[time-nuts] GPS Modulation and 10 MHz Delay Lock

paul swed paulswedb at gmail.com
Wed Oct 3 13:38:44 UTC 2012


I think it was this thread.
But the actual chip rates derive from a 10.22999999543 clock.
We call it 1.023 Mhz but its not. In time-nuttery it matters.
Doesn't that make the use of the signal a bit messy?
Regards
Paul
WB8TSL

On Tue, Oct 2, 2012 at 10:30 PM, Jim Lux <jimlux at earthlink.net> wrote:

> On 10/2/12 3:39 PM, johncroos at aol.com 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<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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