The Night Mail

[Tony_S]

12 hours ago, Winslow Boy said:

Perhaps one of more 'technical' minded ER's could explain it in lay man's terms. 

I wondered how they corrected for relativistic effects. It seems that they adjust the clock  oscillator frequency in the satellite so it gives the “correct time” as one on earth. The orbiting atomic clock experiment was suggested in the 1950s in order to test for relativistic effects. So,when GPS satellites were launched they knew they would need a correction. My physics background didn’t include big stuff like planets and satellites. Chemical physics was more concerned with tiny physics. 

[Compound2632]

22 minutes ago, Tony_S said:

I wondered how they corrected for relativistic effects. It seems that they adjust the clock  oscillator frequency in the satellite so it gives the “correct time” as one on earth. 

 

The corrections are applied in software - there's quite a bit of post-processing which results in a prediction of the correction to be applied in real time. 

[Steamport Southport]

2 hours 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!)    

 

Magnifico!

 

I wish I could understand that but I'm just a poor boy from a poor family

[Compound2632]

3 minutes ago, Steamport Southport said:

I wish I could understand that but I'm just a poor boy from a poor family

 

There are really just two rules in physics:

1. What goes up must come down.
2. What goes in must come out.

Experience shows they're of general applicability.

[Tony_S]

1 hour ago, Compound2632 said:

 

The corrections are applied in software - there's quite a bit of post-processing which results in a prediction of the correction to be applied in real time. 

I thought the clock frequency was reduced to make it tick at the same frequency as a stationary clock on earth but that software correction was applied for things like non circular orbits but as I mentioned I only recall  what I have read in popular science stuff, I never studied orbital mechanics, just tiny stuff. All those scientists and engineers make something that corrects for special and general relativity and drivers still turn left into railway lines or onto beaches. 

Edited April 22 by Tony_S

[Compound2632]

3 minutes ago, Tony_S said:

I thought the clock frequency was reduced to make it tick at the same frequency as a stationary clock on earth but that software correction was applied for things like non circular orbits but as I mentioned I only recall  what I have read in popular science stuff, I never studied orbital mechanics, just tiny stuff. All those scientists and engineers make something that corrects for special and general relatively and drivers still turn left into railway lines or onto beaches. 

 

As far as I'm aware, no physical adjustment is made to the clocks in the satellites. The frequency corrections are made in software, along with all the other corrections you mention. It's the comparison with the master clocks on the ground that enables this correction to be made. 

[jamie92208]

2 hours 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

Why just stick to the Ozarks and Brossleyyou can't forget the upper Spen Valley or Acre Road in South Leeds. 

 

Jamie

[Happy Hippo]

14 minutes ago, jamie92208 said:

Why just stick to the Ozarks and Brossleyyou can't forget the upper Spen Valley or Acre Road in South Leeds. 

 

Jamie

We just set them up, which lets you score the winning shot.

[polybear]

2 hours 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

 

Does this mean that SWMBO now has access to all your ER posts?

[New Haven Neil]

50 minutes ago, Tony_S said:

All those scientists and engineers make something that corrects for special and general relatively and drivers still turn left into railway lines or onto beaches. 

 

 

ahh, Ashland.  Go to 12.20.

 

[Tony_S]

I don’t think general or special relativity errors will need to be taken into account this afternoon as my appointment to have my hearing test is just along the A13 in Leigh and I knows where I am going. Hopefully sitting in a box being observed won’t make me into Schrödingers cat and I will be alive on exit as it doesn’t involve any random radioactive events.  I hope the test results confirm my own feelings as I turn the default volume up by 1 notch when I insert the hearing aids. The test will look at more than just overall volume and loss,at higher frequencies can be adjusted. 

[SM42]

Last time I had a hearing test, I mentioned that it was extra hard as the, I presume, cooling fan in the top of the box was rattling/ humming. 

 

"What fan?" Asked the tester. 

 

"That one," sez  I pointing to the whirling thing that seemed to have an issue with its bearings. 

 

" Oh" they said, "I can't hear it."

 

" Well I can and it was downright distracting "

 

The other distraction was my pulse.

 

I still managed to pass OK even with the pum, pum, pum,  whirr, rattle backing track

 

Andy

Edited April 22 by SM42

[Winslow Boy]

1 hour ago, Compound2632 said:

 

There are really just two rules in physics:

1. What goes up must come down.
2. What goes in must come out.

Experience shows they're of general applicability.

But not necessarily in the same direction or the same size.

[Winslow Boy]

48 minutes ago, Tony_S said:

I don’t think general or special relativity errors will need to be taken into account this afternoon as my appointment to have my hearing test is just along the A13 in Leigh and I knows where I am going. Hopefully sitting in a box being observed won’t make me into Schrödingers cat and I will be alive on exit as it doesn’t involve any random radioactive events.  I hope the test results confirm my own feelings as I turn the default volume up by 1 notch when I insert the hearing aids. The test will look at more than just overall volume and loss,at higher frequencies can be adjusted. 

Oh you must be special as when I had it done I got to sit in a dimly lit cubicle with the rather attractive audiologist. I then had to have an appointment to check my eyes.

[J. S. Bach]

4 hours 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

Don't forget West Virginia!

 

Their new state motto:"One big, happy family. Really.".

Edited April 22 by J. S. Bach

[Dave Hunt]

4 hours ago, polybear said:

 

Does this mean that SWMBO now has access to all your ER posts?

 

Thank the Good Lord, no. If she had I wouldn't be typing this due to:

a. Not being very well after being ambushed by a baseball bat or similar, and..

b. Not being able to get into the house to use any of my electronic thingies.

 

Dave

[Tony_S]

3 hours ago, Winslow Boy said:

Oh you must be special as when I had it done I got to sit in a dimly lit cubicle with the rather attractive audiologist. I then had to have an appointment to check my eyes.

When I got there the audiologist said I’m Nick, Jen doesn’t work Mondays. I said it was ok, it was my hearing that was faulty not my eyes. Then he announced the soundproof box was faulty but was able to use the apparatus they have for home visits. 

[Happy Hippo]

30 minutes ago, Dave Hunt said:

 

Thank the Good Lord, no. If she had I wouldn't be typing this due to:

a. Not being very well after being ambushed by a baseball bat or similar, and..

b. Not being able to get into the house to use any of my electronic thingies.

 

Dave

'What is all this stuff on the floor of the garage?'  Asked Nyda when she went in there, earlier this afternoon.

 

'It's the stuff you asked me to take out of the boot of your car, dearest one'.  I replied.

 

It has been quite quiet for the rest of the afternoon.

 

I'm now awaiting my next tasking order which will no doubt get me to sort the 'stuff' out, and get some boxes into the car to make the boot space tidier.

 

 

[Northmoor]

7 hours ago, SM42 said:

Last time I had a hearing test, I mentioned that it was extra hard as the, I presume, cooling fan in the top of the box was rattling/ humming. 

 

"What fan?" Asked the tester. 

 

"That one," sez  I pointing to the whirling thing that seemed to have an issue with its bearings. 

 

" Oh" they said, "I can't hear it."

 

" Well I can and it was downright distracting "

 

The other distraction was my pulse.

 

I still managed to pass OK even with the pum, pum, pum,  whirr, rattle backing track

 

Andy

Bit like my mate years ago who went for an eye test.

"Can you read the bottom line of the chart?"

"What, where it says Made in England?"

 

Or when I did my bike test.

"Can you read the plate on that blue car down there on the left, please sir?"

I replied with what I read.  The tester looked at me like I'd answered him in Norwegian.  "That blue car on the left, there", pointing at a car about three car lengths away.

I gave the right answer, then "Sorry I was looking at the dark blue one down there" (about 100 yards away).

"What, you can read that?  I probably couldn't tell you what car it was".

 

Not quite as good vision now, need reading glasses but still pretty clear.  I remember reading how Freddie Spencer, 1980s motorbike racing genius, once said how leaning into a corner he reckoned he could still resolve individual blades of grass alongside the track.

 

[J. S. Bach]

Night Owl from the Piedmont.

[pH]

Five cars carrying scrap ties on fire in CPKC train passing through London, Ontario:

 

https://x.com/jmccall54/status/1782261594501554619

 

Edited April 23 by pH

[TheQ]

21 hours ago, Compound2632 said:

 

 

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!)    

And don't expect it to look like it did when Barnes Wallace was bouncing golf balls... 

They've got a lot of very shiny newish looking buildings now.

 

Had a personal tour of some parts of the place, when I brought down some references voltage sources and reference resistors to be measured.

 

I've had the same problem with the bike cbt, me assuming the car plate was one much further way. 

Also when the audiology crew were discussing a cricket match outside the soundproof box , the box wasn't very soundproof...

Edited April 23 by TheQ

[Compound2632]

4 minutes ago, TheQ said:

And don't expect it to look like it did when Barnes Wallace was bouncing golf balls... 

They've got a lot of very shiny newish looking buildings now.

 

When I started there, in 1995, the area around what was then the main entrance was recognisable in the film and the ship tank building was still standing, though disused. (The oft hear cry of the library staff was "Oh no the ducks have got in again!") We specified our new labs in 1997 and finally moved in in 2007, by which time that specification was well out of date. (Oh the joys of the PFI process and lack of engagement between designers, contractors, and final users.) Much of the stuff I was involved in has moved again to an Advanced Metrology Lab back in the vicinity of Bushy House, close to where it had been before.

[jamie92208]

1 hour ago, pH said:

Five cars carrying scrap ties on fire in CPKC train passing through London, Ontario:

 

https://x.com/jmccall54/status/1782261594501554619

 

That is amazing.  I presume that they had brought the train to somewhere that thr fire brigade could access it. 

 

Apart from a lot of black Locos with big furnaces in them that I saw in my youth the nearest I ever got to seeing something like that was seeing a Class 47 heading north from Settle with dragging brakes on the loco. The tyres were glowing red. 

 

Jamie

[polybear]

10 hours ago, Northmoor said:

Not quite as good vision now, need reading glasses but still pretty clear.  I remember reading how Freddie Spencer, 1980s motorbike racing genius, once said how leaning into a corner he reckoned he could still resolve individual blades of grass alongside the track.

 

 

Bear recalls an interview with Ron Haslam:

 

"Why did you crash?"

 

Ron:   "The girl in the red dress f****d off...."

 

Apparently he'd been using her as his braking point marker.