Rainer, John is right.
Furthermore, I must just warn you against "home made" syncro.
Actually I must severely warn you about most of the low cost syncro solutions now available.
Many GPS based solutions have proven to be failing. Some stages (car racing) have even been declared null. This is so bad that anyone understands how painful it can be.
Please don't ask any name of brand which sells hardware failing the sync, I don't want to make negative advertising. Just be aware of the problem.
In order to avoid surprises, it is better to rely on specific hardware for sync, if you want or need the perfect TOD.
This is just a warning...don't try on your skin.
Others did it and won't do it ever more.
