GPS Module problem

[Herath]

Recently I bought a GPS module board from *bay. It arrived to my door step without any problem since I asked the seller to describe the item as electronic to get rid of unnecessary attention of custom officers.

I connected the GPS module to PC via a cable I took from a mobile phone data cable. So the module is powered via USB and Tx, Rx of the cable is connected to the GPS module.

I can get a GPS fix at cold start within the specifications. However I have some problems with the module. I am listing them here.

• GPS module never sends heading(Course) value. It is always 0. I tried moving around with laptop. But no use.
• The altitude value is always negetive. Which means I am below the sea level.
• I can't enable VTG message. I can enable/disable every other output message using NMEA input sentences, but not VTG.
• The Longitude and the Latitude does not update very often. I observed the same value for Lat/ Lon for 2 mins although I moved around at abbout 3kmph.

GGA, RMC,GSV ,GSA is output at 1Hz rate.

Below is GGA, RMC sentences from the module.

Code: Select all

$GPGGA,130712.044,072x.xxxx,N,0800x.xxxx,E,1,04,11.7,-71.5,M,,,,0000*xx
$GPRMC,130712.044,A,072x.xxxx,N,0800x.xxxx,E,0.00,0.00,181210,,*xx

These data are from december 18 log. I tried again today with the help of a friend to carry the module. But could not find any helpful thing about this. I tried enabling VTG via SiRFdemo software too. But it too does not work.

I dumped the firmware using SiRFFlash, the firmware is "2.4.13.03-XTrac2.0.2-C3PROD1.0". So it is Xtrac. GPS chip is SiRFStarII Low Power chip. The module has a built in patch antenna.

So any help is appreciated.

[Neo]

If you have used a uBlox chip, You would have never experience such issues (mostly NMEA message version specific) as the chip itself does everything for you and send you few messages with all required information. Easier to handle.

Recently I bought a GPS module board from *bay

What's the brand/model of the GPS module? (I can't remember).

GPS module never sends heading(Course) value. It is always 0. I tried moving around with laptop. But no use.

I suggest you to travel few kilometers, log data and then analyse the track to see what exactly is going on. Make sure you analyse heading value with respect to heading accuracy.

The altitude value is always negetive. Which means I am below the sea level.

This is a NMEA version specific problem. If you have an older firmware, the reported altitudes will be ellipsoidal. You can usually tell whether a GPS receiver is reporting true altitude above mean sea level, or ellipsoidal altitude by examining the NMEA sentences.

The field immediately following the "M", specifies the geoidal separation value. If a GPS receiver is reporting a value there, then that usually indicates that the the field just before the "M" is the altitude above mean sea level. So, in this case, we're fine.

If the field following the first "M" is null then that indicates that the GPS receiver did not report geoidal separation, most likely because it did not know what the value was. So, we can assume that this GPS receiver is reporting ellipsoidal altitudes. Technically, that's in violation of the NMEA protocol, but it's a pretty minor violation.

I can't enable VTG message. I can enable/disable every other output message using NMEA input sentences, but not VTG.

Again a NMEA version case. VTG is available in Version 1.5 (Also on some devices with Ver 2.3). But it is not in 2.0. You need to find which NMEA version that your firmware uses.

The Longitude and the Latitude does not update very often. I observed the same value for Lat/ Lon for 2 mins although I moved around at abbout 3kmph.

Again, I suggest you to travel few kilometers, log data and then analyse the track to see what exactly is going on. I suspect the displacement isn't more than the speed accuracy.

[Herath]

Thanks for the reply..

My GPS receiver is made by Navius. It is a module from NSA-C3. The cost was just 16 dollars. That is why I bought this. If I bought a ublox, I would need an external antenna too(Could not find one with a built in antenna). So that is going to cost about at least 60$ for the GPS and the antenna. That is why I bought this module.

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0581292221
This is the module I bought.

The NMEA protocol is version V2.2 NMEA0183 as for the seller. Its chipset is SiRFstar IIe/LP with Xtrac v2 . Firmware version is "2.4.13.03-XTrac2.0.2-C3PROD1.0". This is what SiRFDemo reports and I could see in firmware dump.


I will try logging more data again. I disabled Static Navigation in the module since I read that it sometimes causes problems with pedestrian use of GPS module.

93. Explain the static navigation parameters.

Since SA was turned off, we now disable static navigation as our default, and we recommend that you do also. When it is enabled, if velocity is below 1.2 m/s for 3 seconds we will freeze the position, and leave them frozen until velocity exceeds 1.4 m/s (so there is a bit of hysteresis in the solution).

So, I think static navigation thing explains why the position was not updated properly.

[Neo]

I disabled Static Navigation in the module since I read that it sometimes causes problems with pedestrian use of GPS module.

It comes as disabled by default, isn't it?

[Herath]

When I checked with SiRFDemo, it is enabled. I disabled it. Seems like when disabled, module does not get a fix for a very long time. But that's only in NMEA mode. If I switch to SiRF mode and use it with SiRFdemo everything works fine.
I could read heading value, velocity updates properly, location updates properly etc. But in NMEA mode, i could not get them. I guess that there is something wrong with the NMEA processing in the module.

[Neo]

I'm not sure what's going on with NMEA on your chip. Are you sure the baud rate is set correctly on the module ?

In some forums, I found some information that may be related to your issue.

First, try to disable "Synchronise Protocol and baud rate" on SirfDemo.

Second thing is to use baud rate 38400. I think this is the baud rate of GPS chip. Try baud rate 4800 as it is the standard baud rate for NMEA.

Thirdly, when NMEA isn't working over SirfDemo, it is required to detach the batteries and do a hard reset. Since yours is USB powered, I don't think this is relevant. But it could be helpful if you remove it from power and keep it a day until all condenser charges are gone.

[Herath]

The default baud rate is 4800bps. I will try using 38400bps. And NMEA does not work over SiRFDemo( I can only see NMEA sentences in debug view, but software does not show any information), only SiRF Binary work. The module has an internal battery. I guess that I will have to remove that too. Although the module is battery backed up, every setting is lost as soon as I remove power to the module. I did not check the battery to see if it is soldered to the board since I need to remove insulation to check it.

I collected more data in the evening. The track made using the GPS points does not seem to be the way I went. It is wrong(impossible points ) at some points.

I also suspect if there are any problem with the connection. But, I receive data without errors, and I can send commands to it properly. the cable I am using can handle 115200bps easily. I tried enabling debug messages but the module does not respond to that command.

I will try to get NMEA working. Since it is easy to work with than SiRF Binary.

Thanks.

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Edit:
I desoldered the button battery on the board. It still does not respond to VTG enable.Anyway, I will leave it that way for about a day and check again.

[Herath]

Resoldered the battery. Can't enable the VTG still. But I get heading value when I pass 1.4m/s velocity limit. . Traveling in bus cause the module to update the location at 1Hz. If I turn off static navigation, values gets really crazy. So, static navigation should be there for greater good.

[behind cover story]
Connected the module to laptop, installed Bluesoleil to get Serial Port Profile on my laptop. Then wrote a little app using .NET to read from GPS serial port and write to Bluetooth Serial port. Then I made a huge hardly portable Bluetooth GPS reciever.

[Neo]

Cool... Good to see some progress with this now even in the hard way.