> Also for GPS timing applications (i.e. the GPS receiver is in position hold > mode) a single satellite is all that's needed. Adding more satellites can > add > a failure alarm function but may not improve accuracy. In my case multipath > is > a big problem so a high elevation mask is needed.
Both one-satellite and all-in-view methods are used for time transfer. But all other things being equal you get the typical sqrt(N) improvement with multiple concurrent SV in view so it's more than just a redundancy trick. Your concerns with multipath are valid; I suspect the more serious the user the more attention is paid to the antenna design and placement (e.g., choke ring antennas, remote sites, dual frequency, etc.). > An idea: When averaging GPS 1 PPS signals the receiver switches satellites as > the currently tracked ones set so there may be a slight change in accuracy > because of the satellites being used. But changing from a satellite that's > about to set i.e. that has a lot of multipath to high in the sky sat should > make for a better 1 PPS. The traditional method of GPS common view avoids this by carefully picking which SV to track at which specific times. Thus there is no switching in the middle of any 13 minute track. I'm not sure what the all-in-view time transfer solutions do for this case. Someone can check FCS, ION, or PTTI abstracts for a paper on this. Somewhere there must be a nice plot showing measured accuracy by SV as a function of angle. This would be a worse issue, except that even since the first GPS receivers, provision is made for elevation mask angle. I'm guessing low elevation is more than just a multipath problem. The ionosphere correction may also be less accurate at low angles. Perhaps Tom Clark can tell us about this. Brooke, one cool thing you could do with your Oncore and a cesium is to change your elevation mask angle, say once a day, or day and a half, going all the way from 0 to 90 over a week or two. That single plot will tell a good story. You might guess that you will get poorer results near 0 (obstructed view or excessive multipath) and also near 90 (not enough satellites in view). But where the sweet spot is, and how many tens of degrees wide it is will be revealing. /tvb _______________________________________________ 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.
