A surprise

Following a morning with retinas according to Richard H. Masland, we moved on at lunch time to the workings of GPS systems.

It seems that without my having thought about it at all, the GPS system inside a fancy watch is now good to less than 5 metres. My suggestion was that this must be down to the watch being able to detect from what direction a satellite signal is coming from, rather in the way of the radio receivers of my youth which were fitted with rotating bar aerials and which were used for the off-shore navigation of small boats - by triangulating on three coastal transmitters, rather in the way of Captain Cook with his three stars, astronomical tables, sextant and chronometer.

However, later perusal of wikipedia (see reference 1) says that I was quite wrong. What actually happens is that each GPS satellite transmits suitable time and position signals, from which, with the aid of its chronometer, the receiver can compute the distance of the satellite. Given four such satellites, it can then triangulate its own position.

Which, according to the back of my envelope means being able to inspect radio signals in time intervals of the order of 1,000th of a second. Which sounds quite impressive to me.

The considerable costs of all this were paid for in the first instance, according to wikipedia, by cold war imperatives. But which later turned into a gift from the USA to the rest of the world.

Reference 1: https://en.wikipedia.org/wiki/Global_Positioning_System.