NMEA messages necessary for ntp server

Hi, my name is Nori Abe from Japan. I’m creating a ntp server on RasPi 4B.
The ntp server receives GPS and PPS signal from GNSS sensor, Spatial Dual
made by ADVANCED NAVIGATION, and corrects its time.

Once booting up my RasPi, ntpq -p status shows

remote           refid      st t when poll reach   delay   offset  jitter
============================================================================
*SHM(0)          .GPS.           14 l    1   16  377    0.000    7.196   1.457
oPPS(0)          .PPS.            0 l   19   64  377    0.000   -2.369  10.883
+ntp-a3.nict.go. .NICT.           1 u   15   64  177   13.840   41.227  21.948

It looks like OK.

But after a while, it shows

   remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
xSHM(0)          .GPS.           14 l    8   16  377    0.000    9.534   0.708
xPPS(0)          .PPS.            0 l   26   64  377    0.000    0.085   5.913
xntp-a3.nict.go. .NICT.           1 u   11   64  377   13.936   58.650  12.450

It’s not good I think.

I know my GNSS sensor does NOT send GPGSV, which tells satellites status.
I’m wondering such missing NMEA message affect the bad status.

Could you teach me what NMEA messages does ntp server needs for correct
behavior? If you have any good documents, I’d be appreciated.

Thank you,