gps speed input

[Josh T]

Iv got a problem running a Hmgs1a10cp hummingbird gps speed input. It can be setup as 2000ppk, 4000ppk, 8000ppk or 50000ppk. Iv tried 2000ppk and 8000ppk with the correct calibration and it will jump around with koeo from 0-3kph. With engine on and reving it stopped on the flat it spikes to 50 or 60kms. I tried wiring it straight to the rear mounted battery to rule out interference from under the dash and its in di1 as gp speed with pullup on and a rising edge with a calibration of 800 at the moment. Its a g4+ storm. 

Iv run similar gps speed inputs before on g4+s and they were fine, this is the first with internal antenna though. But i have put in different spots with no change.

[Scott]

Hi Josh,

I'm not familiar enough with GPS outputs to understand the relevance of the ppk levels. However if starting the engine makes the reading change while the GPS is stationary all I think that could be happening is a problem in the electrical interference or power supply to the GPS.

Monitoring the DI channel frequency would likely give some clues here. Using PCLogging and the time plot would give a good view for analysis of the input frequency.

Scott.

[Adamw]

Hi Josh,

I'm not familiar enough with GPS outputs to understand the relevance of the ppk levels.

I know with speedo signals it's common to refer to PPM (pulses per Mile), so I suspect PPK = Pulses Per Kilometer...

Many GPS receivers have trouble detecting "stationary" especially when signal is poor so this might be the case here. You will need the antenna in best possible location (typically middle of roof) to reduce this effect. 

Many daq systems that use gps for speed will also have some other logic to reduce this issue such as comparing gps speed to lat G or some threshold setting such as "anything below 5kmh is considered zero", I can't think of a way to duplicate logic like this in Pclink.

[Josh T]

Yes pulse per a kilometer. I have basically rulled the link out as the problem, i scoped the output with and without it plugged into di1and found it erratic bothways. It did have a non square wave spike intermittently when reving it which id say was interference but it was also getting a square wave output intermittently while sitting still. I also powered it up in another car which didn't get the spike but did get it outputting intermittently too. Id say you're right about it not realising its stationary. Ill give hummingbird a call and sed what they have to say. Cheers for the help.