Manitoba Geocaching Association

lizardo · Joined: 02 Sep 2005 Posts: 497 Location: Winnipeg

Am I wrong or does the elevation entry in a waypoint have any bearing on the accuracy of your position? I have noticed since using GSAK for transferring my waypoints into my Meridian, that I need to enter the elevation on site at times in order to get an accurate reading. I used to go to Google Earth to gather the elevation info of the area, but once I started using GSAK, I stopped bothering. Could it be that the extra time it takes for me to enter the elevation onsite allows the GPSr the time to receive a more accurate reading? I could be wrong, but I am sure that the Waypoint moves when the elevation is adjusted even to Manitoba's meager altitude.

Because of the reflective position of Manitoba versus the Equator where (I think?) most of these satellies reside, would elevation not play a part in accuracy? Or should I turn off 3D and live in the Homer Simpson 2D world? I seem to only get WAAS readings when I am in 3D mode, I have yet to see WAAS pop up in 3D.

Thoughts?

ertyu · Joined: 25 Apr 2005 Posts: 556 Location: Winnipeg

Elevation should have no bearing on 2D positioning. Also remember that GPS is very poor at producing accurate elevation data.

lizardo · Joined: 02 Sep 2005 Posts: 497 Location: Winnipeg

I have been messing with this quite a bit today. I really just needed to do some more research. I found that elevation has no bearing on the final position. At least as far as I have found. Thanks ertyu.

TurdleEggs · Joined: 31 Jul 2005 Posts: 687 Location: Ste Genevieve

Do you actually get WAAS adjusted signals? My Garmin Etrex Legend sure doesn't! I understood there were only two WAAS satellites - one over the Atlantic and one over the Pacific - pretty much over the Equator (I don't have a globe in front of me so that may not make any sense) and you'd be lucky if you got WAAS on the EastWest coast in Canada.

ertyu · Joined: 25 Apr 2005 Posts: 556 Location: Winnipeg

I get WAAS once and a while. They appear on the east and west horizons so they are often not in view, but this is the prairies.

junglehair · Joined: 05 Apr 2005 Posts: 374 Location: Winnipeg

The elevation should have no effect on the accuracy of your position. Your GPS determines your position by calculating the distance that you are from each satellite. Each satellite sends out a unique radio signal which is detected by the GPS. There is a time delay between when it is sent from the satellite and when it is received by the GPS unit. That time delay is what is used to calculate the distance that the signal travelled. Remember your high school physics class? (distance = velocity * time). The signal travels at the speed of light.

Now think about drawing a sphere at that distance around a satellite. The GPS unit knows that you are located somewhere on the surface of that sphere. Once it has calculated the distance to 3 or more satellites, it can determine your position as the location where the spheres intersect. This is similar to triangulation, except there are no angles involved so it's called trilateration (ok, more information than you needed to know, right?)

The GPS satellites actually orbit the earth twice each day. They are constantly moving, which is why your accuracy will change depending on the time of day. You will achieve the best accuracy when the satellites you are tracking are evenly dispersed across the sky.

The WASS satellites are two geo-stationary satellites. That means that they are always located in the same position over the earth. They are at the equator - one over the Atlantic Ocean and one over the Pacific Ocean. It is unlikely that you will be able to pick up a signal from the WAAS satellites in Manitoba.

Hope that helps.

lizardo · Joined: 02 Sep 2005 Posts: 497 Location: Winnipeg

Holy Smoke, Junglehair! Well done. That is basically what I was able to determine. But thank you for that. Yours was easier to understand than most of the text I was able to find.

I am getting WAAS with some regularity. This morning for instance in St. Vital Park. I have a Magellan Meridian. The signal I have been getting is really strong even when I am in the bush. My unit usually shows less than 10m EPE when WAAS isn't working, and good old Magellan always zeros in on the right location (It's just my eyes that sometimes aren't so good). There's probably some kind of radiation leak in my GPSr that is causing this and I will likely develop some sort of a tumor soon.

Lizardo

Peter and Gloria · Posted: Sep 07, 2005 6:25 am Post subject: RE: Elevation Issues

As long as we're not in a dense forest, both our Magellean Sportrac Map and our Geko 201 consistantly show that they are receving the WAAS signals.

junglehair · Joined: 05 Apr 2005 Posts: 374 Location: Winnipeg

It's cool that you can pick up the WAAS signal. I'll have to check next time I'm in Winnipeg.

TurdleEggs · Joined: 31 Jul 2005 Posts: 687 Location: Ste Genevieve

I didn’t seem to be receiving WAAS so I turned it off to save battery. After hearing other success stories, I'll turn it back on again and see if I have any luck. Great explanation Junglehair.

Peter and Gloria · Posted: Sep 09, 2005 9:03 pm Post subject: RE: Elevation Issues

For any Garmin owners:

Did you know that you are receiving WAAS when you see "D"s on the Signal Strength Bars. The manual says :The "D" stands differential connections and denotes a WAAS Satellite.

ertyu · Joined: 25 Apr 2005 Posts: 556 Location: Winnipeg

Along the lines of WAAS, Anik F1R was successfully launched and it is carrying a WAAS payload for Canada.

TurdleEggs · Joined: 31 Jul 2005 Posts: 687 Location: Ste Genevieve

I got WAAS. I re-enabled WAAS on my Etrex Legend and it started working for the first time yesterday evening. 3m accuracy. I've had it turned on for the last three weeks or so, so I'm not sure what changed yesterday, or maybe conditions were just right.

MHz · Joined: 20 Apr 2005 Posts: 292 Location: Morris

I'm glad other people are seeing the WAAS signal here. Someone, who shall remain nameless, didn't believe I was recieving it, so I didn't say any more on the topic to anyone but happily enjoyed WAAS on a somewhat regular basis. Good to see I'm not loosing my mind.

I am wondering if it is WAAS or a differential ground station. I know the U of Manitoba has a Differential Ground station setup on the roof of the Wallace Building (Geology) on the Fort Garry campus. The person (a GIS researcher and prof) who arranged for it says that it's there for research and commercial use only and you have to pay a subscription to a 3rd party company (whose name escapes me right now) to use it.

I also heard many rumors of a ground station being setup in Niverville, MB for farming use but never got that rumor confirmed.

-- Edited by MHz at 08:45, 2005-09-29

grnbrg · Posted: Sep 29, 2005 2:47 pm Post subject: RE: Elevation Issues

MHz writes:

I am wondering if it is WAAS or a differential ground station.

Unless you know it's DGPS, because you've got a separate DGPS signal reciever plugged into your GPSr, or have paid extra to have it included (ie: a survey or aviation grade GPSr) it's WAAS.

DGPS requires the reception and integration of a discrete correction signal that is broadcast from a terrestrial source, which requires extra (expensive to very expensive) hardware. WAAS corrections piggy-back on the existing GPS satellite signal, so it's merely a question of firmware.

I know all my "D" readings have been WAAS, because I only see them when I have a lock on satellite 35, the WAAS master sat.

grnbrg