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February 1, 2011
about 2 years ago
After some poking around with this board, I have found some quirks and bugs I’d like to share:
The 20Hz feature is of limited use. With 100ms apart, two packets arrive back to back with absolute zero space between. This means the first in each pair is at least 50ms late. If you configure it for 10Hz, packets are properly spaced 100ms apart. This quirk is true for both NMEA and binary.
GGA is the only message that is sent faster than 1Hz. All other NMEA types are sent at 1Hz, regardless what update rate you configure.
SkyTraq’s current document Binary Command Set totally skips the navigational output format, it only handles the binary input config messages. I had to dig long and deep to find an earlier version (1.10) that explains the format of the 0xA8 message. Highly recommended; it’s more efficient in that it has all the greatest hits, all wrapped up into one packet.
Some users on the forum has complained about a static looking nav solution, where not even the least significant digit moves in lat/lon. No GPS is that good! It bothered me too, until I found the fix. SkyTraq calls it “pinning”, and is the threshold in position and velocity change that must occur before it’s reported as such.
By default this feature is turned off (message 0x39 turns it on/off). However, the pinning parameters must also be zeroed out, even if the feature is disabled! Bug! Use message 0x3B and set all to zero. Now you will see the normal “random” drift. You may also want to check out message 0x3C and set car or pedestrian dynamics, for optimal filtering.
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