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SiRF III Bluetooth adventures

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I spent the last week learning about my new Holux 240 bluetooth receiver. I downloaded a number of different applications trying to figure out how to enable WAAS support. In the process I thought that these bluetooth units just did not include WAAS capability or they did not provide a means of turning it on. In the end I found that it is just the lack of good software and documentation and they are really not hard at all to use.


In short, the bluetooth units are a bit special because they have a true serial port between the two chips inside the receiver; the GPS chip and the bluetooth chip. This seial data rate defaults to 38,400 bps normally. Many of the advanced features and status displays for the SiRF III chip require the use of SiRF binary mode and are not available using the NMEA command set. Various programs will let you send and receive the binary commands, but they typically don't use the binary commands for data display.


The SiRFDemo program from SiRF is intended to be used with the SiRF demo boards and not end user devices. For whatever reason they decided that when you switch it to binary mode, the seial data rate is changed to 57,600 bps. But in a bluetooth unit, only the GPS chip sees this command and changes the seial data rate. The bluetooth chip remains at the old seial data rate and all communications is garbled. The operation of the receiver is corrupted until the battery drains and the unit "forgets" its new setting.


The way to use the bluetooth receivers with SiRFDemo is to not use the menu command to switch to binary mode, but to manually send the NMEA command to change modes *WHILE KEEPING THE SERIAL DATA RATE THE SAME*. In my unit this command is "PSRF100,0,38400,8,1,0". The menu command "Action - Transmit Serial Message" allows you to enter this string and it will append the $ and checksum. Be sure to select NMEA as the "Protocol Wrapper". You can even store this command in a file so you don't have to remember it. From then on your unit will be in binary mode until it is set back to NMEA with the appropriate binary command or the battery is drained.


Several commands are required to put the unit into SBAS or WAAS mode. They can be executed from the menus. Under "Navigation" set "DGPS Mode" to "Automatic" with the default timeout. Set "DGPS Source" to "SBAS channel". Set "SBAS Control"; "SBAS PRN" to "Auto Scan", "Mode" to "Testing" and "Timeout Source" to "Default". Then, if you have an SBAS SV insight, you should see a SV in channel 12 with a PRN greater than 32. When the signal is used for corrections it will show in the "Navigation View" window as "3d+DGPS"


Another note on the Holux 240 SiRF III receiver and SBAS that I figured out with some advice from Dennis Gröning. Seems that the SiRFDemo program won't display the actual corrections from the SiRF III receivers, but they are calculated and are sent over the comm port.


Dennis suggested that I turn message 27 off (DGPS Status) and turn Message ID 29 on (Nav Lib DGPS Data). I had tried this before, but saw no difference in the dsiplay, which when MID 27 is used shows a constant 14.27 meter correction for all SVs.


Looking at the logged commands I saw that MID 29 does indeed indicate that corrections are enabled and different corrections are shown for different SVs. However, I did not see a correcton for *every* SV being used in the fix.


If I restart SiRFDemo with MID 27 disabled and MID 29 enabled, nothing shows up in the DGPS view window as if SiRFDemo does not report MID 29. I believe in one of the GPSPassion forum threads, Carl from SiRF said that the SiRF III does not report corrections even when they are calculated and applied. I guess they *are* reported, but not with MID 27.


If you want full info on how to use SiRFDemo to control and monitor a bluetooth GPS receiver, go to http://www.gpspassion.com/forumsen/topic.a...amp;whichpage=1 and check out both pages. That is the thread I will be updating as I find more info.

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