NTP behavior on Solaris.

Hello,
We have 4 NTP servers (Solaris 9). All clients are getting date/time from these servers. These servers are getting data/time from 3 GPS devices. Over past weekend, GPS license was expired on 2 GPS devices and its time went back to 1999, while time remains same. One was still fine. Due to this, all clients went back 19 years back. Immediately we added two new new GPS devices and restarted NTP daemon on all NTP servers and all clients. Below is the output of updated IPs (after issue was corrected)

time-serv1 # cat /etc/inet/ntp.conf
server 192.168.xx.xx
server 172.28.42.xx
server 172.28.34.yy

driftfile /var/ntp/ntp.drift
statsdir /var/ntp/ntpstats/
filegen peerstats file peerstats type day enable
filegen loopstats file loopstats type day enable
filegen clockstats file clockstats type day enable

time-serv1 #
time-serv1 # ntpq -p
     remote           refid      st t when poll reach   delay   offset    disp
==============================================================================
*gps-clock3.	 .GPS.            1 u  715 1024  377     3.52    1.718    2.14
+172.28.42.xx   .GPS.            1 u  697 1024  377    44.37   -0.865    1.16
+172.28.34.yy   .GPS.            1 u  820 1024  377    70.02    0.865    1.01
time-serv1 #

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Is there setting, which can be applied on NTP servers or individual clients and tell it, do not sync with bad ones, because sudden 19 years drop doesn't make sense. Why it couldn't not have synced with good GPS device and picked date from bad ones ? There was one good, out of 3.
Any advice please ?

Thanks