[time-nuts] Indoor GPS for GPSDO
Murray.Greenman at rakon.com
Wed Feb 18 01:24:06 UTC 2009
You might as well forget using an indoor antenna for GPSDO use. For best performance, you need an antenna which sees the top of the sky (say down to 30° elevation) really clearly, and ideally you want an antenna designed for timing applications, which favours high angle satellites.
This is quite different to navigation use, where low angle satellites give better angular calculations.
I'd say if you can't achieve 24 hours without a holdover event, you need to improve the antenna. How long it takes to perform the survey is another matter, and seems to depend on the GPSDO.
I've experience with some of the hot new receivers (such as the ublox LEA-5) with good indoor navigation performance, but even with this type, I would not expect the timing to be perfect without a good antenna in a good location. You should also be able to achieve 7 - 8 satellites in the fix most of the time, with at least 40dB average CNR.
My Samsung GCRU-D achieves these figures with no difficulty using a Novatel GPS-600 antenna and 30m of cable. Performance of my Trimble NTGS50AA is also similar, on a commercial hockey-puck designed for timing reception. The HP Z3801 has only a 6-ch receiver, and on an old Furuno marine quadrifilar antenna (designed for navigation) performs less well, and I have short holdover events every few days. All antennas have a clear view of the top of the sky.
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