Re: [time-nuts] GPS altitude somewhat wrong?

[Van Horn, David]

Using the thunderbolt here.

I only asked because a co-worker spotted the altitude and thought it was 
"wrong" for boulder.

-Original Message-
From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Mark Sims
Sent: Thursday, June 9, 2016 10:56 AM
To: time-nuts@febo.com
Subject: [time-nuts] GPS altitude somewhat wrong?

You have to be careful with a lot of modern GPS receivers.   Many implement 
some sort of "position pinning".   If they do not detect significant movement,  
they either stop updating the coordinates  or heavily filter it...  you do not 
see the actual computed location.  On some receivers you have to move well over 
10 meters before the position un-pins.
--
> CEP(95) of 1.5 meters over 1,000 seconds. 
>   
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Re: [time-nuts] GPS altitude somewhat wrong?

[Gary E. Miller]

Yo Hal!

On Thu, 09 Jun 2016 02:12:53 -0700
Hal Murray  wrote:

> g...@rellim.com said:
> > While you are waiting check out the attached scatter plot.  Now
> > THAT is a good $25 GPS!  Beats the heck outta any Garmin.  CEP(95)
> > of 1.5 meters over 1,000 seconds.  
> 
> What type of GPS was that?

Skytraq  NS-HP.  With the S2525F8-RTK chip in it.

http://navspark.mybigcommerce.com/ns-hp-rtk-capable-gnss-receiver/

I see they have a new firmware update, they have been tweaking it
as it is a new-ish chip.  Time for me to re-flash.

RGDS
GARY
---
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
g...@rellim.com  Tel:+1 541 382 8588


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Re: [time-nuts] GPS altitude somewhat wrong?

[Gary E. Miller]

Yo Mark!

On Thu, 9 Jun 2016 16:56:12 +
Mark Sims  wrote:

> You have to be careful with a lot of modern GPS receivers.   Many
> implement some sort of "position pinning".   If they do not detect
> significant movement,  they either stop updating the coordinates  or
> heavily filter it...  you do not see the actual computed location.
> On some receivers you have to move well over 10 meters before the
> position un-pins. --
> > CEP(95) of 1.5 meters over 1,000 seconds.


Yes, but this model Skytraq is designed for survey work.  So that mode
is selectable.

RGDS
GARY
---
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
g...@rellim.com  Tel:+1 541 382 8588


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Re: [time-nuts] GPS altitude somewhat wrong?

[Van Horn, David]

-Original Message-
From: time-nuts 
[mailto:time-nuts-bounces+david.vanhorn=backcountryaccess@febo.com] On 
Behalf Of Michael Perrett
Sent: Wednesday, June 8, 2016 4:33 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPS altitude somewhat wrong?

A couple of things come to mind:
1) Is this a single measurement or an average over at least 24 hours?

60+ hours

2) Did you get your elevation via the receiver survey mode (recommended)?

This is what's currently being displayed in LH after a 60 hour survey

3) How close is your "nominal" elevation measurement and what makes you think 
it is truth?

Damifino.  :)

4) The vertical component of the GPS position solution is typically 50% worse 
accuracy and a lot noisier than the horizontal measurement. If you have a good 
horizontal measurement it is unlikely you have a "wrong answer"
on elevation since your receiver is using the same data, just solving the 
equation for a different variable.

5) What is your satellite mask angle? The geometry (hence accuracy) degrades as 
an increasing function with mask angle. Suggest for the survey mode you use as 
low a mask angle as possible (typically 5 to 10 degrees).

Currently set to 5 degrees, which I know is low, but I wanted to see what the 
whole sky map looks like.
I will dial it up as we get things settled in.


Finally, your 214' error is outrageous. For a surveyed position the answer 
should be with +/- 10'.

Ok..



Michael Perrett

On Wed, Jun 8, 2016 at 1:33 PM, Van Horn, David < 
david.vanh...@backcountryaccess.com> wrote:

>
> I have just installed a Thunderbolt here to get our time and frequency 
> equipment all on the same page.
> As I was looking at the display on Lady Heather, I was noticing that 
> the GPS altitude seems rather wrong.
> We are in Boulder CO, which is nominally 5430' and the antenna is 
> about 20' off the ground.
> The display (near overdetermined position) reads 1589.72991 meters or 
> 5216 and change in feet.
> Altitude is a big deal around here. :)
>
> I suppose 214' isn't that outrageous, but it does bring me to a question:
>
> How accurate is the altitude number really?
>
> Thanks.
>
>
> --
> David VanHorn
> Lead Hardware Engineer
>
> Backcountry Access, Inc.
> 2820 Wilderness Pl, Unit H
> Boulder, CO  80301 USA
> phone: 303-417-1345 x110
> email: david.vanh...@backcountryaccess.com david.vanh...@backcountryaccess.com>
>
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[time-nuts] GPS altitude somewhat wrong?

[Mark Sims]

You have to be careful with a lot of modern GPS receivers.   Many implement 
some sort of "position pinning".   If they do not detect significant movement,  
they either stop updating the coordinates  or heavily filter it...  you do not 
see the actual computed location.  On some receivers you have to move well over 
10 meters before the position un-pins.
--
> CEP(95) of 1.5 meters over 1,000 seconds. 
>   
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Re: [time-nuts] GPS altitude somewhat wrong?

[Van Horn, David]

I'm satisfied that it's reasonably accurate now. 
It is interesting how much elevation can change without really being obvious to 
a person on the ground. 



-Original Message-
From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Michael Perrett
Sent: Wednesday, June 8, 2016 5:28 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPS altitude somewhat wrong?

I just checked Google Earth and the elevation of your office is 5260', only 
about 24' off of your GPS estimate if that is your location.
Michael

On Wed, Jun 8, 2016 at 1:33 PM, Van Horn, David < 
david.vanh...@backcountryaccess.com> wrote:

>
> I have just installed a Thunderbolt here to get our time and frequency 
> equipment all on the same page.
> As I was looking at the display on Lady Heather, I was noticing that 
> the GPS altitude seems rather wrong.
> We are in Boulder CO, which is nominally 5430' and the antenna is 
> about 20' off the ground.
> The display (near overdetermined position) reads 1589.72991 meters or 
> 5216 and change in feet.
> Altitude is a big deal around here. :)
>
> I suppose 214' isn't that outrageous, but it does bring me to a question:
>
> How accurate is the altitude number really?
>
> Thanks.
>
>
> --
> David VanHorn
> Lead Hardware Engineer
>
> Backcountry Access, Inc.
> 2820 Wilderness Pl, Unit H
> Boulder, CO  80301 USA
> phone: 303-417-1345 x110
> email: david.vanh...@backcountryaccess.com david.vanh...@backcountryaccess.com>
>
> ___
> time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to 
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
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>
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Re: [time-nuts] GPS altitude somewhat wrong?

[Hal Murray]

g...@rellim.com said:
> While you are waiting check out the attached scatter plot.  Now THAT is a
> good $25 GPS!  Beats the heck outta any Garmin.  CEP(95) of 1.5 meters over
> 1,000 seconds.

What type of GPS was that?


-- 
These are my opinions.  I hate spam.



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Re: [time-nuts] GPS altitude somewhat wrong?

[Angus]

I've had that query before too, but in that case it turned out to be
that by default some receivers/software report Mean Sea Level and
others Height Above Ellipsoid, or both.

Angus.


On Wed, 8 Jun 2016 20:33:58 +, you wrote:

>
>I have just installed a Thunderbolt here to get our time and frequency 
>equipment all on the same page.
>As I was looking at the display on Lady Heather, I was noticing that the GPS 
>altitude seems rather wrong.
>We are in Boulder CO, which is nominally 5430' and the antenna is about 20' 
>off the ground.
>The display (near overdetermined position) reads 1589.72991 meters or 5216 and 
>change in feet.
>Altitude is a big deal around here. :)
>
>I suppose 214' isn't that outrageous, but it does bring me to a question:
>
>How accurate is the altitude number really?
>
>Thanks.
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Re: [time-nuts] GPS altitude somewhat wrong?

[Gary E. Miller]

Yo Tom!

On Wed, 8 Jun 2016 19:26:21 -0700
"Tom Van Baak"  wrote:

> From one NW GPS farm to another... I'm willing to help you debug
> yours.

Nothing to debug.  I know if I move the antenna I can do better, but
it does what I need, and it is near the server that needs it. ntpd
caculates precision -22 and the jitter is usually less than 0.5 uSec.
More than good enough for my purposes.  It makes a dandy Stratum 1 
chimer.

And 16 meters is not good at all.  That is 52 feet!  Over just two
hours.  I bet that gets 2x or more worse over 24 hours.  If I cared
about location accuracy I would throw away that GPS. Any modern GPS
should do way better than that.

> > A Garmin 18x reports:
> >Altitude Err:+/- 264 ft 
> 
> Something is terribly wrong with your setup. The Garmin 18x is much,
> much better than this. I know because the 18x was one of the GPS
> receivers I brought along on a recent mobile clock experiment.

Yes, with good antenna placement it can be better, but the 18x also
degrades much quicker than newer GPS when the antenna placement is places
badly.

> in Tucson: http://leapsecond.com/great2016a/2016a-garmin-18x-2.gif

what year is yours?  Looking at mine, I see it is actually an 18, not an
18x, so a 12 year old design .  Worse than the 18x.

> With clear sky view, the peak to peak is under +/- 8 m, and the
> (1-sigma) standard deviation is 3 m. Even at the hotel lobby, with
> obstructed sky view, the (1-sigma) standard deviation stayed under 7
> m. Your 18x number, +/- 264ft (+/- 80 m), is 10x to 25x worse than
> this. It doesn't feel right.

My antenna is between two 2 story houses at ground level against a tall
hill.  It is lucky to get much of a signal at all.  I know it would work
better in aother location, but it needs to be there next to my main
servers.

> Off-list, can you send me a day of NMEA from your 18x? Not gpsd
> output; but the raw serial ascii data from the receiver. I'd like to
> get to the bottom of this. We'll all learn something.

I can get you pseudo NMEA, the Garmins work much better in binary mode.
When they work in binary mode. :-)

I'll start to grab that now.  I'll have a 12 hour scatter plot in
the morning.

While you are waiting check out the attached scatter plot.  Now THAT
is a good $25 GPS!  Beats the heck outta any Garmin.  CEP(95) of 1.5
meters over 1,000 seconds.

RGDS
GARY
---
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
g...@rellim.com  Tel:+1 541 382 8588


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Re: [time-nuts] GPS altitude somewhat wrong?

[Tom Van Baak]

Hi Gary,

>From one NW GPS farm to another... I'm willing to help you debug yours.

> A Garmin 18x reports:
>Altitude Err:+/- 264 ft   

Something is terribly wrong with your setup. The Garmin 18x is much, much 
better than this. I know because the 18x was one of the GPS receivers I brought 
along on a recent mobile clock experiment.

Here is a plot of 2 hours of GPGGA raw altitude data while stationary in Tucson:
http://leapsecond.com/great2016a/2016a-garmin-18x-2.gif

With clear sky view, the peak to peak is under +/- 8 m, and the (1-sigma) 
standard deviation is 3 m.
Even at the hotel lobby, with obstructed sky view, the (1-sigma) standard 
deviation stayed under 7 m.
Your 18x number, +/- 264ft (+/- 80 m), is 10x to 25x worse than this. It 
doesn't feel right.

Off-list, can you send me a day of NMEA from your 18x? Not gpsd output; but the 
raw serial ascii data from the receiver. I'd like to get to the bottom of this. 
We'll all learn something.

Thanks,
/tvb

- Original Message - 
From: "Gary E. Miller" <g...@rellim.com>
To: "Van Horn, David" <david.vanh...@backcountryaccess.com>
Cc: <time-nuts@febo.com>
Sent: Wednesday, June 08, 2016 3:24 PM
Subject: Re: [time-nuts] GPS altitude somewhat wrong?

Yo David!

On Wed, 8 Jun 2016 20:33:58 +
"Van Horn, David" <david.vanh...@backcountryaccess.com> wrote:

> I suppose 214' isn't that outrageous, but it does bring me to a
> question:

Looking at my GPS farm, fair but not great skyview.

A Garmin 18x reports:
Altitude Err:+/- 264 ft   

A u-blox 8:
Altitude Err:+/- 36 ft 

And an adafruit HAT (u-blox 6?:
Altitude Err:+/- 69 ft 

On a Skytraq I have seen +/- 12 feet.

> How accurate is the altitude number really?

Depends.  :-)

RGDS
GARY
---
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
 g...@rellim.com  Tel:+1 541 382 8588


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Re: [time-nuts] GPS altitude somewhat wrong?

[Jim Harman]

On Wed, Jun 8, 2016 at 4:33 PM, Van Horn, David <
david.vanh...@backcountryaccess.com> wrote:

> Backcountry Access, Inc.
> 2820 Wilderness Pl, Unit H
> Boulder, CO  80301 USA
>

Google Earth has the elevation of this address as 5272' which is a good
deal closer to your reading...


-- 

--Jim Harman
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Re: [time-nuts] GPS altitude somewhat wrong?

[Van Horn, David]

Some of that is probably the difference between the geoid (what your surveyed 
maps report height relative to) and the WGS84 ellipsoid (what your GPS reports 
heights relative to).  At Boulder that difference is only about 15 meters, 
though.

Generally with VDOP < 2 and a reasonably modern receiver the accuracy of a GPS 
altitude measurement should be better than about 20 meters.
I'm not sure if the Thunderbolt counts as such.

How flat is Boulder?  Do you have a proper surveyed elevation of your location?

I do not.

The readout is plausibly right, but it is displaying to the nearest tenth of a 
millimeter which I thought was somewhat optimistic.  :)

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Re: [time-nuts] GPS altitude somewhat wrong?

[Michael Perrett]

I just checked Google Earth and the elevation of your office is 5260', only
about 24' off of your GPS estimate if that is your location.
Michael

On Wed, Jun 8, 2016 at 1:33 PM, Van Horn, David <
david.vanh...@backcountryaccess.com> wrote:

>
> I have just installed a Thunderbolt here to get our time and frequency
> equipment all on the same page.
> As I was looking at the display on Lady Heather, I was noticing that the
> GPS altitude seems rather wrong.
> We are in Boulder CO, which is nominally 5430' and the antenna is about
> 20' off the ground.
> The display (near overdetermined position) reads 1589.72991 meters or 5216
> and change in feet.
> Altitude is a big deal around here. :)
>
> I suppose 214' isn't that outrageous, but it does bring me to a question:
>
> How accurate is the altitude number really?
>
> Thanks.
>
>
> --
> David VanHorn
> Lead Hardware Engineer
>
> Backcountry Access, Inc.
> 2820 Wilderness Pl, Unit H
> Boulder, CO  80301 USA
> phone: 303-417-1345 x110
> email: david.vanh...@backcountryaccess.com david.vanh...@backcountryaccess.com>
>
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> To unsubscribe, go to
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>
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Re: [time-nuts] GPS altitude somewhat wrong?

[Chris Caudle]

On Wed, June 8, 2016 3:33 pm, Van Horn, David wrote:
> How accurate is the altitude number really?

Probably not the question you really want to ask.

Try "what does GPS mean by altitude? Altitude relative to what standard?"

http://www.esri.com/news/arcuser/0703/geoid1of3.html

-- 
Chris Caudle


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Re: [time-nuts] GPS altitude somewhat wrong?

[Henry Hallam]

Per 
https://www.topoquest.com/map.php?lat=40.02486=-105.24468=nad27=2=auto=d=zoomin=m
the address in your signature is close to the 5250 ft (geoidal)
contour.

Henry

On Wed, Jun 8, 2016 at 3:18 PM, Henry Hallam  wrote:
> Some of that is probably the difference between the geoid (what your
> surveyed maps report height relative to) and the WGS84 ellipsoid (what
> your GPS reports heights relative to).  At Boulder that difference is
> only about 15 meters, though.
>
> Generally with VDOP < 2 and a reasonably modern receiver the accuracy
> of a GPS altitude measurement should be better than about 20 meters.
> I'm not sure if the Thunderbolt counts as such.
>
> How flat is Boulder?  Do you have a proper surveyed elevation of your 
> location?
>
> Henry
>
> On Wed, Jun 8, 2016 at 1:33 PM, Van Horn, David
>  wrote:
>>
>> I have just installed a Thunderbolt here to get our time and frequency 
>> equipment all on the same page.
>> As I was looking at the display on Lady Heather, I was noticing that the GPS 
>> altitude seems rather wrong.
>> We are in Boulder CO, which is nominally 5430' and the antenna is about 20' 
>> off the ground.
>> The display (near overdetermined position) reads 1589.72991 meters or 5216 
>> and change in feet.
>> Altitude is a big deal around here. :)
>>
>> I suppose 214' isn't that outrageous, but it does bring me to a question:
>>
>> How accurate is the altitude number really?
>>
>> Thanks.
>>
>>
>> --
>> David VanHorn
>> Lead Hardware Engineer
>>
>> Backcountry Access, Inc.
>> 2820 Wilderness Pl, Unit H
>> Boulder, CO  80301 USA
>> phone: 303-417-1345 x110
>> email: 
>> david.vanh...@backcountryaccess.com
>>
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Re: [time-nuts] GPS altitude somewhat wrong?

[Gary E. Miller]

Yo David!

On Wed, 8 Jun 2016 20:33:58 +
"Van Horn, David"  wrote:

> I suppose 214' isn't that outrageous, but it does bring me to a
> question:

Looking at my GPS farm, fair but not great skyview.

A Garmin 18x reports:
Altitude Err:+/- 264 ft   

A u-blox 8:
Altitude Err:+/- 36 ft 

And an adafruit HAT (u-blox 6?:
Altitude Err:+/- 69 ft 

On a Skytraq I have seen +/- 12 feet.

> How accurate is the altitude number really?

Depends.  :-)

RGDS
GARY
---
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
g...@rellim.com  Tel:+1 541 382 8588


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Re: [time-nuts] GPS altitude somewhat wrong?

[Henry Hallam]

Some of that is probably the difference between the geoid (what your
surveyed maps report height relative to) and the WGS84 ellipsoid (what
your GPS reports heights relative to).  At Boulder that difference is
only about 15 meters, though.

Generally with VDOP < 2 and a reasonably modern receiver the accuracy
of a GPS altitude measurement should be better than about 20 meters.
I'm not sure if the Thunderbolt counts as such.

How flat is Boulder?  Do you have a proper surveyed elevation of your location?

Henry

On Wed, Jun 8, 2016 at 1:33 PM, Van Horn, David
 wrote:
>
> I have just installed a Thunderbolt here to get our time and frequency 
> equipment all on the same page.
> As I was looking at the display on Lady Heather, I was noticing that the GPS 
> altitude seems rather wrong.
> We are in Boulder CO, which is nominally 5430' and the antenna is about 20' 
> off the ground.
> The display (near overdetermined position) reads 1589.72991 meters or 5216 
> and change in feet.
> Altitude is a big deal around here. :)
>
> I suppose 214' isn't that outrageous, but it does bring me to a question:
>
> How accurate is the altitude number really?
>
> Thanks.
>
>
> --
> David VanHorn
> Lead Hardware Engineer
>
> Backcountry Access, Inc.
> 2820 Wilderness Pl, Unit H
> Boulder, CO  80301 USA
> phone: 303-417-1345 x110
> email: 
> david.vanh...@backcountryaccess.com
>
> ___
> time-nuts mailing list -- time-nuts@febo.com
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Re: [time-nuts] GPS altitude somewhat wrong?

[Michael Perrett]

A couple of things come to mind:
1) Is this a single measurement or an average over at least 24 hours?
2) Did you get your elevation via the receiver survey mode (recommended)?
3) How close is your "nominal" elevation measurement and what makes you
think it is truth?
4) The vertical component of the GPS position solution is typically 50%
worse accuracy and a lot noisier than the horizontal measurement. If you
have a good horizontal measurement it is unlikely you have a "wrong answer"
on elevation since your receiver is using the same data, just solving the
equation for a different variable.
5) What is your satellite mask angle? The geometry (hence accuracy)
degrades as an increasing function with mask angle. Suggest for the survey
mode you use as low a mask angle as possible (typically 5 to 10 degrees).

Finally, your 214' error is outrageous. For a surveyed position the answer
should be with +/- 10'.

Michael Perrett

On Wed, Jun 8, 2016 at 1:33 PM, Van Horn, David <
david.vanh...@backcountryaccess.com> wrote:

>
> I have just installed a Thunderbolt here to get our time and frequency
> equipment all on the same page.
> As I was looking at the display on Lady Heather, I was noticing that the
> GPS altitude seems rather wrong.
> We are in Boulder CO, which is nominally 5430' and the antenna is about
> 20' off the ground.
> The display (near overdetermined position) reads 1589.72991 meters or 5216
> and change in feet.
> Altitude is a big deal around here. :)
>
> I suppose 214' isn't that outrageous, but it does bring me to a question:
>
> How accurate is the altitude number really?
>
> Thanks.
>
>
> --
> David VanHorn
> Lead Hardware Engineer
>
> Backcountry Access, Inc.
> 2820 Wilderness Pl, Unit H
> Boulder, CO  80301 USA
> phone: 303-417-1345 x110
> email: david.vanh...@backcountryaccess.com david.vanh...@backcountryaccess.com>
>
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[time-nuts] GPS altitude somewhat wrong?

[Van Horn, David]

I have just installed a Thunderbolt here to get our time and frequency 
equipment all on the same page.
As I was looking at the display on Lady Heather, I was noticing that the GPS 
altitude seems rather wrong.
We are in Boulder CO, which is nominally 5430' and the antenna is about 20' off 
the ground.
The display (near overdetermined position) reads 1589.72991 meters or 5216 and 
change in feet.
Altitude is a big deal around here. :)

I suppose 214' isn't that outrageous, but it does bring me to a question:

How accurate is the altitude number really?

Thanks.


--
David VanHorn
Lead Hardware Engineer

Backcountry Access, Inc.
2820 Wilderness Pl, Unit H
Boulder, CO  80301 USA
phone: 303-417-1345  x110
email: 
david.vanh...@backcountryaccess.com

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