Astro-Intertial Systems

[Zipper730]

I understand on a basic level that such a system basically augments an inertial navigation system by using star-tracking data to produce more accurate coordinates.

Some systems like those on the SR-71 tracked three-stars, others such as those on ICBM's tracked one star. To avoid wading into classified information, I'm curious about the following...

• While I know the system will usually lock onto a star while on the ground, how does it lock onto additional stars in the air?
• What happens if a maneuver causes it to lose track of either one of three stars, or the one star it's using?

 

[tyrodtom]

With the range of the SR-71, it never just depends on one, or just three stars to update it's inertial system. The system had about 60 stars in it's computer.

It may or may not be 3 stars at a time, but it's not going to be the same 3 stars the whole mission. It constantly switching to different stars as moves across the earth.

If you'll look at pictures of old WW2 bombers, you'll notice a clear hemispherical bubble, somewhere on the top of most of them. The navigator would take a sextant, and take star sights at night, or sun sights during the day, , those angles, and a very accurate watch ( chronometer ) would tell you where you were on the planet. Same method as used on ships for a couple of hundred years. Of course all this depended on being able to see the stars, and iD them correctly, or being able to see the sun.

The SR-71 navigational computer kept track of these star positions through a clear opening in the top of the fuselage. .At high altitudes the SR-71 operated at, you don't have to worry about clouds .

 

[Zipper730]

With the range of the SR-71, it never just depends on one, or just three stars to update it's inertial system. The system had about 60 stars in it's computer.

It may or may not be 3 stars at a time, but it's not going to be the same 3 stars the whole mission. It constantly switching to different stars as moves across the earth.

Sorry for the miscommunication, what I meant was one, or three stars at a time.

What I was curious about was the way it would switch from one star, or one set of stars, to another in mid-flight

 

[parsifal]

I don't know. however star positions are fairly easily determined manually when navigating by the use of a star globe or star chart, coupled with a set order of mathematical (or perhaps more correctly a series of arithmetical calculations) to determine your position on the earths surface. Navigators in the modern sense have been doing this stuff since the 15th century, though accurate longitude couldn't be done except when a reliable chronograph became available in the mid 18th century.

In the southern hemisphere we have no pole star, but we have a number of constellations that remain relatively stationary or close to due south. If you have the known position of three or more stars and you are able to determine the position of the horizon you can then triangulate your position on the map. .

In the northern hemisphere you have the Pole star which does not move from its declination or elevation. This provides a ready made referencing point for anything north of the equator.

The difficulty with navigation at night is that it is difficult without radar to determine the position of the horizon. That's why you normally take your fixes at dusk or predawn, selecting stars that are on the or close to the east in the mornings and the west in the evenings. You need to find the position of the horizon when getting a fix using a sextant. in an aircraft this difficult because of the a/c is at altitude, which has to be allowed for in terms of the declination and bearing of the star to the horizon, and because the aircraft is travelling in the dead of night often and cant wait for dusk or dawn to find the horizon. , A lot of aircraft navigation pre-radar was done by dead reckoning....estimating the position of the horizon, or by picking up on major geographical features like mountains or rivers. Later, emitted radio beacons allowed aircraft to vector onto a target using radio beams. when the beams intersected, it was time to drop the bombload.

When I was a trainee we had to still demonstrate proficiency in the manual systems (ie using the sextant and manual sight reductions using the almanac and a pencil with slide rule), in order to qualify for our navigation patch, but the more modern navigation systems were available and the norm. I don't claim to know exactly how they worked, but some of them I was told used the same methodologies as the sextant and chart manual systems. If they could find a star, and then the horizon, it was easy for them to determine position of the a/c. problem is to get the gizmo to do that. .

The most modern positional systems of course use satellites, and these are more accurate, easier to use and quicker than any on board navigation system using predictive modelling. The technology used to make them work is relatively simple and very reliable.

 

[Zippythehog]

The modern GPS or GNSS works similarly. It's no secret that the satellites emit a unique signal. The receiver almanac has the unique code in it and can tell distance from the satellite (based on a pseudo-random code). After receiving and fixing distance from 5 or 6 satellites, the receiver finds the point where the 5 or 6 distances intersect. That's where the receiver sits.
You can get a fix with as few as 4 however, there is no way for the receiver computer to double check the integrity of the calculations and reliability of the signals.
All requires a hyper accurate time reference.

 

[Zipper730]

I don't know. however star positions are fairly easily determined manually when navigating by the use of a star globe or star chart, coupled with a set order of mathematical (or perhaps more correctly a series of arithmetical calculations) to determine your position on the earths surface. Navigators in the modern sense have been doing this stuff since the 15th century, though accurate longitude couldn't be done except when a reliable chronograph became available in the mid 18th century.

I was merely curious how aircraft such as the V-Bombers, the B-58, XB-70, and SR-71 did it.

The modern GPS or GNSS works similarly. It's no secret that the satellites emit a unique signal. The receiver almanac has the unique code in it and can tell distance from the satellite (based on a pseudo-random code). After receiving and fixing distance from 5 or 6 satellites, the receiver finds the point where the 5 or 6 distances intersect. That's where the receiver sits.

Understood

You can get a fix with as few as 4 however, there is no way for the receiver computer to double check the integrity of the calculations and reliability of the signals.
All requires a hyper accurate time reference.

From what I remember, variables such as distance, the time it takes for the speed of light to make it to the receiver, and the change in the speed of light (has to do with changes in gravity from space to earth's surface or the receiver elevation) need to be factored in.

 

[Mike Flanagan]

The AGM-28 B Hound Dog Missile carried by the B-52 up into the Mid 50's had an Inertial guidance platform. It had an Astro tracker built into the front Nose of the Pylon that attached to the Missile to the B-52. But the Astro Tracker stayed with the pylon when the Missile was launched. It only went about 650 Miles so it only used the Astro Tracker to confirm it position be fore launch. I think it only used 1 star or the sun at a time' Wasn't a Guidance troop just a Hanger Rat and Combined Systems High Bay troop.

 

[swampyankee]

The modern GPS or GNSS works similarly. It's no secret that the satellites emit a unique signal. The receiver almanac has the unique code in it and can tell distance from the satellite (based on a pseudo-random code). After receiving and fixing distance from 5 or 6 satellites, the receiver finds the point where the 5 or 6 distances intersect. That's where the receiver sits.
You can get a fix with as few as 4 however, there is no way for the receiver computer to double check the integrity of the calculations and reliability of the signals.
All requires a hyper accurate time reference.

GPS signals include the time references. As an aside, some specialized GPS receivers, used most notably by geophysicists and seismologists use phase comparisons and can get sub-millimeter relative positions which they use for tracking the motion of tectonic plates.

 

[Zippythehog]

Sub-millimeter?! I knew the system can get a hyper accurate location fix if left in one place long enough, but, wow.