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#173593 - 05/20/09 06:45 PM Re: GPS accuracy at risk in coming years. [Re: ]
Blast Offline
INTERCEPTOR
Carpal Tunnel

Registered: 07/15/02
Posts: 3593
Loc: TX
Quote:
Another tax? Don't make me go dig up Thomas Jefferson and give him a minigun, Man.


+10 to that!

I have plenty of shovels and access to a lab where dead bacteria have been successfully reanimated. We could probably pull this off though it might lead to a zombie apocalypse.

-Blast, who has been practicing headshots on microorganisms.
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#173598 - 05/20/09 09:15 PM Re: GPS accuracy at risk in coming years. [Re: ]
DannyL Offline
Member

Registered: 02/22/08
Posts: 103
Loc: SE Alaska
Originally Posted By: IzzyJG99
Originally Posted By: Tyber
Well this could be a funding issue. Considering that GPS is a free service (provided you have a reciver) what if we add a $10 tax to everything sold that uses GPS and put the funding towards updating and maintance. Considering that almost EVERY phone, that is sold in this day in age has a GPS reciver on it, they could realy increas the budget..

but that is the rambelings of a mad man.


Another tax? Don't make me go dig up Thomas Jefferson and give him a minigun, Man.



He has a point.
We already pay a "tax" for the "free" 911 service. Nothing in life is free and I really expect something like this to happen in the future to support the GPS network. Just about everything today can use the GPS system, hell, even the Alaska Marine Highway ferries use GPS guidance.
I personally expect a 10.00 GPS "fee" when you buy something, and for cell phones a 1.00 month GPS maintanance fee.

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#173600 - 05/20/09 09:28 PM Re: GPS accuracy at risk in coming years. [Re: ]
MartinFocazio Offline

Pooh-Bah

Registered: 01/21/03
Posts: 2155
Loc: Bucks County PA
Originally Posted By: IzzyJG99
Originally Posted By: Tyber
Well this could be a funding issue. Considering that GPS is a free service (provided you have a reciver) what if we add a $10 tax to everything sold that uses GPS and put the funding towards updating and maintance. Considering that almost EVERY phone, that is sold in this day in age has a GPS reciver on it, they could realy increas the budget..

but that is the rambelings of a mad man.


Another tax? Don't make me go dig up Thomas Jefferson and give him a minigun, Man.


You DO understand how the GPS system - and the Air Force - and the Entire Military gets the money they need to run stuff, don't you?

You DO understand that "user fees" for things like, oh, say National Parks, has long been a mantra of those who don't like federal taxes. So call it a $10 user fee for those folks who want to use the GPS system.

Oh, and while you're at it, let's add a $10 user fee for the Interstate Highways.

Might as well add a User Fee of $10 for each time someone calls 911 - gotta keep those calls making money and it's not fair to those who don't need 911 to have to pay for it.

Oh, and I guess if we're going to pay the 911 center a user fee, then let's make sure you get billed by the call for fire services...I guess since most people's houses don't burn, that user fee would be at least $6,000.

And that's gotta bring up a whole question of keeping taxes low in so many other areas. You know, those lights on radio towers - the ones that keep planes from crashing into them? Yeah, we should charge a user fee of $5.00 a flight for people who use airplanes, and I guess radios too since people who don't listen to radios don't need the towers, so why should they pay anything.

OH! Wait, I totally forgot! Credit Card Regulations! Yeah, that just passed, and I bet that took a lot of office work and stuff. But people who don't use credit cards shouldn't have to pay for all that lawmaking stuff...so why not charge a "credit card user fee" of say $10 a year to cover the costs of keeping those companies from being more greedy.

Am I being silly? A little, but I think that it's a clear that we all certainly expect a lot for our taxes, which are - compared to the bulk of the developed world - very low.

In Finland for example, the tax on a new car is 100%. That's right. A $20,000 car has a $20,000 tax. In Denmark, income tax is 52%. Yeah, you get free college, health care, cash for each of your kids, a pension for when you're old...but you pay for it.

The situation with the Air Force isn't so much one of funding - it's poor management and that's a situation that spans the whole of the USA. We've lost the big picture, the long view, the sense of duty now for the future. The fact that we even have to question the stability of the GPS system - that's more than sad, it's terrifying, because the GPS system - hell the whole space-based engineering world - was absolutely the domain of the USA, and the best we can do these days is to fly 20 year old space craft up to fix 19 year old spacecraft. That's pathetic. This GPS story is pathetic. I'm disgusted.



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#173606 - 05/20/09 10:36 PM Re: GPS accuracy at risk in coming years. [Re: ]
wildman800 Offline
Carpal Tunnel

Registered: 11/09/06
Posts: 2828
Loc: La-USA
I received this message today through the corporate network.

Since leaving New Orleans, presently in Galveston Bay headed to Houston; I've noticed 10m (normal) to 100m (not good) accuracy.

Normally, our units provide 2m accuracy.
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#173611 - 05/20/09 11:23 PM Re: GPS accuracy at risk in coming years. [Re: ]
Russ Offline
Geezer

Registered: 06/02/06
Posts: 5338
Loc: SOCAL
More likely at that moment in time you were within view of a high number of GPS satellites. That's one reason I really like the newer high sensitivity receivers -- more satellites are in view.
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#173628 - 05/21/09 03:17 AM Re: GPS accuracy at risk in coming years. [Re: Blast]
Am_Fear_Liath_Mor Offline
Carpal Tunnel

Registered: 08/03/07
Posts: 3078
DGPS may provide a solution for keeping GPS accuracy reasonable (within 1-5m CEP with a < 100km baseline) which will be somewhat better than the satellite based WAAS or EGNOS PRN as long as the baseline is within a suitable range to the ground station. DGPS receivers such as the CSI MBX3 are pretty expensive and bulky so here is a link to a home brew solution for a low cost solution;

http://web.abnormal.com/dgps/bix1/ (makes for a nice home electronics project cool)

Your GPS model will need RS232 port comms rather than USB though to recieve the RTCM-SC104 message.

Even the cheap Garmin eTrex Units can have improved accuracy by ensuring WAAS or EGNOS PRN is enabled for differential accuracy as long as the firmware is up to date. But I don't think the eTrex models have HDOP and velocity filtering though or even the ability to discard any satellite PRN below a certain angular height above the horizon.





Edited by Am_Fear_Liath_Mor (05/21/09 03:24 AM)

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#173645 - 05/21/09 02:03 PM Re: GPS accuracy at risk in coming years. [Re: ]
Am_Fear_Liath_Mor Offline
Carpal Tunnel

Registered: 08/03/07
Posts: 3078

Quote:
It's interesting that you bring that up. I just punched up my GPS unit and it's giving me my cords. I'm getting, if Google Maps is translated the coords correctly, literally 1 meter accuracy. That's up from last month when I tested it and the accuracy was about 10 meters.


To determine the accuracy of your GPS you will need some software such as 'SA Watch' and run your GPS for around 24-48hrs whilst logging the GPS data on a PC. This will give you an extremely accurate position of you location i.e. less than a few decimetres using statistical averaging.

http://www.huntting.com/sawatch/


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#173646 - 05/21/09 02:07 PM Re: GPS accuracy at risk in coming years. [Re: Am_Fear_Liath_Mor]
Russ Offline
Geezer

Registered: 06/02/06
Posts: 5338
Loc: SOCAL
My Garmin GPS V has an "average" function. Turn it on and let it run, and the location info refines itself over time.
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Better is the Enemy of Good Enough.
Okay, what’s your point??

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#173661 - 05/21/09 06:12 PM Re: GPS accuracy at risk in coming years. [Re: Blast]
CAP613 Offline
Journeyman

Registered: 06/22/05
Posts: 87
Loc: W. PA
A bit off topic but having to do with sats.

I was talking to someone at the FAA in D.C. He told me that the Air Force does not want to put SAR-SAT on the two birds that they where going to. I do not know if tis is a move on there part but I have known the person who gave me the info for 50 years I do believe that the info is true.
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#173664 - 05/21/09 06:24 PM Re: GPS accuracy at risk in coming years. [Re: Russ]
Am_Fear_Liath_Mor Offline
Carpal Tunnel

Registered: 08/03/07
Posts: 3078
I had some interesting results with the 'SA Watch' and Google Earth software used in conjuction with a Garmin eTrex Camo GPS SW ver 3.7 (Discontinued but with the latest firmware).

The Garmin eTrex is the older standard chipset and not the high sensitivity H or HC models etc.

WAAS (EGNOS in Europe) was enabled and the NMEA 0183 output (4800,8,N,1) was selected. The eTrex was situated on a south facing window sill and could observe only half the sky with some large trees obscuring the southerly view. So not the best sky view. The Garmin eTrex was still able to view 6-7 SVs (many of them to the North which was obscured so considerable multipath must have been at play). The EGNOS PRN was detected and DGPS positional computations were being carried out by the eTrex unit. (typically the error estimation by the eTrex was typically around 3-6 metres with SA view showing a CEP @ 95% around the same value. Not bad considering the poor sky view)

The Garmin eTrex supports the NMEA 0183 sentences GPGGA, GPGLL, GPGSA, GPGSV, GPRMB, GPRMC, GPRMT, GPVGT, GPWLP, GPBOD

None of these message sentences in the NMEA format indicate the Datum as indicated in the NMEA sentence reference i.e GPDTM

http://www.tronico.fi/OH6NT/docs/NMEA0183.pdf

This is where there appears to be a problem with the eTrex unit. When ever the Datum is changed a significant error may occur if the eTrex is communicating its position to another device if the other electronic mapping/recording/navigational device isn't aware of a datum change by the eTrex GPS.

Initially the eTrex was set to the OSGB36 datum for UK Ordnance Survey - the eTrex generated a stable 8 digit grid reference accurate to 10m, which was spot on (excellent considering this is about as accurate as any 1:25,000 scale mapping work would ever require and considering the poor sky view and multipath error that must have been occurring)

What wasn't so great was when I went to use Google Earth (which I assume uses the WGS84 datum) with the eTrex using the OSGB36 datum. The position was consistently out by 90m to the east. At first I was at a loss as to what was happening, then realised the difference must be down to the difference in the Google Earth WGS84 datum and the OSGB36 datum. As soon as I reconfigured the Datum on the eTrex to the WGS84 datum, the Google Earth pinpointed the window on my house on the aerial photo.

If Garmin aren't going to include the $GPDTM output message when the eTrex talks to another device application such as external electronic mapping applications then I would have expected the output NMEA sentences to have remained referenced to the WGS84 datum. So if using your eTrex in conjuction with Google Earth on your 3G enabled netbook, you need to ensure that the eTrex has the WGS84 datum selected. As always make sure that both your map and GPS are using the same Datum, whether the map is paper or electronic. This is probably more important than losing a couple of SVs in the next 5 years from the constellation where general navigation work is required. Survey work, now that might be a whole different ball game.


Edit - An update on the eTrex Camo accuracy





The blue portion in the Observed GPS horizon is the actual sky view and the orange portion would have been subject to multipath error. As you can see there wasn't a good view of the sky and the accuracy has suffered as a result. Most of the observed measurements 72% had an HDOP > 2.0

Actual accuracy is around 20 metres CEP @ 95% rather than the earlier assumption of 3-6 metres. Again I still would actually consider this to be an excellent result considering the poor HDOP due to the limited sky view and the inherent multipath errors.










Edited by Am_Fear_Liath_Mor (05/22/09 12:50 AM)

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