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GPS. WAS: ASIs

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Michel

Joined: 10 Jan 2006
Posts: 966
Location: Norway

Posted: Fri Feb 09, 2007 10:10 am Post subject: GPS. WAS: ASIs

On Feb 9, 2007, at 4:22 PM, Noel Loveys wrote:

 	  Quote: 	
		   How much do you guys trust the altitude feature of your GPS?


	


Unless it is differential GPS, about 300 feet, Noel. And it's not 

likely to change. Something to do with the geoidal height table, 

somewhat coarse in your "worldwide" instrument and the fact that the 

angles made by the satellites over the horizon give poor triangulation 

in the Y-axis (altitude).



Cheers,

Michel

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dave

Joined: 22 Sep 2006
Posts: 1382

Posted: Fri Feb 09, 2007 10:35 am Post subject: GPS. WAS: ASIs

I have a 196  garmin and find it to be fairly accurate (  +/-   50 feet) 

most of the time.



I have had at time erratic readings of 200 to  800 feet fklying over 

Georgian Bay here in Ontario.

Why ?    No idea here , but it only happens  afew times but once over land 

again  it seems to be ok.  ?



Dave

---

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Posted: Fri Feb 09, 2007 11:35 am Post subject: GPS. WAS: ASIs

Probably the infamous "Georgian Bay Triangle".  



But at a guess, I'd say you are seeing some reflecting

signals off the water confusing your GPS a bit.  I

don't know for sure, but the GPS freq doesn't

penetrate water well and may bounce off.  All the

signals are weak to begin with, so it could be one 

reason.



VOR and other signals can bend as they cross shore

lines too and can cause some position error.  When I

flew across the bigger ponds, we had to watch for

this.  Only was a problem when far enough out to

receive just one navaid.



Kurt S.  S-5



--- dave <dave(at)cfisher.com> wrote:



 	  Quote: 	
		   I have a 196  garmin and find it to be fairly

 accurate (  +/-   50 feet) most of the time.

 

 I have had at time erratic readings of 200 to  800

 feet fklying over Georgian Bay here in Ontario.

 Why ?    No idea here , but it only happens a few

 times but once over land again it seems to be ok.  ?

 

 Dave


	




 

Don't pick lemons.

See all the new 2007 cars at Yahoo! Autos.

http://autos.yahoo.com/new_cars.html

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Sun Feb 11, 2007 4:25 pm Post subject: GPS. WAS: ASIs

The further you deviate from the equator the worse it gets.  I'm at around

48 N give or take and it's not too bad.  There was one day as I mentioned in

an earlier post where my location (static) was jumping around several miles.

On the map source topo maps I have for Canada one end of the pond I use to

launch from is programmed in as 20' ( it's closer to 8') the middle of the

pond is listed as close to 30' and the far end is 24'.  Imagine here in

Newfoundland water doesn't meet it's own level!



With more than six satellites I will trust the speed indication if it read 0

static before starting.



Still like maps and usually I'll spend the time dead reckoning before the

flight just to stay familiar with it.  Then again I still carry and use,

every chance I get, a slide rule.  Amazes some people what can be done

without batteries.



Noel



[quote] --

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Noel Loveys
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Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Sun Feb 11, 2007 4:28 pm Post subject: GPS. WAS: ASIs

Loran C.....  No problems with a good fix.  They were supposed to add

altitude to it some time ago but don't think they ever did.  Needs a long

wire antenna though

Noel



[quote] --

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Beemer

Joined: 26 Aug 2006
Posts: 87
Location: Middle Georgia

Posted: Mon Feb 12, 2007 6:20 am Post subject: GPS. WAS: ASIs

Noel, 

I don't remember which GPS you have, but could you be getting out of WAAS

range if you go north far enough?



Bradley



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Middle Georgia

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Mon Feb 12, 2007 6:01 pm Post subject: GPS. WAS: ASIs

All satellites in the GPS constellation are in geostationary orbit.  By

definition that means they must also be directly over the equator.  The

further away form the equator you are the greater the angle of incidence of

any signal reaching your receiver.  The greater the angle of incidence the

greater the possibility of error both vertical and horizontal.



I believe there are WAAS transmitters in northern Labrador (Voisey's Bay).

They were installed to facilitate heli-borne geo-magnetic surveys.



I had a friend who was the navigator of a ship doing exploration surveys all

over the world.  He told me in the high arctic and in the Antarctic the GPS

was just another big expensive paperweight.  



Noel



[quote] --

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jcorner(at)shaw.ca
Guest

Posted: Mon Feb 12, 2007 7:53 pm Post subject: GPS. WAS: ASIs

Noel



Your statement in your last post are quite incorrect and I could not  

let them pass without comment.



First as I understand it there are 6 orbits of 4 satellites each  

inclined at approximately 55 degrees to the equator.

This gives world-wide coverage, including the north and south poles,  

with a minimum of 4 satellites in view at all times.



WAAS stations are currently all located in continental US or Alaska  

and provide coverage in those areas.

The farther north areas of Canada do not benefit from  this  

augmentation service nor does Europe.

You may be referring to differential GPS locations in Labrador.



Your final comments I believe apply to Loran coverage at the poles.



Cheers



Jim

On 12-Feb-07, at 7:01 PM, Noel Loveys wrote:



[quote] 



 All satellites in the GPS constellation are in geostationary  

 orbit.  By

 definition that means they must also be directly over the equator.   

 The

 further away form the equator you are the greater the angle of  

 incidence of

 any signal reaching your receiver.  The greater the angle of  

 incidence the

 greater the possibility of error both vertical and horizontal.



 I believe there are WAAS transmitters in northern Labrador  

 (Voisey's Bay).

 They were installed to facilitate heli-borne geo-magnetic surveys.



 I had a friend who was the navigator of a ship doing exploration  

 surveys all

 over the world.  He told me in the high arctic and in the Antarctic  

 the GPS

 was just another big expensive paperweight.



 Noel



> --

Guy Buchanan

Joined: 16 Jul 2006
Posts: 1204
Location: Ramona, CA

Posted: Mon Feb 12, 2007 11:06 pm Post subject: GPS. WAS: ASIs

At 06:01 PM 2/12/2007, you wrote:

 	  Quote: 	
		  All satellites in the GPS constellation are in geostationary orbit.  By

definition that means they must also be directly over the equator.  The

further away form the equator you are the greater the angle of incidence of

any signal reaching your receiver.  The greater the angle of incidence the

greater the possibility of error both vertical and horizontal.


	


GPS orbit altitude: 10,988 nautical miles (Geo is about 22,300 statute miles.)



Ref: http://www.losangeles.af.mil/smc/pa/fact_sheets/gps_fs.htm

Guy Buchanan

K-IV 1200 / 582-C / Warp / 100% done, thanks mostly to Bob Ducar.



Do not archive

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Michel

Joined: 10 Jan 2006
Posts: 966
Location: Norway

Posted: Tue Feb 13, 2007 10:22 am Post subject: GPS. WAS: ASIs

On Feb 13, 2007, at 4:51 AM, Jim Corner wrote:

 	  Quote: 	
		   Your final comments I believe apply to Loran coverage at the poles.


	


Well, I don't think so, Jim, Loran C is a hyperbolic system and you 

only need to be within range of the stations. Nothing to do with the 

poles. I think our friend Noel was mistaking GPS for many other 

satellites, like communication ones, which are indeed geostationary 

over the equator. Such as broadcasting satellites and the INMARSAT 

maritime communication satellites.

In Norway, which goes up to latitude 72 N on mainland (hello Torgeir! 

 TV sateliites and INMARSAT are not covering those latitudes. Even 

here, in the south, all TV dishes are nearly horizontal.



The GPS, pretty much like its predecessor, the Transit Satellite 

System, is making a birdcage around the earth. I think the GPS doesn't 

have true polar orbits but the Transit had. And because of that, the 

frequency of a fix opportunity was actually greater the further north 

or south, one would come.



Noel, of course Loran C is much better than Decca, which was 1943 

technology and phased out a few years ago. Decca and Loran C are very 

reliable as a relative positioning system. I.e. fishermen could come 

back and fetch their nets in the thickest fog if the had plotted the 

position. But it was not too accurate as absolute position, i.e. 

relative to latitude and longitude. Something to do with lower 

frequencies that are influenced by land masses, etc.



The Omega system was also working on a very low frequency and therefore 

not so accurate. But it was the first system to use the idea of a 

differential signal. It meant that existing maritime VHF coastal 

stations were registering the current offset and sent it back to the 

ships as to increase the accuracy of their position.



WAAS is also a differential system and while it is only covering North 

America, differential GPS has been used for a long time in e.g. Europe. 

But then, it was only a local service that you had to subscribe to. 

E.g. in the North Sea offshore industry, they use differential GPS 

where a correction is sent to the platforms or pipe-laying vessels. The 

problem is: There isn't really a standard format as for WAAS.



Sorry to be a bit lengthy on the subject but I have worked with 

maritime navigation systems for many years. The bottom line is: All of 

them beat the old sextant! 



Cheers,

Michel



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Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Tue Feb 13, 2007 1:59 pm Post subject: GPS. WAS: ASIs

I guess you can't believe everything you are told.  That includes the GPS

constellations being geostationary.  The site says it is a 12 hr orbit.

They don't go into accuracies or more directly inaccuracies in any specific

part of the world.  After all you really don't want Bin Laden's crew using

GPS against yourself.



Noel



[quote] --

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Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Tue Feb 13, 2007 4:14 pm Post subject: GPS. WAS: ASIs

Michel:



You are right I was confused, not the first or last time.  I was of the idea

that the GPS constellations were interspersed between the com sats.... One

of which is making background noise for me at this very minute.



As for the sextant... If you have a good clear sky.... If you have a good

watch then at least you don't need batteries   Just don't ever ask me to

shoot a fix for you!  You may find yourself in much warmer climes speaking a

strange language.  Come to think of it if you throw a dart at a spinning

globe you will probably get a better fix than I can give you with a sextant.



Noel



[quote] --

_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

jcorner(at)shaw.ca
Guest

Posted: Tue Feb 13, 2007 6:09 pm Post subject: GPS. WAS: ASIs

On 13-Feb-07, at 11:22 AM, Michel Verheughe wrote:



 	  Quote: 	
		   


	


Michel, the reason I suspected he was referring to loran is that I  

think you will find  that the nearest Loran station or slave is a  

long,  long way from the poles.

Canada doesn't have any mid continent stations and the coastal  

stations are at lower latitudes.



 	  Quote: 	
		  

 On Feb 13, 2007, at 4:51 AM, Jim Corner wrote:

> Your final comments I believe apply to Loran coverage at the poles.



 Well, I don't think so, Jim, Loran C is a hyperbolic system and you  

 only need to be within range of the stations. Nothing to do with  

 the poles. I think our friend Noel was mistaking GPS for many other  

 satellites, like communication ones, which are indeed geostationary  

 over the equator. Such as broadcasting satellites and the INMARSAT  

 maritime communication satellites.

 I




	




As I understand it, the main difference between WAAS and what  

commonly called differential GPS is that WAAS broadcasts its  

corrections via stationary satellites where differential GPS   

stations communicated error signals directly to a special receiver,  

thus useable over a much smaller range, but perhaps more accurate.

 	  Quote: 	
		   WAAS is also a differential system and while it is only covering  

 North America, differential GPS has been used for a long time in  

 e.g. Europe. But then, it was only a local service that you had to  

 subscribe to. E.g. in the North Sea offshore industry, they use  

 differential GPS where a correction is sent to the platforms or  

 pipe-laying vessels. The problem is: There isn't really a standard  

 format as for WAAS.



 Sorry to be a bit lengthy on the subject but I have worked with  

 maritime navigation systems for many years. The bottom line is: All  

 of them beat the old sextant! 


	




I have to agree the old sextant was difficult in turbulence, as was  

transposing time differentials from the loran scope to the charts!



My nav experience was in the early 60's in P2V7's over the Pacific



Cheers



JIm



KF2- 582   1100+ hrs



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MichaelGibbs(at)cox.net
Guest

Posted: Tue Feb 13, 2007 9:27 pm Post subject: GPS. WAS: ASIs

Noel sez:



 	  Quote: 	
		  All satellites in the GPS constellation are in 

geostationary orbit. By definition that means 

they must also be directly over the equator.


	


Noel, you are a one-man dis-information machine. 

  According to Wikipedia 

<http://en.wikipedia.org/wiki/Global_Positioning_System>:



"...The GPS design calls for 24 SVs [satellites] 

to be distributed equally among six circular 

orbital planes. The orbital planes are centered 

on the Earth, not rotating with respect to the 

distant stars. The six planes have approximately 

55° inclination (tilt relative to Earth's 

equator) and are separated by 60° right ascension 

of the ascending node (angle along the equator 

from a reference point to the orbit's 

intersection).



"Orbiting at an altitude of approximately 20,200 

kilometers (12,600 miles or 10,900 nautical 

miles; orbital radius of 26,600 km (16,500 mi or 

14,400 NM)), each SV makes two complete orbits 

each sidereal day, so it passes over the same 

location on Earth once each day. The orbits are 

arranged so that at least six satellites are 

always within line of sight from almost anywhere 

on Earth.



"As of January 2007, there are 29 actively 

broadcasting satellites in the GPS constellation. 

The additional satellites improve the precision 

of GPS receiver calculations by providing 

redundant measurements. With the increased number 

of satellites, the constellation was changed to a 

nonuniform arrangement. Such an arrangement was 

shown to improve reliability and availability of 

the system, relative to a uniform system, when 

multiple satellites fail."



Mike G.

N728KF

MichaelGibbs(at)cox.net
Guest

Posted: Wed Feb 14, 2007 7:30 am Post subject: GPS. WAS: ASIs

Noel sez:



 	  Quote: 	
		  I guess you can't believe everything you are told.


	


That's because so many people just make stuff up and/or repeat stuff 

without taking 5 minutes to learn how it really works.  Google is 

probably the most incredible educational tool ever invented and 

that's not even what it's for!  If everyone made a quick visit to 

<http://www.snopes.com> or <http://www.howstuffworks.com> before 

speaking up...  oh well, it won't happen.  



 	  Quote: 	
		  ...They don't go into accuracies or more directly inaccuracies in 

any specific part of the world. After all you really don't want Bin 

Laden's crew using GPS against yourself.


	


That's what Selective Availability (SA) is for.  The U.S. military 

can degrade GPS accuracy with a flip of a switch and their own 

receivers can compensate.  On the other hand, even at the degraded 

accuracy the bad guys will still get pretty close to you...



Mike G.

N728KF

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