# [time-nuts] Need advice for multilateration setup

Jim Lux jimlux at earthlink.net
Thu Mar 26 14:56:23 EDT 2015

```On 3/25/15 7:27 PM, Robert Watzlavick 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.
>
> 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.  The idea
> is to use a transmitter in the rocket and have 4 or more ground stations
> about a mile apart each receive the signal. Multilateration based on
> TDOA (time difference of arrival) measurements would then be used to
> determine x, y, z, and t.  With at least 4 ground stations, you don't
> need to know the time the pulse was transmitted.  The main problem I'm
> running into is that most of the algorithms I've come across are very
> sensitive to the expected uncertainty in the time measurements.  I had
> thought 100 ns of timing accuracy in the received signals would be good
> enough but I think I need to get down less than 40 ns to keep the
> algorithms from blowing up.  My desired position accuracy is around 100
> ft up to a range of 100k ft.
>

The key is that you don't need *real time* position.. a few seconds or
minutes delay is probably ok, right?

So transmit a PN code modulated onto a carrier from your rocket at some
convenient frequency that's legal.  Drive the PN shift register from
your carrier, divided down, so there's an integer number of carrier
cycles per chip.

Receive that signal and digitize it on the ground at a suitably high rate.

Post process the sampled data to recover the timing of the PN (and carrier).

To compensate for the receiver variability, simultaneously transmit a
signal with a different PN code, at the same frequency (roughly) as the
rocket's transmitter..  The receiver will receive both, but the signal
from your ground reference transmitter isn't moving, so you can use the
"non-rocket" signal as a calibration reference.

The transmitter can be very cheap.
The receiver is going to be the pricey part, depending on how it's
implemented.  A sort of "brute force" approach would be to use a USRP
and a portable PC at each receiver site.

>
```