Precision Measurement 🙂
The BBC had a heartwarming story today about an ‘Ice Explorer’ satellite. The story was meant to be heartwarming because poor Professor Duncan Wingham had already built the satellite once and seen it blown up at launch. Now he watched the rebuilt version lifted to orbit in a mere 16 minutes. However my heart was warmed not by his triumph over adversity – a typical media take an any endeavour – but by the mission’s reliance on precise electromagnetic measurement. Something my own team are getting quite good at.
As far as I can tell, the Cryosat 2 measures sea ice thickness by measuring the time delay between radar pulses reflected from the top surface of sea ice, and the surface of the sea detected in between patches of floating ice. All this while traveling at thousands of kilometres per hour 700 kilometres above the Earth’s surface. The heart of this measurement is the ability to detect a time delay corresponding to radar waves traveling an additional 50 centimetres or so. In order to resolve this extra distance with a resolution of a centimetre or so, the satellite must be able to make timing discrimination of just 30 picoseconds. WOW! At a height of 700 km, a centimetre represents just one part in 70 million of the travel time. Very clever.
Very clever, but actually we have done something not too dissimilar. As part of our efforts to determine an accurate value for the Boltzmann constant we have recently worked out the diameter of a 120 mm diameter spherical resonator with an uncertainty of just ±10 namometres. This is a measurement uncertainty of roughly 1 part in 10 million. This is not quite as good as Cryosat, but this is an absolute measurement – something Cryosat doesn’t need to do. We can detect changes at level around 10 times better than this!
Our latest triumph, has been to detect the effects of the antennae we use to make our measurements! This is quite a trick. To measure the diameter we insert two small antennae (just tiny straight pieces of wire) into our sphere which send and receive microwave signals. We send out different frequencies of microwaves and work out the diameter from the frequencies at which the microwaves bounce backwards and forwards most strongly within our sphere. However all kinds of tiny defects have a small effect on the result. We can measure these, but how do we detect the effect of the probes we are using to measure with? If we remove the probes, then we can’t measure anything! The trick for achieving this is subtle and depends on a profound understanding of what is happening inside the sphere. We have been able to determine that our probes change our estimate for the radius by around 2 parts in a million – this is way more than anyone else had ever assumed. In other words, the probes we use to measure the diameter change our estimate of the diameter by around 240 nanometres. We are able to detect this and correct for it with an uncertainty of only around 10 nanometres.
I would bet money that some of NPL’s advanced metrology is somewhere inside Cryosat 2. I don’t know how or where. But I would also bet money that some of the metrology my team and I are developing in our project will also be inside something equally cool and clever in 10 years time. But at the moment I just don’t know what!