Doppler Radar Navigation Equipment.

Limitations

The functional operation and accuracy of Doppler velocity sensors is affected by many factors, including aircraft velocity, attitude and altitude above terrain. It is also affected by environmental factors, including the type of terrain the radar is illuminating, and precipitation in the atmosphere.[5]

As the aircraft moves, the backscattering coefficient changes within the beam width, and this causes a shift and some skewing of the Doppler spectrum, and hence an error in the measurement of velocity.[2] A major limitation of using DVSs for navigation is that they typically suffer from accumulated error. Because the guidance system is continually integrating velocity with respect to time to calculate position ''(see dead reckoning)'', any measurement errors, however small, are accumulated over time. This leads to 'drift': an ever-increasing difference between where the system thinks it is located and the actual location. Due to integration a constant error in velocity results in a linear error in position.

See also

References
  1. "Doppler Velocity Sensor APN-200 Error Model And Flight Test Results" (PDF). Defense Technical Information Center. 8 March 1974. Archived (PDF) from the original on March 25, 2020.
  2. Fried, Walter R. (Summer 1993). "History of Doppler Radar Navigation". Journal of the Institute of Navigation. 40 (2): 121–136. doi:10.1002/j.2161-4296.1993.tb02299.x.
  3. International Communication Union. "Feasibility of MSS operations in certain frequency bands" (PDF). Itu.int.
  4. Kayton, Myron., Fried, Walter R. (1997). Avionics navigation systems (PDF) (2nd ed.). New York: Wiley. ISBN 0471547956. OCLC 34798180. Archived from the original (PDF) on 2018-12-22. Retrieved 2019-01-02.{{cite book}}: CS1 maint: multiple names: authors list (link)
  5. Mike., Tooley (2017). Aircraft Communications and Navigation Systems, 2nd ed. Wyatt, David. (2nd ed.). London: CRC Press. ISBN 9781317938347. OCLC 1006392205.
  6. "Technical Standard Order (TSO)-C65a, Airborne Doppler Radar Ground Speed and/or Drift Angle Measuring Equipment (For Air Carrier Aircraft)". Federal Register. 2012-09-04. Retrieved 2019-02-18.
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