I have installed Ubuntu 12.10 version on my PC. I am running NTP service having NTP server as GPS.

I found that when we start NTP service by ntp start command, PC is able to sync with GPS as i get '*' symbol before GPS IP when i run ntpq -p command. This remains good for some time and then the * symbol is removed which means that PC is not synchronized to that server. Now, by running command ntpq -p it shows that all parameter are OK but as '*' is removed, slowly offset goes on increasing.

     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*192.168.100.33  .GPS.            1 u    7   16    1    2.333   23.799   0.808

     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*192.168.100.33  .GPS.            1 u   14   16    3    2.333   23.799   0.879

     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*192.168.100.33  .GPS.            1 u   11   16    7    2.333   23.799   1.500

     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*192.168.100.33  .GPS.            1 u    8   16   17    2.333   23.799   2.177

below are the last 4 ntp status when sync is lost with GPS

==============================================================================
 192.168.100.33  .GPS.            1 u    1   16  377    2.404  1169.94   1.735

     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 192.168.100.33  .GPS.            1 u    -   16  377    2.513  1171.80   0.898

     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 192.168.100.33  .GPS.            1 u   15   16  377    2.513  1171.80   0.898

Since, GPS is already available, PC never re-synchronize itself to GPS later ON. I have to restart the ntp service and then PC synchronizes to GPS and '*' symbol arrives.