Why Multi-Band (L1/L5) per Garmin Engineer

[gpsfan]

An interesting take on L1/L5 by a Garmin engineer here https://www.garmin.com/en-US/blog/outdoor/why-multi-band-qa-with-garmin-engineer-jared-bancroft/

So very much like SBAS it's a way to increase reliability more than accuracy, unlike what most people (want to) think ;-)

It's not until E6/HAS 448bps arrives with its RT broadcast of PPP data that standalone accuracy will truly be sub-meter, with a best of 20cm.

PS - also check out his "tongue-in-cheek" comment about people who think the receiver can show the actual accuracy of the position : " Accuracy is generally thought of as the error from the true point. However, if the receiver knew the exact error from the true point, it would just give you the true point."

[Jeff 5050]

A given in the error budget of the GPS signal is space is refraction of the signal entering and transiting the atmosphere.  There are generic constants applied that account for a good part of that refraction but they are just that, generic.  One significant benefit of multi band is that being different frequencies means the refraction index can be determined to a closer degree.  Not only do the different layers all have different refractive indexes they also change in reaction to different external events like day/night transitions, solar storms and seasonality to name a few.  The capability has existed for a very long time for a multi band receiver to look at the arrival times of the signals, calculate how each has been refracted and come up with a more accurate index to apply to the nav solution.  I don't know if the "old" error budget has a lot of relevance today but in the 90's the refraction issue was a significant part of the error budget in the nav solution.  For what it is worth!

Jeff

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[Sean Barbeau]

Thanks for sharing, good article!

I found this article for oneNav in GPSWorld talking about building an L5-only receiver for mobile devices interesting too:

https://www.gpsworld.com/l5-only-receiver-designed-for-mobile-phones/

It discusses various advantages of L5 signal design and complexity of the required steps to build an L5-only acquision. It also mentions that mobile devices currently acquire L1 and then L5, which matches my observations of a Pixel 5 and Galaxy S21 - in GPSTest L1 and E1 show up first, the device acquires a fix, and then a few seconds later L5 and E5a will show up in the Status view.

Jeff - And yes, to my knowledge in theory eliminating ionospheric interference by leverage both L1 and L5 carriers from the same satellite should still help with accuracy in the mobile world! Although I'm not sure any mobile devices is actually doing that today - I'm not sure if it's a function of antenna quality.

Sean

To view this discussion on the web visit https://groups.google.com/d/msgid/gpstest_android/72de6fd0-d4f1-51ee-8351-2aede246a9dd%40gmail.com.

[gpsfan]

Yes I'd linked to the oneNAV solution in the 3DML topic here https://groups.google.com/g/gpstest_android/c/ToBk2zgfdz8 - What they're doing is a different from "our" receivers that sort of "stack up" all the signals available and try to figure out how to best use them. I'm not getting the sense that so far L5 or E5 are being used "differently" than the L1 feeds from Glonass, Beidou, etc...to get more accuracy.

As for deriving iono and tropo corrections by analyzing the L1/L5 no one is doing that in RT right now on mobile solutions AFAIK. SBAS broadcasts these corrections anyway but I haven't seen any IRL experiements showing improvements in accuracy on the horizontal plane, but there are some for the elevation.

uBlox did get 1 meter accuracy on the uBlox 7P chipset with what they called "carrier phase tracking" but they gave up on that with uBlox 8 and 9. It's just more convenient to use RTK, as you immediately get centimeter accuracy. It's like night and day compared to any non assisted positioning. Even an RTK Float solution will give you decimeter accuracy but you need raw observations for that and...other 3 or 4 smartphones it's conspicuously absent so which developer is going to bother supporting Android raw observations in their app ?

[Lodro Gyamtso]

the person who wrote the garmin article has low knowledge or telling lies to the costumers

before 40 years, YES there was only 2 signals, L1 and L2, the first for civilian and the second for military.

please read this article
https://www.gps.gov/systems/gps/modernization/civilsignals

But the last 16 years (2005) we have 2 civilian signals, L1C/A and L2C. From 2015 we have 3 civilian signals (L5) and from 2018 we have 4 civilian signals (L1C). Yes, not all of them has 100% finished, but the L2C has. So, into year 2021 we can use L1C/A, L2C, L5, and L1C. WHY garnim don't use from 2005 the L2C ?

The military has at least 2 signals L1M and L2M, and civilian has L1A/C and L2C. Sorry garmin, there are 4 signals and not 2. (m= means military and c letter means civilian)

Companies who makes cars with auto pilot like Tesla, use L2C from the beginning. Soon and the new L5.

Those are that i have found. Sorry garmin .....