All FrSky users...
A little while ago Chris Bott lent me some FrSky sensors to play with:
- Current & Voltage Sensor
- 3 Axis Accelerometer
- LiPo Cell Voltage Sensor
- GPS Module
I stuck all these (except for Cell Voltage) into my Maxthrust Riot to test them out.
All worked well but the GPS Module does have one issue, the amount of time between connecting the flight battery and it getting a GPS lock.
After some research I found out that the GPS module involved is a UBlox NEO-6M unit which has a 1 Hertz update rate once it has a GPS lock.
The NEO-6M also has provision for a memory backup battery to be fitted, but FrSky have decided not to include this battery on their implementation. The battery is normally a small Lithium "button" battery.
This means that every power cycle is a "Cold Start" in GPS terms where the module has to scan for available satellite signals. Cold Starts can take up to 10 minutes even in nice clear space like our field. I know 'cos I've stood on the flightline waiting for the GPS to lock!
Adding the memory backup battery means that the module can do a "Warm Start" where it looks for the satellites that it already knows first, and will almost certainly find them if you are flying in the last location as last time.
Chris gave me permission to open up the module and look at fitting a backup battery.
I cut off the heatshrink covering and sure enough found a UBlox NEO-6M module, without a battery.
The backup battery pin on the module (Pin 22) was indeed connected, but to Ground (0v)...!
No other unused module pins were connected to ground, so it looks like FrSky had really tried to make it difficult to connect a backup battery. This meant I had to completely desolder the whole surface mount module (hot air rework station), cut the ground connection to Pin 22 because it was UNDER the module and replace the module.
This done I found a tagged Lithium button cell which just happened to straddle the module and a ground pad and the Pin 22 pad. Attached it to the top of the module with double sided tape (not really needed but made soldering it on easier) re-heatshrinked and reconnected it to the aircraft.
I took the fuselage round the local park (tiny, but clear sky view) and sat down for the GPS to lock. Results:
- 1st Power Cycle - Cold Start - About 4 minutes
- 2nd Power Cycle - Warm Start - 13 seconds!
- 3rd Power Cycle - Warm Start - 17 seconds!
- Power off for 5 minutes and wait
- 4th Power Cycle - Warm Start - Didn't get chance to time! Too quick!
So that looks like a good hack to me!
Will try it at the field today at lunchtime. Expect the first cycle to take a little longer because there may be different satellites in view.
Here's some pictures of the module and hack...
