Ground Proximity Warning System for my Experimental Velocity Aircraft

Question

I'm working to put together a quasi GPWS (Ground Proximity Warning System) for my experimental airplane... a four seat Velocity. The purpose is essentially to provide a timely warning (like someone screaming into my headset) that I'm about to land the airplne without first extending the landing gear. My idea so far is this - there is an RS232 output from my aviation GPS receiver that contains latitude/longitude, ground speed, destination & distance from destination, and the GPS calculated elevation (referenced to Mean Sea Level (MSL). I've got a Raspberry Pi on order... my plan is to use the Raspberry Pi to interpret the GPS serial stream and to run the following test.... If I am within, say, 3 miles of my destination AND I'm below, say, 300 feet, AND I'm below my maximum gear extension speed (120 knots) AND my landing gear isn't extended - WARNING! WARNING! I'd be interested in the thoughts of this community. The biggest challenges I see (outside of learning linux and whatever programming language makes sense), are things like: - How to determine my altitude Above Ground Level (AGL) from the MSL output of the GPS (I'll need a terrain elevation database, or perhaps a database of airports in the US including their altitude in MSL), - How much should I worry about GPS altitude error? - Do I need to plan for any Raspberry Pi driven display in the aircraft? - This won't be my only gear up warning system... but... if it becomes my primary system, what should I be worried about? Thoughts anyone? My Raspberry Pi should arrive this week... I'm ready to go. 
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shabaz · Answer

Hi Dave, It sounds an interesting project. I'm no expert in this area, but I can help with letting you know of some general considerations, in case you can incorporate into your design. 1. Simplicity - the less to fail, the better. e.g. maybe consider a barometer, and less code, e.g. a microcontroller running without an OS 2. temperature - worth considering all parts to be at least industrial temp, and consider how it will be protected from extremes of temp and moisture. In particular, ensure your sensors, power supplies, etc., work at the expected temperatures. 3. Reliability - rely on locking connectors intended for industrial or military use 4. Redundancy (I guess you're familiar with this since you mention it won't be your only system) 5. Some self test/checking mechanism Personally, I wouldn't use a R-pi for this. I would pick an industrial temp range microcontroller and write the minimum code needed to interface to the sensors (e.g. GPS and/or barometer) and provide visual and audio indication). A serial interface is quite easy with a microcontroller without requiring an OS. Your use-case doesn't need multiple threads of execution or much shared variables, so a simple interrupt based scheme is a better idea I think. A database could be stored in FLASH (many microcontrollers have (say) 128kB or more of FLASH which is possibly more than enough to store hundreds of airfield locations maybe). I suppose you're thinking of storing a co-ord, and then using math to figure out if you are within a certain radius of it. For a GPS sensor, I would have a redundant one too. Also, screening (metal case), supply decoupling and filtering against RF on supply and interfaces too, since I'm guessing you may have a transmitter or there may be some powerful one in the vicinity, and you don't want them to affect your electronics.

Ground Proximity Warning System for my Experimental Velocity Aircraft

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