GPS needs service??
I have the same problem with a BK117, The antenna is mounted on the Hyd Cowling and the GPS comes up with "GPS needs service" With some investigatton we found that another operator had the same problem with their BK wit the antenna mounted in the same spot. They repositioned the antenna on the cowling between the exhaust diffusers pretty close down to the tail boom and this seemed to cure the problem.
I believe the garmin units can get confused with some multi blade helicopters interfering with the recieved signal from satelites, with the antenna so close to the head and blades this causes a pulsing distortion of the recieved GPS signal that causes the service fault to come up. It seems that getting as much vertical space between the antenna and blades is the go and also as far away from the hub as possible.
We have a 400 fitted to an AS365 and the antenna is mounted on the top of the fin with no problems in that install.
PM me and I will see if I can find some photos of the BK antenna mount