Tkamiya,
Mine was only 'really' insensitive at the beginning when it was still figuring out that it was in the US rather than in Japan. After a bit of time running off of an outdoor antenna it stored the data and got a minimum of five or six sats fairly consistently after that, which is enough to maintain the time with a high level of precision. Thats why they picked the Furuno timing unit which is known for that ability.
Now my GPSDO (when its on) is running off of an antenna slapped on top of an AC unit, outside but underneath an overhanging roof with its metal gutter. It works fine there. It also works okay (not great, but it maintains lock) with the antenna indoors as long as the antenna has a good ground plane.
No, its not as sensitive as my Skytraq GPS which is much newer. That never loses lock even with a passive indoor antenna. Its incredibly sensitive.
Nor is it as sensitive as my ublox GPS which is also quite new.
Its probably around the same as my Mio with sirfstar III chip, a PND, a car GPS.
It likes to have a good sky view if possible.
Its gotten better with time, likely because it trains itself as to its surroundings, case, position, etc.
I should leave it on all the time, but haven't been because mine is still not in a permanent case.
I am probably going to encase it in an extron case I have, sharing the inside of my video distribution amplifier's case.
That seems like the best plan for now.
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There are some TruePosition GPSDO videos on YouTube now!
The first one is by YouTube user CircuitValley who also is a user here.