Somewhere on the upper surface of every B-2 Spirit is a small round window that has nothing to do with the crew. Behind it sits a telescope that looks for stars. While the bomber flies, that system measures the angles to known stars and uses them to work out where the aircraft is, without asking anyone on the ground or in orbit for help.

A popular Instagram reel sums it up as the B-2 navigating by the stars “just like ancient sailors”, with a system an enemy “simply can’t jam”. That is essentially right. But the system is older, cleverer and more widespread than the reel suggests, and the comparison with sailors only goes so far.

Two Halves of One System

An inertial navigation system, or INS, is the backbone. It measures every acceleration and rotation of the aircraft with gyroscopes and accelerometers and integrates them, second by second, into a position. It needs no outside signal at all, which makes it ideal for a stealth bomber. Its weakness is drift: tiny measurement errors add up over hours, so the longer an aircraft flies on inertial navigation alone, the further its computed position wanders from the truth.

The star tracker is the cure. Because the positions of bright stars are known precisely for any given moment, measuring the direction to two or three of them, together with an accurate clock, pins down where the aircraft must be and which way it is pointing. Each star fix resets the accumulated error of the INS. Put the two together and you get a system that is smooth and continuous like an INS, and does not drift like one.

From a 1950s Cruise Missile to the B-2

The B-2’s system has a surprising ancestor. The bomber uses the NAS-26 astro-inertial navigation system, whose design goes back to Northrop’s SM-62 Snark of the 1950s, an intercontinental cruise missile that had to find its way across thousands of kilometres with no help from the ground. Northrop, which built both the Snark and the B-2, simply kept refining the idea. Our story of the Snark, including the one that went missing over the Amazon, is here: The Snark: America’s Nuclear Cruise Missile That Flew Into the Amazon.

The best-known descendant is the one in the SR-71 Blackbird. Its Nortronics NAS-14V2 sat behind the reconnaissance systems officer’s cockpit and looked up through a quartz window. Crews nicknamed it “R2-D2” after the Star Wars robot it vaguely resembled. According to the Smithsonian, it could fix the Blackbird’s position to within about 90 metres, or 300 feet. It held a catalogue of 64 stars and could find them in daylight as well as at night.

Seeing Stars in Daylight

The hard part is not the maths but the light. In daylight, sunlight scattered by the atmosphere drowns out the stars. Aircraft star trackers get around this with a long, narrow, heavily baffled telescope that looks at only a tiny patch of sky, and by flying high, where there is far less air above them to scatter the light. NASA engineers working on modern daytime star trackers go further, filtering out visible light and looking for stars in the near infrared instead.

That is also the system’s honest limitation. A star tracker needs a clear line of sight to the sky. Below a thick cloud deck it sees nothing, and the aircraft has to rely on its inertial platform until it gets a view again. For a high-flying bomber or a reconnaissance jet operating above most weather, that is rarely a serious problem.

Why It Cannot Be Jammed

GPS works by listening to faint radio signals from satellites more than 20,000 kilometres away. Those signals can be drowned out by a jammer on the ground, or imitated by a spoofer that feeds a receiver a false position. A star tracker listens to nothing that an enemy controls. It receives starlight and transmits nothing, so there is no signal to jam, nothing to spoof and nothing for an enemy to detect. For an aircraft whose whole purpose is to reach a defended target unseen, that is exactly the right property.

This is why astro-inertial navigation has quietly come back into fashion. Northrop Grumman’s LN-120G, fitted to the RC-135 reconnaissance fleet, combines stellar, inertial and GPS navigation in one unit, and militaries everywhere are looking for ways to keep flying when GPS is jammed, as it routinely is over Ukraine and the Middle East.

Like Ancient Sailors? Fact-Checking the Reel

“The B-2 uses stars to navigate.” True. Its NAS-26 system takes automatic star sightings to correct its inertial navigation.

“Just like ancient sailors.” Partly. The principle is the same one a navigator used with a sextant and a chronometer: measure the angle to known stars at a known time and work out where you are. Sailors measured a star’s height above the horizon by hand, usually only at twilight, when both the stars and the horizon were visible, and then worked out the fix with tables and a pencil. The B-2 does it automatically, in daylight, against a stabilised inertial platform rather than the horizon, and recalculates continuously.

“One of the few navigation systems an enemy can’t jam.” True, with a nuance: it is not the B-2’s only system. The bomber also uses GPS and its inertial platform. The star tracker is what keeps it accurate when GPS cannot be trusted.

What the reel leaves out. The B-2 is not unique. The SR-71 did it in the 1960s, later B-52s carried an automatic astro tracker that replaced the navigator’s hand-held sextant, and the idea itself was developed for a cruise missile before the B-2 was even on the drawing board.

Sources: Smithsonian National Air and Space Museum, Time and Navigation, “Nortronics NAS-14V2 Astroinertial Navigation System”; The War Zone, “SR-71’s R2-D2 could be the key to winning future fights in GPS-denied environments”; The Aviation Geek Club, on the SR-71 Astroinertial Navigation System; Wikipedia, “Northrop Grumman B-2 Spirit”; Glenn’s Computer Museum, “B-52 MD-1 Astro Navigator”; NASA, “NASA Engineer Advances New Daytime Star Tracker”.