standard that is used as a time reference can be coordinated through the U.S. Army
Precise Time and Time Interval (PTTI) coordinator. The U.S. Army PTTI coordinator
will obtain the calibration service required from the U.S. Observatory. To obtain this
service, send a statement of calibration requirements and location where needed to:
Director, U.S. Army TMDE Activity, ATTN: U.S. Army PTTI Coordinator, AMXTM-S,
Redstone Arsenal, A1 35898-5400.
g. Global Positioning System (GPS). The GPS is a space based radio navigation
system which provides position, velocity, and time worldwide. GPS should be fully
implemented by the mid-1990's. The system will include 24 satellites. Each satellite
will carry two rubidium and two cesium atomic standards. Each GPS satellite
broadcasts two carrier frequencies called L1 and L2 (where L1 = 1575.42 MHz and L2 =
1227.6 MHz). A GPS based counter and frequency workstation have been planned to
replace the VLF time and frequency method in use now. The system will provide higher
accuracy and better worldwide coverage than the VLF system in use.
12. Frequency Calibration System
a. Standard Substitution. Calibration can be performed when necessary by:
(1) Substituting the crystal oscillator in a counter for the quartz oscillator.
(2) Substituting an oscilloscope for the frequency difference meter (an unknown
signal can be synchronized with a known signal and the stability computed from the drift
b. VLF Signal Use. A good discussion of VLF receiver operation is contained in the
VLF receiver manual; however, the following points should be emphasized.
(1) All-daylight paths should be used for maximum accuracy (all-night paths may
be used at lesser frequency). An all-daylight path is defined as one on which daylight
be performed during diurnal shift transitions.
(2) Phase plots should be repeatable from daylight to daylight, often in parts in
1011 Proper tracking can be checked at any time by advancing or retarding the phase
serve approximately 2 to 5 s from its equilibrium tracking position and observing
whether or not the servo moves back to this position. It is recommended that the servo
be checked by both advancing and retarding. If the plots are not repeatable (i.e., have
wide variations), consider the possibility of "long way" propagation. "Long way"
propagation means that the VLF receiver receives both a signal by the shortest route
and a signal travels the "long way" around the world arriving 180 degrees from the
direction of the shortest route. Check for "long way" propagation by switching
frequencies and checking for repeatable phase plots. Fluctuations in phase plots are
also caused by solar flares, local electrical interference, and local atmospheric