Doppler Radar NavigationLate in 1955 a Lockheed P2V Neptune equipped with a revolutionary new means of aerial navigation prepared for a flight from San Diego California to the Naval Air Station at Key West Florida. Easier to try out this new navigational equipment the aircraft navigator cranked in the coordinates of the point of departure and the desired ground track he then pushed the start button and sat back to observe. Throughout the flight the equipment automatically and continuously displayed ground speed, drift angle, the ground track also ground miles, aircraft latitude, and aircraft longitude. The navigational information for this flight was provided accurately without the aid of ground facilities, without regard to weather conditions, without celestial references, without reference to airspeed and without knowledge of the speed or direction of the wind. In this flight from San Diego to Key West Florida, the aircraft arrived at its destination with a navigational error of less than 1% of the total distance traveled.
APN-67 is a complete integrated automatic navigator which provides a ground speed, drift angle, ground position, ground track, and other outputs for autopilot, automatic Astro compasses, central reference system, dead reckoning craters, and ground stabilized radar displays. Other Ryan systems such as the APN 122V in 1897 provide only ground speed and drift angle, additional data being generated as desired by the tie in with various navigational computer equipment. These Doppler systems were planned and designed to satisfy diversified requirements and they employ interchangeable units through the greatest practicable extent. As a result, there is a Doppler navigational set further to the requirements of almost every aircraft.
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Present military missions can be performed with degrees of accuracy and automaticity, previously unknown and new tactics are being born from the imagination of those who have recognized the far-reaching capabilities of Doppler navigational techniques. Accurate navigation is required for a flight anywhere over the entire surface of the earth. In conditions of poor visibility, in high winds, in turbulent flying conditions, over Compton, and over the roughest thing, over every type of land including the polar regions. The navigational equipment must have no practical low altitude limit and no practical high altitude limit. It must be adaptable to all see from the zero and negative speeds of helicopters and vertical takeoff and landing aircraft through the supersonic speeds of the most modern high-performance aircraft.
Limitations Of Navigational TechniquesNow consider the shortcomings of navigational techniques that existed prior to the introduction of Doppler navigation. Celestial navigation requires good visibility and takes time-consuming impetus navigational computation.
Manual dead reckoning is time-consuming inaccurate and requires the undivided attention of the navigator. Automatic dead reckoning navigation on the basis of estimated wind and true airspeed status removes some but not all at the manual computations and time lag. Furthermore, errors in the airspeed data and the wind estimate caused the automatic dead reckoning system to exhibit a large error.
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The use of radio navigation demands an extensive network of ground stations, during the time of war or emergencies these radio stations undoubtedly would be silenced in both friendly and enemy territory.
Navigation by means of radar mapping is obviously impracticable or remote for the Earth's surface. None of the foregoing techniques are sufficiently accurate or automatic to meet the exacting requirements of modern Aeronautics. At the present state of the art of aerial navigation, these requirements can be met only by using the techniques of Doppler self-contained navigation. The basic principle employed in doppler navigation is one of measuring the Doppler frequency shift of microwave energy. Continuous-wave rather than puffs type techniques are used in Ryan's Doppler navigational sets in order to maintain high accuracy at all altitudes and release the most extreme altitude requirements.
How Doppler Radar Works?Now let us examine this new utilization of the Doppler effect a natural phenomenon which has been long neglected. To provide a connection to the ground, the microwave energy generated by the CW Doppler set is concentrated in two narrow beams of radiation. A portion of the energy is transmitted along each beam at frequency FT, energy is reflected back into the antenna at a frequency FR, the reflection of energy results from the dispersion of the incident energy caused by the roughness of the Earth's surface. With beam directed half the frequency FR of the reflected energy is lower than the frequency of transmission because of the component of the aircraft speed away from the ground along the direction of the beam.
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The difference in frequency is proportional to the component of aircraft speed along the beam axis. The higher the aircraft speed, the larger the frequency difference, this frequency shift phenomenon known generally as the Doppler effect is the basic principle used in Doppler ground velocity set to measure the speed of an aircraft in flight.
With one microwave beam, only one component of the aircraft speed can be measured. However, with two beams of radiation and barometric altitude data, it is possible to compute the aircraft heading speed, drift speed and drift angle builder. Additional beams can be added to meet the special requirements of helicopters and airships. It is the application that determines the number of beams required.
The ground velocity data obtained with the Doppler set is the basic output from which other vital navigational data is generated. It can be directly integrated to provide ground miles or it may be used in a navigational computer along with heading information for the determination of the north-south and east-west components of the aircraft's velocity, then if desired the north-south and east-west speeds data and automatically integrated to obtain the north-south and east-west distances travel which in turn can be converted to the latitude and longitude coordinates of the aircraft's present position.
Knowing the present position and the desired destination the best course and shortest systems to that destination can be automatically computed and displayed. To meet the requirements of a certain application was the ground speed may be combined with true drift speed for automatic determination of the wind speed and the wind heading. Accordingly, Doppler navigational set which provides navigation without reference to wind data actually permits the automatic computation and display of the wind speed and wind direction.
Equipment Used In Doppler RadarNow let us look at some of the equipment developed to meet the needs of modern aerial navigation. Before looking at specific sets, let's examine certain characteristics which are common through every Doppler navigational set developed by Ryan. Continuous-wave energy is generated by an extremely rugged and highly stable klystron oscillator, the frequency of its output is in an interference-free band centered at 13,300 megacycles. For klystron which is usually located between two microwave crystal mixers not only serves as a rugged microwave transmitter but also acts like a little oscillated coupling small portions of its power into the crystal mixer.
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Now you begin to see the simplicity of the CW concept there is no need for IS amplifiers, local oscillators, or automatic frequency control circuit. The klystron and the crystal mixers are mounted on a featherweight but rugged antenna a very unique concept and design. This antenna unit virtually has 100% reliability and there are no adjustments or moving parts. Further, it requires virtually no maintenance.
The microwave plumbing is served in place within a system that separates the transmitting and receiving halves of the antenna notes the frequency used to provide the two microwave beams associated with each reflecting surface. There is two speed for transmission and foo for acceptance. The antenna is sealed by an integral radome section, one section served as a window for the outgoing energy and the other section served as a window for the ground returned. The antenna has an integral shroud rigidly connecting it's reflecting surfaces with the radome section. This shroud is trimmed to match the contour of the aircraft.
All Ryan antenna system has super simplicity, high reliability, and adaptability to the face available in various types of aircraft. Another characteristic of Ryan 2W-86 set is the orderly arrangement apart within functionally organized plugin subassembly. These subassemblies feature advanced X circuitry and pictorial identification apart. Hetch circuitry is also used for the interconnection of sub-assembly troublesome interconnecting tables that are minimized. This unit typifies the orderliness of the design concepts used by Ryan. The equipment is designed for rapid maintenance. Much design effort has been expended to minimize the training requirement.