Electronic Weapons: Slide Rules And Sextants Survive In The Sky


December 6, 2021: In late 2021, Russian video appeared that showed a heavy bomber in flight with one of the crew using s slide rule, apparently to calculate course and/or fuel consumption rate. Most pilot training, especially for the crews of long-range aircraft, includes instruction on how to use special slide rules for such calculations in the event of problems with the electronic navigation and flight management instruments that do this automatically. For generations, ever since long-range flight became possible, manual tools were used for these calculations, along with a special bubble sextant to obtain the location of an aircraft. On the surface the original sextant is used for this but in the air, there is no fixed horizon to base these calculations. The bubble-sextant creates an artificial horizon that enables aerial navigation that shows position within ten kilometers or less. Surface sextant navigation is even more accurate and electronic navigation, especially using GPS, is accurate enough for landing aircraft.

Media tends to refer to EMP (Electromagnetic Pulse) created by a distant nuclear weapon detonation as the reason for these manual backups. EMP causes widespread and irreparable damage to microelectronics and that usually means the aircraft will have problems staying in the air, with navigation a distant second. Over the last few decades, it has become more and more difficult to build aircraft that are invulnerable to EMP. More frequently there are failures of navigation equipment for several other reasons, justifying continued training in how to revert to old-school techniques. This includes dead-reckoning and use of landmarks below.

The U.S. Air Force holds exercises periodically to test flight personnel on their ability to handle this. For example, in late 2018 the U.S. Air Force turned off GPS (satellite-based navigation) over a portion of Montana where the recently expanded Powder River training area allowed warplanes to use airspace up to 16,000 meters (51,000 feet). Ten heavy bombers (B-52, B-1B and B-2) flew in while the GPS was off and tested their skills at navigating without GPS. That means going old school via maps, manual calculations, dead reckoning, maps, radar, and visual clues on the ground. The last item gets tricky from 15,000 meters up and is especially difficult when clouds are present. Not mentioned were the use of the bubble-sextant and slide rules, but they are also part of the backup plan, especially if you are over open water.

While most warplanes still have the pre-GPS INS (inertial navigation system) equipment, long-range flights require the unjammable INS to be updated periodically to adjust for the inherent “drift” in any INS system. This can be done by returning to World War II methods of navigating over water in long-distance aircraft. That would mean using a sextant to get the current (approximate by GPS standards) position. That does not help large aircraft using INS as a backup but INS is adequate for giving an aircraft time (an hour or more) to make an emergency landing. Long range bombers do not have that option because INS becomes less accurate the longer you use it. That’s because the gyroscopes and accelerometers used have a tiny inaccuracy (drift) built in which grows over time since the last accurate update.

INS has become smaller, cheaper, and more accurate since GPS was introduced in the 1990s and, for shorter range aircraft (fighters and fighter-bombers), INS is an adequate substitute for GPS. But for heavy bombers, which sometimes must fly halfway around the world, the INS drift can grow to the point where you must use pre-GPS techniques. Before GPS there were satellite-based navigation systems for updating INS systems but these were superseded by the faster, cheaper, and more accurate GPS. Realistic training for a wartime GPS outage is difficult for long-range bombers because they can’t just turn off the GPS and practice. Commercial aircraft are everywhere these days and at all altitudes. Thus, the need for a large bit of airspace that can exclude commercial aviation for a while. During such an exercise the air force monitored the airspace with ground radar to make sure no aircraft wandered in unannounced while the GPS is disabled up there. This “no GPS” training must be done periodically to validate the training heavy bomber crews are given.

Meanwhile the navy and ground forces have also been reviving and practicing pre-GPS navigation. Since introduced in the early 1990s GPS has revolutionized movement in unfamiliar areas and nowhere was this more apparent than in the military, where so many personnel on the ground, flying aircraft and steering ships operate in unfamiliar surroundings. Yet after a decade, old methods of navigation using compass, sextant and maps began to fade away in the military. After a few years of that a growing number of commanders and their subordinates realized that those satellites could fail or be jammed and the only alternative would be to fall back on the old ways. Training was revised. Ground troops were reintroduced to pre-GPS training in which troops were taught how to move cross country using a compass and map or simply told to reach a barely visible distant land feature by using landmarks they could see. Naval academies reintroduced celestial navigation and the use of the sextant. Pilots, especially those flying helicopters, were taught the old “visual orienteering” methods.

It was found that without GPS the younger troops, sailors and pilots could still do it old school and seemed to relish the challenge. In one U.S. Navy exercise officers and sailors aboard a destroyer successfully used pre-GPS methods to guide their ship across 2,500 kilometers of open water from Japan to Guam and arrive within seven kilometers of where GPS would have taken them. That was close enough to Guam, which is 59 kilometers long, to visually navigate to the naval base and dock. Similarly, there was a recent incident in Central Asia that was caught on a cell phone video and widely distributed, showing a Kazakh Mi-8 helicopter landing on a highway and asking people in the next vehicle to show up where they were on a map. A truck driver pointed to where the city was that they were looking for and the helicopter went on its way. The Mi-8 crew were on a training exercise to see if they could navigate without GPS and back in the old days that sometimes involved landing and asking for directions.

On the ground troops are being reintroduced to the compass, paper maps and orienteering. Most managed to figure out how it worked before GPS when the joke was that the most dangerous man on the battlefield was a second lieutenant with a map.




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