Add htpdate as an option for sync time
In some cases ntp is not accessible due to firewall restrictions. Time is critical for doppler correction of the satellites so we need to have an alternative to ntp.
A good alternative is htpdate which sync time from http request headers from several web servers online.
by default it uses "www.pool.ntp.org", "www.ntp.br" and "www.wikipedia.org". This service set the time to +-0.5s.