Back in the 1960’s, pre-GPS times, the SR-71 had an astro-inertial navigation system. The system took fixes on bright stars and computed the aircraft’s position from them. It worked during daytime (!), at least at 85,000 feet. Not sure if it would work at the more mundane altitudes general and commercial aviation is operating.
A modernized version of such a system could be an alternative to quantum inertial navigation in the short term?
Peter wrote:
Not sure how cruise missiles could use that
In the same way that cruise missiles use vision for navigation. They have a magnetic map and compare the local magnetic field with what the map shows.
Searching the web gives several hits. This is a good one, although I seemed to misremember what vehicle was used – in this case an ordinary aircraft was the test vehicle.
Peter wrote:
It sounds like they are just implementing a three axis accelerometer, which should also work. Actually I wonder why one needs the yaw gyros?
If you yaw and then accelerate, an accelerometer can’t detect the yaw and accelerates you in the wrong direction.
Astro nav gives you a super accurate heading. It can be simply a template with holes which is servo-aligned with the sky; that was used by US photo reconnaisance ramjet drones in the 1960s. I don’t know how they flew to a given position but presumably they had accelerometers which by then (ex Polaris ICBM) were quite accurate. Astro is not usable at say FL400 or below.
Interesting re magnetic anomaly navigation but I would expect the accuracy to be awful. I can’t see cruise missiles using this unless they carry nukes 
I’d still like to know how the quantum system achieves pico-G acceleration resolution. Some constants in nature e.g. the ratio of electron mass to proton mass, have been measured to this sort of precision, but for normal physical stuff e.g. voltage is production-doable to about 0.01% (100ppm) or lab-doable to about 10ppm, and 10ppm is 7 orders of magnitude short.
an accelerometer can’t detect the yaw and accelerates you in the wrong direction.
I meant a three axis one. A single accelerometer is obviously useless
AFAIK all accelerometer implementations measure acceleration only along one axis. If you had one which tells you the G along any axis then adding a 3 axis yaw sensor would give you navigation.
That’s the trouble with all these “New Scientist” style popular science articles
Even a three axis accelerometer cannot detect rotation. It can MAYBE detect a shift in gravity direction (provided there is no other acceleration), but it still doesn’t work in the yaw direction. All complete inertial systems have 6 axis : 3 for acceleration, 3 for rotation (gyro).
Yes I agree. 3 accelerometers alone are no good because any rotation will screw up the solution and will not be detected. So yaw gyros are needed to detect rotation.
Good to know everybody else has been there before 
The need for gyros can also be explained from the general physical principles: an aircraft as a rigid body has 6 degrees of freedom. Even though flight dynamics equations do impose 2 (?) constraints on these, these constraints are parameterised by aircraft configuration (actual mass, thrust, positions of control surfaces, etc.) and wind, and are therefore hard to compute precisely, but can be used as approximations. If one sensor out of 6 fails, it won’t render the INS totally inoperative, but will degrade its accuracy. It may even tolerate a double failure, but the degradation will be a lot worse. On the other hand, one can improve INS accuracy by incorporating magnetometer, airspeed and barometric altitude data.
So perhaps the MEL on any aircraft flying IFR should be a sextant.😁
Attach it to a chip and some sort of AI software and algorithms, and Bob’s your uncle.
How hard can it be? Who needs GPS?
Mind you a gold edition sextant is about €11,000+ these days.🙃
Well, most VFR aircrafts already have a 1-axis accelerometer (the ball) and a combined 3 axis gyro (2 in the horizon, 1 in the heading indicator)
Those are very low grade sources though, compared with what one is trying to enhance.
