The Night Mail

[New Haven Neil]

18 hours ago, jjb1970 said:

I had a friend who did a lot of work on the Derbyshire case. Accepted wisdom for many years was that the ship split apart because of a structural design fault (frame 65?) and it was all covered up by the government and others for various reasons. When they found the wreck and did a lot more informed analysis the reason for the loss was most likely failure of the fwd hatch covers and although the ship did split around the frame in question this was most likely because of excessive stresses as the ship went down and not the cause (I caveat because in any analysis there is some uncertainty). My mate did a lot of the structural analysis and was made a target of hate, legal threats, threats of violence etc for being part of a cover up. Now if people want to disagree with findings and analysis that's their prerogative,  but knowing the people concerned I have zero doubt that they did an impartial analysis using the best analytical tools and naval architectural knowledge available.

 

As I think you know I had a vested interest in all that - having served on her, and my dad was standby 2EO as she built, and a close friend and neighbour was on board with his newlywed wife..... and I was supposed to be first trip extra fourth on her when she sunk, due to being late I went to the Staffordshire instead. I have the book that built the theory of the bulkhead failure, and it rang true to me - she had huge cracks there that were a constant worry and a lot of attention was spent trying to repair them and the unknown cause.  The cruciform pipe castings above the cargo pumps fractured too as a result of the heaving, so it all felt fine to this very junior engineer at the time.  It was quite a shock after the second enquiry, but neither myself or dad (who was still alive then) would ever direct any malice at the authors of the original conclusions, far from it - they did the best they could with what was known at the time.

Edited April 21 by New Haven Neil
speelnk

[TheQ]

18 hours ago, BR60103 said:

Reported in our newspaper today that places in the Middle East are interfering with GPS as a defensive measure. 

(hope this can be read)

 

When they parked a yank aircraft carrier near the Clyde a couple of years ago, a notice to mariners went out, GPS temporarily degraded in that area.

[Tony_S]

7 minutes ago, TheQ said:

When they parked a yank aircraft carrier near the Clyde a couple of years ago, a notice to mariners went out, GPS temporarily degraded in that area.

One thing that interested me was that GPS systems have to correct for relativistic effects as the clocks  in orbit run faster than stationary ones on the surface. 

[Dave Hunt]

Had a trip out to Dorothy Clive gardens today and enjoyed both the flora and lunch outside at the cafe. I managed a reasonable amount of walking but after just half an hour during which there was some uphill stretches my legs felt like jelly. I hadn't realised properly until now just how unfit I have become because of not doing any proper exercise for years while my spine was giving me grief. Now I've had it largely sorted, though, I'm determined to get  reasonably fit again. It'll be a bit of an uphill battle but a necessary one.

 

Dave

[New Haven Neil]

36 minutes ago, TheQ said:

When they parked a yank aircraft carrier near the Clyde a couple of years ago, a notice to mariners went out, GPS temporarily degraded in that area.

 

Still used Decca Navigator in my time - which as I'm sure you in particular know, was based on H2S radar technology IIRC!

[New Haven Neil]

Derbyshire, nee Liverpool Bridge.  Big ol gurl.

 

 

I still get survivors guilt, lost four good friends on her.

[Hroth]

40 minutes ago, New Haven Neil said:

 

Still used Decca Navigator in my time - which as I'm sure you in particular know, was based on H2S radar technology IIRC!

 

It was related to Gee/Oboe, which were hyperbolic navigation systems devised for "precision" bombing during WW2.  Decca differed by using two signal phase comparison rather than the pulsed timing of the RAF systems and was first deployed by the RN for precision minesweeping during the D Day operations.

 

H2S was an aerial bombing radar system, mainly used for targets outside the range of Gee or Oboe.

 

https://en.wikipedia.org/wiki/Gee_(navigation)

https://en.wikipedia.org/wiki/Oboe_(navigation)

https://en.wikipedia.org/wiki/Decca_Navigator_System

https://en.wikipedia.org/wiki/H2S_(radar)

 

Edited April 21 by Hroth

[jamie92208]

A quick good evening from a house where French has been the main language since yesterday lunchtime as our longest known French friends are staying. We've known them since 1996.  Last night's digestif was the last of the Glengoyne cask strength tonights was a good Glenlivet. 

 

Jamie

[Winslow Boy]

2 hours ago, Tony_S said:

One thing that interested me was that GPS systems have to correct for relativistic effects as the clocks  in orbit run faster than stationary ones on the surface. 

Yes I understood that was so. Something to do with the fact the atomic clocks are so accurate that they can't since with the ones in orbit. I don't know how they 'record' time on satellites but I assume they do in order to calculate there position etc. Perhaps one of more 'technical' minded ER's could explain it in lay man's terms. 

[AndyID]

1 hour ago, Winslow Boy said:

Yes I understood that was so. Something to do with the fact the atomic clocks are so accurate that they can't since with the ones in orbit. I don't know how they 'record' time on satellites but I assume they do in order to calculate there position etc. Perhaps one of more 'technical' minded ER's could explain it in lay man's terms. 

It's counter to human intuition, but clocks (including atoms) are actually affected by speed and gravity. While it seems obvious that a second is a second everywhere, it really isn't and that has been proved by many experiments.

[J. S. Bach]

10 hours ago, PhilJ W said:

Posted on wrong thread.

Since when has that stopped anyone before?????

[J. S. Bach]

Night Owl from the Piedmont.

[polybear]

8 hours ago, TheQ said:

When they parked a yank aircraft carrier near the Clyde a couple of years ago, a notice to mariners went out, GPS temporarily degraded in that area.

 

One reason may be to reduce the accuracy of any incoming GPS guided weapons

 

4 hours ago, J. S. Bach said:

Since when has that stopped anyone before?????

 

Not this Bear, that's for sure.....

[New Haven Neil]

10 hours ago, Hroth said:

 

It was related to Gee/Oboe, which were hyperbolic navigation systems devised for "precision" bombing during WW2.  Decca differed by using two signal phase comparison rather than the pulsed timing of the RAF systems and was first deployed by the RN for precision minesweeping during the D Day operations.

 

H2S was an aerial bombing radar system, mainly used for targets outside the range of Gee or Oboe.

 

https://en.wikipedia.org/wiki/Gee_(navigation)

https://en.wikipedia.org/wiki/Oboe_(navigation)

https://en.wikipedia.org/wiki/Decca_Navigator_System

https://en.wikipedia.org/wiki/H2S_(radar)

 

 

D'oh, I knew that really, I was tired, honest! 🙄 Picked the wrong one...should have thought about it longer as it would have become obvious by the modus operandi.  Thanks for the correction.  I always thought that the Decca lines spoiled charts!

Edited April 22 by New Haven Neil
add a t

[jjb1970]

11 hours ago, New Haven Neil said:

 

As I think you know I had a vested interest in all that - having served on her, and my dad was standby 2EO as she built, and a close friend and neighbour was on board with his newlywed wife..... and I was supposed to be first trip extra fourth on her when she sunk, due to being late I went to the Staffordshire instead. I have the book that built the theory of the bulkhead failure, and it rang true to me - she had huge cracks there that were a constant worry and a lot of attention was spent trying to repair them and the unknown cause.  The cruciform pipe castings above the cargo pumps fractured too as a result of the heaving, so it all felt fine to this very junior engineer at the time.  It was quite a shock after the second enquiry, but neither myself or dad (who was still alive then) would ever direct any malice at the authors of the original conclusions, far from it - they did the best they could with what was known at the time.

 

I always understood the sensitivities of survivors or families of those who lost loved ones and usually make allowances when I see them in the media on various issues. It's easy to criticise someone acting very unreasonably after a tragedy but it's easy to say that if we weren't affected. It's why even as an avowed japanophile (or whatever the correct term is) I never criticised the old boys in Cumbria or extended family in Indonesia who had/have a visceral hatred of Japan as while I do not agree I also recognise the reasons for it and it's not for me to tell camp survivors or those who lost loved ones in the most brutal ways to get over it. Unfortunately when the media get involved rational thought and analysis goes out of the window, and these things attract a cadre of the professionally angry and aggrieved who had nothing to do with the incident or any involved but who see it as their duty to make it personal. That's not aimed at those who want to improve things and who are passionate about safety, but having seen a few of these people it's clearly more than an interest in safety to some. Most of those involved in investigations are painfully aware of the human side of the incidents they're investigating and feel a powerful obligation to do what's right for those lost and left bereaved.

[jjb1970]

On satellite navigation, there are now quite a few systems, as well as GPS the Russian GLONASS, Chinese BDS and European Galileo all offer global coverage and there are some more narrow systems. All the ships I was on had both GPS and GLONASS systems in case one or the other went down. They were all developed by state actors primarily for non-civilian/commercial reasons but have hugely benefitted society despite that.

[Compound2632]

25 minutes ago, jjb1970 said:

On satellite navigation, there are now quite a few systems, as well as GPS the Russian GLONASS, Chinese BDS and European Galileo all offer global coverage and there are some more narrow systems. All the ships I was on had both GPS and GLONASS systems in case one or the other went down. They were all developed by state actors primarily for non-civilian/commercial reasons but have hugely benefitted society despite that.

 

Galileo has been designed primarily as a commercial system, with enhanced features for those commercial organisations willing to pay fees. 

 

7 hours ago, AndyID said:

It's counter to human intuition, but clocks (including atoms) are actually affected by speed and gravity. While it seems obvious that a second is a second everywhere, it really isn't and that has been proved by many experiments.

 

A second is a second is a second in the inertial frame of the clock. The clocks in the satellites, doing their 90-minute orbits around the Earth are moving relative to the clocks on the ground, so as seen from the ground, the frequency of their 'ticks' is blue or red shifted relative to that of the clocks on the ground - the clock going past in its satellite is like the pitch of the siren of the ambulance wizzing past you. This is all explained by Einstein's theory of Special Relativity and the appropriate corrections applied by the algorithm your receiver uses. Also, the clocks in the satellites experience a weaker gravitational pull than the clocks on the ground, being further from the centre of the Earth (the acceleration due to gravity is a bit less up there), which results in a slight shift in their tick rate. This is all explained by Einstein's theory of General Relativity and again, the appropriate corrections are applied.

 

As I'm sure you all know, the second of the SI system is defined in terms of the 'ticks' of a caesium atom - specifically the frequency corresponding to the energy gap between its two ground state hyperfine energy levels, such that this frequency is defined to be 9,192,631,770 Hz (the hertz being the reciprocal of the second, 1 s = 1/9,192,631,770 oscillations of the radiation corresponding to this energy level difference.) In order to build a primary frequency standard or clock - one realising this definition, and against which other clocks can be compared - one has to tickle the caesium atoms with microwaves at the right frequency to make them giggle. This is best done with about a million or so atoms, so that the giggling is loud enough to be heard. Also, the best results are got by tossing them up and checking that they're still giggling when the come back down about a second later - the so-called caesium fountain. (For the technically-minded, this means your microwave source has to have superb short-term stability.) If you had a bunch of caesium atoms at room temperature, they'd be all over the shop by the time they came down again, so they have to be got really cold so that they stay huddling together - within a few millionths of a degree of absolute zero. (This is done by sapping their kinetic energy using laser light.) So, instead of a single caesium atom at rest, we have a great crowd of them pushing and shoving and crying 'whoopee' as they ride the fountain, all of which gives rise to niggling little shifts in that hyperfine energy level splitting. These effects have all been studied to death, with the result that after thirty years of development, the best caesium primary frequency standards realise the SI second with an accuracy of around one part in ten to the power sixteen, or about a millionth of a gnat's whisker compared to the diameter of the Earth.

 

Anyway, the point of this digression is twofold. Firstly, the clocks in the GNSS satellites are not as accurate as this but need to be sufficiently stable to hold their tick rate between synchronisations to the timing ground stations. These, again, are not as accurate as the primary standards but are syncronised to them, via the international timescale UTC. (The complexities of which I won't bore you with here.) Secondly, the General Relativity effect has to be taken into account both within the caesium fountain itself (the frequency shift is about one part in ten to the power sixteen per metre change in height above the centre of the Earth and the fountain is about 0.3 m high) and when comparing the tick rates of these primary clocks. The UK's ones are at the National Physical Laboratory in Teddington, SW London, a few metres above sea level, while the USA's are at the National Institute of Standards and Technology's campus in Boulder, Colorado, over 1.6 km above sea level. 

 

If you are interested and in the area, I would thoroughly recommend booking a place at the National Physical Laboratory's Open Day on 20 May, when some of my former colleagues will be showing off the caesium fountain primary frequency standards. (Spoiler alert: it's all happening in a high vacuum system under several layers of magnetic shielding, so you can't actually see the atoms going up and down!)    

[Winslow Boy]

16 minutes ago, Compound2632 said:

 

Galileo has been designed primarily as a commercial system, with enhanced features for those commercial organisations willing to pay fees. 

 

 

A second is a second is a second in the inertial frame of the clock. The clocks in the satellites, doing their 90-minute orbits around the Earth are moving relative to the clocks on the ground, so as seen from the ground, the frequency of their 'ticks' is blue or red shifted relative to that of the clocks on the ground - the clock going past in its satellite is like the pitch of the siren of the ambulance wizzing past you. This is all explained by Einstein's theory of Special Relativity and the appropriate corrections applied by the algorithm your receiver uses. Also, the clocks in the satellites experience a weaker gravitational pull than the clocks on the ground, being further from the centre of the Earth (the acceleration due to gravity is a bit less up there), which results in a slight shift in their tick rate. This is all explained by Einstein's theory of General Relativity and again, the appropriate corrections are applied.

 

As I'm sure you all know, the second of the SI system is defined in terms of the 'ticks' of a caesium atom - specifically the frequency corresponding to the energy gap between its two ground state hyperfine energy levels, such that this frequency is defined to be 9,192,631,770 Hz (the hertz being the reciprocal of the second, 1 s = 1/9,192,631,770 oscillations of the radiation corresponding to this energy level difference.) In order to build a primary frequency standard or clock - one realising this definition, and against which other clocks can be compared - one has to tickle the caesium atoms with microwaves at the right frequency to make them giggle. This is best done with about a million or so atoms, so that the giggling is loud enough to be heard. Also, the best results are got by tossing them up and checking that they're still giggling when the come back down about a second later - the so-called caesium fountain. (For the technically-minded, this means your microwave source has to have superb short-term stability.) If you had a bunch of caesium atoms at room temperature, they'd be all over the shop by the time they came down again, so they have to be got really cold so that they stay huddling together - within a few millionths of a degree of absolute zero. (This is done by sapping their kinetic energy using laser light.) So, instead of a single caesium atom at rest, we have a great crowd of them pushing and shoving and crying 'whoopee' as they ride the fountain, all of which gives rise to niggling little shifts in that hyperfine energy level splitting. These effects have all been studied to death, with the result that after thirty years of development, the best caesium primary frequency standards realise the SI second with an accuracy of around one part in ten to the power sixteen, or about a millionth of a gnat's whisker compared to the diameter of the Earth.

 

Anyway, the point of this digression is twofold. Firstly, the clocks in the GNSS satellites are not as accurate as this but need to be sufficiently stable to hold their tick rate between synchronisations to the timing ground stations. These, again, are not as accurate as the primary standards but are syncronised to them, via the international timescale UTC. (The complexities of which I won't bore you with here.) Secondly, the General Relativity effect has to be taken into account both within the caesium fountain itself (the frequency shift is about one part in ten to the power sixteen per metre change in height above the centre of the Earth and the fountain is about 0.3 m high) and when comparing the tick rates of these primary clocks. The UK's ones are at the National Physical Laboratory in Teddington, SW London, a few metres above sea level, while the USA's are at the National Institute of Standards and Technology's campus in Boulder, Colorado, over 1.6 km above sea level. 

 

If you are interested and in the area, I would thoroughly recommend booking a place at the National Physical Laboratory's Open Day on 20 May, when some of my former colleagues will be showing off the caesium fountain primary frequency standards. (Spoiler alert: it's all happening in a high vacuum system under several layers of magnetic shielding, so you can't actually see the atoms going up and down!)    

Well you learn something new. As I said I knew it was something atomic but 'fountains' and 'giddy atoms' who would have thought. Can you tell science isn't really by fortay.

 

The uneducated of Manutopea 

[Dave Hunt]

I always thought that special relativity was the reason that there are lots of funny people in remote parts of the Ozark mountains and suchlike.

 

Dave

[Compound2632]

2 minutes ago, Dave Hunt said:

I always thought that special relativity was the reason that there are lots of funny people in remote parts of the Ozark mountains and suchlike.

 

No, rather the opposite. The problem in the Ozarks is that they're all in the same inertial frame and don't move around relative to one another nearly enough. 

[Happy Hippo]

3 minutes ago, Dave Hunt said:

I always thought that special relativity was the reason that there are lots of funny people in remote parts of the Ozark mountains and suchlike.

 

Dave

Or Broseley:  Those folk south of the River are something else🤣.

[Winslow Boy]

1 minute ago, Compound2632 said:

 

No, rather the opposite. The problem in the Ozarks is that they're all in the same inertial frame and don't move around relative to one another nearly enough. 

Leads to lot of 'Bobs Ur Uncle' that does.

[Dave Hunt]

I’ve just bought Jill a new iPad because her old one is now Neolithic and can’t update so more and more web sites won’t let it play. Unfortunately when I fired it up my iPhone was in my pocket so they immediately fell in love and started to share passcodes etc. I’ve now got to set about dissuading it from pursuing this love affair and become independent. Isn’t technology wonderful sometimes?

 

Dave

[skipepsi]

1 minute ago, Dave Hunt said:

I’ve just bought Jill a new iPad because her old one is now Neolithic and can’t update so more and more web sites won’t let it play. Unfortunately when I fired it up my iPhone was in my pocket so they immediately fell in love and started to share passcodes etc. I’ve now got to set about dissuading it from pursuing this love affair and become independent. Isn’t technology wonderful sometimes?

 

Dave

No is the short answer. As for Apple's resource wasting  model of capitalism the less said the better.

[PhilJ W]

1 hour ago, Dave Hunt said:

I always thought that special relativity was the reason that there are lots of funny people in remote parts of the Ozark mountains and suchlike.

 

Dave

 

1 hour ago, Winslow Boy said:

Leads to lot of 'Bobs Ur Uncle' that does.

In the Ozark's Bob's not just your uncle.