# [time-nuts] Need advice for multilateration setup

Magnus Danielson magnus at rubidium.se
Sat Apr 4 03:07:33 EDT 2015

```Hi Bob,

The actual receiver logic is that you have some sampling point in time,
the tracking phase of a channel is being sampled. As you do for multiple
channels, the relative phase of each channel is sampled.

In order to extend this phase into a pseudo-range, one needs to guess
how many integer multiples of the code there is from each GPS to the
receiver. A bunch of multiples is assumed from the orbit, as there is at
least the delay of the shortest distance, and then you can make a rough
estimate by the sub-code phase of the birds, as they hook up like a set
of clock-work gears. That gives you a first approximate guess, which
might be wrong, but as we try to make it fit, we can solve this equation
and out pops a first rough estimate, from that we can then maintain a
correct guess from then on.

For your rocket, you have a known stable situation at the launch-pad.
That cuts out the guess-work, as at that point, you can assume that
there is no multiple as your measurement nodes are within range.

Cheers,
Magnus

On 04/04/2015 05:51 AM, Robert Watzlavick wrote:
> To head off a bunch of replies - I think I stumbled upon what is being
> suggested.  To extract the pseudorange, you have to figure out the
> offset of the locally generated PN code against the one that is
> received. In this reverse GPS case, I assume each ground station would
> have to start their local PN codes at the same time?  Then you would be
> able to get the pseudoranges at each ground station and use those values
> for the multilateration equations.  You still would have an uncertainty
> of one clock cycle since the phases of the local clocks at the stations
> wouldn't be aligned but several folks have suggested ways around that.
>
> -Bob
>
> On 04/03/2015 10:12 PM, Robert Watzlavick wrote:
>> I have an amateur radio license (mostly CW/HF and some VHF/UHF
>> experience) and I've written some driver software for an IQ
>> demodulation board but I have to admit, I would have no idea how to
>> begin setting up that system as initially described by Attila and
>> expanded by you and others.  I have a rudimentary understanding of the
>> modulation schemes involved but I don't fully understand how the
>> various codes mentioned fit in. I've poked around a bit at some
>> articles on PN codes and I can see how data would be transmitted but I
>> think I'm missing something key that allows you to extract positions,
>> velocities, etc. out of the various links.  I think I have some more
>>
>>
>> Thanks,
>> -Bob
>>
>> On 04/03/2015 06:08 AM, Magnus Danielson wrote:
>>> I think this is a good idea, and it is relatively straight-forward to
>>> do.
>>>
>>> You can observe both code and carrier phase this way, given that the
>>> transmitting radio is coherent with the code generation clock.
>>> Doppler also pops out of the tracking station.
>>>
>>> A good coding-gain reduces the need for a strong transmitter.
>>>
>>> The issue might be the allowed width of the signal being transmitted,
>>> forcing the chipping rate down.
>>>
>>> Cheers,
>>> Magnus
>>
>

```