CO 400t Compass & WAAS (Two Questions)?

[+Ratsneve]

1. If you leave the Compass on Auto (Off is only other choice) and it starts building up an error how is this detected or known while using the CO? If you turn the Compass off (suggested to conserve battery life) does the "other" compass continue to function error free while so long as you are moving? Hmmm... If the Compass is turned back to Auto (On) and calibrated but now it has built up a lot of error does this conflict with the GPS operation when moving? When you come to a stop does the compass swing off course demonstrating its error and then goes back on course once you start moving again?

 

2. It has also been suggested to turn WAAS Off to Normal (no WAAS) to conserve battery life. I'm wondering how important WAAS actually is in North America? I have it On now inside my wood house showing 10 satellites tracking with a 14 ft accuracy.

 

BTW, 9 of those sats are showing a "D" at their base. What does that mean? Are there other codes--I haven't seen any others--only the "D". Sometimes the bar is white. Sometimes the sat is gray. Sometimes the sat is gray flashing on and off.

 

Okay, I'm showing 11 blue bars with all but sat 51 showing a "D" and sat 02 showing no blue bar and the accuracy at 14 ft. I'm turning off WAAS now without moving the CO and we'll see what happens...

 

Okay, after around 10 minutes there are 11 sats showing bars. 02 is a white bar and its sat is flashing on and off working its way into acquisition? Sat 51 has disappeared left the scene. None of bars have "D" so that must be some indication of the sat having WAAS capability for the GPS user? The accuracy got better for a time down to 13 ft but has gone out to 18 ft suddenly--now back to 14 ft. Whew... So WAAS doesn't affect accuracy at all here??

 

Do weak batteries, after the first warning of low batteries affect any of the GPS functions besides the backlight?

 

Going back to WAAS now... After around 10 minutes the "D" on the blue bars have not returned? I wonder why? 10 sats tracking but 48, which is new, is white bar but sat is flashing. Sat 02 has stopped flashing but remains gray and has no bar. Ha! The "D"s just all came on for 9 of the sat blue bars. Accuracy has varied from around 17 to 14 ft. Elevation has varied 7 ft. Why did the "D" (WAAS?) designations take so long to return?

 

If you are willing to shed any detail knowledge on this I'll certainly be interested. In summary though it looks like neither the stationary compass nor WAAS matters at all. Thank you.

[+twolpert]

First your questions on the compass.

 

The compass has nothing at all to do with accuracy. To understand what the compass does, you have to think a bit about the information that the GPSr shows you.

 

In geocaching, the "red pointer" tells you which way you should go in order to reach the cache. In effect, the red pointer shows you the difference between the direction you are going and direction you should be going. If the red pointer points straight ahead, you are going in the right direction; if it points to the left, you should move more toward the left; and so on.

 

Similarly, if you have the unit in track up mode, the map is always oriented so that your track (the direction you are moving) is at the top of the map.

 

Both of these things require the unit to know which way you are going. With the compass off, the unit depends on satellite information. If you are moving, the unit uses repeated position samples to determine direction of motion. If you are not moving, this information is not available. You can no longer rely on the red pointer or the map orientation. When the compass is in auto mode, the unit relies on the compass at low speeds and on satellite information at higher speeds. In particular, when you are standing still, the unit relies on the compass to determine which way you are facing. This influences the red pointer as well as map orientation in track up mode. If the compass is not calibrated, or if you do not hold the unit level, the direction you are facing is not reliably known. The red pointer may swing, map orientation may change at random, and so forth. Regardless of visible symptoms, the information is not reliable in this case either.

 

Bottom line: If the compass is off and you are not moving, you cannot rely on anything which requires the unit to know your direction of travel. Things return to normal when you being moving again. If the compass is on and calibrated, and you hold the unit level, you can rely on this information with the understanding that it is now based on the direction you are facing. If you are moving slowly, you also need to be aware that the unit may switch between satellite and compass without warning. This can cause some confusing readings, especially in challenging conditions.

 

Next, your questions on WAAS.

 

WAAS is a combined ground/satellite method of correcting for errors due to localized atmospheric conditions. Information gathered from ground stations is transmitted to properly equipped GPSrs via a separate set of satellites. The unit then uses this additional information to correct for local (more or less) atmospheric conditions. This requires the unit to do extra computations, which is why it consumes power.

 

The "D" in the satellite displays indicates that WAAS corrections are in use. In order to use WAAS, you must turn it on. At least one of the separate WAAS satellites must be in view. After acquiring the normal GPS constellation and the WAAS satellite, the unit must have time to incoporate any WAAS-based corrections into its computations. The combination of acquisition time and computation delay accounts for the delay you see when you enable WAAS.

 

The utility of WAAS for recreational devices is open to debate. At any given time, you may or may not see any difference in position or reported EPE. Remember that WAAS is only good for correcting errors due to local atmospheric conditions. Those errors are normally a relatively minor component of any inaccuracy in your position. Things like the geometry of the normal GPS satellite constellation, the set of satellites your GPSr can see (based on proximity to steep hills, cliffs, tall buildings, etc.), and the reception conditions (for example, tree cover) are much more important.

 

Variations in your reported position (and EPE) over time are normal in a recreational GPSr, especially with a high-sensitivity receiver. The GPS satellites are not geosynchronous. The set of satellites your receiver can see varies with time. Multi-path reception (caused by tree cover, proximity to surfaces which reflect radio signals, etc.) can cause abrupt shifts in your apparent position. The reported EPE is a probabilistic interpretation of the possible error in your position. Typically, EPE will vary from 12-15 feet up to about 30 feet. Anything over 30 feet means either very bad reception conditions or (possibly) that you have entered the twilight zone of Colorado position errors. In the latter case, power-cycling the unit will cause the EPE to drop to a more normal level -- and will cause your position to readjust to reality.

 

Bottom line: WAAS probably won't help a lot. A lot of cachers leave it off (except possibly when placing a cache).

[+Ratsneve]

First your questions on the compass.

Bottom line: If the compass is off and you are not moving, you cannot rely on anything which requires the unit to know your direction of travel.

 

Next, your questions on WAAS.

Bottom line: WAAS probably won't help a lot. A lot of cachers leave it off (except possibly when placing a cache).

Thank you for the great info Tom. Until there is some reason to be concerned about battery consumption I'm going to leave both features on.

 

I haven't seen any compass error so far on my CO but it will be very interesting to follow a course with the compass in a vehicle and see what the compass might do when you hit a red light and the GPS directional updates stop.

 

Does the CO use all WAAS corrections or only GPSs within some radius of my location? Atmospherics here in central Oregon might be more important then atmospherics in Bose, Idaho?

 

I wonder what the "D" actually stands for, "ground Data"? Also, do you know what a white bar instead of blue means; and also, what a grayed-out GPSs vs a grayed-out GPSs that is blinking means? I think the white bar means that even if the single strength is high enough the data is unreliable and the GPSr doesn't use it. This might relate to the satellite being light gray but solid. Flashing means the stellite or rather the GPSr is either gaining or losing acquisition?

 

Thanks again.

Edited June 30, 2008 by Ratsneve

[+SparkyInCali]

The "D" stands for differential meaning the Waas system has indicated the is a postional difference between what you are recieving and what is correct.A blue bar means the unit is locked onto that satllite signal and it is being used in the positional calculation.This satellite will be solid grey on the screen.A white bar is usually linked to a blinking satellite meaning the unit has located the signal for this satellite but it is not being used in the position calculation.

[+twolpert]

Does the CO use all WAAS corrections or only GPSs within some radius of my location? Atmospherics here in central Oregon might be more important then atmospherics in Bose, Idaho?

Essentially, you're going to see corrections which are appropriate to your location. Corrections are based on information provided by wide-area reference stations (WRS) on the ground. These are scattered throughout North America. If you're interested in more details on WAAS, the Wikipedia article isn't bad. And the FAA has a simplified explanation. Information about the location and currents status of the North American WRS can be found on the map page at the Standford test master station (TMS) web site.

[+Ratsneve]

Essentially, you're going to see corrections which are appropriate to your location. Corrections are based on information provided by wide-area reference stations (WRS) on the ground. These are scattered throughout North America. If you're interested in more details on WAAS, the http://en.wikipedia.org/wiki/WAAS Wikipedia article isn't bad. And the FAA has a http://www.faa.gov/about/office_org/headqu...aas/howitworks/ simplified explanation. Information about the location and currents status of the North American WRS can be found on the map page at the http://waas.stanford.edu/tmslive/ Standford test master station (TMS) web site.

Thank you for the basic info as it relates to my CO screen--I don't think I'll delve deeper write now certainly but I removed the html code to reveal the site locations so I can print this out and keep it.

 

I'm afraid there is one other area of interest with the CO that actually fits in nicely here. That is the air and water temperatures and barometric pressure readouts the CO has. Air temp I know is way way off and it doesn't relate to holding the unit in the hand. A tech at Garmin last week thought these temperatures and the pressure all came from the GPSs and you mention these WRS (stations) so they may be part of this too? If true can these temperatures be made more meaningful--actual ground ambient temperature where the GPSr is located and is the barometric pressure readout accurate?

 

I was thinking that an external temperature probe was a solution but that does not seem to be the case right now. If you can montior heart rate and cadence why not temperature too?

 

Thanks.

[+twolpert]

The ambient air temperature readings on the Colorado are notoriously inaccurate -- to the point of being completely useless.

 

There has been debate about the accuracy and usefullness of the built-in barometer. If you search the forums for the words Garmin and barometer, you'll see several long threads by bigots on both sides of the argument.

 

The following assumes that you are using the device to read altitude (Variable Elevation mode), not barometric pressure (Fixed Elevation mode). My personal take is that it's only as good as the calibration. Most people don't have the information necessary to manually calibrate it when working in the field (accurate altitude or air pressure at their current location). In any case, if you move far enough (or wait long enough), you will see barometeric pressure changes unrelated to altitude change. As a result, most people leave it in the "auto calibrate from GPS" mode. What this means is that you are more or less continuously calibrating the device using the altitude reading provided by the GPSr. In this mode, you may get slightly better instantaneous altitude information in challenging reception conditions. But in general, the altitude reported by the barometer is going to be very close to the altitude reported by the GPSr itself. Except on the satellite page (where you always see GPSr elevation) and when selecting a point on the map (when you see DEM elevation from the map data), all elevations displayed by the unit are those reported by the barometer.

 

As far as I know, the only interaction between WAAS and the barometer is that WAAS corrections provide better GPSr-based altitude information. This helps make the barometer more accurate in auto-calibration mode.

 

g-o-cachers' wiki has excellent information on the barometer.

[+Ratsneve]

The ambient air temperature readings on the Colorado are notoriously inaccurate -- to the point of being completely useless.

 

There has been debate about the accuracy and usefullness of the built-in barometer. If you search the forums for the words Garmin and barometer, you'll see several long threads by bigots on both sides of the argument.

 

The following assumes that you are using the device to read altitude (Variable Elevation mode), not barometric pressure (Fixed Elevation mode). My personal take is that it's only as good as the calibration. Most people don't have the information necessary to manually calibrate it when working in the field (accurate altitude or air pressure at their current location). In any case, if you move far enough (or wait long enough), you will see barometeric pressure changes unrelated to altitude change. As a result, most people leave it in the "auto calibrate from GPS" mode. What this means is that you are more or less continuously calibrating the device using the altitude reading provided by the GPSr. In this mode, you may get slightly better instantaneous altitude information in challenging reception conditions. But in general, the altitude reported by the barometer is going to be very close to the altitude reported by the GPSr itself. Except on the satellite page (where you always see GPSr elevation) and when selecting a point on the map (when you see DEM elevation from the map data), all elevations displayed by the unit are those reported by the barometer.

 

As far as I know, the only interaction between WAAS and the barometer is that WAAS corrections provide better GPSr-based altitude information. This helps make the barometer more accurate in auto-calibration mode.

 

g-o-cachers' wiki has http://garmincolorado.wikispaces.com/Altimeter+and+Elevation excellent information on the barometer.

Thank you. I think that wraps up this end of it for the time being.