a380900
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Does GPS Take Altitude Into Account For Distance?

Sat Jul 24, 2004 12:50 pm

What the topic says.
 
goboeing
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RE: Does GPS Take Altitude Into Account For Distance?

Sat Jul 24, 2004 1:16 pm

No. GPS, unlike VORs, gives the actual distance rather than the slant range.

Nick
 
fspilot747
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RE: Does GPS Take Altitude Into Account For Distance?

Sat Jul 24, 2004 2:30 pm

If I'm not mistaken, GPS does take into account the barometric pressure reading (from the altimeter) to verify that the system is recieving accurate signals from the satellites.

Other than that, as Goboeing said, you don't get the slant range error that you would do with VOR.
 
erj-145mech
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RE: Does GPS Take Altitude Into Account For Distance?

Sat Jul 24, 2004 3:23 pm

GPS does not get pressure readings from the aircraft static system. Totally independant from the pitot/static system. GPS gets its altitude information from the galaxy of satellites.
 
liamksa
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RE: Does GPS Take Altitude Into Account For Distance?

Sat Jul 24, 2004 4:39 pm

When only 4 satellites are in view (need 5 for RAIM), some GPS systems use the pressure altitude from the txpdr (or an approved altimeter) as the 5th 'distance measurement'.

It's called barometric aiding but using all satellites is preferred.
 
Vorticity
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RE: Does GPS Take Altitude Into Account For Distance?

Sun Jul 25, 2004 5:24 am

Some GPS receivers can be programmed for a number of different solutions, including both 3D and 2D. Typically in my experience always programmed for a full 3D position. The distance to any other location can be calculated however you wish.

The GPS solution is based entirely on the measurements from the GPS Satellites, or including Differential GPS ground sites if you are using Differential GPS. You can then mix and match position information from GPS and any other instruments later if you wish. Some hardware packages include GPS integrated with other instruments already. An example is Honeywell's SIGI (Space Integrated GPS/INS).
Thermodynamics and english units don't mix...
 
SlamClick
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RE: Does GPS Take Altitude Into Account For Distance?

Sun Jul 25, 2004 11:03 am

If you fly all the way around the world at 42000' you only add 45.6 nautical miles to the journey at sea level.

I'm not too worried about it.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
liamksa
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RE: Does GPS Take Altitude Into Account For Distance?

Sun Jul 25, 2004 8:53 pm

G'day Vorticity

From the ICAO document titled “GUIDANCE MANUAL FOR THE TRAINING OF HUMAN RESOURCES ON THE CNS/ATM SYSTEMS” http://www.icao.int/icao/en/ro/nacc/meetings/2002dcacar1/cardca1-wp16APXa.pdf. -

"Barometric aid - Process that employs altitude information to simulate a GNSS satellite situated directly over the receiver antenna (reduces by one the number of satellites needed to perform a given function)."

(Bold added).

AFAIK three satellites are needed for a 2D fix, four for a 3D fix and five for RAIM. However with the appropriate equipment which can take advantage of barometric aiding you can get by with one less for each of the required functions.

What do you reckon?
 
airplay
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RE: Does GPS Take Altitude Into Account For Distance?

Sun Jul 25, 2004 9:51 pm

GPS does not get pressure readings from the aircraft static system. Totally independant from the pitot/static system. GPS gets its altitude information from the galaxy of satellites.

Wrong. You have obviously never operated (or maintained) an airborne, approved GPS based navigation system in IFR conditions. Interface to an altitude source is a certification requirement for IFR GPS installations.

The source can be baro-corrected altitude or pressure altitude, but the latter requires baro-correction information to be input on cue prior to GPS approaches.

Excerpt from FAA Advisory Circular AC-138 (AIRWORTHINESS APPROVAL OF
GLOBAL POSITIONING SYSTEM (GPS) NAVIGATION EQUIPMENT FOR USE AS A
VFR AND IFR SUPPLEMENTAL NAVIGATION SYSTEM):

Pressure/Barometric Altitude Inputs: An appropriate input of pressure and/or barometric altitude must be provided to the GPS equipment.

A minimum of three satellites must be in view to determine a two dimensional position, if altitude is known, and four are necessary to establish an accurate three dimensional position.


AC 20-138 has recently been superceded by AC 20-138A (AIRWORTHINESS APPROVAL OF GLOBAL NAVIGATION SATELLITE SYSTEM (GNSS) EQUIPMENT) which now states:

If the GNSS equipment requires barometric corrected (or pressure) altitude data for certain operations (identified in the GNSS installation instructions), the installation should provide an automatic altitude input from the air data system to the GNSS such that additional pilot actions are not required.

This leeway is afforded to accomodate any new technology (like WAAS) that allows for greater accuracy indepent of altitude sensors. However, for now there aren't many WAAS approaches around, so all installations, including those with WAAS capability still have altitude interfaces.

If you fly all the way around the world at 42000' you only add 45.6 nautical miles to the journey at sea level.

I get 25 miles....


[Edited 2004-07-25 15:02:36]
 
MD11Engineer
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RE: Does GPS Take Altitude Into Account For Distance?

Sun Jul 25, 2004 11:50 pm

Following the rules of spherical geometrics, you need at least three satellites to get a fix for your position (degrees longitude and latitude) on the surface of a sphere. To add altitude, you´ll need a fourth satellite signal. The more satellites you receive (depending on their geometrical position, as far apart as possible), the more exact will your fix be. GPS calculatet altitude will always be less accurate than the position fix (my handheld Garmin can give me a position fix of down to 10 meters, but the altitude can only be calculated to about 100 meters).
Another thing concerning IFR flight is that above the transition altitude, you´ll be flying acc. to flight levels, which are in fact levels of equal pressure. The exact ALTITUDE above sea level of those flight levels varies with the local barometric pressure, e.g. you can be flying at FL 200, but at one point over earth be at only 18000 ft ASL, while a few hundred miles away you might still be at FL200 and be 22000ft ASL.
GPS calculated altitude gives you the geometrical altitude as distance from the center of earth minus earth diameter to sea level. So you could never fly acc. to flight levels using a pure GPS calculated altitude, even if you disregard it´s lower accuracy.

Jan
Je Suis Charlie et je suis Ahmet aussi
 
Vorticity
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RE: Does GPS Take Altitude Into Account For Distance?

Mon Jul 26, 2004 2:13 pm

Wrong. You have obviously never operated (or maintained) an airborne, approved GPS based navigation system in IFR conditions

Again, the key word there is GPS based... again, in the strictest sense, GPS is soley a calculation based on GPS satellites. You can create a system encorperating more measurements to get a more robust representation of your position. When it really matters, rarely is GPS used as the sole instrument (in my experience).

Interestingly enough, if you get a cool enough GPS receiver board (not one that you typically buy at a retail store), you can program some interesting settings. As was said earlier, in order to get a 3D fix on your position, you need 4 satellites, but what do you do if you have 5 satellites? You can program to compute a solution based upon all 5 sats, or 6, 7 whatever the case may be.... or you can tell it to calculate based on the best 4.

I think to answer the original question. When you ask if it accounts distance, is that distance to a location? I think depending on the software, your distance to any location can be calculated many ways. Again, GPS alone provides you with your position. Additional software calculations can be made to calculate your distance to any other location. Those calculations can be made many ways. But you can get your GPS solution in both 3D and 2D to varying degrees of precision.

Additional interesting fact about GPS. There are rules about GPS receivers, so that they could not be used to guid ICBM's and so forth. Many GPS receiver boards cutoff at 60,000 ft. However some will continue to provide a solution at that attitude, as long as your velocity isn't that of a missile  Smile It's a wild world of GPS out there in the finer details.
Thermodynamics and english units don't mix...
 
timz
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At 42000 Ft

Tue Jul 27, 2004 5:15 am

"If you fly all the way around the world at 42000' you only add 45.6 nautical miles to the journey at sea level."

"I get 25 miles...."

Hopefully we all agree the answer is about 42000 feet times 2 times pi? Which is 43.43 nm.
 
airplay
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RE: Does GPS Take Altitude Into Account For Distance?

Tue Jul 27, 2004 6:19 am

Hopefully we all agree the answer is about 42000 feet times 2 times pi? Which is 43.43 nm.

Yep..I had a bit of finger trouble in my calculation.
 
SlamClick
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RE: Does GPS Take Altitude Into Account For Distance?

Tue Jul 27, 2004 9:38 am

Okay, new rule: Never do arithmetic in front of an audience. Maybe I should have added "about" to it.

Happiness is not seeing another trite Ste. Maarten photo all week long.
 
FredT
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RE: Does GPS Take Altitude Into Account For Distance?

Fri Jul 30, 2004 1:37 am

You need one satellite more than would be required by spherical geometry alone. One extra is required for the timing, as you don't have an atomic clock in your GPS receiver.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
MD11Engineer
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RE: Does GPS Take Altitude Into Account For Distance?

Fri Jul 30, 2004 2:57 am

Funny why they block them that you can´t use them on an IXCBM. A simple cruise missile would be much easier to build for terrorists or agressive governments and it would work with a normal GPS receiver.

Jan
Je Suis Charlie et je suis Ahmet aussi
 
timz
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RE: Does GPS Take Altitude Into Account For Distance?

Fri Jul 30, 2004 7:35 am

"You need one satellite more than would be required by spherical geometry alone."

So he's right for the wrong reason-- three satellites is geometrically enough, but you need the fourth for timing.
 
RupesNZ
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RE: Does GPS Take Altitude Into Account For Distance?

Sat Aug 14, 2004 12:03 am

Talking of cruise missiles - here is project that is/was being worked on by a fellow NZ'er

http://www.interestingprojects.com/cruisemissile/diary.shtml
 
User avatar
Starlionblue
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RE: Does GPS Take Altitude Into Account For Distance?

Sat Aug 14, 2004 1:56 am

Cute. He is obviously a fan of the V1. Similar look and a pulsejet. Hey, it is probably the simplest possible cruise missile you can construct.

Very interesting.


I must also say that this page where the dude talks about aerodynamics is very good at putting it in simple terms. http://www.interestingprojects.com/cruisemissile/airframe02.shtml

[Edited 2004-08-13 19:01:42]
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