PPS signal for time correction

My name is Nori Abe from Japan. I’m creating a ntp server on PasPi 4B
with PPS time correction function. The PPS signal is provided by GNSS
sensor, Spatial Dual manufactured by ADVANCED NAVIGATION.

My problem is

(1)time offset value does NOT go below 100 microsecond
(2)ntpq -p command status shows “x” 10min after displaying “o”

I’m wondering if the PPS signal is NOT good electrically.

Here is the wave form of my PPS signal.

My concern is

(1)Vmin is not good? (It’s 0.97V, Not 0V)
(2)I have to specify rising-edge or falling-edge my RasPi sees.

Please advise me if you have any good documents which tells exact
conditions PPS signal is required.

Thank you,