Author Topic: A-GPS? (Read 3353 times)

andyoxon · « **on:** 02 February, 2014, 12:46:29 am »

Do people have this set to on, if tracking with android/mobile, and what is it exactly? I thought it was optional, but apparently Strava app might need it.

frankly frankie · « **Reply #1 on:** 02 February, 2014, 09:50:45 am »

Can't imagine why. I believe A-GPS uses phone cells to achieve a quick'n'dirty initial location, which then allows the normal GPS system to find itself faster, when starting from cold. Presumably also takes over indoors or anywhere where the sats are obscured but there's still a phone signal.

fhills · « **Reply #2 on:** 02 February, 2014, 11:50:26 am »

On this "helping the GPS to locate you faster" by using info such as location services etc, is there any way to tell any non-phone-connected GPSs that you are near a certain place to give it a kick/help it out. So even if you are slightly lost, or have just got off a plane etc, you could tell it to "find near"by for instance selecting a waypoint you had in the device - ie when landing in Rome click on the Forum or Roma Termini railway station. Warning - this may be a very stupid question but it has idly occurred to me when sometimes waiting for my Etrex 20 to figure out where in Europe I've taken it for a dirty weekend. I kind of like the little friend so am perfectly willing to help it out in a spirit of friendship.

citoyen · « **Reply #3 on:** 02 February, 2014, 11:56:04 am »

This doesn't quite answer your question, fhills, but the latest generation of Garmins allow you to save locations you use regularly, in order to help the GPS lock on quicker when you're starting from one of those locations.

fhills · « **Reply #4 on:** 02 February, 2014, 01:03:47 pm »

ok thanks citoyen.

Clearly it wasn't such a daft question

I'll live with the limitations of the Etrex 20 in this regard - lots more is good about it.

TimC · « **Reply #5 on:** 02 February, 2014, 01:53:50 pm »

The old generation of Garmins - Forerunner 201 et al - used to ask if you'd moved far since the last activation if they struggled to get a signal, and then would let you enter a geographical position. The latest devices, if regularly updated (by connection to a Internet-connected computer, for instance) think they are clevererer because they should have a cluemap to help orient the search, and so they don't ask for help. Sometimes they should!

fhills · « **Reply #6 on:** 02 February, 2014, 03:11:04 pm »

Thanks for that TimC.#

Seems my Etrex 20 (and presumably the 30) fall between two stools as it were.

What's the longest I should have to wait for a fix?

I do seem to remember that once or twice I've given up and switched the poor dear off. Was also concerned that it would use a lot of battery - fair to assume that this unit's "where the *** am I" tizz uses a fair bit of power?

Other times it seems quite fast.

Clearly if landing anywhere the first thing you should do is switch the GPS on.

fuaran · « **Reply #7 on:** 02 February, 2014, 05:08:19 pm »

Assisted GPS (A-GPS) can refer to a few different things.
It can be using nearby mobile phone cells or wifi networks to figure out the approx location.
Or it can be downloading the satellite almanac, over wifi or the phone connection.
Maybe also setting the time accurately (using NTP).

It usually helps a lot, much quicker for finding your position. Especially for phones/tablets, most of them seem to have poor GPS receivers. Though A-GPS will probably use a bit of data, so you might not want to use it if on a metered phone connection.
It can also give an approx location if indoors.
A-GPS won't actually help much with overall accuracy (assuming you are outside, and have a good view of the satellites).

Most of the Etrex models let you say you have moved a long way. Usually by going to the satellite screen, then press menu, then "New location". Then you can pick your approx location from a map. Don't know if this works on the Etrex 20/30?

frankly frankie · « **Reply #8 on:** 03 February, 2014, 09:48:50 am »

Quote from: fhills on 02 February, 2014, 03:11:04 pm

What's the longest I should have to wait for a fix?

Just moving it from UK to Rome, and assuming it's been used as a GPS within the last week (for example while travelling to the UK airport) - and assuming you're giving it a decent sky view in Rome (might be difficult) - I wouldn't expect it to take much longer than it usually does. That would still qualify as a 'warm start' and at least some of the expected satellites should still be above the horizon. I suspect a poor sky view wouldn't help.

I do think it's worth while if travelling a longer way - to the far east for example - to have the GPS locked on in the UK as late as possible before departing, so that it has an up-to-date almanac [edit - and the clock is synched]. Also, it doesn't help if waiting for a lock in a new location, if you loom over it and peer at the screen - that just blocks the signals.

Greenbank · « **Reply #9 on:** 03 February, 2014, 09:56:46 am »

https://en.wikipedia.org/wiki/Time_to_first_fix explains the basics of it. Obviously the more modern GPS devices have more clever stuff to help reduce TTFF. But somewhere up to 15 minutes is to be expected if the device has been off for a long time, moved a long way since last use, etc.

I know my eTrex H took 10 minutes or so to get used to being back in the UK a few days after being used in South America.

I'll find out soon as my Edge 705 has been off for over 6 months and I'll be starting to use it again soon.

fhills · « **Reply #10 on:** 03 February, 2014, 10:03:10 am »

Thanks Frankie and Greenbank - it's possible that the time I remember it taking some time it had actually been switched off in the UK for some time.

citoyen · « **Reply #11 on:** 03 February, 2014, 10:14:47 am »

Also worth noting that the Edge 510 and Etrex 10/20/30 use GLONASS as well as GPS, so you might get a quicker/stronger fix on your Etrex 20 if you make sure GLONASS is enabled.

https://support.garmin.com/support/searchSupport/case.faces?caseId=%7Ba3bcf150-1fa1-11e1-73d0-000000000000%7D
[edit: link seems not to work but try googling for "Garmin GLONASS"]

frankly frankie · « **Reply #12 on:** 03 February, 2014, 02:52:18 pm »

It looks impressive but I seriously doubt if the additional found sats actually do much.
Unfortunately, it's hard to prove because, while there's a menu option to turn GLONASS off leaving GPS on, there isn't an option to do it the other way round. American kit, pah.
Switching GLONASS on and off didn't seem to have any impact on the 'accuracy' estimate, either with a clear sky view or with a very restricted one. Certainly not with EGNOS enabled.

citoyen · « **Reply #13 on:** 03 February, 2014, 03:11:12 pm »

I don't really understand how EGNOS works but I can imagine it would do more to improve GPS accuracy than just adding a secondary satellite system. I can see how adding GLONASS might enable a quicker fix in some circumstances though.

Do you know which Garmin (or other) units are EGNOS enabled?

andyoxon · « **Reply #14 on:** 03 February, 2014, 04:41:03 pm »

Thanks. Partly in response to my crashing Strava app, their support said "You must have A-GPS turned on in order to record any activity." I haven't asked why, but it didn't seem to explain the app crashing after 40km, when I took a photo. Neither was I totally convinced by 'running out of memory' if operating MyTracks simultaneously with Strava* as a failsafe.

* given up on this for the time being.

fuaran · « **Reply #15 on:** 03 February, 2014, 05:22:13 pm »

Quote from: citoyen on 03 February, 2014, 03:11:12 pm

I don't really understand how EGNOS works but I can imagine it would do more to improve GPS accuracy than just adding a secondary satellite system. I can see how adding GLONASS might enable a quicker fix in some circumstances though.

Do you know which Garmin (or other) units are EGNOS enabled?

EGNOS uses a network of ground stations across Europe. These measure the accuracy of the GPS or Glonass signal, and how much it is offset due to atmospheric conditions etc. This is then sent to the EGNOS satellites, which send the details to your GPS receiver. So it usually does improve accuracy a bit.
Note the EGNOS satellites are geostationary (above the equator), so to receive them you need a fairly clear view to the south (especially if you are at further north).

I think just about all current Garmin models are EGNOS enabled, plus most models from the last five years or so. At least the 'outdoor' models, not sure about the running watches, or in-car models.
Note the specifications might list it as "WAAS", which is the equivalent USA/Canada system.

frankly frankie · « **Reply #16 on:** 03 February, 2014, 06:29:35 pm »

In the screenshot above, the first five bars show a small 'D' superimposed - which indicates that WAAS/EGNOS is enabled and allegedly correcting those signals. The 'D' stands for 'differential' I think.
You can also see (from the circular display) that this GPS is 'travelling' south and so is likely to have a good sky view in a southerly direction - if it were travelling north the rider's body would attenuate those signals.
Older GPSs that only say 'WAAS' in the menus, will still pick up EGNOS OK. It may be a minor battery hit though, and not really necessary on a road bike - I normally leave it turned off, only turned it on for the sake of this 'full house' screenshot.

Ningishzidda · « **Reply #17 on:** 04 February, 2014, 07:22:40 am »

http://www.nbmg.unr.edu/Staff/pdfs/Blewitt%20Basics%20of%20GPS.pdf

Differentiated

Reading through that pdf, it could mean "Dependent".

citoyen · « **Reply #18 on:** 04 February, 2014, 10:24:21 am »

Quote from: fuaran on 03 February, 2014, 05:22:13 pm

This is then sent to the EGNOS satellites, which send the details to your GPS receiver.

This is the bit I don't quite get. EGNOS uses different satellites to GPS? And the correction data is sent you your receiver, not applied to the location signal transmitted by the satellite? I am blinded by science.

Anyway, I've looked into it some more and the Edge 510 isn't EGNOS/WAAS enabled. I presume therefore that the 800/810 isn't either. The Etrex range is EGNOS/WAAS enabled though.

Out of interest, I did a little comparison of satellite reception between my Edge 510 and venerable ForeRunner 405CX, and even with GLONASS switched on, there doesn't seem to be any difference in accuracy between them, though I suspect the Edge will perform better in built-up areas, with or without GLONASS - I'll test this theory when I next go for a lunchtime run by taking both with me and seeing how the tracks compare.

Looking forward to this all changing again in a few years when Galileo is up and running... if that ever happens.

Greenbank · « **Reply #19 on:** 04 February, 2014, 11:17:23 am »

Quote from: citoyen on 04 February, 2014, 10:24:21 am

This is the bit I don't quite get. EGNOS uses different satellites to GPS? And the correction data is sent you your receiver, not applied to the location signal transmitted by the satellite? I am blinded by science.

Read the "History" section here: https://en.wikipedia.org/wiki/Differential_GPS

It seems to be a constant game of "it's too accurate, lets mess it up a bit, but that's global so lets fix it locally with more satellites and ground stations..."

EGNOS is 4 satellites and a bunch of ground stations. According to Wikipedia[1] it's mostly useful for aviation users as the EGNOS satellites are very low in the sky for northern latitudes.

1. https://en.wikipedia.org/wiki/European_Geostationary_Navigation_Overlay_Service

TimC · « **Reply #20 on:** 04 February, 2014, 11:33:39 am »

Selective Availability, which was the only deliberate downgrading of GPS accuracy, was removed in 2000. By then, WAAS systems of DGPS had pretty much negated the accuracy downgrade in the continental US and Europe. About that time, aviation GPS systems were quoting less than 10m accuracy. Nowadays it's around 3m. That's why we very often offset from airway centrelines - you can be certain that if two GPS-guided aircraft were heading towards each other at the same altitude on an airway, they would collide 100 times out of 100 - and it wouldn't be a glancing blow.

Kim · « **Reply #21 on:** 04 February, 2014, 03:08:02 pm »

It was the removal of Selective Availability that made useful consumer GPS applications like car navigation systems possible. That's why they all appeared in the noughties, in spite of the underlying technology being available in the 90s.

90s civilian GPS receivers were great for navigating at sea and on aircraft, but wouldn't be precise enough to automagically navigate a car through an urban environment, let alone tell you which side of the road you were on.

I had the opportunity to join some Vodaphone engineers on a signal-mapping expedition in 1996. They were using high-end GPS receivers of the time, but without differential augmentation (which they were equipped for, but not using for that particular survey), the resulting data was full of random offsets. A few years later my original yellow eTrex was at least an order of magnitude more accurate, and those are pretty flaky compared to modern chipsets in all but ideal circumstances.

TimC · « **Reply #22 on:** 04 February, 2014, 04:27:07 pm »

I still have (in my techno-vault in the loft) a Lowrance hand-held GPS from around 1992. I used to use it in the air as it was rather better than the onboard kit we had on most RAF Hercules at that time - Omega and Loran C were our long-range aids, though a very few Special Forces Hercs had P-coded GPS by then. The Lowrance took an age to start up, could only accept 10 lat-long waypoints (agonisingly painful to input) and only interrogated up to 8 satellites - and it cost a fortune - but it was a revelation in accuracy, even though that was about 30-100m.

Kim · « **Reply #23 on:** 04 February, 2014, 05:42:53 pm »

A friend of mine's dad worked as a geologist in the oil industry, and had an early Garmin handheld (I suspect a GPS 45). As a group of nerdy teenagers in the days when GPS was rarely seen outside of Tomorrow's World and ship/aircraft cockpits, we spent a couple of hours wandering round the local fields with it, and decided it was witchcraft. Never imagined I'd have one on my bike.