Altimeter Question

[edelsite]

I see that many of the new units have barometric altimeters built into them. What I've noticed is that even the cheaper GPS's still give elevation info. What is the difference between the elevation info on my basic Meridian and the info given with a unit containing a barometric altimeter? Is it just a matter of elevation accuracy?

[+Rubberhead]

The original intent was for the unit to provide altitude even when the unit isn't receiving a signal.

[edelsite]

That makes sense...never thought about that.

[Glenn W]

Because the GPS satelites occupy similiar orbitial altitudes, your reported vertical fix is much less accurate than your horizontal fix. The barometer is supposed to help firm up the vertical accuracy. I prefer to use the topo maps loaded on my 60CSx for elevation, so I haven't played with the barometer much.

[edelsite]

That brings up another question of mine...if you load top software, does the unit use the topo info to give elevation readings, or does it still rely on the GPS signal?

[Glenn W]

That brings up another question of mine...if you load top software, does the unit use the topo info to give elevation readings, or does it still rely on the GPS signal?

 

No the topo software does not influence the reported elevation. I simply read the elevations from the labeled contour lines. For my purposes, this is accurate enough.

Edited June 26, 2006 by Glenn W

[Jerry Bransford]

I am a pilot and can verify a GPS receiver's ability to derive altitude information from its 3D fix from satellites is marginal at best. I could be flying level but the altitude displayed on my GPS would bounce around +/- 500' to 750' so it's not accurate enough when you need the correct altitude.

 

Only those GPS receivers that actually measure the barometric pressure can be relied on for accurate altitude indications.

Edited June 27, 2006 by Jerry Bransford

[zookc]

I am a pilot and can verify a GPS receiver's ability to derive altitude information from its 3D fix from satellites is marginal at best. I could be flying level but the altitude displayed on my GPS would bounce around +/- 500' to 750' so it's not accurate enough when you need the correct altitude.

 

Only those GPS receivers that actually measure the barometric pressure can be relied on for accurate altitude indications.

 

And then only if in a non-pressurized aircraft cabin.

See the following thread for additional discussion on use in pressurized aircraft.

http://forums.Groundspeak.com/GC/index.php?showtopic=136426

[peter]

I am a pilot and can verify a GPS receiver's ability to derive altitude information from its 3D fix from satellites is marginal at best. I could be flying level but the altitude displayed on my GPS would bounce around +/- 500' to 750' so it's not accurate enough when you need the correct altitude.

Assuming that you've got a good 3D fix (preferably 5 or more satellites with decent geometry), then discrepancies of that magnitude are much more likely due to the inaccuracy of the plane's barometric altimeter than the GPS. The plane's altimeter reading depends on a standard model of the atmosphere - in particular making assumptions on the lapse rate or how rapidly the temperature falls with increasing altitude and also what the current sea-level pressure is at your current location. When landing at an airport you can get an update as to the current reading there and adjust the Kollman window on the altimeter so it'll be very accurate for the landing, but depending on how closely the atmosphere happens to coincide with the standard assumed model that day it can still be off by hundreds of feet once you're at an altitude of thousands of feet AGL. Despite this accuracy issue, pilots should still use the barometric altimeter to set their flight altitude since the idea is to maintain vertical separation from other planes that are flying in the same atmosphere and therefore subject to the same error. As long as everyone is off by the same amount the planes will stay separated.

 

BTW, I rarely see a discrepancy greater than about 40' when comparing a GPS altitude using at least 5 satellites while on an airport runway or other location with an accurate surveyed elevation.

[peter]

Because the GPS satelites occupy similiar orbitial altitudes, your reported vertical fix is much less accurate than your horizontal fix.

Not quite. The tri-lateralization calculation used for GPS position will work fine for altitude even is all the satellites are at exactly the same orbital height (and they're pretty close to that in practice). But the best accuracy is obtained when you have satellites on either side of your position. So for a good east/west longitude it's best to have satellites both to the east and to the west and for good latitude accuracy you want satellites both to the north and south. Similarly the best altitude accuracy would be with satellites both above and below us but unfortunately we can't receive any signals from those that are below us. But the altitude accuracy is still pretty good, just not quite as good as the horizontal.

 

BTW, long-term measurements also show that east/west accuracy is a bit better than north/south when at middle latitudes since the satellites will be evenly placed east & west, but will tend to be mainly in the southern sky as seen from the northern hemisphere.

[edelsite]

I've read just enough to understand what WAAS is and how it works. From what I've read, it seems WAAS was designed to give aircraft extremely accurate position indication. If that is a fact (please correctly if I'm wrong), it seems that the GPS elevation indication should be every bit as precise as a barometric altemeter. It may not read the same elevation as a barometric altemeter because of changing weather conditions, but it should still be as precise. Please correct me if my thinking here is wrong.

[Jerry Bransford]

I am a pilot and can verify a GPS receiver's ability to derive altitude information from its 3D fix from satellites is marginal at best. I could be flying level but the altitude displayed on my GPS would bounce around +/- 500' to 750' so it's not accurate enough when you need the correct altitude.

Assuming that you've got a good 3D fix (preferably 5 or more satellites with decent geometry), then discrepancies of that magnitude are much more likely due to the inaccuracy of the plane's barometric altimeter than the GPS.

Are you a pilot by any chance? I'm betting you are not and would bet you certainly have never flown as a PIC using a GPS to navigate by.

 

Trust me on this, I know the aircraft was not gaining and losing 500' of altitude as the GPS indicated I was. These were fairly instantaneous GPS indications of altitude variation in calm weather and in smooth level flight. As a pilot, I can dadgum sure tell if I'm gaining and losing 500' in altitude... the stomach is infallible in those situations. In other words, it "ain't" me or my altimeter that is causing those GPS indicated altitude variations. And it's also well known by both pilots and the FAA that GPS derived altitudes are inherently unreliable and because of that, are illegal to use for determining altitude. It is the GPS indicating those inaccurate altitude changes, not the aircraft, not the weather, and not my flying. It's a well known issue with GPS receivers and flying... trust me.

[peter]

I've read just enough to understand what WAAS is and how it works. From what I've read, it seems WAAS was designed to give aircraft extremely accurate position indication. If that is a fact (please correctly if I'm wrong), it seems that the GPS elevation indication should be every bit as precise as a barometric altemeter.

WAAS improves both the horizontal and the vertical accuracy for users who are within the geographic footptint of the WAAS ground stations and who are getting correction data from at least one of the WAAS satellites. While the FAA was more concerned about improving the vertical accuracy, the use of WAAS correction data inherently also improves the horizontal accuracy. You are correct that the altitude accuracy of a GPS receiver with good reception (generally true for airplane use) will usually be superior to a barometric altimeter unless the current calibration for the barometer is known for the particular location. The latter condition holds true for landings at an airport where the barometric pressure at the ground is monitored but doesn't hold true when the plane is far from such an airport or high above the ground.

 

In addition to the improved accuracy provided by WAAS, probably an even more important addition to standard GPS for aircraft approaches is the RAIM (Receiver Autonomous Integrity Monitoring) technique to identify abnormal situations where the GPS position/altitude measurements should *not* be trusted which could be due to multipath, local interference, a faulty satellite, etc. WAAS is one method by which the receiver can get sufficient redundant GPS data to make the RAIM determination that the data is either valid or questionable. In the latter case no GPS approach should be attempted until the problem is resolved or an alternate type of approach can be made instead.

[peter]

Trust me on this, I know the aircraft was not gaining and losing 500' of altitude as the GPS indicated I was. These were fairly instantaneous GPS indications of altitude variation in calm weather and in smooth level flight.

How many satellite signals were you receiving at the time, approximately what did the geometry look like, and are you using a WAAS/RAIM equipped GPS receiver?

 

I have never observed such rapid jumps in indicated altitude *when getting good GPS reception*. It certainly happens at times, but in my experience only when there are clear indications that the reception is questionable.

 

OTOH, I have frequently seen situations where the indicated altitude given by a properly functioning barometric altimeter is off by hundreds or even a thousand feet or more because of atmospheric conditions that differed markedly from the standard model. No, those variations are not nearly instantaneous - especially not in conditions that are conducive to a smooth flight.

 

As to the FAA rules, I already indicated why those call for continued use of barometric altimeters. Absolute accuracy is less important that consistency when the goal is to keep aircraft separated vertically.

[Jerry Bransford]

Trust me on this, I know the aircraft was not gaining and losing 500' of altitude as the GPS indicated I was. These were fairly instantaneous GPS indications of altitude variation in calm weather and in smooth level flight.

How many satellite signals were you receiving at the time, approximately what did the geometry look like, and are you using a WAAS/RAIM equipped GPS receiver?

 

I have never observed such rapid jumps in indicated altitude *when getting good GPS reception*. It certainly happens at times, but in my experience only when there are clear indications that the reception is questionable.

 

OTOH, I have frequently seen situations where the indicated altitude given by a properly functioning barometric altimeter is off by hundreds or even a thousand feet or more because of atmospheric conditions that differed markedly from the standard model. No, those variations are not nearly instantaneous - especially not in conditions that are conducive to a smooth flight.

 

As to the FAA rules, I already indicated why those call for continued use of barometric altimeters. Absolute accuracy is less important that consistency when the goal is to keep aircraft separated vertically.

What do you fly, what ratings do you have, and how often have you been PIC flying an aircraft with a GPS system? RAIM is not required, my GPS receiver does not have it and WAAS is not available in all locations. Any pilot who has never seen their GPS indicated altitude jumping up and down during level flight doesn't have many hours PIC.

[peter]

How many satellite signals were you receiving at the time, approximately what did the geometry look like, and are you using a WAAS/RAIM equipped GPS receiver?

RAIM is not required, my GPS receiver does not have it and WAAS is not available in all locations. Any pilot who has never seen their GPS indicated altitude jumping up and down during level flight doesn't have many hours PIC.

I take it from your response that you have essentially no idea how good (or bad) your reception was at the time that you observed anomalous altitude readings. No, RAIM is not required, but it would both automate and improve the process of determining how reliable the GPS data is. In its absence it becomes that much more important to use other available means of checking such as looking at the number and positions of the satellites being received.

[Jerry Bransford]

How many satellite signals were you receiving at the time, approximately what did the geometry look like, and are you using a WAAS/RAIM equipped GPS receiver?

RAIM is not required, my GPS receiver does not have it and WAAS is not available in all locations. Any pilot who has never seen their GPS indicated altitude jumping up and down during level flight doesn't have many hours PIC.

I take it from your response that you have essentially no idea how good (or bad) your reception was at the time that you observed anomalous altitude readings. No, RAIM is not required, but it would both automate and improve the process of determining how reliable the GPS data is. In its absence it becomes that much more important to use other available means of checking such as looking at the number and positions of the satellites being received.

I'll take it from your repeated lack of response to my questions that you are NOT a pilot and thus have no personal knowledge of flying with GPS other than what you read and what you cut and paste from various information sites. If you are not a pilot, then where do you get off on stating how GPS is used on approaches unless all you're doing is Cutting and Pasting information?

 

You have dodged my repeated questions on what your PERSONAL experience is with flying with GPS each time. I'll take it you are thus unqualified to provide input on flying with GPS as a pilot and thus can only provide secondary information based only on what you read. At any rate, you should realize that at altitude, 8-12 satellites is a minimum that will be received so good 3D locks are the norm. Generally speaking, you always get superb GPS reception at altitude.

 

One last time... are YOU a rated pilot and do YOU PERSONALLY fly with GPS so you are qualified to comment on what happens at altitude? Otherwise, stop implying you are a pilot and know anything about how a GPS operates at altitude.

 

Knowing GPS as you do is one thing but implying you are an expert on them in the cockpit too is a bit much.

Edited June 28, 2006 by Jerry Bransford

[edelsite]

In addition to the improved accuracy provided by WAAS, probably an even more important addition to standard GPS for aircraft approaches is the RAIM (Receiver Autonomous Integrity Monitoring) technique to identify abnormal situations where the GPS position/altitude measurements should *not* be trusted which could be due to multipath, local interference, a faulty satellite, etc. WAAS is one method by which the receiver can get sufficient redundant GPS data to make the RAIM determination that the data is either valid or questionable.

 

That's interesting, is this RAIM function an additional calulation process added to aircraft GPS receivers or is it an additional signal that is received and processed?

Edited June 28, 2006 by edelsite

[+152Driver]

Are you a pilot by any chance? I'm betting you are not and would bet you certainly have never flown as a PIC using a GPS to navigate by.

Who gives a crap if he's a pilot or not?! I think you should check your pilot ego and read/listen to what Peter is saying, the man is right.

And it's also well known by both pilots and the FAA that GPS derived altitudes are inherently unreliable and because of that, are illegal to use for determining altitude. It is the GPS indicating those inaccurate altitude changes, not the aircraft, not the weather, and not my flying. It's a well known issue with GPS receivers and flying... trust me.

You seem to be arguing a non-existant point here. The reason the FAA doesn't try to incorporate GPS derived altitudes is that the current system of navigation (vertically at least) is based on Pressure Altitudes, not True Altitudes. GPS altitudes are True and aren't correctible for local pressure variations (at least in any models I've seen). From what I've seen any Altitude that a panel mounted GPS shows is taken from the Transponder encoder which takes the altitude from the altimeter.

It isn't that GPS's can't be accurate enough but 1) either the current system would have to change or 2) keep the current system and try to incorporate/create a new GPS type of product. But I don't see that happening since "if it ain't broke, don't fix it".

I'll take it from your repeated lack of response to my questions that you are NOT a pilot and thus have no personal knowledge of flying with GPS other than what you read and what you cut and paste from various information sites. If you are not a pilot, then where do you get off on stating how GPS is used on approaches unless all you're doing is Cutting and Pasting information?

I don't think it matters what Peter's experience is in the cockpit, you need to read what he's saying and see that he's right. Do you give as much attitude to a mechanic that fixes your airplane since he isn't a pilot?

One last time... are YOU a rated pilot and do YOU PERSONALLY fly with GPS so you are qualified to comment on what happens at altitude? Otherwise, stop implying you are a pilot and know anything about how a GPS operates at altitude.

Knowing GPS as you do is one thing but implying you are an expert on them in the cockpit too is a bit much.

I've been quite impress how many times you've been able to incorporate "I'm a pilot" into your responses. At this point I'm really curious to your background regarding this topic.

What do you fly, what ratings do you have, and how often have you been PIC flying an aircraft with a GPS system?

So what would your answers be to your own questions and how that relates to this topic. At this point I'm expecting Chuck Yeager himself to spring from behind this screen name.

 

-------

 

Peter, I agree with your points and comments. I've only tried a handheld GPS in an airplane and was quite suprised at the accuracy (as far as I could verify). I can't remember any specifics but I just remember commenting on my surprise at it's accuracy.

 

I recently bought a GPS (allows me to stop borrowing my folks), specifically it's a eTrex Vista. Is there anyway to turn off the barometric altimeter?

[kb9nvh]

I've done some flying with my GPS and, in my experience, I can tell if I'm climbing or descending so rapid altitude changes can be felt. If the GPS indicates fairly rapid altitude changes when the altimeter on the plane indicates none then I belive the altimeter. I think the key here is rapid change (30 seconds). Certainly the pressure altimeter is subject to anything that causes a pressure change but those things happen gradually especially if your trying for level flight.

 

Not to take sides here but I've seen my GPS altitude vary by quite a bit when sitting still on the ground with pretty clear view of the sky. I dont trust GPS altitude for more than about 75 ft in my experience and gps altitude is way worse than horizontal position in my experience. That said, dont trust your planes altimeter for landing without first adjusting for local pressure conditions.

 

The new crop of gps's with pressure sensors only show pressure altitude so some folks have not seen a strictly GPS elevation displayed. Seems there's some confusion on GPS elevation and pressure altitude in some threads I've recently read when it comes to gps's with pressure sensors.

 

Just my two cents on this subject.

 

I am a pilot and can verify a GPS receiver's ability to derive altitude information from its 3D fix from satellites is marginal at best. I could be flying level but the altitude displayed on my GPS would bounce around +/- 500' to 750' so it's not accurate enough when you need the correct altitude.

Assuming that you've got a good 3D fix (preferably 5 or more satellites with decent geometry), then discrepancies of that magnitude are much more likely due to the inaccuracy of the plane's barometric altimeter than the GPS. The plane's altimeter reading depends on a standard model of the atmosphere - in particular making assumptions on the lapse rate or how rapidly the temperature falls with increasing altitude and also what the current sea-level pressure is at your current location. When landing at an airport you can get an update as to the current reading there and adjust the Kollman window on the altimeter so it'll be very accurate for the landing, but depending on how closely the atmosphere happens to coincide with the standard assumed model that day it can still be off by hundreds of feet once you're at an altitude of thousands of feet AGL. Despite this accuracy issue, pilots should still use the barometric altimeter to set their flight altitude since the idea is to maintain vertical separation from other planes that are flying in the same atmosphere and therefore subject to the same error. As long as everyone is off by the same amount the planes will stay separated.

 

BTW, I rarely see a discrepancy greater than about 40' when comparing a GPS altitude using at least 5 satellites while on an airport runway or other location with an accurate surveyed elevation.

Edited June 28, 2006 by kb9nvh