[dogma1138]

Outside of military uses when GNSS can be jammed INS is pointless when GNSS is available.

INS that is accurate is very expensive to build and maintain. INS that isn’t reliant on external inputs including from a magnetic compass for calibration is even more so.

[cm2187]

The SR71 was using stars to navigate!

[HPsquared]

That worked in the SR71's case because, flying high as it did, the sky above is black and you can see stars during the day.

[ampdepolymerase]

You need clear view of the sky and big lenses (you can always use more compact metamaterial lenses and infrared imaging for cloudy days but they are as expensive if not more than INS). Star charts also drift over the years and you will have the same problem as magnetic true north where the database needs to be updated regularly. Fine for airliners, not so great for skyhawks.

[adolph]

An automatic celestial navigation system for navigating both night and day by observation of K-band or H-band infrared light from multiple stars.

https://patents.google.com/patent/US7349803B2/en

In this celestial map, the bodies of the solar system are placed so exactly that those versed in astronomy could calculate the precession (progressively earlier occurrence) of the Pole Star for approximately the next 14,000 years. Conversely, future generations could look upon this monument and determine, if no other means were available, the exact date on which Hoover Dam was dedicated.

https://www.usbr.gov/lc/hooverdam/history/essays/artwork.htm...

[JKCalhoun]

I'm imagining a navigation system by trying to pattern-match the earth terrain with a downward-facing camera from the plane.

A suitably large database of satellite photos covering various conditions, day, night might work for all cases but cloudy (when the plane is above/in the clouds).

[fbanon]

Already exists: https://en.wikipedia.org/wiki/TERCOM Since the 1950s!

[JKCalhoun]

Oh yeah, contour-matching. I remember this being touted with regard to U.S. cruise missiles in the 80's, 90's.

Guess radar + countour is more reliable than camera + imagery.

(I'll crawl back to the 1800's now.)

[maxden]

They've used that landing on Mars as well, Terrain Relative Navigation.

[cm2187]

Yeah I guess that wouldn’t work during takeoff and landing. But for the most common airliners (a320 to a380 and boeing equivalent): are there clouds above their cruising altitude? By day I presume you could use the sun.

[p_l]

Some early jetliners had special cupola for star navigation, but it soon fell out of use because NDB, VOR, DME and other radio systems were easier and better.

[adolph]

Exactly what I was thinking about:

https://www.thedrive.com/the-war-zone/17207/sr-71s-r2-d2-cou...

https://www.thedrive.com/the-war-zone/41287/r2-d2-spotted-on...

[PaulHoule]

They were teaching cadets at the Air Force Academy celestial navigation in the 1980s.

This guy

https://en.wikipedia.org/wiki/Guy_Murchie

taught celestial navigation to navigators flying across the Atlantic in WWII.

[rjsw]

Operation Black Buck [1] was done using INS [2] as GNSS was not yet available.

[1] https://en.wikipedia.org/wiki/Operation_Black_Buck [2] https://en.wikipedia.org/wiki/Delco_Carousel

[roenxi]

If something unforeseen happens an the GNSS systems go down for whatever reason we might be set up for a terrifying few months of disruption. Coronavirus was bad enough, and most of the disruptions were voluntary to some extent.

[scoopertrooper]

If all the various GNSS constellations went down simultaneously we'd definitely be in the shit. Not just because we lost GNSS, but we'd also probably be fighting a war or Kessler syndrome would be in effect. Either way, losing GNSS would be just one of many problems!

[p_l]

We still have VOR network and other legacy systems. Some are no longer integrated into parts of the avionics but you can still plan routes and use them to fly. Plus Primary Surveillance Radar will see you anyway.

[bronco21016]

I wonder how long it will take to get the VOR network serviceable. Many of them are no longer maintained.

[p_l]

some dropped off maintenance, but not all. NDB is pretty much dead outside of few rare spots, though, and not everywhere had DME even at best time. But nav points used with GPS still often are based on VOR locations.

[ghaff]

And even top-grade military INS does external checkpoints where possible (although it's probably pretty accurate in any case).

[mr_overalls]

Not saying you're wrong, but. . . who actually knows what actual top-grade INS is doing these days? Those kinds of capabilities would probably be Top Secret or SCI.