NavList:

Thanks Dan for your contribution .

Yes, HP made wonderful Calculators.

A couple of years after the HP-35 came the HP-25 and very shortly thereafter, in late 1976 as I can recall, then came the HP-25C (Continuous memory programmable) with the same 49 steps of programming memory as the HP-25 and with the same very limited number of Storage Registers.

For Celnav, the most powerful functions then implemented were :

- The bidimensional Rectangular to Polar function, labelled g⇾P on the HP-25C and renamed R⇾P on subsequent HP models. And :

- The bidimensional Polar to Rectangular function, labelled f⇾R on the HP-25C and renamed P⇾R on subsequent HP models.

For the first time, it was possible to solve for the full navigational triangle (i.e. both Intercept and Azimuth) without recourse to the sine and cosine functions, which made programs definitely shorter with the RPN language (see attachment).

For lack of programming space on the HP-25C, the only minor flaw was that angles were to be entered as ° ' " and not as ° and decimal arc minutes.

Then came first alpha-numerical calculator with the HP-41C series, with its ultimate HP-41CX model  : 319 Storage Registers switchable into programming steps. This was huge RAM space for the time. .

Through dedicated User defined programmed sub-routines the HP-41C made it possible to implement 2 important functions in 3D Space derived from the R⇾P and P⇾R functions here-above, namely the Rectangular to Spherical R⇾S function and the Spherical to Rectangular S⇾R function.

Instead of the classical mathematical Spherical Coordinates, the generally implemented 3D coordinates are (LAT, LON, Radius Vector) for which the "elevation" coordinates vanish - i.e. are equal to zero - for all points in the "Equator". This can be turn into a significant simplification for subsequent computations.

Personal note : I have always wondered why Mathematicians keep clinging to their "classical" elevation variable reckoned from the North Pole instead of the Equator.

By the mid and late eighties, and while the original HP-41CX had its built-in limitation of 319 registers of user-defined RAM, User programmable extensions were being developed by various pioneer Companies.

Among other developers, W&W Software GmbH eventually came with a Modified Quasi-ROM  "Rambox II" 2 x 64k extension, fully User programmable which immensely enhanced the HP-41CX possibilities.

Again, having 128k of Programmable Q-ROM or equivalent was then almost unthinkable for the time.

In your list of references of HP-41C Emulators I would like to specifically mention :

- The HP-41CY emulator ported onto the HP-48GX by HrastProgrammer ( https://www.hrastprogrammer.com/emulators.htm ) where the original HP-41CX + Rambox II is enhanced with an extra over 2,000 storage registers just in place of the initial built-in 319 Register RAM limitation. To say the least, HrastProgrammer is an amazing wizard here. In a HP-48GX, with the appropriate memory card extensions, you can now have in hand virtually all the software developed for the HP-41C Series.

- The HP-41X + Rambox II emulator by Jean-François Garnier ( https://www.jeffcalc.hp41.eu/emu41/index.html ) running on Ms-Dos at incredible speeds, up to 800 times faster than the original HP-41C, which definitely overcomes one the well known HP-41C series limitations.

Antoine M. "Kermit" Couëtte