Ntp gps rs232 on alix+pfsense

[ohifra]

Hi guys , I need your help.
In the services section - > ntp I noticed that in pfsense there is the possibility to install a GPS module for the creation of a stratum 1 NTP server.
Now I pfsense 2.3 , standard version not nano , updated last night 05/04/2016 on a alix board apu1d4 .
I want to know me which GPS module take , inside the alix but also outside, in order to create the ntp server with pfsense .

I saw on the net that there are some devices that act as time server function but cost much .

many thanks in advance

ps I found this receiver rs232 on ebay
http://www.ebay.it/itm/Nautica-Ricevitore-Antenna-GPS-Evermore-SA-320-seriale-RS-232-Navigazione-RS-232-/371442035464?hash=item567babab08mMHwzCBtLTU7oD-4h-XzfmA
think will be fine ?