Fantastic technology! Sounds like right out of Star Trek!
I suppose it seems more of a jump for those of us old fogeys that haven't grown up with it. Today I came across a device that uses differential spectral absorption of haemaglobin to detect brain haematomas. It looks just like a tricorder. Or is that Tricorder T? J Dave Long From: [email protected] [mailto:[email protected]] On Behalf Of Mike Borgelt Sent: Tuesday, 26 March 2013 6:09 PM To: Discussion of issues relating to Soaring in Australia. Subject: Re: [Aus-soaring] GPS logging At 03:31 PM 25/03/2013, you wrote: I've been asked how it is possible to receive weak signals if they are below background noise which is the case with GPS and other satnav systems. The answer is that you need to know what you are looking for. Each GPS or GNSS satellite sends out repeatedly a digital code unique to that satellite. So if you want to look for SV19 say, you need to know its code. The receiver picks up the code + background noise and you XOR (exclusive OR) that, a logic operation with the code itself and after some repetitions the code appears out of the signal(ie the noise averages out to zero, the signal doesn't). Now this is also how the range to the satellite from your receiver is figured out too. When you initially XOR the code with the received signal it is unlikely that you will be in the same phase as the code so if you get nothing you shift the phase and keep trying until the signal appears. The phase shift gives you a time offset which, given that the speed of light is constant gives you a range to the satellite. This is all complicated by Doppler shift, the apparent shift in the radio frequency of the signal due to the relative motion between you and the satellite. Once you are locked to a satellite you can look at how much the radio signal frequency has shifted from what you would expect from the rotation of the Earth and the motion of the satellite and derive your radial velocity to the satellite. Do this with enough sats and you get your motion relative to the Earth in 3 dimensions and your position in 3D. 4 sats is the minimum for a 3D fix. Note that the velocity is not a successive subtraction of one position from another, it is straight off the Doppler shift and is very accurate - like millimeters per second in each axis. Once you have a lock on a satellite you can then extract signals you don't know about from the signals plus noise you receive as you know that the signal is repeated and you know the frame length so again the noise averages to zero after enough repetitions and additions and the signal is apparent. You need this to know where the satellites are so your position can be calculated in your receiver. Some very, very smart people invented this system. As Arthur C. Clarke said "any sufficiently advanced technology is indistinguishable from magic". If you think the above is magic the stuff we are working with now is advanced magic. Mike Borgelt Instruments - design & manufacture of quality soaring instrumentation since 1978 www.borgeltinstruments.com <http://www.borgeltinstruments.com/> tel: 07 4635 5784 overseas: int+61-7-4635 5784 mob: 042835 5784 : int+61-42835 5784 P O Box 4607, Toowoomba East, QLD 4350, Australia
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