DR Computer (Remember that?) - Help wanted
 

If I can't get a answer here I don't know where else to look.

The Problem

One of the slides among my collection of DR computers is different from the rest. It looks like this:

http://s372.photobucket.com/albums/o...l/OddSlide.jpg

Drift lines are there, speeds are there, but the cross lines are not arcs in the normal manner, and there are those 'odd' Away-To arrows along each side. It is a two sided slide with speeds 180 - 370 on one side and 380 - 570 on the other. The maximum drift shown on the slide is 2° either side. The computer element associated with it is the Computer DR Mk 4A (high Speed version).

The Question?

What was the slide used for and, if possible, how was it used. A bonus would be to know for which systems/ops it would be used. Any indication or suggestion as to where I might continue the research would be welcome. (I tried 3456 but nothing came up in the editions available to me).

Keg

The TO and AWAY are for the Automatic Doppler TO/FROM Navigation Computers that give only cross track and to/from data in the airplot that is kept whilst the doppler system tries to keep up!

Missing Wings
 

I do apologize in advance. I am really sorry.

To me it looks like it is missing its wings.

Have'nt seen one in years, but I think it is a " Mear's slide", used to correct astro shots for acceleration errors caused by changes in speed or heading during the period of the shot.

Yep - It is indeed a "Mears Slide" . Used on the Vulcan (and I don't know what else) Invented, I think by Wally Mears - later OC 44 Sqn. During astro shots the the Captain flew, Co pilot did the timing, the Nav Radar took the shots, the AEO watched heading and speed changes and worked out corrections using said slide and the Plotter did the sums! the "TO" and "FROM" refer to corrections towards or away from the point on the earths surface below the body being shot.

Why can I remember that but not where the car keys are??

Thank you all very much for the information. This is getting most interesting. Now I want to know more. Where would you advise me to go next, RAF Museum or Cranwell?

Keg

I don't think anyone in the modern RAF will know much about astro navigation, and this is such an obscure corner of RAF history that I doubt the museum will know much either . But if you want to know more - here goes, this should kill an hour or two:

Astro navigation is based around measuring the elevation of a heavenly body, ie the angle between it and the horizon. On ships this is easy enough to do with an optical arrangement that superimposes the body on the horizon and you read off the angles. In an aircraft at night finding the horizon is not so easy so sextants were devised with built in ways of defining the horizon(tal), This was done with a spirit level like arrangement called a bubble sextant or a more modern one with a "pendulous reference". In each the operator kept the instrument level using the reference and "shot" or measured the elevation of the body. In an effort to cancel out errors these instruments did not take an instantaneous measurement but took 60 shots over a minute and averaged out the result using a clockwork device.

During the minute "shot" the aircraft obviously has to be stable. Any accelerations will act on the bubble or the pendulum to give a false horizontal and thus an erroneous elevation of the body. Fore and aft accelerations manifest themselves as speed changes during the shot and left and right errors as heading changes.

This is what the Mears Slide was designed to compensate for. You could set it with the speed and heading changes and it would tell you corrections to be applied to the measured elevation.

To plot an astro shot you had to assume a position where you would be at the required time. It would be the nearest whole number of degrees of lat and long. You then entered a lot of tables in thick books and extracted the elevation and azimuth (direction) of the body if you were at that assumed position at the calculated time. When you took the shot the difference between the calculated elevation and the measured one was called an intercept and plotted as a position line on the map as nautical miles towards or away from the assumed position (in the direction of the azimuth). The Mears Slide corrections were applied to the intercept towards or away from the body.

That would all have been easier in front of a blackboard but I hope it makes sense.

Now that is out of my brain perhaps there will be room for how to work my Christmas I Pod!

Timelord - A masterly description, but you forgot to mention the goat.

I thought about it!

I recall the days of navigators practising their astrological weirdness whilst I was on the Tin Triangle. Much muttering and consulting ancient tablets of tables and star charts at the planning stage; crystal balls, incense and pigeon entrails probably featured as well. Finally they would produce their plan and off we would go to the aircraft, with the navigators clutching their lodestones, quadrant staffs and other tools of their mysterious art. I just carried their sandwiches...:(

The more imaginative of the breed would use a 2 sextant method, which involved hopping (a challenge for our more cuddly nav radars) between port and starboard sextant. All we dumb co-pilots had to do was to count them down to the precise moment at which to start the clockwork, then advise them of the speed change through the 'shot'. With a bit of practice you soon learned which one of a series to cock up, reducing a 7 shot set to a mere 3...;)

After the athletics and clockwork were completed, various mysterious numbers would be chanted over the intercom, together with prayers to their gods of astrology. Much muttering, pencil chewing, scribbling and abacus work would then follow - and about 10 minutes later they would come to a vote and proudly announce "Turn one degree port"!

Actually, the good navigators were astonishingly accurate with astro, particularly if Doppler was available. I last encountered astro in 1979; when I came back to big aircraft after 4 years elsewhere I was amazed at how poor the young navs were at astro, compared to the V-force hairies. One day, or rather night, an ancient mariner nav instructor took the first tourist nav on our crew out to the jet at Akrotiri, to practise their black art of the occult. After about an hour, they returned to the bar to announce that we were, in fact, 500 nautical miles due east of Moscow...:hmm: However, a few pencil on beer mat calculations then followed, the error was resolved and off we all trooped to Chris'. Again!

I guess the sextants and all those mysterious books have long since disappeared, ever since GPS/LINS came upon the scene?

That said, one of the most important bits of kit on the SR 71 was an automatic astro tracker - and if it was good enough to navigate with that at Mach 3.5+......

....now there is some memories of a young Officer 'R'...and the tirals and tribulations of argunig the position of East Sale, a location (funnily enough) we would all manage to land at following 6 or so hours bumbling around the southern ocean.

For a while in the 70s I flew on Shacks with Frank Easto (sp?) - who I'm sure will be remembered by some here.
His enduring claim to fame was that he proved by Astro Navigation that Gan was misplotted on charts by a mile or two. (can't remember the exact error)
sv

Timelord

Without commenting on the knowledge of those in today's RAF you should know that in the fully amended and up to date version of the RAF Flying Manual AP3456 the whole of Part 3 of Volume 7 is devoted to Astro Navigation and Part 3 is itself divided into three sections dealing with the topic.

Quote:

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"...in the fully amended and up to date version of the RAF Flying Manual AP3456 the whole of Part 3 of Volume 7 is devoted to Astro Navigation and Part 3 is itself divided into three sections dealing with the topic."

---

I'll bet that's a best seller these days, John....:hmm:

I'm keen to find out, does any fleet regularly use astro these days??

TL

BEags

Stay away from the bookies!

Timelord

I suspect the odd one does but probably not for navigation purposes...

Timelord- why so reticent about the goat? A case of don't kiss and tell?

Side valve - I do indeed remember Frank Easto. As a mere pilot with an interest in navigation, I got him to teach me the basics of astro during an SAR detachment to Gan. The two weeks just flew by.
The other classic story of F E was on an anti submarine exercise.
Radar " Small radar contact Bearing 215 range 5 miles"
Captain "Action stations, bone domes on, 2600 revs, turning on"
Frank "Can you maintain heading, I'm in the middle of a 3 drift wind."

An essential component in Astro was AP 1629 (IIRC) the Air Almanac. It was issued every 6 months (or maybe 4) and tabulated the sunrise/sunset, SHA for Aries, the Sun, Moon, and planets. Some years ago they Almanac tables were reduced to sunrise/sunset only making it impossible to even take heading checks unless one used ephemris or a casio mini-computer that would enable you to work out accurate alt/az for any navigational body.

The E3 crews used these to enable them to conduct heading checks on the kollsman sextant. However an E3 surveillance orbit was not suitable for taking heading checks.

On the Vulcan, in the 1970s, a technique called fix-monitored azimuth (FMA) was used to correct the heading reference gyros (HRS). Basically any azimuth error could be corrected from an accurate fix. FMA could be used to justify not doing an astro heading check which of course was difficult at low-levels, below cloud, in a Nimrod.

Astro was taught at Nav School in the late 80s but probably dropped soon after.

Gentlemen,

Thank you all very much indeed. Your responses exceeded my expectations. I am indebted to you.

Regards

Keg

Mears Slide
 

Hello, I was playing on my computer and came across your question relating to a slide for the standard track and speed computer. Your correspondents were right. It is a device for correcting astro observations for acceleration errors and was invented by me in 1960 and was in use in the RAF at least until 1980. If you are still interested in moe info, I should be happy to oblige.

wamears