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Author Topic: EM406A GPS receiver not locking.  (Read 1715 times)

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Offline yash25Topic starter

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EM406A GPS receiver not locking.
« on: July 06, 2010, 12:29:18 PM »
Hey,
I bought an EM406A GPS receiver from Sparkfun some time ago for a GPS controlled robot I'm building.
(http://www.sparkfun.com/commerce/product_info.php?products_id=465)

Now, I built the basic circuit to read the NMEA sentences on the hyper terminal, i.e just the PC and the module, no microcontrollers involved.
I connected " RECEIVER-> LEVEL CONVERTER CIRCUIT(2.85V to 5V) -> MAX232 circuit"

Now, when I power this circuit up, the NMEA sentences start appearing on the screen, but the module doesn't get locked at all. I kept the circuit switched on for around half an hour but to no avail. (Also the led on the receiver is continually turned on. )
I keep getting sentences like these

"
$GPGGA,164310.088,,,,,0,00,,,M,0.0,M,,0000*57
$GPGSA,A,1,,,,,,,,,,,,,,,*1E
$GPRMC,164310.088,V,,,,,,,060710,,*2E
"


Isn't the above format the correct NMEA format?

So could there possibly be a problem in the circuit I've connected (in spite of the fact that I'm getting proper NMEA sentences) , or could there be a problem in the module itself?
Or do I need to change my location to a more open area so I can get better signals ( although I'm currently in a 5th floor apartment)?
Thanks !!!
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Offline madsci1016

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Re: EM406A GPS receiver not locking.
« Reply #1 on: July 06, 2010, 02:31:27 PM »
Try it outside with open sky, just for the heck of it. This will see if it works at all, or is just a signal indoors issue.

Offline yash25Topic starter

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Re: EM406A GPS receiver not locking.
« Reply #2 on: July 06, 2010, 02:33:57 PM »
Yes. I guess that's what I'll have to try...
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Offline yash25Topic starter

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Re: EM406A GPS receiver not locking.
« Reply #3 on: July 06, 2010, 02:43:09 PM »
Hey, I have another doubt though.

Is it okay if I simply remove the level shifter only for the transmit(TX) line of the GPS receiver.

In other words , is it okay if I directly connect the TX line of the receiver to the MAX232 input line? Because the MAX232 datasheet says that it will recognize voltages as low as 2V on it's inputs. So I simply remove the level shift from 2.8 volts to 5volts...

will this modification help in any way?
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Offline madsci1016

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Re: EM406A GPS receiver not locking.
« Reply #4 on: July 06, 2010, 02:45:17 PM »
Hey, I have another doubt though.

Is it okay if I simply remove the level shifter only for the transmit(TX) line of the GPS receiver.

In other words , is it okay if I directly connect the TX line of the receiver to the MAX232 input line? Because the MAX232 datasheet says that it will recognize voltages as low as 2V on it's inputs. So I simply remove the level shift from 2.8 volts to 5volts...

will this modification help in any way?

You are getting serial data, so your setup works fine now. Yes the 2.8 to 5V shifter is probably overkill, but it's not your problem, or you would not be getting NMEA sentences at all.

Offline yash25Topic starter

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Re: EM406A GPS receiver not locking.
« Reply #5 on: July 06, 2010, 02:52:13 PM »
I know...Guess I'm just getting a bit superstitious after a whole day's work on this without result...I guess fresh air and patience is the only option for me!
I'll get back to you after the field work!
Thanks a million!
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Offline Webbot

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Re: EM406A GPS receiver not locking.
« Reply #6 on: July 06, 2010, 05:17:38 PM »
You need to look for a change in this message:-
$GPRMC,164310.088,V,,,,,,,060710,,*2E
ie when the 'V' changes to an 'A'

I normally get a 'cold boot' fix from my EM406A within about 40 seconds (in England).
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Offline yash25Topic starter

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Re: EM406A GPS receiver not locking.
« Reply #7 on: July 07, 2010, 12:06:45 PM »
Working!
I tried it by taking it near a window in my apartment and it got locked!
However , the coordinates I'm getting are slightly incorrect. For example, I'm getting the coordinates as 1901.1383 N(in the GPRMC sentence) while my actual co-ordinates are 1903.0000...
Is this error expected? (Especially since I still am indoors technically, just near a window)


Also, I'm planning to take the module out of the house, but I can only do that through a laptop, and my laptop doesn't have the hyperterminal (vista, so...)... Can anyone recommend any other terminal software  for vista?
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Offline waltr

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Re: EM406A GPS receiver not locking.
« Reply #8 on: July 07, 2010, 12:50:25 PM »
Tera Term Pro works on my Windows 7 PC so it should work on Vista.
Download it from download.com.

Offline madsci1016

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Re: EM406A GPS receiver not locking.
« Reply #9 on: July 07, 2010, 03:01:40 PM »
You can copy the hyperterm.exe from win xp and run it on any vista or win 7 computer, if you like.

for the coordinates. your issue usually is from the mismatch in how you convey coordinates.

Is it decimal  degrees? dd.ddddd

Is it decimal hours? ddhh.hhhh

Is it decimal minutes? ddhhm.mmmm

Googiling shows the default format for that sentence is decimal minutes, but the GPS module might be configured for something else.

Offline yash25Topic starter

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Re: EM406A GPS receiver not locking.
« Reply #10 on: July 10, 2010, 11:59:32 AM »
Thanks folks.
Turns out I was reading the data in the wrong format!
Accuracy is pretty good...about 10meters indoors...good enough right?
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Offline Admin

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Re: EM406A GPS receiver not locking.
« Reply #11 on: July 13, 2010, 07:05:03 AM »
Accuracy is pretty good...about 10meters indoors...good enough right?
For indoors thats fairly accurate!

 


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