Navigation by Driftmeter

The P-3 Orion was well equipped for long range, transoceanic navigation.  Redundant systems provided the accuracy and reliability required for antisubmarine warfare.

The aircraft was equipped with the ASN-42 Intertial navigation sysems.  This was a first generation system with all of the flaws of its era.  While the unit was fairly reliable, it was not accurate enough to provide the precise navigation to, say, prosecute an ASW contact or penetrate an ADIZ.  (ADIZ=Air Defense Identification Zone,  in which entering aircraft must identify themselves and location prior to entry.  Fairlure to do so mightlead to interception and a flight violation.

Another long range navigation system was LORAN (LOng RAnge Navigation).  LORAN was (and is) a land based hyperbolic radio navigation system.  The system operates on two or more pairs of stations.  Operating at 1.75-2 MHz, each station sent out precisely timed pulses   The set measures the time of arrival delay between pulses from the two stations in a pair, which is plotted on special LORAN nav charts to provide a hyperbolic "line" of position.

Reliable, fairly accurate, and easy to use under most conditions, LORAN was the primary long range navigation syste in the P-3A.  It didn't hurt that the receiver and display were located coveniently near the navigator's station.  There were some issues, though.

• Geometry of the station pairs was critical in determining accuracy.  In particular, navigating the base leg (e.g., near a line from the two stations) was particularly innacuate.  The LORAN station in the Azores (because of the island configuration) was prticularly troublesome for aircraft coming in from the US.

• During nighttime, the LORAN pulses were often reflected from the ionosphere.  This both gave the long range needed, but could be difficulty to interpret, since a single pulse couold be displayed as multiple pulses depending on how many bounces there were in the propagation path.'

Celestial navigation was of course always available, at least at night. It did require a lot of work to produce a fix of one's position.  The process usually worked something like this:

• Synchronize your chronometer with radio station WWV

• Pick a time for the actual sight

• Using that time and an estimate of position, look up the Local Hour Angle in the Air Almanac

• Using the LHA to enter HO 249 sight reduction tables. 

  • One volume "Selected Stars" has the optimal stars already picked out for an optimal fix (e.g., stars equally spaced around the horizon

  • If condictions don't allow this (clouds in a quandrant, for example), use Sight Reduction Tables to pick and of the 50- odd navigational stars

  • Carry out the sightings A celestial fix consists of three separate observations of two minutes each, separated by four minutes. 

    • Each sight consists of 

      • entering the precomputed altiude (angle above horizon) and azimuth.  

      • Check the star field for the target star.  Some navigators told me that they just shot the brightest star in the field, but this was irresponsible in my opinion.  There were handy mini-star charts depicting what the star fields shold look like if you were unfamilar

      • At the planned time, center the bubble on the star and click the start button.  Keep the bubble centered on the star for the two minutes of the observation.  THe field goes dark to signal the end of the sighting

      • Note actual altitude and enter on the sight reduction sheet

      • Repeat two more times.

      • Plot each observation, using the calculated azimuth and observed altitude.  You will get three lines of position, which will hopefully cross at a poiont, or small triangle,  This is your fix.

Celestial navigation is different during daylight.  Observing the Sun gives you only a singleline of position.  Combining this with your calculated dead recking poition, you can computer an "estimated postion".  Sometimes this is the best you can do, but may not be good enogh for some conditions, like making and ADIZ penetration.   Under some unusual circomstances youcan actually get a fix during the day.  For example, one can observe the moon ifit's in the sky.  Computation and observation is a little more complicated, though.  If its a crescent moon, you may want to center the illumated cresent in the bubble. Also, youhtave tocompensate for the fact that the moon is not an "astronomical"  by applying a correction factor called "paralax in altitude."

What to do if you are limited to sunlines and estimated positions?  Obviously,you need a wayto check the wind.   Use of a driftmeter is one such way.  The P-3 had a primitive verison of this instrument, with rudimentary options and unpressurized.