your supplied photo is an excellent example of a GPS antenna in perfect working order.

I've never seen S/N higher than 20
Typical is 3-4 satellites at 10-15 S/N
If all are under 10, something is wrong
And if you only see 0-3 then the antenna is bad

Remember, weather can impact GPS reception.
So take a number of readings over multiple days to determine if antenna is good or bad.