Author Topic: Garmin 500 taking off?  (Read 1551 times)

Garmin 500 taking off?
« on: June 28, 2017, 09:03:43 pm »
Still using my 500 even though I have the 520. It was working fine to work today, then I started a new lap to ride the three miles to the shops (sad, I know). Along the way, I noticed I was doing 100's of mph. Started a new lap on the return leg, which was fine. Uploaded the file onto Garmin Connect when I got home, and it shows a line heading into the Atlantic, and the three mile lap is 983 miles. Did an aeroplane pick up the signal or something, or the satellite it was connecting to suddenly change orbit?
Bikes are for riding, not cleaning!

Feanor

  • It's mostly downhill from here.
Re: Garmin 500 taking off?
« Reply #1 on: June 28, 2017, 09:31:13 pm »
Just one bad fix many miles from where it should be.
Happens from time to time.
Just edit out the spike.

Re: Garmin 500 taking off?
« Reply #2 on: June 28, 2017, 09:35:40 pm »
Wow. In all the years, that's never happened. Cheers!
Bikes are for riding, not cleaning!

Re: Garmin 500 taking off?
« Reply #3 on: June 29, 2017, 05:50:01 am »
It's happened to me (once) with an Etrex 30 - no idea why, and never been repeated
R10000 x 2   RRtY x 7    SR x 7    E = 128

Kim

  • Timelord
Re: Garmin 500 taking off?
« Reply #4 on: June 29, 2017, 01:55:17 pm »
Used to happen all the time when GPS receivers were less sensitive and had fewer channels.  A bit of tree cover would often be enough to attenuate the signal beyond the point of reliability, and they were almost unusable in urban conditions due to multipath.

These modern ones that can maintain a fix inside buildings are pure witchcraft.  The SNR ratio is the electronic equivalent of maintaining a normal conversation while standing next to a motorcycle revving its wossnames off.
Careful, Kim. Your sarcasm's showing...

Salvatore

  • Джон Спунър
    • Pics
Re: Garmin 500 taking off?
« Reply #5 on: June 29, 2017, 02:32:10 pm »
Such behaviour was even more commonplace in the days of Selective Availability (SA) (pre May 2000). I once found myself whizzing up the Scottish coast at the speed of a jet fighter.

http://www.gps.gov/systems/gps/modernization/sa/

[warning: possibly misremembered/misinformed stuffs] Even though the US military had GPS units which were immune to SA and could give an accurate fix, there weren't enough to go around during the 1st Gulf War, so they had to turn off SA and use civilian units, which made it rather pointless.
Quote
et avec John, excellent lecteur de road-book, on s'en est sortis sans erreur

Phil W

Re: Garmin 500 taking off?
« Reply #6 on: June 29, 2017, 03:11:27 pm »
The US GPS satellites broadcast on more than one frequency. The military GPS handheld units use a different broadcast frequency to the civilian signal. The SA was only ever applied to the civilian signal. It was Bill Clinton who signed the order to remove SA, though whether it was related to Gulf wars I have no idea but the first one was back in 1990-91, so a while before it was turned off.  We do of course now also have the European built and owned GPS satellites orbiting around up there.

Kim

  • Timelord
Re: Garmin 500 taking off?
« Reply #7 on: June 29, 2017, 03:12:08 pm »
I thought SA just gave you a hundred odd (horizontal) metres of noise.  In the 1990s GPS receivers were firmly in the category of 'expensive toys for grown-ups' so I only got to use them on two occasions[1], and accuracy was in that sort of range.

I suspect unexpected jet fighter behaviour was down to the performance limitations of the kit of the time, rather than selective availability itself.  Unless the military were playing wargames and mucking about with the signal.  I expect that was a lot more common when civilians weren't using GPS for almost everything.


[1] One was a survey-grade thing rackmounted into a Vodaphone engineering car, the other an early handheld Garmin model which took forever to get a fix.
Careful, Kim. Your sarcasm's showing...

Re: Garmin 500 taking off?
« Reply #8 on: June 29, 2017, 07:29:00 pm »
It was Bill Clinton who signed the order to remove SA, though whether it was related to Gulf wars I have no idea
Even before SA was turned off, it was largely cancelled out by WAAS/EGNOS. I thing the main driver for turning it off was that ships, civil aircraft etc were using GPS, and they didn't want expensive/embarrassing accidents.
These days, they can jam the GPS signal for selected areas instead (there are occasional warnings in the hiking press if it's jammed in the UK).

Phil W

Re: Garmin 500 taking off?
« Reply #9 on: June 29, 2017, 09:23:08 pm »
I thought SA just gave you a hundred odd (horizontal) metres of noise.  In the 1990s GPS receivers were firmly in the category of 'expensive toys for grown-ups' so I only got to use them on two occasions[1], and accuracy was in that sort of range.

I suspect unexpected jet fighter behaviour was down to the performance limitations of the kit of the time, rather than selective availability itself.  Unless the military were playing wargames and mucking about with the signal.  I expect that was a lot more common when civilians weren't using GPS for almost everything.


[1] One was a survey-grade thing rackmounted into a Vodaphone engineering car, the other an early handheld Garmin model which took forever to get a fix.

So SA shifts you 100 metres one way then 100 metres the other way for fixes a second apart. So 200 m / s which translates to 720 km / h. Sounds like fighter jet type speeds to me.

Kim

  • Timelord
Re: Garmin 500 taking off?
« Reply #10 on: June 29, 2017, 09:25:20 pm »
Yes, but surely the speed display on the SA-era GPS receiver will low-pass filter that out (just as they do with the <3m fluctuations today).  And it won't whizz you up the coast.
Careful, Kim. Your sarcasm's showing...

Phil W

Re: Garmin 500 taking off?
« Reply #11 on: June 29, 2017, 09:35:06 pm »
It was Bill Clinton who signed the order to remove SA, though whether it was related to Gulf wars I have no idea
Even before SA was turned off, it was largely cancelled out by WAAS/EGNOS. I thing the main driver for turning it off was that ships, civil aircraft etc were using GPS, and they didn't want expensive/embarrassing accidents.
These days, they can jam the GPS signal for selected areas instead (there are occasional warnings in the hiking press if it's jammed in the UK).

EGNOS didn't exist till long after  SA was turned off and WAAS only in test mode at the time coming on stream (test mode) end of 1999 and not used in aviation till 2003. I had a consumer GPS before SA was turned off and WAAS wasn't an option though I seem to remember differential GPS being an option at the time.

Phil W

Re: Garmin 500 taking off?
« Reply #12 on: June 29, 2017, 09:36:15 pm »
Yes, but surely the speed display on the SA-era GPS receiver will low-pass filter that out (just as they do with the <3m fluctuations today).  And it won't whizz you up the coast.

My GPS certainly didn't filter it out and it did appear in the data. But I was mainly interested in position rather than sped in those days so it wasn't an issue.

Kim

  • Timelord
Re: Garmin 500 taking off?
« Reply #13 on: June 29, 2017, 09:41:34 pm »
So the speed display was pretty much useless?  (I don't know, I spent half an hour playing with a Garmin in about 1996 and most of that was working out the UI.)
Careful, Kim. Your sarcasm's showing...

Phil W

Re: Garmin 500 taking off?
« Reply #14 on: June 29, 2017, 09:44:57 pm »
So the speed display was pretty much useless?  (I don't know, I spent half an hour playing with a Garmin in about 1996 and most of that was working out the UI.)

You think Garmin is bad now? Surprised you need to ask.

Phil W

Re: Garmin 500 taking off?
« Reply #15 on: June 29, 2017, 09:47:31 pm »
You have to remember the chips in use were also low sensitivity and often lost the signal (or got a duff data) in many places that we wouldn't even consider a problem these days. So rogue positions and  other stuff was the norm.

Kim

  • Timelord
Re: Garmin 500 taking off?
« Reply #16 on: June 29, 2017, 10:38:57 pm »
So the speed display was pretty much useless?  (I don't know, I spent half an hour playing with a Garmin in about 1996 and most of that was working out the UI.)

You think Garmin is bad now? Surprised you need to ask.

No, I think Garmin's amazingly good now.  I don't think I've even seen an obviously bollocks speed reading on my e30 (earlier models were still very prone to multipath in urban areas).  You still get duff positions in the tracklog on startup or when losing signal, but in normal use they're rare enough that it's noteworthy.

The GPS in my phone is previous generation.  You still actively think about whether it's got a decent view of the sky, but with the e30 it's so reliable I've come to expect it to just work (except when you're under a load of concrete, or on Class 220/221 trains, which are active RF jamming devices).
Careful, Kim. Your sarcasm's showing...