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Old 09-25-2018, 04:45 PM   #1
Titanium Spork
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GPS and/or GLONASS and/or WAAS ?

Just got a shiny new GPSMap 64s, woohoo, now I can give back to OpenStreetMap (thanks Trail Boss)! Oh, and not get lost and stuff. That's also a plus.

Experimenting with the reception on my windowsill on a rainy Montreal day I find that the unit reports a consistently better accuracy with GPS+GLONASS compared to GPS-only. WAAS doesn't seem to make a difference.

Some guy on the Internetz said that using GLONASS and WAAS reduces your battery life, but I dunno if it's significant enough to be a consideration.

Thoughts? Experiences?
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Old 09-25-2018, 05:19 PM   #2
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Re: WAAS

I have no idea if there are any ground-based stations within range of the High Peaks for it to make a difference.


My phone receives both GPS and GLONASS (no control over which one it does or doesn't use). Battery life is fine even over the course of a ten-hour hike (phone has a beefy 3610 mAH capacity).


My old Garmin Rino 530 HCX doesn't support GLONASS. Battery life is excellent (days) and accuracy is fine although sometimes bested by my phone.


Enjoy your new 64s!
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Old 09-25-2018, 05:28 PM   #3
Neil
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I was told using WAAS would reduce battery life. As supposedly does using the electronic compass, recording a tracklog (especially with points very close together) and using the screen with "route up" instead of north up, due to constant re-writing.

The 64 is a nice model.

Edit: Found this re battery life.
http://pureoutside.com/blog/extend-gps-battery-life/
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Last edited by Neil; 09-25-2018 at 05:44 PM..
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Old 09-27-2018, 05:01 PM   #4
ndoggac
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GPS + Glonass will provide you with additional satellites you can use for computing your position. With GPS you might have 8-12 in view, but add in Glonass and you might have up to 18-22 possible satellites in view. This helps a lot with shading scenarios like tree cover, or canyons/valleys. It gives you a better chance of computing your position even when you have an obstructed view of many satellites. Also, generally speaking, more satellites used in your solution = more accurate position, but not always.

WAAS is just one example of a Space Based augmentation system (SBAS). You can read the wiki for more examples in Europe/India/Japan/etc. All current SBAS only provide corrections for GPS satellites, no other GNSS constellations like Glonass/Galileo. Measured performance for GPS only positioning is on the order of 4m vertical, and 2.5m horizontal 95% of the time. Using WAAS should improve your accuracy to approx 1.5m vertical and 0.8m horizontal. You probably won't notice the difference when hiking, and the receiver's estimated accuracy may not even reflect the improved accuracy (eg. it always says 10 foot). However, WAAS was built for aviation purposes, and although it does improve accuracy a good deal, its main focus is integrity. Ensuring a notification to the airplane within 6 seconds if a GPS satellite starts sending a bad signal, or there is some other source of error like space weather (ionospheric error).

SBAS corrections are not the same as RTK corrections, and you do not have to be close to a base station to realize the accuracy improvements like you do for RTK. RTK will get you down to several centimeters of accuracy, but you typically have to be within ~20-30km of the base station. The only time distance from a WAAS base station matters is if you go far out into the ocean, outside the ionospheric correction model that WAAS provides, and that would typically only affect your vertical accuracy.

The SBAS satellites that provide the correction messages are in a geo-synchronous orbit, so they will always be in the southwestern sky (in the ADKs). They can also act as additional ranging sources to aid in shading scenarios, but they will be at lower elevations in the sky (~10-30 degrees) and all in the same section of sky, so their benefit might be limited. Also, most hiking receivers capable of WAAS will only track one GEO for corrections, and will likely not utilize it for ranging. See your satellite tracking screen for details of what your receiver is doing. SBAS GEO satellite numbers are 120-158 (NMEA: 33-71). Presently, the WAAS GEOs are 131(44), 135 (48), 138(51). PRN 133 (46) coming next year.

Both Glonass and WAAS will incur additional power consumption because the receiver is using energy to track and process the additional satellite signals. WAAS probably more so because you also have to process the WAAS correction messages and apply them to your measurements (additional CPU and memory usage). The power consumption has gotten much better with modern GNSS chipsets, but it will likely still be a noticeable difference.

TLDR: For hiking GPS only position is fine and will utilize the least power. If tree cover or geographic shading prevents you from getting a position, then turn on Glonass. If you want maximum accuracy and assurances that GPS satellite signals are good, turn on WAAS.

Last edited by ndoggac; 09-27-2018 at 05:13 PM.. Reason: added stuff
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Old 10-13-2018, 12:45 PM   #5
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@ndoggnac

If RTK base stations need to be no more than 20-30 km away for proper operation, is there a similar max distance for WAAS base stations?

In other words, if there's no WAAS base station within, say, 100 km of your position, is there a noticeable benefit to using WAAS? I have trouble imagining how they'd pull off the trick of ensuring the same degree of accuracy/benefit if you were 25 or 250 km from the nearest base station.


Can you help me interpret the satellite numbering scheme in Locus Map? Do the satellite numbers shown in the first image (Locus Map) correspond to the ones shown in the second image (FAA Satellite Position and WAAS Status).






Or is there some sort of offset required as you (I assume it was you; same moniker) described in this post.

I'm trying to understand if any of these satellites are sending WAAS info and if my Android phone is using it. I see several matches (2, 5, 12, 13, 29, etc) but assume those are vanilla GPS? I believe the WAAS satellites have a higher numbering scheme? I don't know if the ones numbered in the 60's and 80's (seen in the Locus Map image) have anything to do with WAAS. By their shape I believe they're GLONASS.

PS
I know the icon's shape indicates if it's GPS, GLONASS, Beidou, or Galileo (circle, square, triangle, rhombus).
http://docs.locusmap.eu/doku.php?id=...uide:tools:gps
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Last edited by Trail Boss; 10-13-2018 at 12:55 PM..
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