
SR-71's “R2-D2” Could Be the Key to Winning Fights in GPS Denied Environments - loriverkutya
https://www.thedrive.com/the-war-zone/17207/sr-71s-r2-d2-could-be-the-key-to-winning-future-fights-in-gps-denied-environments
======
ckozlowski
Neat article. The Pentagon has been aware of this for a while now (though as I
understand, the Navy only recently brought back celestial navigation into it's
curriculum) but what's neat to me is how we might have had say, one backup
method for navigation before falling back to manual methods (usually INS,
which the U.S. was a leader in). But modern computers allow for pulling
together a vast array of sources like the NAVSOP system described.

One method I thought was neat: I play a lot of military flight sims, with my
favorite aircraft being the AJS-37 Viggen from the 1970s. A lot of aircraft of
this period used TACAN for navigation supplemented by INS for missions over
friendly territory. But not the Swedes. They used INS, with fixes being made
by the pilot using it's ground radar to identify features on the ground that
would correspond on a map. The idea was that you'd program your flight plan
into the computer, with your waypoints being say, a bit of land that jutted
out on a coastline, or a bridge, or a tiny lake. When you're flying, as your
approach the feature, there would be a little "+" where the feature should be,
which may or may not have drifted. The feature would be visible on the radar,
and since the pilot knew it had to be centered on it, he would correct the
waypoint and skew it back onto the feature it should be on. That would then
update the rest of the waypoints in the system, correcting the drift.

This is modeled in the game, and so when I'm going to hit a target, I'll have
a nav fix some 20-30km from the target as the actual thing I'm hitting might
not be identifiable on the radar. But my INS will be corrected just shy of the
target, and I can be reasonably sure it won't drift too much when I arrive in
the area. We can hit points in darkness or poor weather with no visibility, no
GPS, no night vision. It's not fantastic, but it's reasonably capable, and
requires no outside input.

The Swedes later added TERCOM to the jet in the 90s (same thing early models
of the Tomahawk used to navigate) which means the INS drift is now largely
corrected automatically, but if this for some reason isn't working (in the
game you can perform a TERCOM fix anywhere, but in real life the amount of
ground maps that could be stored was limited, so if you were _wildly_ off
course it wouldn't be able to match where you were), we still have the radar
nav fix backup.

Neat stuff. =)

~~~
e12e
I now see spec op teams (farmers and fishermen) who's job it is to change
landmarks with dynamite and diggers...

~~~
ahartmetz
That sort of did happen. In WW2 they used soot to cover lakes which apparently
were useful for night navigation because they reflect moonlight.

------
brownbat
People often focus on the potential of quantum computing, but generally focus
far too little on the potential of quantum sensing.

We revolutionized the precision of timekeeping with atomic clocks. Quantum
inertial sensors could precisely track your location with very little drift
and no satellite constellation maintenance.

[https://www.newscientist.com/article/mg22229694-000-quantum-...](https://www.newscientist.com/article/mg22229694-000-quantum-
positioning-system-steps-in-when-gps-fails/)

~~~
DoctorOetker
sych a device would still require synchronization, it promises lower drift,
but to eliminate drift you will always need to recallibrate to synchronize
with reference points (stars, beacons, GPS, ...)

~~~
sgt101
I wonder what kind of improvement you can get by using several of the devices
at once? Could some sort of consensus monitoring at least tell you what kind
of margin of error you are working with ? I guess for some applications being
100m out is fine (for example an Astute in open ocean), but getting it that
wrong in, for example, the Med or the Sound of Skye is probably a problem.

~~~
planteen
Standard error decreases by the square root of the number of samples. So an
order of magnitude improvement would need 100 sensors!

~~~
mattkrause
That’s true when averaging together iid samples, but you could probably do
something smarter if you know about the likely errors and the sensors’
covariance. For example, if three of the four sensors agree, you should
probably completely disregard the fourth, rather than averaging it in.

~~~
robotresearcher
> you could probably do something smarter

Assuming measurements are independent and errors are Gaussian, the Kalman
filter is this smarter thing. Variants exist for nonlinear systems.

[https://en.wikipedia.org/wiki/Kalman_filter](https://en.wikipedia.org/wiki/Kalman_filter)

------
Animats
The USAF would prefer more accurate inertial systems. Modern bombing runs are
often near the ground, not at high altitude where the stars are visible.

DARPA is working on this.[1][3] They're trying to get MEMS gyros up to
navigation grade, and funding work on something called "single atom
interferometry".[2]

[1] [https://www.darpa.mil/program/adaptable-navigation-
systems](https://www.darpa.mil/program/adaptable-navigation-systems) [2]
[http://web.stanford.edu/group/scpnt/pnt/PNT07/Presentations/...](http://web.stanford.edu/group/scpnt/pnt/PNT07/Presentations/14.%20Kasevich_PNT-
Symposium.pdf) [3] [https://www.darpa.mil/program/micro-technology-for-
positioni...](https://www.darpa.mil/program/micro-technology-for-positioning-
navigation-and-timing)

------
jessriedel
Of course, celestial navigation doesn't work in the daytime. It also seem
weird to write an article aimed at readers who have never heard of it, but not
mention the existence of terrain contour matching, which is widely used on
cruise missiles and works any time of day.

[https://en.m.wikipedia.org/wiki/TERCOM](https://en.m.wikipedia.org/wiki/TERCOM)

~~~
the8472
> Of course, celestial navigation doesn't work in the daytime.

Even with a simple sextant you can use the sun. And the SR71's system did
operate during daytime using starlight.

~~~
ams6110
I was wondering about that. I guess at the altitudes the SR71 operated at, at
least the brighter stars would be visible.

~~~
the8472
They don't have to be visible to the human eyes, just visible to instruments.
With band filters and a narrow field of view you can get a stronger signal
that sticks out from the the approximately uniform sky background. I mean the
altitude helps, but it's not necessary.

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dingaling
I don't see why they have to reach back and summon the mystic name of the Habu
when astronav is still flying on the B-2 and Trident SLBM. It's not as if we
have fogotten that the technology exists, it's just that GPS is non-mechanical
and more deployable, cheaper and robust.

Side project I've wanted to do for years: smartphone AR app that you point at
the night sky and it does the star recognition and calculations automatically.
But I've no illusions that it would be much less practical than just using the
onboard GNSS receiver.

~~~
sansnomme
The issue is light. Your typical smartphone camera in its default package
simply cannot get enough light to create a stars-against-dark-background image
regardless of how much machine learning and signal processing you throw at it.
It's much easier when you have a massive roof mounted lens. There are ways to
improve gain and boost the optical signal but there are limits to what's
possible in a smartphone without hardware additions or modifications (even
with dual cameras like modern flagships).

~~~
joshvm
You could probably do it with a Pixel (or something else with night sight) and
astrometry.net.

~~~
sansnomme
Doesn't work, unless you have a tripod and use a significant exposure time.
Perhaps. It's nowhere near "AR" levels. If you are going to go that far, save
yourself some trouble and get a proper astrophotography setup. Also,
astronomy.net is overkill for this.

~~~
joshvm
Not for AR perhaps, but on a phone is doable.

See for example (not Night Sight though):
[https://ai.googleblog.com/2017/04/experimental-nighttime-
pho...](https://ai.googleblog.com/2017/04/experimental-nighttime-photography-
with.html)

~~~
sansnomme
Precisely as I said. If you are going to get a tripod rig and long exposure
then might as well save some effort and just do a full astrophotography system
(star trackers are cheaper than you think) instead of trying to squeeze more
light from tiny phone cameras.

Fwiw the OP's point of reference in the child post is Google Sky Map. The
Pixel's AI post is so far from the UX required for AR that there's honestly
not much point to it. If you are going to do night time photography, go get a
lens or at least a photomultiplier. The Pixel's AI camera is a nice gimmick
for improving nighttime photography but using it for AR live celestial
navigation is a bit like key chording a Gameboy to control a XBox.
Theoretically usable given enough patience and time but way out of the horizon
from its intended UX. I have tried out the Night Sight before and it's nowhere
close to Point and Shoot for astrophotography.

You can build a celestial navigation system pretty easily for e.g. your car.
You don't even need a lot of computational power. A Raspberry Pi would be
overkill but considering how cheap the RPi Zeros are, might want to consider
using one.

------
tomohawk
Before the days of GPS in the cockpit, a relative of mine was a 747 captain
doing routes across the Pacific. On one flight, all of the gyros went offline.

He had been trained in the US Air Force in celestial navigation, so he was
able to use that to get the aircraft within range of a beacon, and land
safely.

~~~
gcb0
wouldn't a simple compass have sufficed if they weren't off route?

~~~
gonzo41
There'd probably be too much metal / electrical interference for a regular
compass to work.

~~~
gruez
Planes don’t have built in compasses?

edit: yes they do
[https://en.wikipedia.org/wiki/Flight_instruments](https://en.wikipedia.org/wiki/Flight_instruments)

------
inetsee
<rant> This is one of the most obnoxious websites I have ever encountered. It
popped up an ad covering the content. I clicked on the close ad "x" like I
always do. Then another box popped up asking if I never wanted to see this ad
again, and I replied "yes". Then another box popped up asking why I didn't
want to see this ad. I replied, and another box popped up saying the ad had
been closed by Google. That box remained over the content for 3 or 4 seconds.
That box eventually faded, but literally 2 seconds later, another ad popped
up, covering the content, and I clicked the back button. I will never click on
a story from that website again. </rant>

~~~
inetsee
<unrant> I must confess that when I went to the website containing this
article, I was using a second computer that I rarely use, which did not have
an adblocker installed. My primary computer has Firefox Privacy Badger
installed, and when I went back to the article, using my adblocked machine,
the experience was much less obnoxious.</unrant>

What I HIGHLY recommend now is that if you don't have an adblocker installed,
do so. Privacy Badger, UBlock Origin, etc, whatever you prefer. I still see
ads, but the experience is infinitely less obnoxious.

~~~
darkpuma
I'd like to point out that with uMatrix set to block _all_ javascript,
including first party, the article still renders just fine. The only thing
missing is the youtube embeds, which are replaced by hyperlinks to the youtube
videos.

I find this to be the case more often than not. Most people here would
probably be surprised how often javascript is totally unnecessary.

------
KaiserPro
Now that storage and processing power are cheap and plentiful, there are a
range of options for navigation

1) using visual keypoints ala [https://scape.io](https://scape.io)

2) a much older idea, used in cruse missiles: radar contour mapping.

3) decca or loran

3.5) passive radar

4) inertial dead reckoning(sensors are much, much better nowadays).

5) laser painting way points.

6) multi spectral, multi viewpoint cameras with slam/visual odometery (with a
database of coast lines for pinpointing land fall.)

or better yet, a combo of all of them. The more data points the better.

------
caycep
IT's funny how my understanding of warfare changes over time. As a youngling,
it's all about sleek shapes and speed and boom boom (that may be a criticism
of how society raises men as well).

Now, it's more about how one side's nerds outthinks the other side in terms of
sensors, data, etc. Even in the macho world of fighter planes (i.e. JSF and
how everyone is thinking of detecting signals vs. hiding signals).

It's a weird juxtaposition - the techworld and silicon valley's general ethos
about changing the world, democratizing tech, etc, vs. nerds coming up w/ ways
to better kill each other...

------
ycombonator
I have always wondered which discipline is responsible for designing the rigs
that various components are bolted on later ? Are there any online resources
to learn this craft
?[https://imagesvc.timeincapp.com/v3/foundry/image/?q=60&url=h...](https://imagesvc.timeincapp.com/v3/foundry/image/?q=60&url=https%3A%2F%2Fs3.amazonaws.com%2Fthe-
drive-staging%2Fmessage-editor%252F1514914884271-a19700234000d4.jpg)

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exabrial
Whenever someone asks me to explain how GPS works, I give the example given in
the article: Sailors used to track the position of stars in the sky. Since
stars are hard to see during the day, and you can see them through walls, we
put some artificial stars in the sky that we can identify easy and we put
enough of them there's always a few above you.

~~~
rocqua
Not the best of analogies.

With stars, you measure angle above the horizon and compass heading of the
star. With GPS you are measuring time-delay of a signal. Those are two very
different ways of navigating.

------
dproblem
Summary: You an mix the coordinates obtained from celestial navigation (stars)
with signals of opportunity (Electromagnetic sources like FM) to find the
position without GPS.

Problems: Anytime a military advertisement happens (LN-120G Stellar-Inertial-
GPS from Northup G.) - if you don't see the total price of the unit, know that
it was a waste of money and resources.

------
DoctorOetker
does anyone know if astro navigation systems deduce time from star transits
(or rather generalized star transits, so they don't need to actually be
momentarily collinear, i.e. a database of stars with their distances
included)?

~~~
joshvm
The SR-71 carried a clock accurate to a hundredth of a second [1]. It used
that, combined with current observed locations of the brightest stars in the
field of view to determine location (with reference to a catalogue).

More info here:
[https://books.google.co.uk/books?id=6svmtOFa1JIC&pg=PA65&lpg...](https://books.google.co.uk/books?id=6svmtOFa1JIC&pg=PA65&lpg=PA65&dq=sr-71+chronometer+accuracy&source=bl&ots=pwGcwkO8ZI&sig=ACfU3U3YySJdhfKYta20kOoI6LyXU72nrw&hl=en&sa=X&ved=2ahUKEwjt-u6mpqrhAhUTsHEKHXKcCFkQ6AEwCnoECBsQAQ#v=onepage&q=sr-71%20chronometer%20accuracy&f=false)

What do you mean by a transit in this case?

~~~
DoctorOetker
it is possible to deduce location of the earth on its orbit around the sun,
and hence the time by star transits:

[https://en.wikipedia.org/wiki/Transit_instrument](https://en.wikipedia.org/wiki/Transit_instrument)

consider for example 2 very distant stars, and a closer star in the middle,
and all of them in a plane perpendicular to the ecliptic. as the earth
rotates, the closest star seems to move left/right with respect to the
"vertical" defined by the more distant stars. from such a star transit you
know the earth has just passed through the plane defined by these stars, and
hence know the earth's position, hence the time.

in the past catalogues for transits were optimized for human use, i.e. close
to each other, ... but a modernized i.e. software solution should not need
such proximity, and could make use of all the visible stars to make a more
accurate assessment.

I was kind of asking if there were any free open source software, it would be
kind of neat to have an emergency laptop (with some optics) loaded with
openstreetmap and astronomical positioning software, ...

~~~
rocqua
I might be off here, but it seems like resolution would be a real issue.

The variation you are looking for has a period of 1 year. To get hour accuracy
out of that you'd need to be able to distinguish 356*24 different relative
positions of these stars. It seems to me like camera's aren't going to be
supplying that kind of data.

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mycall
Boots on the ground could use laser communications to satcom for coordinate
corrections, bypassing the locale drift, or better yet, retransmitting over
the spoofed signal with higher strength.

~~~
nradov
It's tough to align a laser precisely enough to reliably hit a satellite. The
necessary equipment is too heavy and fragile for ground troops. Plus it
doesn't work through clouds.

------
paulsutter
GPS signals aren't cryptographically signed. It's a bizarre oversight since it
could prevent spoofing. I guess it's just another example of the tradition for
zero-security software.

If nothing else, the current interest in cryptocurrency will train a new
generation of developers to trust no message.

~~~
swixmix
This exists for the military ->
[https://en.m.wikipedia.org/wiki/Selective_availability_anti-...](https://en.m.wikipedia.org/wiki/Selective_availability_anti-
spoofing_module)

Also, there are other methods to find position data.

~~~
paulsutter
is there any reason not to sign all gps signals?

~~~
coldacid
How often would keys be rotated, and can that be done from the ground? And
then, the nightmare of ensuring that updated certs for the keys get rolled out
to all the devices so they don't all panic and say that the GPS isn't working
at all anymore. For that, cell phones would probably be the easiest despite
jerkass behaviour of manufacturers and service providers. Embedded GPS
updating would be an absolute nightmare.

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s4vi0r
Luv too celebrate imperialism, very cool

~~~
arama471
What?

