GPS Problems

[mm123521]

My GPS works great in my twin 6S SprintCat - very consistent readings between 70 and 71 MPH.

The same GPS will not give consistent readings in my twin 6S Rigger, the readings are always low. It is NOT the boat. I can look at the boat and know it is running in the 80's or low 90's, and the GPS reads 62MPH or some other number in the 60's or 70's. Sometimes I get believeable readings, but not often, and never repeatable. I can run the boat for one lap and get 60MPH then, with NO setup changes, it reads 93 MPH on the next lap, and 65MPH for the next lap.

Both boats are twin 6S.
Both use the same batteries, GensAce 5300Mah or several other brands, one for each motor.
Both use the same T180 ESC's
The SprintCat is running Castle Creations 1520, 1600KV motors.
The Rigger is running Castle Creations 1518, 1800Kv motors.

In the SprintCat, I lay the GPS right over the top of all my battery wires and it works great every time.

In the Rigger, I have tried putting the GPS inside the hull, near the battery wires.
I have tapped the GPS to the outside of the tub on the bow of the boat.
I have tapped it to the stern of the boat.
I have tapped it to thr right sponson.
I have tapped it to the left sponson.
I have mounted it with an inch of foam rubber under it, in case shock was messing it up.
No matter where I put it, it gets bad readings.

GPS is a Garmin Etrax H

It could be electrical interference from the motors or ESC's - but both boats have very similar propulsion systems.
It could be the rooster tail blocking the satelite reception?
I have read that GPS recievers are limited to 4 G's of acceleration, so they cant be used to make homemade guided missiles- is the high speeds and accelerations messing it up? A rigger will easily hit 4 G's of acceleration in a fast, tight turn.

I have run the boat in 600 feet long, straight runs.
I have run the boat in 600 foot diameter, smooth circles, and the readings are still inconsistent.

Has anyone used a GPS in a fast rigger?
Anyone with similar experience?

Its making me mad, because I cant fine tune the boat without accurate speed readings!!

HELP!

Mark

[m4a1usr]

GPS is a Garmin Etrax H

It could be electrical interference from the motors or ESC's - but both boats have very similar propulsion systems.
It could be the rooster tail blocking the satelite reception?
I have read that GPS recievers are limited to 4 G's of acceleration, so they cant be used to make homemade guided missiles- is the high speeds and accelerations messing it up? A rigger will easily hit 4 G's of acceleration in a fast, tight turn.

I have run the boat in 600 feet long, straight runs.
I have run the boat in 600 foot diameter, smooth circles, and the readings are still inconsistent.

To be sure of what is effecting your GPS some observations are required to narrow it down.

1st. Temperature and atmospheric conditions effect the GPS signals. On cloudy days a GPS can have range limitations. Temperature inversions in strata are known to cause havoc as well.

2nd. Consistant signal or sat lock is important. If your GPS is switching from previous locked on sat's (before any accleration changes) to new ones, the logrythmic computation errors will increase. You need at least 3 solid sat's locked on to begin a run.

Sources of GPS signal errors
Factors that can degrade the GPS signal and thus affect accuracy include the following:

Ionosphere and troposphere delays - The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.

Signal multipath - This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.

Receiver clock errors - A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.

Receiver Accuracy Errors - This is known as CEP or Circular Error Probable and in essence is a known (by the computing software) range of distance error where speed and distance calculations will be performed. Its better to google this for an explanation as the answer is somewhat complex.

Orbital errors - Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.

Number of satellites visible - The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.

Satellite geometry/shading - This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.

Intentional degradation of the satellite signal - Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.

Hope this helps some.

John

[mm123521]

John,

temperature = various, some cool overcast days, some clear and hot.
I have made 40 runs trying to tune up the rigger.
There are no buildings, for multipath.
I get 6 to 8 satelites, at least 5 have full signal strength.

This is a very pronounced problem, the max speed readings are off by 25MPH sometimes, and always low.

Like I say, It runs great, every time in my SprintCat @ about 70MPH.
The GPS works great on my bycicle and in the car.

Its something different about the Rigger.
All the stuff you listed would affect the GPS in the car or bike or the SprintCat.
It's just unreliable in the rigger

Mark

[m4a1usr]

You dont happen to have a second GPS do you? Maybe a fellow club member could loan you theirs? Or at least be out on the water the same day? At this point the only obvious possiblities are 1) The rigger is faster, 2) it is more manuverable. I doubt that GPS is influenced by any electrical signals given off by whats inside the boats. The L1 frequency that civilian GPS rec operate at is high enough to be out of the possible EMF given off by what we use. Something to compare it against is what you need. I'm sure you are already aware of that.

John

[mm123521]

John,
That sounds like a good idea.
I'll try to get someone to loan me a different GPS.
Our club is small and there is only one other GPS that I have seen.
I would gladly go buy another GPS, if it would solve the problem, for sure.

I was thinking electrical interference was the problem, so I mounted the GPS to the sponson to get it as far from the electrics as it could be. That also put it up high where it would get good signals -- but no help.

Also tried mounting it upside down, but it didnt help.
Also tried putting tin foil under it to maybe block electrical interference from the motors.

These things run at about 1500MHZ -- the rooster tail would block that frequency????

Mark

[Griff]

this could be an expensive idea, but, do you have an iPhone? or one that can download a gps app, i just downloaded an app for mine called awesome hud, free, and records max speed, just an idea, of course no liability here if you decide to do this and get your phone wet thats down to you, having said that I am going to risk my phone next time out, after putting it in some ziplock bags and taping it well.
like I said it's an idea
Tom

[cyberhoops]

Is the rigger carbon fiber? That will block GPS reception. Ahh.. never mind. I re-read the first post and realize you taped it outside the hull as a test.