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•   A red LED visual time signal flash of 100ms duration on the UT second.
•   An audible 1kHz tone burst on the UT second with minute identification compatible with the existing VNG time-signal format (shortened or extended 1kHz tone-bursts).
•   The same 1kHz tone-burst is available as an electrical output signal (6dBm or 3Vpp balanced 600W).
•   A logic-level fast rise-time pulse output (50W BNC connector).
•   An optional computer interface via an RS232 serial port (DCE at 9600Baud). The 1pps timation pulse is provided on the DCD line of this port for computer synchronization purposes.
•   An optional LED drive current output to allow the remote connection of an LED flasher.

•    The status and health of the unit, whether it is still acquiring UTC, whether it has detected an error or inconsistency.
•   UTC date, day-of-week and time on the second.
•   TAI, MJD (TAI) and Local Mean Sidereal Time.
•    Current latitude, longitude and elipsoidal height with respect to the WGS84 system.
•   GPS satellite status, number of satellites visible, dilution-of-precision data.
•    Battery voltage, temperature and other data.

Other important functions/characteristics are:

•   Precise time logging for up to 20 events which can be signalled by an external contact closure (eg pushbutton switch) or logic-level input signal.
•   Optional RS232 serial port connection for sending time signals to a computer or to allow the GPS receiver unit to be used in its native mode controlled directly by a computer.
•   A backlight to the LCD display for night-time use (can be illuminated continuously or for a preset time).
•   The "VNG" time signal waveform is synthesized directly by the microcontroller using an on-chip DAC converter; this waveform could be changed to a different standard without requiring hardware circuit alterations.
•   The microcontroller program can be easily upgraded via a JTAG interface without removal of the chip. 

What will it eventually look like?

The GPS Time Signal Generator will consist of circuitry installed in a small rugged plastic box powered by an internal battery or by an external DC source of between 6 and 36 VDC so as to constitute a self-contained precise-time UT clock and time-signal generator. To permit extended use in the field the total power consumption will be in the vicinity of 400milliwatt or 600milliwatt when the display backlight is illuminated.

The GPS TSG will be constructed on a double-sided printed circuit card to which most of the controls and indicators are directly soldered. (This is to minimise assembly time.) The design uses a Trimble LASSEN SQ GPS receiver module which is 26mm square and has a power consumption of 100milliwatts. This unit was selected because it has the lowest price, lowest power consumption and smallest clock-alignment resolution of any currently available GPS engine and has a precise 1pps output. A Cygnal C8051F015 microcontroller is interfaced with the GPS module to interpret the position, time and status information and generates the "VNG" time signal waveform directly using on-chip DAC converters. Most of the functionality of the unit is implemented in the microcontroller which is programmed in industry standard 8051 assembler code. The remaining electronic circuitry is designed with reliability, resistance to transient damage and low power consumption in mind.

Issued by Gary Hovey for the VNG Users Consortium 30/June/2004