Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
Does GPS Take Altitude Into Account For Distance?  
User currently offlineA380900 From France, joined Dec 2003, 1112 posts, RR: 1
Posted (10 years 2 months 2 days 11 hours ago) and read 5170 times:

What the topic says.

18 replies: All unread, jump to last
 
User currently offlineGoboeing From United States of America, joined Jun 2000, 2698 posts, RR: 14
Reply 1, posted (10 years 2 months 2 days 11 hours ago) and read 5127 times:

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

Nick


User currently offlineFSPilot747 From United States of America, joined Oct 1999, 3599 posts, RR: 12
Reply 2, posted (10 years 2 months 2 days 10 hours ago) and read 5103 times:

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.


User currently offlineErj-145mech From United States of America, joined Oct 2001, 306 posts, RR: 1
Reply 3, posted (10 years 2 months 2 days 9 hours ago) and read 5083 times:

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.

User currently offlineLiamksa From Australia, joined Oct 2001, 308 posts, RR: 0
Reply 4, posted (10 years 2 months 2 days 7 hours ago) and read 5065 times:

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.


User currently offlineVorticity From United States of America, joined May 2004, 337 posts, RR: 5
Reply 5, posted (10 years 2 months 1 day 19 hours ago) and read 5012 times:

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...
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 6, posted (10 years 2 months 1 day 13 hours ago) and read 4988 times:

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.
User currently offlineLiamksa From Australia, joined Oct 2001, 308 posts, RR: 0
Reply 7, posted (10 years 2 months 1 day 3 hours ago) and read 4952 times:

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?


User currently offlineAirplay From , joined Dec 1969, posts, RR:
Reply 8, posted (10 years 2 months 1 day 2 hours ago) and read 4943 times:

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]

User currently offlineMD11Engineer From Germany, joined Oct 2003, 14026 posts, RR: 62
Reply 9, posted (10 years 2 months 1 day ago) and read 4932 times:

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


User currently offlineVorticity From United States of America, joined May 2004, 337 posts, RR: 5
Reply 10, posted (10 years 2 months 10 hours ago) and read 4878 times:

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...
User currently offlineTimz From United States of America, joined Sep 1999, 6836 posts, RR: 6
Reply 11, posted (10 years 1 month 4 weeks 1 day 19 hours ago) and read 4831 times:

"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.


User currently offlineAirplay From , joined Dec 1969, posts, RR:
Reply 12, posted (10 years 1 month 4 weeks 1 day 18 hours ago) and read 4820 times:

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.


User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 13, posted (10 years 1 month 4 weeks 1 day 14 hours ago) and read 4795 times:

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.
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 14, posted (10 years 1 month 3 weeks 5 days 22 hours ago) and read 4728 times:

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.
User currently offlineMD11Engineer From Germany, joined Oct 2003, 14026 posts, RR: 62
Reply 15, posted (10 years 1 month 3 weeks 5 days 21 hours ago) and read 4718 times:

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


User currently offlineTimz From United States of America, joined Sep 1999, 6836 posts, RR: 6
Reply 16, posted (10 years 1 month 3 weeks 5 days 16 hours ago) and read 4697 times:

"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.


User currently offlineRupesNZ From New Zealand, joined May 2004, 105 posts, RR: 0
Reply 17, posted (10 years 1 month 1 week 5 days ago) and read 4568 times:

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 currently offlineStarlionblue From Greenland, joined Feb 2004, 17044 posts, RR: 66
Reply 18, posted (10 years 1 month 1 week 4 days 22 hours ago) and read 4557 times:

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]


"There are no stupid questions, but there are a lot of inquisitive idiots."
Top Of Page
Forum Index

Reply To This Topic Does GPS Take Altitude Into Account For Distance?
Username:
No username? Sign up now!
Password: 


Forgot Password? Be reminded.
Remember me on this computer (uses cookies)
  • Tech/Ops related posts only!
  • Not Tech/Ops related? Use the other forums
  • No adverts of any kind. This includes web pages.
  • No hostile language or criticizing of others.
  • Do not post copyright protected material.
  • Use relevant and describing topics.
  • Check if your post already been discussed.
  • Check your spelling!
  • DETAILED RULES
Add Images Add SmiliesPosting Help

Please check your spelling (press "Check Spelling" above)


Similar topics:More similar topics...
Stored Aircraft - What Does It Take To Un-store posted Mon Jul 24 2006 22:14:13 by Dmanmtl
Taking Atlantic Jetstream Into Account posted Fri Oct 28 2005 18:36:35 by Julesmusician
What Does It Take To Get An Alert Area? posted Sun Feb 27 2005 02:15:05 by 2H4
What Does It Take To Start An Airline? posted Thu Dec 16 2004 03:34:06 by COAMiG29
Is GPS/INS The Way Forward For Large Aircraft posted Fri Jul 16 2004 15:13:53 by Kiwi1273
How Does A Plane Take Off? posted Sat Apr 24 2004 13:12:14 by BEG2IAH
Minimum Take Off Distance For The 757 posted Wed May 16 2001 07:32:43 by ILUV767
Max Distance For The Concorde? posted Tue Mar 28 2006 17:39:51 by 747LUVR
VFR Only Panel Mouted GPS For IFR Enroute posted Mon Dec 19 2005 18:48:04 by Qxeguy
Max. Cruising Altitude For Extra-long Flights posted Fri Nov 25 2005 14:16:29 by Emrecan

Sponsor Message:
Printer friendly format