> Tom, > > > > For this to be true, you need to have equal tracking for each vehicle so > > that > > > you integrate equal amount of positive and negative shifts from each > > vehicle. > > > Better yeat is naturally to compensate each measure for the mainpart of > > the > > > Sagnac effect. If you know it is there, you can compensate for it since > > you > > > fairly well know your position and the satellites orbit well enought for > > the > > > remaining error to be negliglable. > > > > I think we're talking two different things here. > > The FMT guys -- with fixed transmitter and fixed > > receiver -- don't experience any Sagnac effect. > > Yes, we are talking about different things and I changed midflight. > But no, you are not correct that they don't experience any Sagnac effect. > They do if their transmission goes east-west direction. Their transmission path > will leave a non-zero area when projected onto the equatorial plane.
Magnus, OK, let me rephrase what I meant - yes, the FMT transmissions do experience a Sagnac effect (a time synchronization propagation delay), as would any other form of EM transmission: RF or optical. But the key thing here is that since the FMT event is a *frequency* measurement, any constant path delays cancel. Because transmitter and receiver are stationary, the Sagnac effect is constant. Any fixed phase offset drops out of a frequency calculation. And so the Sagnac effect has no effect on FMT. > If you say that it could silently be ignored I would agree, but mostly because > there seem to be some discussion about the actual frequency and stability of > their transmissions to start with. It turns out to be rather small, about 200 ns for a full 40 000 km around-the-earth trip at the equator. But no matter how great or small the effect is, it cancels when you do you frequency measurement. A good analogy is measuring a time/frequency receiver against your house standard. If you are measuring time accuracy, then the cable length makes a big difference and it has to be taken into account for accurate results. On the other hand, if you are measuring frequency accuracy any cable will do, long or short, as long as you don't change the cable during the test. > The fiber optic gyro is actually a good explanation here. The transmitter and > receiver sits VERY FIXED in the same frame. However, as the optical wave goes > through the fiber, any turning of that fiber-roll will be effected by the > Sagnac effect and much like a coil, the total area (turns x average area per > turn) will be a direct measure on how sensitive they are. Direct laser gyros > work the same. They sit in commercial airliners, tactical robots and the space > shuttles to give a few examples. The more sensitive ones has a constant shift > in them since they feel the earth turning and the most sensitive ones also > measures the resonances in that turning. Cool. What sort of sensitivity does one get out of these things? Do you need three of them; one for each axis? > So, Sagnac effects even those measurements, but for most practical parts it can > be ignored I guess. > > > For traveling clock synchronization experiments, > > yes, one does have to take Sagnac into effect if > > you're at the sub-microsecond level. > > > > The other thing -- please check your books for > > me -- is that the GPS clocks don't deal with the > > Sagnac effect at all. It's the receivers that have > > to apply the correction. > > Indeed. The receivers are the only one being able to correct for it, since it > is the receivers position (or rather, projection onto the equatioral plane) > which will determine the effect. Infact, it will be different between eached > tracked vehicle. If say Poul-Henning (which if is fairly south of me) and I > would compare clocks, we would have almost the same Sagnac effect between the > sats we track, where as if you and I would compare clock (you are mostly west > of me) we would experience very different Sagnac effects from a common vehicle. Right. The GPS receiver knows where the SV is but the SV cannot know where the receiver is. So Sagnac corrections must be done by the party with all the information: the GPS receiver. The SR and GR corrections, on the other hand are done in the SV since the altitude/velocity information is known to the SV (by design or even by dynamic measurement). The other reason the SR and GR corrections must be made in the SV is because they affect the GPS clock frequency; it's a redshift. > This is how the Sagnac effect works for us. The Kaplan book(s) just breifly > touches the subject on the level saying it is there. The Parkinson & Spilker > books goes much deeper into it with alot of nice plots and graphics spending > about 10 pages or so on the subject. I have read it before, but it was a few > years ago. > > Cheers, > Magnus Thanks, I'll look into the Parkinson & Spilker book. I re-read my sources on the Sagnac effect also when I did the Mt Rainier relativity experiment last year. Since that was a round-trip road experiment, the net Sagnac effect was zero. But during various parts of the trip it would (predicted, not measured) amount to about +/- 100 ps depending on how far west or east I was from home. /tvb _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
