Navigational Computer, Radar, Doppler, XN-1/APN-67 US Navy

Now here's a very interesting instrument for our aeronautical navigation historians:

The XN-1/APN-67 was a development-stage (XN-1 = Experimental Navy, Series 1) navigational computer used in a Doppler radar system in the early 1950's, designed by Ryan Aeronautical, tested in the US Navy's P-2 Neptune, intended for the P6M-1 Seamaster, and eventually used during "Project Magnet" in the R5D Skymaster and the Lockheed NC-121.  

The computer measures ~8 inches wide, ~13 deep, and ~5 inches high. It is in good condition given its age. Only a few knobs function, likely requiring an electrical input.  For those familiar with the Air Position Indicator unit that appeared in B-29's and B-50's, the dial on the right is nearly identical to that unit.

We have pieced together information from the following sources about the APN-67 system: 

From SURVEY OF SELF-CONTAINED NAVIGATION SYSTEMS W. E. Frye E. V. Stearns RM-756-1 26 December 1951 Revised 20 May 1952

The Naval Research Laboratory has under development a doppler navigation system, AN/APN-67, which uses continuous wave X-band radio transmission from fixed and unstabilized antennas with computer compensation to replace gyro stabilization. A Kearfott gyro vertical is used in the com~uter as a vertical reference and the General Electric G-2 compass is used as a heading reference, the latter having an accuracy of about 0.5°. The equipment is being redesigned for production by Ryan Aeronautical Company with the expectation that it will weigh about 175 lbs. The accuracy specified for this equipment is 2 percent of the distance traveled.

From A NEW MAGNETIC SURVEY AIRCRAFT FOR THE UNITED STATES NAVY HYDROGRAPHIC OFFICE
by Henry P. S t o c k a r d and Floyd B. W o o d c o c k

Project m a g n e t , the United States Navy Hydrographic Office’s Airborne Geomagnetic Survey Program (*), was established in 1951 to provide urgently needed geomagnetic data over the world’s ocean areas. Survey operations commenced in mid-1953 using a P2V Neptune aircraft. The P2V was retired from survey use in June 1954 after having flown approximately 60 000 nautical miles of survey track over the North Atlantic Ocean. In the fall of 1955 and R5D Skymaster aircraft became operational and resumed the survey of the North Atlantic. With this aircraft the survey of the North Atlantic from the equator to 70° N latitude was completed, the Mediterranean Sea was surveyed, and approximately 20 000 nautical miles of survey track were flown over the Pacific Ocean. 
In late 1957, a WV-2 Super Constellation aircraft was assigned to Project m a g n e t . This aircraft is well adapted for long over-water flights. It is capable of flights in excess of 4 000 nautical miles and is generously endowed with such necessary crew comforts as a complete galley, tables, reclining seats, and bunks, all of which make the long and tedious survey flights more endurable. The WV-2, with its excellent range capabilities, provides access to hitherto inaccessible regions of the world.

Accurate dead-reckoning capability is provided by an AN/APN-67 Doppler Radar Navigation System. This system, utilizing headings supplied by an N1 compass, computes and indicates ground speed, drift angle, altitude, longitude, course, and distance traveled.

From FLIGHT, 12 April 1957

AN/APN-67 Navigator Ryan Aeronautical Company, San Diego 12, California. THE U.S. Navy Bureau of Aeronautics has specified AN/APN-67 for the Martin P6M SeaMaster. A continuous-wave Doppler radar coupled with a computer, it uses no ground references or aerological data, but gites the pilot continuous indications of latitude and longitude position, ground speed, ground mileage flown, drift angle and ground track. As an offshoot of this equipment Ryan have also developed a helicopter hovering aid (AN/APN-97) which presumably uses Doppler signals to tell the pilot whether or not his machine is stationary above ground or sea. The APN-67 has been used during Operation Deep Freeze in the Antarctic and during Project Magnet, which was designed to measure the direction and intensity of the earth's magnetic field. The U.S. Navy contract for APN-67 is valued at $5m.