VNG Users Consortium
The VNG UC GPS Time Receiver is a self
contained precise time UT clock and time signal generator
- being GPS based, this
receiver will operate anywhere in the world. It
is a small, affordable package powered by internal batteries or any external
DC source, suitable for extended use in the field or at a fixed site such
as a laboratory or observatory. The Time Receiver will output time
and position information as audio and electronic output in a wide range of
formats and displays key parameters on a two line LCD. It can also
be used to log (store and display) the time of a number of external contact
closure or logic events.
The main output formats are:
The 20 character x 2 line Liquid
Crystal Display, together with a green LED and an amber LED, is used to
display the following information:
- A red LED visual time signal flash of 50 ms duration on the
- An audible 1kHz tone burst on the UTC second with minute identification
compatible with the existing VNG time signal format (shortened or extended
1 kHz tone bursts) is available from an internal loudspeaker or from
the front panel phone jack.
- The same 1 kHz tone burst is available as an electrical output
signal (6 dBm or 3 Vpp) transformer coupled into a balanced 600 ohm line.
- A open drain FET output and current limiting resistor provides
for the remote connection of an LED flasher or alternatively a potential
free synchronisation signal for other clocks.
- A logic level fast rise time pulse output (50 ohm BNC connector).
- An optional computer interface via an RS232 serial port (DCE
at 9600 Baud). The 1 pps timation pulse is provided on the DCD line
of this port for computer synchronization purposes, including existing
timing equipment and computer programs such as the KIWI system. The
serial port also allows the Trimble GPS receiver unit to be used in its
native mode controlled directly by a computer.
Other Timing Applications
- UTC date, day of week and time on the second.
- TAI, Modified Julian Day (TAI) and Local
Mean Sidereal Time.
- Current latitude, longitude and ellipsoidal height with respect
to the WGS84 system.
- The status of the unit, whether it is still acquiring UTC,
or has detected an error or inconsistency.
- GPS satellite status, number of satellites visible, dilution
of precision data.
- Battery voltage, temperature and other data.
Additional Technical Information
- Precise time logging (resolution 1 microsecond) for up to 20
events which can be signalled by an external contact closure (e.g. pushbutton
switch) or logic level input signal. In this mode the device supports
traditional scientific (geophysical, astronomical, etc.) observing techniques.
- The receiver will output a separate 1 pulse per minute marker
(in addition to the VNG code output) to enable synchronisation of multiple
external 'slave' timing devices (e.g. based on crystal oscillators).
- The electronics and batteries are in an extruded aluminium box
225 x 165 x 52 mm.
- The GPS Antenna is a small separate module (26 mm square - see
photograph above) which can be mounted in a suitable position to receive
- Choice of internal power (4 standard 'D' cells) or an external
DC source of between 6 and 30 V, reverse voltage and transient protected.
The total power consumption is approximately 400 milliwatt or
600 milliwatt when the display backlight is illuminated.
- The backlight to the LCD display can be lit continuously or
for a preset time. The LCD bias voltage is automatically adjusted
for low ambient temperatures.
- The GPS Time Receiver uses a Trimble TM
LASSEN SQ GPS receiver module with a power consumption of 100 milliwatts.
This module was selected because it has the best clock alignment resolution,
lowest power consumption and the lowest price of any currently available
GPS engine and has a precise 1 pps output. A Cygnal
TM C8051F015 microcontroller is interfaced
with the GPS module to interpret the position, time and status information
and generate the "VNG" time signal code 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. It
is is constructed on a double sided printed circuit card to which most
of the controls and indicators are directly soldered for reliability.
The circuitry is designed with reliability, resistance to transient damage
and low power consumption in mind.
- The "VNG" time signal code 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
- The microcontroller program can be easily upgraded via a JTAG
interface without chip removal.
(Information provided by Dr Gary Hovey and David Herald
- July 2005)