An update on Garmin and EGNOS

In my previous post, I ranted about the fact that my new Garmin GPSMAP 60Cx would not use EGNOS signals, whereas my old GPS 60 would. However, this weekend I took my GPSMAP with me on a hike, and at a certain post I noticed that I had indeed obtained a differential fix (and a EPE of about 3 m).

Today, this happened again, but now I had my camera handy to catch the unit in the act (see left). The image clearly shows a differential fix, where Garmin channel #33 (PRN 120) is providing the differential corrections. The second thing that stands out, is that the bar for channel 33 is solid (rather than open). On my GPS60, the EGNOS bar would usually be open, but I would still get differential corrections. It happened to me only once on the GPS60 that the EGNOS bar was solid. After asking around on usenet, I appeared that an EGNOS' satellite bar is solid only if the EGNOS satellite sends actual ranging data for itself,
in addition to the correction data for the other satellites.

It seems that the GPSMAP 60Cx uses EGNOS perfectly, but only if the EGNOS satellite sends out ranging data for itself in addition to correction data for the other satellites. Does anyone have any experience on this with WAAS? Do WAAS satellites do this?

Garmin and EGNOS

A little while ago, I bought a Garmin GPSmap 60Cx to replace my Garmin GPS 60. The GPSmap 60Cx provides (as the name implies) mapping functionality, and it also has the very sensitive SiRF III GPS receiver that the GPS 60 did not have.

The sensitivity of the GPSmap 60Cx is spectacular compared to that of the GPS 60: I can get a fix indoors, and outdoors the EPE rarely ever gets above 5 meters CEP.

However... whereas my GPS 60 happily picked up the EGNOS (WAAS for Europe) signals at Garmin channels 33 and 39 (and sometimes 37), the improve the accuracy to 1 to 2 meters CEP, the GPSmap 60Cx bluntly refuses to use these signals. The unit recognises _very_ strong signals on channels 33 and 39, but then decides not to use it after about 30 seconds. After some discussion and searching on the internet, it seems that EGNOS currently still sends out a "testing" flag in its correction signals. The GPS receiver in the GPS60 would happily use the signals anyway, but the SiRF III firmware in the GPSmap 60Cx actually honours the testing flag, and thus (correctly by design) refuses to use them.

That may all be very well according to the standards, but my experience with EGNOS has been very good on the GPS 60. I would therefore like to use it on the GPSmap 60Cx as well, and I'm willing to take the risk of these signals not being 100% reliable.

Now, it seems that it is possible to force some chipsets into using testing signals anyway, as posted here. However, it is not clear whether this will work with the SiRF III chipset as well. In addition, these commands seem to work on a serial link only, while all I have is a USB cable, and my laptop has no serial port.

We'll see...

And so it starts...

Well, well... This is my first blog post.