Multi-Band Difference?

Multi band means that the GPSMAP can receive the two signals sent by a single satellite. In the past satellites only sent signals over a single frequency. Later, a second frequency was added to improve accuracy. So a multi-band GPS unit can receive both of these frequency coming from the same satellite.

Combine this with multi-GNSS, the ability to use satellites from multiple GNSS systems (GPS, Galileo, Beidou, QNSS and IRNSS), and accuracy should improve further because there is more data available.

How much more accuracy? That is difficult to say in absolute terms because there are so many factors influencing the quality of the signals. The 67i should perform better in difficult conditions than the 66i. It certainly performs better than the Oregon 700 I had, which only had GPS and Glonass and no multi-band.

Thanks, JungleJim. 

I didn't know GPS satellites were transmitting on two frequencies.  That clears up most of my confusion about the Garmin explanation on the web site. And also why I like this forum.

But it still doesn't answer my primary question.  Is the improved accuracy worth $200?  I know the value of $200.  It's the other half of that question I'm asking about.  As you say, it would be hard to answer it in absolute terms and probably impossible to answer it in absolute terms meaningful to me. 

That's why I'm hoping for pairs of side-by-side simultaneous tracks in different locations.  That would make the accuracy difference and the effect of terrain and canopy meaningful to me. Even better would be tracks along a precisely mapped curve. 

I suspect it would also be worthwhile for Garmin to provide track pairs like that to get shoppers past the decision block and speed sales of both single-band and multi-band devices.

Thanks again, Jungle Jim.  That Youttube link you posted https://youtu.be/GuDkjCqiag4?t=768 is the most helpful thing I've seen on the accuracy improvement from mutli-band processing.   It's a helpful contribution.

www.garmin.com/.../

Here you can find another comparison between 66i and 67i of someone who used both devices to track a couple of hikes.

Great contribution, Jungle Jim. 

For my backcountry navigation purposes on foot or horseback, the multi-band feature showed no advantage on the comparison tracks at the link you posted (https://www.gpsrchive.com/Discussion/viewtopic.php?t=2450).

Two hikes shown there are both excellent comparisons of multi-band vs. no-multi-band. Both were multi-hour hikes through relatively vertical terrain with some heavy canopy.  All devices were comparably mounted on the same hiker.  The tracks are downloadable.  I would have posted the .gpx files here if that were possible.  I wish it were.

I stepped through the tracks on Basecamp.  The 66i (without multi-band) recorded only three track points that were outliers.  All three were in sequence from 6 May 2023, 19:23Z to 19:24Z.  The worst of the three was about 115m off.  They added 0.1 mile to the recorded length of the hike.  I did not see any comparable outliers from the tracks recoded by the 67i or the 66sr, both of which have multi-band.  For record keeping purposes, you could easily delete those three outliers in Basecamp.  They show up immediately in track properties since the hiker's steady 3 mph pace jumps to 64 mph.  Even in the field, those track points would have been obvious outliers on the 66i display.

A couple of things to note in the comparison:

Looking at the images posted at the web site, consider that the test was done with all devices set to their default "auto" tracking resolution according to the description.  From the .gpx files, you see that the default resolution on the 66i is about one-third the resolution of the 67i and the 66sr.  But that has nothing to do with accuracy.  It's just a difference in the default setting, which a user can change.

All devices did amazingly well at recording elevation.  It was almost suspiciously accurate.  They were typically within 50 feet elevation of one another and within 80 feet of what the Garmin Topo NA US 2022.20 map showed.

If knowing your current or average speed in real time is important, then the occasional outlier recorded by the 66i without multi-band would be a problem for that since one or two extreme outliers can have a big effect on cumulative distance and speed.  As I said above, that's only an issue in real time.  Afterwards, it's easy to delete outliers in Basecamp to get fairly accurate distance and speed calculations.

The tests were not done in expedition mode.  I would like to know if the 67i can use multi-band in expedition mode.  Expedition mode on the 66i overrides certain user option settings.  I would also like to see a similar comparison between the 67i and 66i where both are in expedition mode.  At least once a year, I go on a hike where I need to use expedition mode.  I've had two 66i devices.  Both recorded absurd elevations in expedition mode.  I don't mean occasionally absurd.  I mean consistently absurd.  That has led me to believe that this is either a firmware bug or a documentation failure.  By that, I mean that if the 66i is not designed to record elevation in expedition mode, that limitation should be documented and it should not record ridiculous numbers in place of the elevation.  I've reported it multiple times to Garmin through several past firmware revisions.

Hi jlg2, thanks for sharing your insights. You might want to share them in the topic I linked to as well, as there is more activity there than here...

But also take into account that the 66i only supports GPS (and WAAS/EGNOS, for what it's worth), and the 66sr and 67 also support multiple satellite systems (GPS, BeiDou, Glonass for non-inreach devices, Galileo, Glonass, QNSS and IRNSS). So any difference in accuracy is not only due to multi-band (multiple frequencies for same satellite system) but also due to multi-gnss (multiple satellite systems).

As for expedition mode, I do not hike that long so I have got no experience with it

Couple of general comments. 

Large, fast "outliers" are characteristic of multipath. That would be expected in "vertical" terrain. If anything would help with this, I would guess that multi-GNSS would have more of an effect than multi-band. More satellites from which to choose means less likelihood of choosing to use weaker signals, some of which will be multi-path echoes. But it's all speculation - no way to prove it one way or the other.

When comparing altitude readings to reality, you must be careful about device settings. If you choose to include barometric altitude, you have to be careful about how you calibrate it. Generally, properly calibrated barometric altitude is more accurate than GPS-derived altitude. The geometry makes the probable error much higher for vertical distance than for horizontal distance. And, after all, it's a consumer-grade GPSr.

Of course, unless you periodically recalibrate, weather will affect the barometric altitude reading. And if you allow automatic "correction" vis GPS, you have to try to figure out if that helps or hurts. 

Although it does not help with the tracks, you are much better off looking at the contours on a high-resolution DEM map.

Thanks, trwolpert.  You're correct.  But you didn't understand what I intended by "absurd."  In the sense that I used the word, no error from changing barometric pressure would be absurd.  I meant "absurd" in a narrower sense, as in "it couldn't occur in any terrain to any human who survives the changes recorded."