>From a friend: Yes, emphatically! It was established more than twenty years ago, by theory and experiment, that simple GPS receiving antennas yield position determinations with errors of many millimeters, and time-synchronization errors of equivalent magnitude (i.e., many millimeters multiplied by the speed of light), for two reasons:
(1) The locus of constant phase of the antenna's received-signal output is not spherical, even when the antenna is in an anechoic environment. In other words, the antenna has no well-defined phase-center. The apparent phase center is typically a strong function of the elevation angle, and also varies substantially with the azimuth, from which a signal is received. (2) In the real world, an antenna's received-signal output is a superposition of the desired signal (received directly from a satellite via free-space TEM propagation) and one or more undesired signals, received via reflection and/or scattering from surfaces/objects near the antenna. The strongest of these undesired signals has usually been reflected from the ground. The phase of the composite signal, i.e., the superposition of desired and undesired signal components, differs from the phase of the desired signal. The magnitude of the difference can be greater than a radian, and can vary so slowly, or so _systematically_, that time-averaging fails to eliminate its effects. An ideal GPS receiving antenna would respond to signals received directly from satellites without introducing direction-dependent phase variation; and would NOT respond to reflected or scattered signals. A highly directive antenna such as a well-designed parabolic "dish" reflector with very low sidelobes could do this for just one satellite at a time, but simultaneous observations of multiple satellites are required to cancel receiver-related errors; so the antenna that's usually regarded as ideal has a "upper-hemispheric" gain pattern. That is, its gain (both magnitude and phase) is uniform for the whole sky, but its gain is zero below the horizon. The gain must be nil below the horizon because it is practically impossible to prevent reflection from the surface of the ground, and because ground-reflection effects cannot be modeled and accounted for theoretically unless the electromagnetic properties of the ground material(s) can be characterized and the topography determined at the millimeter level. In some GPS geodetic measurements, sub-millimater accuracy has been achieved by covering the actual ground with a usefully large sheet of metal that was planar and horizontal within a small fraction of one millimeter. However, a sufficiently large and precise artificial "ground plane" is expensive and cumbersome, so it is seldom used. Artificial ground-planes having diameters less than a meter are not uncommon, but careful experiments have shown that they are not satisfactory for submillimeter work. In antenna theory there is a well-known Fourier-transform relation between an antenna's gain as a function of direction, and the spatial distribution of RF current on the surface of the antenna. It follows from Fourier-transform properties that, for the gain of an antenna to cut off very sharply at the horizon, the antenna must be spatially large. Sharp horizon-cutoff is most efficiently achieved by extending the antenna vertically (perpendicular to the horizon plane). Sharp horizon-cutoff can also be achieved by extending the antenna horizontally for all azimuths, in other words, by making the antenna in the form of a very large, circular, disk. One-dimensional, vertical extension requires less material (mass, weight, and cost) than two-dimensional, horizontal extension. The referenced "bullet" antenna you mention has small extent both vertically and horizontally; so it is effectively useless for millimeter-level work. Best, -John =============== > > You are not going to get anywhere near sub-mm levels without doing L1/L2 > measurements with a geodetic grade receiver and thermally stabilized > antenna (and receiver/cable). With a patch antenna (which is in a lot of > timing antenas) on a geodetic L1/L2 receiver you can see 1 meter errors! > _______________________________________________ > 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. > > _______________________________________________ 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.
