The GPS Monitor/Display

A useful display for Motorola Binary GPS modules, and a GPS fix quality monitor for the VNGBOX

There are of course many projects around that will display GPS information on an LCD screen, so why is this one any different? Well, in most respects it is not, but it does have some special features:

It operates only with GPS units that support Motorola Binary Protocol as their native format, and support the '@@Ea' combined position/status/data message. These units are frequently used for automotive and timing applications.
It will initialize the GPS module, so it does not need to be set up separately before connection (this type of module transmits no messages until instructed to do so).
By monitoring the fix quality (insufficient satellites, acquisition, 2D fix, 3D fix) it is able to provide a hardware output which warns when the fix quality is poor, and thus the 1PPS output of the module is unreliable. This feature is used in conjunction with the VNGBOX.
By monitoring the receiver parameters (antenna shorted or open, poor geometry, invalid almanac) the unit is available to display and thus further warn of poor fix quality.
The GPS module can still be monitored and controlled by a PC connected to the unit, for example running the Motorola WINONCORE software.
It could potentially be built into a small box with display and GPS module, for a portable (say car mounted) clock and GPS display.
While no use in any other application, the GPS Monitor/Display also includes a Morse identification generator. Used in conjunction with the VNGBOX, the unit can insert an eight-character Morse ID into the data stream to allow the transmissions to be identified. This is done every 10 minutes. The message can be sped up or slowed down to fit the available time, and can also be user programmed.

The GPS Monitor/Display Time & Date display

Description

The GPS modules which will operate this unit include the Navman Jupiter-T and the Motorola Oncore. These modules are specialized 'timing engines', highly appropriate for GPS locking applications. They also include the TRAIM feature which will track and disable satellites yielding poor timing data, and can also disable the 1PPS pulse if the fix quality is poor.
The GPS Monitor/Display unit displays UTC time all the time, accurate to within a fraction of a second. The delay to display is typically about 250ms, and is caused by a combination of delays at the GPS module (which can be adjusted) and about 80ms delay in serial transmission. The fix type and any GPS problems are also always displayed.
Other displays are cycled every eight seconds, and include date, latitude, longitude and fix quality (DOP, number of satellites used, number in view).

System Arrangement

The way the two micros are interconnected will be very confusing without the following diagram:

System Block Diagram

The 1PPS (seconds) signal from the GPS connects to the sampling input of the VNGBOX, and allows the micro to accurately sample the phase of the 10MHz clock from the OVEN OSC. The EFC outputs in turn control the frequency of the OVEN OSC, to keep it exactly on 10MHz. The 10MHz clock is used to operate both micros. Using the PC software REFMON4, the VNGBOX system performance can be monitored via the RS232 interface on the right.
The GPS Monitor/Display monitors the serial communications from the GPS module, and when first powered up, also commands the GPS module to set it going. At the same time, the GPS module signals can also be independently monitored. and if necessary the GPS module set up, by a PC connected via the left RS232 interface, and running the Motorola WINONCORE program.
The connection from the GPS module TXD to the right RS232 transmitter (where it is ORed with the VNGBOX transmitter signal), allows the PC running REFMON4 to monitor both the GPS and the VNGBOX. It is of course important that the two devices do not transmit at the same time.
The GPS Display micro interprets the GPS data, displays it, and determines whether the GPS fix is good or not. If the fix is poor, such that there is a risk that the 1PPS signal accuracy might be compromised, the HOLD signal is lowered, signalling to the VNGBOX to operate without further updating corrections until HOLD is removed.
The ID signal from the VNGBOX goes high for nine seconds, starting one second after the start of every 10th minute. This signal triggers generation of the Morse ID message at a time when the VNGBOX suppresses its own pulses. The lack of pulses and the presence of the ID message in no way affects the decoding of the time code.
The system diagram shows two computer serial connections, but of course, neither PC connection is necessary for the proper operation of the system, once it is set up. Also, provided a PC is connected to check the status of the system to ensure integrity when it is used as a reference, there is no reason apart from convenience to have either LCD/PLED display. A single PC connected to the right RS232 connector will allow the whole system to be monitored, even if the GPS Display device is not used.

GPS Display (left) and VNGBOX display (right)

Output Connections

The drawing on the left (below) shows how to connect a small speaker in order to monitor both the time code and the Morse ID as an audible message. The time code ticks will be 1kHz, and the Morse a mixture of 1kHz and 500Hz. If you want 1kHz for both, use DATA in place of CODE. If the sound is too loud, increase the value of R31, or use a 1k pot. If using a pot, connect the ends of the pot in place of the speaker, and connect the speaker from the bottom of the pot to the wiper.
If you place a small line transformer across the speaker, you'll have a signal suitable for modulating a simple low powered AM transmitter (say 1mW). Note the comments below about the harmonic content of the tones.
The drawing on the right (above) shows how to key a CW transmitter with both the time code and the Morse ID. If using an SSB transmitter, build a 1kHz sinewave oscillator and key that. Keep the level well down in order not to overload the transmitter.
The square waves of the tone outputs are too rich in harmonics for direct transmission (although since there is plenty of level, a passive low pass filter with 1kHz corner frequency would be suitable). To key an AM transmitter, use the same approach, but keep the modulation depth below 50%.

Display Schematic Processor Pinout VNG-in-a-BOX

Copyright � Murray Greenman 1997-2009. All rights reserved. Contact the author before using any of this material.