GPS Time Correction Option

Updated: 7/03/99



This page documents how to use a Motorola ONCORE GPS receiver for SDR time keeping.

GPS Receiver:

SDR currently supports the Motorola UT+ or GT+ versions of the ONCORE GPS receiver. This receiver is designed for accurate time keeping (<1 millisecond) unlike off the shelf GPS receivers avaiable at retail stores. The receiver can be purchased from TAPR (Tucson Amateur Packet Radio) or Synergy Systems, LLC. The basic receiver, without a box and power supply, costs between $120.00 and $300.00. The units from Synergy seem to be a lot cheaper. Besides the receiver, you will need an antenna. Synergy sells one for under $55.00. A lower cost antenna made by Pete Ouellette can be found at with a single unit costs of $26.95.

The receiver used for SDR code development was purchased from Synergy. Their part number is R3111G1112. Cost as of 4/99 is $108.00. This is without backup battery option. The antenna used was their active GPS antenna; P/N 10001082, cost is $52.00.

More information about the Motorola ONCORE receiver can be found here. I also have a set of manuals for the ONCORE receiver in PDF format that can be downloaded here (2.5 megabyte zip file).

Please see the  GPS Timing System web page for more information. This page has photos of the GPS receiver, interface board and antenna I am offering.


Receiver Power Supply and Output Signals:

If you purchase just the receiver, you will need to supply +5 volts DC to the receiver. A 12-volt DC wall mounted power supply with 250 MA (or more) of output current connected to an LM7805 power regulator will work fine.

The serial output data from the receiver is a TTL level inverted signal. You must run the signal through a RS232 driver before it can be connected to the computers serial port. You can use a LM1488 or equivalent RS232 driver or you can use one of the Maxim parts (MAX232 is one of them) that will produce the correct RS232 voltages from a single +5 volt power supply. If you use the LM1488 part, you will need to supply +-12 volts DC to the chip.

Besides the serial data, SDR requires the 1PPS (Pulse per Second) signal produced by the receiver. This signal is connected to one of the digital inputs on the A/D card or to the DCD RS-232 signal on the Comm port. Using the DCD signal (pin 1 on a 9 pin RS-232 connector or pin 8 on a 25 pin connector) is easier since no extra wiring is required. You will need to convert the TTL signal out of the receiver to RS-232 voltage levels.

If you use the TTL level 1PPS output signal from the receiver you may choose to buffer the signal to protect the receiver. If you do, make sure that you do not invert the data.

Hardware Connection:

To use this GPS option, attach the serial output line to Comm port 1 or 2. Optionally connect the 1PPS TTL signal to pin 32 (Digital IN 0) on the 37 pin A/D I/O connector. You can use pin 13 as the ground for the 1PPS signal.

Note: If you buy a commercial unit (receiver in a box etc) and use the digital input on the A/D card, make sure that the 1PPS output is a TTL (0 to +5 volt) level and not a RS232 (+- ~12 volts) level. Connecting a RS232 signal to the digital input of the A/D card could cause damage to the A/D card.

SDR Settings and Usage:

Under the F6 settings, set the "Set Time Using Comm Port" to either 1 or 2 and set the "GPS Mode" to "Yes".  If the 1PPS is on the DCD line turn the "GPS 1PPS on RS-232 DCD" to "Yes". If you are using the TTL digital input set this menu item to "No".

You should also turn the debug mode on by pressing the "D" key when you're at the main SDR screen. The "D" key toggles the debug mode on and off. After making the changes, exit out of SDR and connect up the GPS receiver. Restart SDR and hopefully it will lock up to the GPS receiver. If you are in the debug mode, you should see a message saying that SDR has lock to the receiver. When SDR locks to the GPS time, you will see "(L)" next to the UTC time display at the bottom of the screen.

After lock up, SDR will check the GPS receiver's time every 10 minutes. If the internal SDR time gets off by +-6 milliseconds or more it will set its time to the GPS receiver's time and then wait another 10 minutes.

SDR tests to make sure that the receiver is tracking 3 or more satellites before the time will be set the first time. After locking, SDR requires that the receiver track 2 or more satellites before using the GPS time. If the debug mode is on and there are less than 2 satellites being track, an error message will be displayed and logged in the SDR.LOG file.

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Larry Cochrane -