On 10/2/12 3:39 PM, [email protected] wrote:
Hello All -
Here is a link that describes the GPS modulation. You do not need the
1 pps to lock the 10 MHz oscillator to the atomic clock in the satellites.
http://www.kowoma.de/en/gps/signals.htm
If you look at the block diagram you see PN code modulates the carrier at
the 1.023 MHz chip rate. This is done by BPSK modulation of the carrier
with the PN code. It can be done simply with a double balanced mixer.
This spreads the signal with PSK at the chip( i.e. code clock) rate.
Note also the modulo - 2 addition of the data to the code sequence. This
called code inversion
modulation. After de-spread of the code in the receiver - the signal is
then simple BPSK and
may be demodulated by a Costas or Squaring Loop to get at the data message.
The obtain precision frequency needed I believe the T bolt simply locks
to the chipping rate
using some form of Delay Lock Loop. It is NOT at PLL. There is no need
what ever to
deal with the 1 pps using this method. The internal 10 MHz oscillator is
controlled by this locking circuit and
is part of the code correlation loop.
That's not quite how it works.. It would work for terrestrial links
where there is no Doppler, but in the GPS case, there is significant
Doppler shift on all the signals. Since the carrier and the chips are
generated from a common source on the spacecraft (the carrier frequency
is a multiple of the chip rate, in fact), you can recover carrier and
chips at the same time.
But.. most receivers these days don't actually have an analog tracking
loop at all. They digitize the input signal (1 bit quantizer) at a rate
that makes the carrier alias down to something convenient (a few hundred
kHz is typical.. you want it far enough away from zero that Doppler
never makes it go negative). In the experimental receiver in SCaN
Testbed flying on ISS it's about 39 MHz sample rate.
Once you've got your one bit samples, you do some sort of combined
Doppler/Code phase acquisition (these days, often using an FFT), then
track both together digitally using some form of NCO. The tracking
loops for all the satellite signals aren't necessarily independent and
might be part of a Kalman filter that estimates all the observables
together.
Finally, from all that, you have an estimate of your local clock offset
and timing offset, and from that you can generate your 1pps, typically
with another NCO (with granularity of your clock rate). Since it's
unlikely that your clock is EXACTLY an even number of cycles per second,
at each second, a bit of error accumulates, until you have an whole
cycle's worth leading to the familiar sawtooth error.
That sawtooth error is predictable, of course, so you can generate a
"time error" estimate for each 1pps pulse (or, even, control a variable
delay to line it up).
The important thing is that in modern receivers, nowhere is there a
signal at the GPS carrier frequency, nor is there a signal at the chip
rate. There *is* probably a signal (with low precision) at the code
epoch (every millisecond), but it's different for each satellite signal,
of course.
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