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SDR and Garmin GPS Operation

It is possible to use a GPS to provide the SDR with an accurate, self setting clock.  SDR can be connected to a Garmin GPS 16HVS.  SDR needs to be have software version 2.20 or newer to support GPS.

SDR Jumpers

There is a jumper inside the SDR that ensures that the SDR provides 12 Volts on RS232 which the Garmin GPS requires in order to function.  If the SDR is reporting “No GPS Detected”, it may be the case that the jumper is not properly setup.

Normally, if an SDR is purchased together with a Garmin GPS, Sutron ensures that the jumpers in the SDR jumpers are properly setup.  However, if the SDR was purchased at a different time than the GPS, it may be the case that the SDR does not have the jumpers setup properly.

There is a way to determine how the jumper is setup without opening the SDR.  It does require a voltmeter.  Firstly, enable the Garmin GPS setting via the front panel.  Then, measure the voltage across pins 5 and 9 of the SDR RS232.  Those are the two rightmost pins (closest to right side panel).  One pin is above the other.


To setup the jumper, the SDR case must be opened.  The RIGHT side of the SDR case needs to be opened.  First remove all eight screws holding the right side of the case.  Then pull hard the right side panel to remove it from the case.  There is a gasket that seals the side panel to the case.  This gasket can act like a glue, preventing the opening of the side panel.  Make sure to pull hard, or insert a screwdriver between the panel and the case to pry the two apart.


Once the right side panel is removed, the jumper is easily accessible.  Place a connector on jumper J8 so that it connects Vbat to the middle pin.


In addition to J8, there is another jumper inside the SDR that needs to be properly configured in order for the SDR to provide power to the GPS.  However, this jumper is properly setup at the factory and it is unlikely to be the cause of the problem.  The jumper in question is J6 (located next to J8).  It should have a connector across pins 2 and 3 or have no connector at all.

Please note that the above instructions do not apply to a SDR prototype.  A prototype is labeled as such on the side of the SDR Case.  Prototypes (also refereed to as Rev A) use jumper J7: connect pins 5 and 6 together, and also connect pins 2 and 4 together.

RJ45 to RS232 Connector

A custom connector is required to get the SDR and the Garmin GPS together.   The connector bridges the RJ45 on the Garmin GPS to the RS232 on the SDR.  The table below provides the wiring diagram for the connector.

Note: The colors on the Garmin GPS RJ45 do not match the colors of the RJ45 to RS232 converter.

Rj45 pin Garmin plug color

RJ45 to Rs232 converter

RS232 on SDR



1 Red




8 to 40V for 16HVS

2 Black




3 Yellow



Remote power on/off

On if <0.3V, Off if open circuit

4 Blue



Port 1 Data in

NMEA input to GPS

5 White



Port 1 Data out

NMEA output from GPS

6 Gray


no connect



7 Green


no connect

Port 2 Data in

RTCM output

8 Violet


no connect

Port 2 Data out



When equipped with a GPS, the SDR will keep UTC time.  UTC (Universal Coordinated Time) is an internationally accept time standard.  UTC will differ from the local time by a number of hours.  The user cannot setup the SDR so that it keeps local time.

If a Sutron Satlink (versions 6.17 and newer) is connected to the SDR via SDI-12 (in addition to the Garmin GPS), note that Satlink will set the clock of the SDR once per day.  Please ensure that Satlink’s local time offset is zero. 

An SDR equipped with a GPS will provide a timing accuracy  of ± 1 second.

GPS Installation and Setup

When first installing the GPS, make sure the GPS is positioned so that it has a clear view of the sky.  Make sure to connect the GPS to the SDR via RS232.  Sutron provides a custom RJ45 to RS232 connector for this purpose  (the diagram for the connector is on page 53).

After connecting the two devices, go the station setup menu on the front panel of the SDR.  Find the entry called Garmin GPS and press set to enable the GPS.  Then go back to the top of the menu, and hit the down button until the GPS status menu is shown.  If SDR is communicating with the GPS, the menu will say

GPS initializing.  If the GPS has acquired a time fix, the SDR will show GPS functioning.

Pressing right in the GPS status menu will provide more details.

If the GPS has locked on satellites and is providing accurate time, the menu will show a message such as

GPS has valid time

5 satellites used in time fix

Pressing down from the detailed status menu will show the last GPS time sync.  The time show is the time when the GPS last had a time fix.  If it has been more than 12 hours since the last valid time fix, the GPS is not working properly and may need to be respositioned (please see page 51).

When installing, t is recommended that the user wait until the GPS has valid time before leaving the station.  If the GPS does not acquire the time in ten minutes, the GPS should be repositioned so that it has a better view of the sky.

Keep in mind that whenever the display is turned on, SDR will power up the GPS.  This helps with GPS positioning.  As long as the display is on, the SDR will provide power the GPS, allowing it to track satellites.  When the display is off, the SDR will power the GPS once an hour for up to 15 minutes.

GPS Positioning

If the SDR is reporting that the GPS cannot get a time fix, it means that the GPS is unable to get a clear view of the sky.  It could also be the case that the GPS is picking up interference.  The best solution is to reposition the GPS.  The GPS needs to have a clear view of the sky in order to properly function.

Place the GPS antenna in the most open space possible.  Do not place it directly under anything nor directly beside something.  Always attempt to achieve a “full sky” view with the antenna.

Place the GPS antenna high up on a pedestal or in a protected location.  Flat surfaces may tend to cover with ice and snow more so than elevated locations.  Keep away from areas where birds may nest.   Placement is very important and great care should be taken in selecting the location.

GPS Operation

Once every hour, the SDR will wake up the GPS.  Once the GPS has acquired a time fix (should not take more than 40 seconds), the SDR will set its clock and put the GPS in low power mode.  Powering the GPS once an hour provides the optimal power consumption.

In addition, whenever the display is turned on, SDR will power up the GPS.  This allows the user to see whether the GPS can acquire a time fix and helps in positioning the antenna.

GPS Errors

If the GPS is either not communicating with the SDR or if the GPS cannot acquire a time fix, the SDR will blink the red LED to indicate that there is a problem.  In addition, the SDR will show a message describing the problem on the front panel.  Once a day, the SDR will write an event to the log indicating that it has GPS problems.

The SDR reporting “No GPS Detected” can indicate that the connector between the GPS and SDR is bad (please refer to the section on the connector on page 53) or that the SDR does not have its jumpers set properly (please see the section on page 52 about SDR Jumpers)

If the GPS cannot get a time fix, please see the section on GPS Positioning on page 51.

If the SDR reports “GPS Comm Failure”, it means the SDR is detecting data on the RS232 line, but that the data is incomprehensible.  It could indicate that the GPS has been improperly configured.  If possible, try using a different GPS module.

If a faulty GPS is connect to the SDR, of it the GSP is not connected to the SDR, the SDR will take a full minute before deciding it cannot talk to the GPS.  Ensure that the SDR is given enough time to talk to the GPS before leaving the station.