If the GPS system ever fails, will the Photographic Zenith Tube be a useful alternative in the measurement of rotational time?
During the months leading to Operation Desert Storm, US Navy navigation personnel patrolling the Persian Gulf at the time were ordered to take sextant measurements every 6 hours at night to determine if the then Iraqi strongman Saddam Hussein had acquired technology to screw up the GPS navigation system. The constellation of 24 navigational satellites in geosynchronous orbit that has since became an indispensable tool in navigation with an error rate of plus/minus 30 feet or less. Are there also other tools – as in astronomical time or rotational time determining tools that we can use to check if somebody’s screwing up with the proper functioning of our GPS system?
The Photographic Zenith Tube – or PZT – is one of the more accurate tools used to measure rotational time. It was designed by Frank Elmore Ross back in 1909 and was adopted for time determination at the US Naval Observatory in 1934. The PZT is a telescope of 15-foot focal length and 8-inch aperture pointing directly to the zenith. A small photographic plate is placed about ½-inch below the lens. A basin of mercury located at half the focal length below the lens reflects the light from the stars, which comes to a focus at the plate. The lens and photographic plate can be rotated 180-degrees as a unit about a vertical axis.
Four exposures of a star were taken, of 20 seconds duration each, with the lens in alternate positions. A motor drives the plate so as to track the star and compensate for its diurnal motion and time pulses are generated by the moving plate carriage. By means of a chronograph it is possible to relate the impulses to some particular clock. The photographic plate is developed and the images are measured. By combining these measures with the chronograph readings, it is possible to determine what the clock read when the star was on the meridian.