April 5, 2011
News - According to Pete: August…
about 2 years ago
As it was said before, GPS vertical accuracy is by system design about 1.5 times worse than horizontal. Surely WAAS in the US and EGNOS in EU can mitigate many of the common errors of GPS (such as satellite clock, broadcast orbit errors and more importantly atmospheric errors). The receiver you are using is a good one, but I have very little opinion for small quad helix antennas (won’t mention the manufacturer here). I would suggest using a good external patch antenna which will give you about 3 to 5dB more gain in normal conditions (when you are airborne). GPS can also be used in differential mode to achieve centimetre accuracy and surely I would consider that if you have a wireless data link at hand onboard your glider. Finally, please check the uBlox protocol manual on how to set the correct Kalman filtering for your application (I think it’s message UBX-CFG-NAV5). In summary, yes I think you can succeed with GPS and a decent antenna.
Coming to logging.. with GPS you have a PPS (pulse per second) which is an analog pulse synchronised to GPS time to a few tens of nanoseconds. Normally you get the PPS and shortly after that (microseconds) a bunch of NMEA sentences relevant to the GPS second that just happened. uBlox has clear application notes to describe the relationship between this analog PPS signal and UART data. If you would be integrating GPS with accelerometers and gyros then you would have to refer everything to a common time frame in order to be able to integrate sensor outputs post mission. Usually GPS time is a good choice and PPS can be used to trigger reads on the SPI/I2C bus of digital sensors. Moreover uBlox “T” series have an external digital input that will time-tag (with the local receiver time, roughly aligned with GPS time) an external event. So would you want to very precisely reference in time analog events I would strongly suggest to use that. One example could be photogrammetry, where pictures need to be precisely time and position tagged. If the only sensor you want to log is the BMP085 then I think that very coarse synchronisation will suffice (e.g. the GPS UART FIFO buffer is empty -> dump a baro reading on the file). I normally use STM32 and Fatfs which gives me a lot of flexibility, but I would say that the openlog is perfectly capable of handling two sensors too.
Hope this helps?
about 3 years ago
The datasheet does not say anything about hardware integration… This kind of modules often have requirements on placement near ground planes or switching logic. For example, the Fastrax UC-430 is also tiny but needs much broader board to work. Another module I know of, the OriginGps ORG14xx, is only 15x15mm and still needs 6mm clearance around and 1.6mm below. Beware of small GPS modules.. their antennas still need ground planes.
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