STABILITY MEASUREMENTS OF Ku-BAND SPREAD SPECTRUM EQUIPMENT USED FOR TWO-WAY TIME TRANSFER David A. Howe National Bureau of Standards

The NBS Boulder Laboratory is in the process of assembling a very high accuracy time transfer system. The system includes a 6.1 meter Ku-band satellite earth station, two transportable earth stations each with a 1.8 meter dish, and commercial modems designed for two-way timing. Elements of the facilities are described in this paper. High-accuracy timing with this equipment using the two-way time transfer technique via a geostationary satellite is d'iscussed. Phase stability measurements of ground facilities in various loop-around schemes using a satellite simulator have been performed in order to determine ultimate stability limits. Allan-variance stability plots are generated for sample times of 1 s to several days at various carrier-tonoise density (C/No) ratios. These plots are compared to theoretical limits obtained from the model of phase jitter given for the spread spectrum modem. The use of small transportable earth stations with full duplex capability provides for signal turn-around at a remote location with stationary satellites in common view with the NBS earth station. It is shown that the portability of the small earth stations readily allows a measurement of signal delays through the ground equipment including the orthomode transducer, feed, and other antenna-related items. Time delays through the various antenna parts have usually not been measured directly but have been estimated in previous papers on two-way time transfer. High-accuracy time comparisons can be performed between NBS and other Ku-band satellite earth stations via geostationary satellites. Better than 1 ns accuracy should be possible using the two-way time transfer technique. The use of transportable, calibrated earth stations makes possible accurate measurement of the signal delay of an involved earth station by means of a calibration. This work has been partially supported by the Rome Air Development Center under Contract F30602-85-0055. Contribution of the National Bureau of Standards, not subject to copyright.