The Mortar Fire Direction Computer was a handheld, ruggedised and weatherproof device issued to British Army 81mm mortar platoons in the early 1980s.
It was a software app (loaded via physical ROM modules/cartridges) running on an HP-41C programmable pocket calculator.
This is a virtualisation and visualisation of that device with several functions implemented. It will let you determine your grid location from the bearings of known points on a map, and simulate the process for calculating azimuth and elevation of a mortar in order to attack a target at a known grid location. If you then choose to fire, it will simulate the countdown during the mortar time of flight. All displays and responses to key presses exactly mimic the real device, except the mortar alignment calculations are arbitrary (safer!).
Fonts from free web download (http://legionfonts.com/fonts/hp41-character-set), not my work.