GPS Altitude Off [Archive]

benstitt

Aug 10, 2009, 04:13 AM

Hi all,

As a Uni project I am trying to measure the turning circle of a model boat travelling around 5m/s (maybe even up to 10m/s if it works..). To do this i was hoping to use the GPS (with antenna) and V3 elogger. I have tested the gps on my bike and it looked accurate, although there were a few bumps in altitude. However, on the water the readings were much less accurate, despite giving ample time for satellites to be found. A particular problem was the incredible spikes of altitude found when plotting on google earth! I will attach a picture when i figure out how..! I cannot seem to find how to turn off the altitude readings :confused: , can anyone help?

Many thanks,

Ben

Mr Boombastic

Aug 10, 2009, 04:56 AM

Can you just plot your data as 2D?

benstitt

Aug 11, 2009, 08:22 AM

I dunno how to do that on google earth... i have just tried to limit the altitude readings and now nothing is coming up unless i am zoomed right out on google earth. :(

billpa

Aug 11, 2009, 01:00 PM

Hi Ben,

Sorry to hear about your issue. I do know the problem - for some reason, when the GPS module thinks the altitude is below sea level, it starts generating very large altitude numbers. These do not appear to be nice negative numbers, but almost random numbers. We have been in contact with the GPS module manufacturer about this, but as yet do not have a solution.

I believe that if you click "Advanced, Set Min/Max values" and set a maximum value for altitude, the large altitude swings should be filtered out when you download data from the logger again. Let me know if this does not resolve the issue for now.

Regards,

Bill, for Eagle Tree

ragamofyn

Aug 11, 2009, 08:37 PM

These do not appear to be nice negative numbers, but almost random numbers.

Perhaps something to do with unsigned integers? Has ET checked to see what the raw data coming out of the GPS is when at a negative altitude?

Mr Boombastic

Aug 11, 2009, 11:14 PM

Just found a problem with my GPS. It has an error of about 100m. My Garmin GPS reads 133m consistently and the ET unit can show -12m or +20m (seems to sit roughly at the same incorrect altitude after switch on). Both the Garmin and ET have 8 satellites. I have the GPS unit mounted above the tailboom on a plastic jiffy box and a remote GPS sensor on the horizontal tail fin. Disconnecting the tail external GPS doesn't make any difference. I know GPS Altitude can be out a little but close would be good.

Is the GPS meant to read MSL or use your starting altitude as a 0 reference?
Can this be re-calibrated?

billpa

Aug 14, 2009, 01:54 AM

Hi Mr. B,

In some cases we display MSL, and in some cases the GPS altitude is zero referenced. Can you let me know where you are seeing the GPS information (PowerPanel, Seagull, OSD, OSD Pro, etc.)?

Regards,

Bill, for Eagle Tree

Mr Boombastic

Aug 14, 2009, 03:49 AM

The Altitude is being view on the OSD. (Software current)

Have not connected to PC with Heli outside to check what is displayed on the computer screen. Will hook up to laptop tomorrow if this will help with diagnosis.

I thought that using GPS would almost necessitate showing MSL as you then utilise Barometric Altitude for a reference to your takeoff points altitude.

BlitzK

Aug 14, 2009, 07:42 AM

I have the same problem with my GPS unit. I hooked it up and drove around in the car (Florida beaches... ~60' ASL max). 3d plotting on google earth was fairly useless because of the huge spikes in altitude. I'm kind of worried that it will do this in-flight as well. I just ordered the baro altitude module, but might not have the room for it in some cases.

While driving around I can set max values to ~75', but that wont help much while flying.

BlitzK

Aug 17, 2009, 03:48 PM

I've noticed my GPS altitude is way off. USGS and my 2 other GPS's show me being at ~89' ASL right now. The V3 GPS is showing 143' in live mode. I unplug usb and plug in battery and power panel shows 163' gps (8 sats).

I thought I saw a setting in the software that when the GPS starts, start it at 0' alt. but I can't seem to find it now.

BlitzK

Aug 17, 2009, 06:35 PM

Here is my latest flight this evening. V3 GPS vs. V3 Alt.

Flying field is 15' ASL.

http://blitzk.com/toys/GPSvsAlt.jpg

ragamofyn

Aug 18, 2009, 12:55 AM

Here is my latest flight this evening. V3 GPS vs. V3 Alt.

At least your numbers somewhat jive. I've been driving around with my v3 GPS and between sea level and slightly below sea level, I'm getting spikes up to 1000+ feet for no good reason. No rhyme, reason or sense. Basically makes the GPS altitude completely useless.

billpa

Aug 18, 2009, 01:14 AM

Mr. B, on the OSD Pro, the GPS is always zeroed, based on the "home" location's altitude.

Ragamofyn et al, sorry for the issue of incorrect alt reporting by the GPS at sea level. We will get this fixed. It's a tough one to debug, since it is location dependent, but I will be adding some low level logging to help track down the issue.

BlitzK, I think the above fix will also address your issue.

Regards,

Bill, for Eagle Tree

ragamofyn

Aug 18, 2009, 11:42 AM

Ragamofyn et al, sorry for the issue of incorrect alt reporting by the GPS at sea level. We will get this fixed. It's a tough one to debug, since it is location dependent, but I will be adding some low level logging to help track down the issue.

Bill, if you need some below-sea level sample data, I'd be more than happy to run some debugs for you against the barometric altimeter expander. Just let me know what you need. :)

Victor

BlitzK

Aug 18, 2009, 03:54 PM

At least your numbers somewhat jive. I've been driving around with my v3 GPS and between sea level and slightly below sea level, I'm getting spikes up to 1000+ feet for no good reason. No rhyme, reason or sense. Basically makes the GPS altitude completely useless.

I got the same results when I drove around... I had to set it to max 85' altG for it to be useful. Obviously this isn't a good solution.

Good to hear a fix is in the works, Bill!

Any way I can get the Baro Alt to be more accurate? With my field being at 15' ASL I doubt I did that much flying below sea level :)

Here is another chart of the same flight as above. with the baro and GPS on the same side...

http://blitzk.com/toys/GPSvsAlt2.jpg

Mr Boombastic

Aug 21, 2009, 06:22 AM

Mr. B, on the OSD Pro, the GPS is always zeroed, based on the "home" location's altitude.

Thank you for your reply.

If I had a choice I would have made it Altitude above MSL. I purchased it expecting it this way and anticipated using the Barometric Altimeter in the Data Recorder for Altitude above the Home reference point.

I understand your intended use for the return home feature. Something though I won't use with a Heli.

Any chance of putting an option in the Software Setup?

billpa

Aug 25, 2009, 01:33 PM

All, I have finally tracked down (and hopefully fixed) the bug with negative GPS altitude. The version is in "pre-beta" testing, located here:

http://www.eagletreesystems.com/LatestApps/Plane/alpha/setup.exe

Note that this version also has changes to improve RPM accuracy, so there may be side effects.

Mr. B, I have added a feature request to get the option added for MSL altitude display with the OSD Pro.

Regards,

Bill, for Eagle Tree

BlitzK

Aug 25, 2009, 02:32 PM

I havnt been able to test that "negative bug" yet (going for a drive soon), but it didn't seem to improve my gps altitude accuracy. My handheld GPS and my cellphone (gps) both say I'm 75-90' (USGS says I'm 83' on the ground... so that sounds right).
7 sats, hdop-1.1, pdop-1.2, vdop-0.9 (they dont display tdop).

my V3 GPS is still reading 143-148 feet.

hdop 1.0 7-8 sats.

billpa

Aug 25, 2009, 09:34 PM

Hi Blitzk,

I'm pretty sure I know why you are seeing a fixed difference between altitudes. Eagle Tree displays the Mean Sea Level (MSL) altitude, and the other GPS units you have probably display the difference between WGS84 ellipsoid and MSL. The latter is generally more reflective of your ground level altitude, but both are approximations. Some GPS's do MSL, and some do the calculation. Generally, at least in north america, this difference is about 60-90 feet.

We've been displaying MSL since 'day one' so I'm a little leery to change it. Do you need the local alt displayed for a particular purpose?

Regards,

Bill, for Eagle Tree

BlitzK

Aug 26, 2009, 07:45 AM

My Garmin GPS is an aviation GPS that displays MSL. My cellphone, I don't know (but it's the same as my garmin, so I will assume it is also MSL). It's also on par with the USGS Topo maps. Google Earth also uses MSL when displaying altitude.

I went driving and it seems the V3 GPS did get closer to the correct altitude, within 20-30 feet or so which is fine.

You said the GPS "zero's" when plugged into the osdpro, can you have a setting in the data recorder to start at zero when not using the osd?

ragamofyn

Aug 29, 2009, 06:30 AM

All, I have finally tracked down (and hopefully fixed) the bug with negative GPS altitude. The version is in "pre-beta" testing...

Bill, does the latest beta software always include all previous fixes? For example, now that 7.86 beta has now been posted, does it also include the high-res RPM and negative GPS altitude fixes from 7.85 alpha?

Thanks,

Victor

billpa

Aug 29, 2009, 02:14 PM

Hi Victor,

Yes, the fixes are always cumulative. 7.86 has all fixes.

Regards,

Bill, for Eagle Tree

Mr Boombastic

Sep 01, 2009, 05:50 AM