Jim, I agree with Bruce. You should be able to get a good fix and adequately stable performance with a modern GPS unit without a clear view to the horizon in all directions.
Before anyone grumbles - let me affirm that a good view of the sky is ALWAYS best! However, provided you know where you are to start with, you should be able to get a good reliable fix eventually, and once you have the site survey and are in position hold, it should give good timing continuously. As an example, I have a Samsung GCRU-D GPS Disciplined Reference running here at work on an INDOOR antenna pointing out through the window to the SOUTH (here at 37°S most of the satellites pass to the north), and although it frequently sees only one or two satellites, never more than five, the 1pps timing is always within 20 nanoseconds or so (max variance 720ns^2) and the 24 hour holdover prediction typically stays well within one microsecond (the spec is 7us). It also has not gone into holdover (i.e. lost fix) for more than two weeks. Needless to say this is a temporary arrangement. On this receiver I am using a Novatel GPS-600 antenna at ground level (quite literally), looking out the window of a 2-story commercial building; and only the SE corner of the sky (about 1/8 of the sky) gives C/N ratios of 35dB or more. Much of the rest of the sky is completely invisible, except from the odd spot probably caused by reflections off cars parked outside. I admit it took several hours to first fix, and the survey took several days to complete. In a more normal arrangement survey would be down to a few hours. If your hole to the sky is large enough, place the antenna as far to the south (if in the northern hemisphere) of the hole as possible, and as high as possible. Clearly an external antenna should make a big difference, and the clearing in the trees you describe should be more than adequate. I'd look at the C/N ratio of the satellites, try a different antenna if you can, and if possible plot the antenna pattern so you can work out ways to improve things. Use a good GPS diagnostic tool. It would be worth trying a quadrifilar antenna or choke ring type which gives better high angle performance. GPS for navigation works best with low angle satellites, but for timing the top of the sky works best, and you can surely see straight up! Regards, Murray Greenman ZL1BPU _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
