Two-Way Time Transfer Modem

Ivan J. Galysh, Paul Landis

Abstract:	NRL is developing a two-way time transfer modem that will work with very small aperture terminals (VSAT), commercial satellites, and an atomic clock. The two-way method has been chosen because of its performance and insensitivity to the position of the receivers and satellites. Precision, stability, accuracy, and versatility are the primary design considerations of this modern. The modem is designed to use many off the shelf components. Design The modem can be broken up into several sections, analog box, digital card, and computer (Figure 1). The analog box handles the interface between the VSAT and the digital card. The 5 MHz and 1PPS interface are also implemented in the analog box. There are two digital and two analog test ports. The two digital test ports allow the user to monitor various signals in the digital card. Signals to be viewed are selectable through software. The analog ports are driven by a pair of 12-bit digital to analog converters (DAC). The DACs are driven by the computer. The second section of the modem is the digital card. The digital card contains the hardware needed to track the carrier and code and make the time of arrival measurements. The card also handles the transmission of data and IPPS. The third section of the modem is a personal computer with a digital signal processor (DSP) that controls the digital card and analog box. The computer in this implementation is a MS-DOS compatible computer. The computer is configured as a keyboard, display, and storage interface for the DSP. The DSP used is Texas Instrument's TMS320C30 that runs at 16 MIPS and up to 33 MFLOPS. The DSP may seem to be overkill, but the reason for using such a powerful processor is to leave room for other functions that may be added later.
Published in:	Proceedings of the 22th Annual Precise Time and Time Interval Systems and Applications Meeting December 4 - 6, 1990 Sheraton Premiere Hotel Vienna, Virginia
Pages:	345 - 348
Cite this article:	Galysh, Ivan J., Landis, Paul, "Two-Way Time Transfer Modem," Proceedings of the 22th Annual Precise Time and Time Interval Systems and Applications Meeting, Vienna, Virginia, December 1990, pp. 345-348.
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