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I have a GPS receiver module connected to an AVR microcontroller and need to detect if it doesn't have an antenna properly attached. Some time ago I found a reference circuit but now that I need it I couldn't find it in my computer neither with a Google search. Could someone point me to any resources for this, or give the guidelines about the basics?

NB.: it's an active antenna.

Thanks!

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  • \$\begingroup\$ If it's an active antenna, presumably there is a way that you could monitor it's presence/power consumption. \$\endgroup\$ Oct 26 '12 at 14:52
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    \$\begingroup\$ What's the GPS receiver module? Did you check the datasheet? Usually they include some sort of reference circuit for detecting antenna presence for active antenna.s \$\endgroup\$ Oct 26 '12 at 15:08
  • \$\begingroup\$ On your GPS module, is the power connection for the antenna brought out separately from the power for the rest of the module? It often is. Keep in mind that an active antenna may be drawing only 15 mA or so. You might also want to actively limit the current to around 50 mA, in case the antenna connection is accidentally shorted out. \$\endgroup\$
    – Dave Tweed
    Oct 26 '12 at 15:13
  • \$\begingroup\$ @TobyLawrence it's the GM-862-GPS from Telit; if there's such reference, I was unable to find. \$\endgroup\$
    – fceconel
    Oct 26 '12 at 18:44
  • \$\begingroup\$ @DaveTweed the power is not internal to the module, I use the typical inductor+resistor to Vcc and a small cap (tens of pF) in series with the antenna input. \$\endgroup\$
    – fceconel
    Oct 26 '12 at 18:46
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Looking at the GM862 hardware guide (located here), Telit indicates that their module has a built-in current measurement circuit for the GPS antenna input that is readable by AT commands. Using this, you should be able to read whether or not current is being drawn which should indicate whether or not your antenna is connected or you're experiencing a short circuit.

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1st clue is RSSI signal level, 2nd possibility is if you can sense change in tiny voltage drop at source output or have a way to monitor current to antenna elsewhere. Fault detection such as open or short requires more complexity than most GPS's provide. So I would rely on RSSI voltage if accessible. (received signal strength indicator)

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  • \$\begingroup\$ I can spare an ADC from the AVR for that; but would it have enough sensitivity? And more important, won't it inject noise to the antenna? Another concern is that the current track from the antenna to the module is very short, if I draw a track from the AVR to the antenna power source it'll be considerably longer and probably will catch even more noise. \$\endgroup\$
    – fceconel
    Oct 26 '12 at 18:51
  • \$\begingroup\$ About RSSI, the problem is that if eventually the device would be temporarily in a place with very low GPS signal (an underground parking lot, for instance) I won't be able to differentiate it from an actual antenna disconnection, which is my main purpose. \$\endgroup\$
    – fceconel
    Oct 26 '12 at 18:53
  • \$\begingroup\$ If you are trying to provide fault detection isolation and notice for operator intervention, RSSI can still be used to show a change in signal levels from connected and disconnected and confirm normal operation since the difference is significant. Is this for unmanned tracking use? \$\endgroup\$ Oct 27 '12 at 1:58
  • \$\begingroup\$ It's for car tracking, and the concern is that the antenna may be accidentaly disconnected during regular maintenance of the vehicle, or by wear & tear. And I need to send the notification only if I'm sure the antenna is disconnected, in my case it's important not to have a false alarm. \$\endgroup\$
    – fceconel
    Oct 27 '12 at 20:20

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