From PopularMechanics by David Hambling
GPS is indispensable these days—but it’s still incredibly fragile.
GPS may have revolutionized the way we navigate, but for years celestial navigation has been undergoing a quiet revolution of its own.
The SM-62 Snark cruise missile, developed in the 1950s, featured the first automated celestial navigation system, a mechanical-electrical device that weighed almost a ton.
Then again, you don’t need much navigation finesse when you’re delivering a 3.8-megaton nuclear warhead.
Decades later, engineers built modernized celestial navigators, now about the size of a microwave oven, into the B-2 bomber and Trident missile.
While celestial navigation survived in niche applications like missiles, GPS has edged out the competition because it’s accurate and cheap.
Even with these safeguards, a GPS signal may simply not be there: There are concerns about whether the satellite system might be hacked, or the satellites themselves knocked out by an enemy or a massive solar storm.
“The best accuracy for celestial navigation with certainty is within a couple of meters,” says Benjamin Lane of the Advanced Position, Navigation & Timing Instrumentation unit at Draper Laboratory in Cambridge, Massachusetts.
“Twenty years ago, these infrared sensors were quite expensive,” says George Kaplan, a consultant for the U.S. Naval Observatory.
The rise of celestial navigation is also helped along with another piece of tech called a “phased optical array,” which does not need to be pointed at a section of the sky like a telescope does.
Lane and his team have already demonstrated a working prototype of such a sensor.
TWINKLE, TWINKLE, LITTLE SATELLITE
That sounds simple, but it isn't.
“Trex has tried infrared detection of the horizon,” says Kaplan.
Others have sensed the azimuth by the polarization of the sky, or by the refraction of starlight, or even angular motion sensors detecting the Earth’s rotation.
The precise time is vital for celestial navigation: an uncertainty of 30 seconds means a distance error of ten kilometers.
Any visible satellite can be used to navigate if its position is known, and there are now plenty to choose from.
“Starlink’s satellites are being made less reflective but still trackable,” says Lane.
The only limit is how accurately a satellite’s position is known.
Fortunately, the North American Aerospace Defense Command (NORAD) tracks many of them with high-precision radar, and can supply the data to military users.
Once these celestial navigation tools are ready for combat, they’ll come in different shapes and sizes.
And with a new handheld device being developed by Special Operations Command, even the humble foot soldier could be steering by the stars.
TRAVIS K. MENDOZA
For another, Kaplan says that even though infrared imagers work through haze and light cloud cover, they cannot see through dense, low clouds.
“Another ‘gotcha’ using LEO satellites is that in daytime the light is coming from above,” says Kaplan.
In 1980, the first portable Rockwell GPS receiver was a backpack unit that weighed 20 pounds.
In 40 years, who knows where the stars might take us.