ILEC SN10 PDA Support
Last Update 12-December-2011
Ultimately, it would be great to have the PDA synchronized with the SN10 so that an input the pilot makes on either unit is immediately and transparently reflected on the other. This requires a lot of software work on both sides, plus hardware to allow bi-directional information flow (mixing GPS and PDA outputs into the SN10 input, etc).
Customers who are using the SN10 report they want to use a secondary display (PDA, ClearNAV, etc) primarily to get a detailed moving map presentation, with the display of alternates and FLARM traffic. With a secondary display this information is always available immediately without page-changes on the SN10. The secondary display can also provide features not in the SN10 including airspace warnings, sectional map, terrain map for obstacles, and OLC and FAI 28%.
As a first step we'll provide the minimum information needed to support this customer need, via NMEA-format output from the SN10 including:
The POLAR sentence gives the glider polar in TRUE speeds. It is
output periodically and if the pilot changes bugs/water or the
This gives the actual polar coefficients(adjusted for bugs,
air density, true speed) as follows:
Coefficients of the polar equations: sink(V) = a*V^2 + b*V + c
Sink and V in meters/second.
Positive V but note sink will be a NEGATIVE number...
'a' is negative, 'b' positive, 'c' negative.
The PDA1 sentence (at a rate of about one per RMC) gives:
- barometric altitude (meters, integer)
- total energy vario (meters per second xx.xx)
Iff the wind calculator has an output, three additional values:
- wind direction (wind is from, true, integer)
- wind strength (kph, integer)
- confidence (integer 0-100 expressing a percentage)
Examples (first sink and no wind, then with a Westerly wind):
The SET sentence (only when an included value changes) gives:
- altimeter setting in mB
Total-Energy Altitude: To compute total-energy altitude (for TE-compensated glide), use GPS track minus headwind for rough TAS, then use this TAS to adjust altitude for kinetic energy. This because we don't output TAS in the NMEA data stream.
This example was generated by ILEC SN10 software version 2.31, using a Volkslogger as GPS/NMEA source. Note that the version 2.31 software has a few miscellaneous diagnostic output lines that appear in the NMEA stream (which will be removed in the next release). Of course, your NMEA input parser ignores anything that is not a valid NMEA sentence with a valid checksum, so this will not cause problems. Also, the SN10 produces a number of non-NMEA diagnostic messages on power-up.
Sample SN10 output with PDA
As explained above, the SN10 sends the corrected polar (including bugs, water, and altitude correction). When the PDA receives this input, it should simply:
To make both the implementation and pilot use easy, no "SN10-mode" or similar controls for pilot. Each time the PDA software is restarted, it behaves as if there is no SN10 in the picture. Whenever the PDA receives the SN10 polar input, it flips into "SN10 polar" mode. If no SN10 input is received in 15 seconds, revert to non-SN10 mode.
It was designed this way to make it simple for the PDA software, and simple for the pilot !
The wind measured by the ILEC SN10 is the wind behind you. Ahead may be something different. In the SN10, it is left to the pilot to decide whether to use the measured wind. Suggested behavior: The PDA software should compare the ILEC SN10's measured wind to the wind setting in the PDA. If it is significantly different, and the PDA has not recently annoyed the pilot, beep and ask whether the ILEC SN10 wind should be copied to the PDA wind setting for PDA final glide calculation. The question should show the ILEC SN10 measurement and confidence, and also the current setting in the PDA.
A common question is:
Would it be possible to include output of SN10 sensor data to PDAs ? It seems like a waste that all this data (airspeed, temperature and even a g-sensor!) is collected, the PDA is wired to the SN10 but doesn´t get to use this input. At 19200bps I guess data rate is high enough to transfer everything?
The SN10 must echo all incoming NMEA sentences. This includes not just the basic GPS sentences, but also FLARM traffic information. We must be very careful not to add too much data, or there is "not enough space in the pipe" and we will start to lose information. FLARM traffic information plus GPS can use most of the available space. So, we would need to add some message filtering/throttling and/or increase the speed (which causes many other problems)... Not so easy as it seems !
Please contact me with any questions at firstname.lastname@example.org !
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