View Full Version : SR71 Astral Navigation ?

Genghis the Engineer
2nd Oct 2003, 01:44
Last week I was privileged to attend the SETP symposium and oral history sessions in LA, and for those who missed it - my deepest commiserations, it was a remarkable event.

During the week, we had two presentations on the SR71's development, testing and operations. At one point one of the test pilots mentioned an "astral navigation" system which he said gave ?}120ft of track at cruise (M=3.2) speeds.

So the question to the house is - what was this remarkable piece of technology, how did it work, and why did we bother with GPS if it was really so good? (I'm guessing that at least part of the last answer is continuous sight of sky, but presumably not all of it.)


(Currently out of town)

2nd Oct 2003, 02:27
One system used was and is an enhanced astral (astro [sic.]) tracker very similiar to that installed in the B-52's before inertial and GPS. Very, very, very simply put it locks onto a star or celestial pattern and tracks by angular offset which is summed with time and altitude and a few other things.
The system is needed for the SR-71 because the speed and altitude at which it operates can put it outside of the parameters needed to keep four or more satelites in "view" for GPS and DME slant range becomes essentially irrelevant.

Clouds have a highly adverse effect on its accuracy so the airlines wouldn't be interested even if it were available.

2nd Oct 2003, 02:40
A navigator in a box! ... or an electronic sextant, perhaps??

2nd Oct 2003, 02:54
Space Shuttle utilises a similar nav system. IMU combined with a star tracking mechanism

2nd Oct 2003, 05:36
AVIONIKER.........Was GPS an option for the SR71 ? me thinks not
but am prepared to be shot down in flames......

Dan Winterland
2nd Oct 2003, 18:24
We had quite a good astro navigation system in a V bomber I used to fly a while back. I't's performance depended on how much beer it had drunk the night before and whether it had new AAs in it's calculator !

The Apollo capsules used an astro-inertial system. The inertial gyros were run down while it was on a steady tradjectory and the astro tracking took over. the gyros were run up before any manouevreing.

2nd Oct 2003, 21:43
You missed my point. GPS wouldn't make sense due to the altitude and speed the plane flys enroute.
Option? GOK what the NASA and government dreamers are up to now.
I have no idea if it's currently installed. I haven't had any direct dealings for the last couple of years.

3rd Oct 2003, 06:27
Given that the GPS constellation is some 11,000 miles up, I wouldn't have thought the SR-71s altitude would have made that much difference to "satellites in view', and the ionospheric refraction errors would probably be less... the speed factor is interesting though.

3rd Oct 2003, 21:56
The GPS takes the four strongest signals. At altitudes above 70K it may be confused by having six or more with equal strength and two or more may be very low on the horizon. The system gets what it sees as an impossible solution.

3rd Oct 2003, 22:43
The astro navigation system is really the only choice for future manned spaceflight. (outside earth orbit). And can be phenomenally accurate over short distance and longer distances. (Unless said star had some anomolay causing "wobble"

(I'm a bit of a geek on this, sorry!)


7th Oct 2003, 07:19
The GPS takes the four strongest signals. At altitudes above 70K it may be confused by having six or more with equal strength and two or more may be very low on the horizon. The system gets what it sees as an impossible solution.

There's no particular reason for any of what you've just said. I have a hand held that supposedly can track 12 satellites and I've not noticed it get confused. The more satellites a GPS can see the more alternative solutions it can make.

There is a limit on altitude and speed for GPS but that is artificially mandated by the US Government to stop them being used for ballistic missiles. I suggest they might have made an exception for their own aircraft.

7th Oct 2003, 07:27
AFAIK aircraft receivers do not use signal strength to determine the optimum navigation solution - the satellite geometry is used. Satellites low on the horizon (good geometry/bad ionospheric errors) will be eliminated by the receiver mask angle. The more satellites in view, the better the integrity monitoring/FDE function.

Weight and Balance
7th Oct 2003, 09:43
All the discussion of GPS is interesting, but not really relevant for the SR-71 for most of its career. SR-71 first flight was December 1964, IIRC. 20+ years before GPS. Automated astral navigation, on the other hand, was developed for cruise missiles (like the Snark) starting in the mid fifties, and would have been a "mature" technology by 1964.