ARINC-424 Shorthand Lat/Long Hemispheres, from Eddie's notes.
It is an elegant idea but that isn't the point. Your FMS uses these oceanic codes to represent various points around the globe so you need to know how to decode the code. Your approach plates use the VOR/DME codes to identify some of the waypoints. Once you've understand how the points are built, you will be able to crosscheck what is on your screen with reality. Note there has been a change to the specification in the North Atlantic to allow for half-degree latitude spacing. This was first implemented in 2014 by moving the "N" a few spaces but so many pilots were misplacing the "N" that they suspended the change for a year. It looks like it is back, using an "H" but as of June 2015 the change is just finding its way into FMS coding. You should always double check the coding translates into the correct latitude and longitude. If you are assigned a waypoint using half-degree latitudes, especially so.
The specification itself comes these days from Rockwell-Collins, costs $504.00, and covers a lot more than what you as a pilot needs. You can, however, piece together the important stuff from other sources, which I've done below.
What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.
The following comes from a Gulfstream manual but it describes the ARINC 424 specification for naming oceanic waypoints. If your aircraft uses this specification, these codes should work for you. Note: most FMS do not employ a code for every single latitude/longitude combination to safe database space, just the more popular ones. The manual didn't consider half degree waypoints. More on that later.
[G450 Aircraft Operating Manual, §2B-31-20, Table 1]
Half-degree waypoints are used in parts of the North Atlantic and the ARINC-424 code has been changed a few times as a result. The latest version is based on an ICAO NAT OPS Bulletin and reported in the following Jeppesen letter.
Photo: Half-degree waypoint examples, from Eddie's aircraft.
As you can see from the photo, the half-degree inputs do indeed work. In the photo below you see the results.
Figure: Example Master Document, Oceanic, from Eddie's notes.
In the example master document we see a series of waypoints written in the ARINC 24 code: 5150N 5240N 5230N 5220N
The pilot recorded the oceanic clearance as given using the more conventional format: 51° North, 50° West, and so forth.
ARINC 424 Shorthand Radial/DME, from Eddie's notes.
[Technical Manual TM-1-1510-218-10, Table 3-15.]
|Daaab||D stands for DME arc waypoint|
|aaa stands for the radial that the fix is on from the reference VOR.|
|b stands for the letter corresponding to the distance from the reference VOR.|
Portions of this page can be found in the book International Flight Operations, Part VIII, Chapter 9.
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
Half Degree Grid Waypoints, Jeppesen letter, 30 July 2015.
Technical Manual TM-1-1510-218-10 Army C12C Aircraft Manual, Date Unknown.