[time-nuts] Need advice for multilateration setup

[Magnus Danielson]

Hi,

On 03/26/2015 01:25 PM, Attila Kinali wrote:
> On Wed, 25 Mar 2015 21:27:35 -0500
> Robert Watzlavick <rocket at watzlavick.com> wrote:
>
>> I'm working on a project that I could use some advice on and also might
>> be of interest to the list.   If it's not appropriate for the list, my
>> apologies.
>
> The gods have apporved of your request. You may speak now.
> ;-)
>
>> I want to develop a tracking system for an amateur rocket that can allow
>> me to track the rocket even if onboard GPS is lost (as is typical during
>> ascent and sometimes during descent) or if telemetry is lost.
>
> Given you can synchronize the clocks of the ground stations well
> enough, then the rest is "easy". Then you can get away with having
> a simple signal generator that only needs an XO. Or you can go
> for a TCXO to make your signal processing life easier.
>
> What you need to do, is actually the same as GPS does: Create a
> direct spread spectrum signal and track it on all ground stations.
> The DSSS has the advantage over the single pulse, that it's more
> resilient against noise and interference. The disadvantage is, that
> you have to have more complicated hardware. One viable way would be,
> that you have precisly synchronized sampling systems (e.g. SDR's like
> the bladeRF which can take an external clock) and then feed the data
> to a PC where you do the heavy lifting. Then you don't need to build
> custom hardware at least.
>
> Also, if the precision by the DSSS signal is not good enough, you can
> apply various tricks from the GPS world, like carrier phase tracking, etc.

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