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
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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