Altitudes: GPS vs Pressure

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rkstumberg

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RKS
Hi there. Any comments on which altitude is more ‘accurate’ ? Most times the ATC pressure altitude setting will show hundreds of feet delta to displayed GPS altitude, as we all know. How do we discuss this with ATC centers, for example, to decide which is more accurate. Droning along across hundreds of miles of cross county, a number of different press settings can be provided by center. Whereas GPS readings remain pretty steady. Comments ? RKS
 
Well, it depends on what you mean by accurate.

If you mean with regard to absolute altitude, then it depends on the conditions.

If you mean with regard to the datum currently being used by ATC, then baro altitude by far.

First off, GPS altitude is much less precise typically than barometric altitude. Your baro alitimeter is typically good to a ten feet or so. While horizontal position in GPS Is pretty accurate, vertical accuracy suffers because of the natures of the geometry with satellites that an aircraft can see in normal operations.

Pressure altitude or local altimeter setting, the whole point is to put all the planes ATC is controlling on the same reference point.

WHen it comes down to when it matters on landing, you switch over to the local altimeter setting which corrects for a temperature, pressure, and gets the actual field MSL rather than the zero ellipsoid datum which may differ.
 
rkstumberg said:
Hi there. Any comments on which altitude is more ‘accurate’ ? Most times the ATC pressure altitude setting will show hundreds of feet delta to displayed GPS altitude, as we all know. How do we discuss this with ATC centers, for example, to decide which is more accurate. Droning along across hundreds of miles of cross county, a number of different press settings can be provided by center. Whereas GPS readings remain pretty steady. Comments ? RKS
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Your aircraft is equipped with a blind altitude encoder. It sends pressure altitude to ATC (Mode C). ATC converts the pressure altitude sent from your aircraft to the proper true altitude based on the altimeter setting being used.

ADSB transmits an aircraft's GPS position, altitude, velocity, and identification.

GPS altitude can be more accurate or less accurate.
 
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Hey, RKS. GPS altitude is usually more consistent, but ATC relies on pressure altitude for standardization and safety. It’s best to stick with the ATC settings they provide, but you can mention the GPS reading as a reference if needed.
 
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Simple way to think about it. It doesn't matter how high planes are, it matters they are all using the same altitude. Pressure altitude gives all planes in the same area a consistent metric for their altitude relative to each other. GPS is not that consistent and has more chance of variation from plane to plane based on speed, receiver etc.
 
flyingron said:
Well, it depends on what you mean by accurate.

If you mean with regard to absolute altitude, then it depends on the conditions.

If you mean with regard to the datum currently being used by ATC, then baro altitude by far.

First off, GPS altitude is much less precise typically than barometric altitude. Your baro alitimeter is typically good to a ten feet or so. While horizontal position in GPS Is pretty accurate, vertical accuracy suffers because of the natures of the geometry with satellites that an aircraft can see in normal operations.

Pressure altitude or local altimeter setting, the whole point is to put all the planes ATC is controlling on the same reference point.

WHen it comes down to when it matters on landing, you switch over to the local altimeter setting which corrects for a temperature, pressure, and gets the actual field MSL rather than the zero ellipsoid datum which may differ.
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Okay flyingron, you just reminded of all the answers. I seems that I devolved into giving too much credit to the geosynchronous satellite system’s accuracy. Thanks !
 
Pressure altitudes keep all the players on the same page.
 
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"What's most accurate" is less important than consistency. ATC and all the other planes around you are using baro altitude. When you talk to ATC or other planes, you should probably do that, too.

The regs specify how often the altimeter needs to be checked/calibrated and to what tolerances is should perform. That goes a long way to ensuring adequate accuracy for the purposes of not flying into other planes or terrain.
 
Jim_R said:
you should probably do that, too.
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Agreed … must fly altitude ref altimeter set to local pressure (at/blw transition altitude).

RKS:
How do we discuss this with ATC ... to decide which is more accurate.
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For separation/complying with ATC (IFR)/VFR cruising altitudes/TPA/etc; flying ‘gps altitude’ … not an option.
 
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Pressure altitude will always be more accurate than GPS... with the caveat, as others have pointed out, that pressure altitude may or may not correspond with the actual altitude unless the altimeter setting is correctly entered.

Here are two altitude plots from the same flight when I was doing some propeller testing, thus the four climbs and descents to record average rate of climb starting after the 10 minute mark, then some time in the pattern, 3 landings, starting after the 40 minute mark. The upper plot is pressure altitude, not adjusted for altimeter setting. The lower plot is GPS altitude The field elevation is 416'. As you can see there is a lot more variation in the GPS track.

1723599304380.png
 
Which is more accurate? Accurate for what purpose?

For ATC and aircraft collision with each other purposes, indicated altitude is used. Indicated altitude is not "accurate" with respect to your true MSL altitude except for very near the altimeter setting reporting point. The higher above the point you are, or the further away, the less accurate it is compared to true altitude. That is why instrument approaches in cold, generally mountainous states have temperature correction limits - because some segments are high enough above the station that the errors can be quite large.

But for ATC purposes with everybody on the same altimeter setting, indicated altitude is pretty accurate, relative to other airplanes. And that's its purpose.

GPS altitude, however, is not affected by temperature and pressure, and is generally pretty accurate as a true altitude, especially with WAAS. Meaning that if a mountain is 5000 feet MSL, and you fly at it at 5000 GPS altitude, you are probably going to hit it right at the top.

In my day job, I actually fly true altitudes all the time. Heck, I did it multiple times today. As we are certifying radio signals, we want to do that at a constant true altitude, not an indicated altitude that would vary each time we come by. So we use GPS altitude for this. For one test today we wanted to be at 8000, which ended up being 7800 indicated. A little later, we wanted to be at 6300, which was 6200 indicated. Later we wanted 16000, which was 15,600 indicated. Generally the difference between the two increases as altitude increases, and specifically height above the reporting station (the airport). This work today was all in Colorado, so our field elevations were 4500 feet and higher.
 
The mechanical altimeter in your airplane is a stand-alone device. It uses static air pressure. I have seen hikers use a similar device that looks a lot like a pocket watch, another stand alone device. GPS, is definitely NOT a stand alone device. It relies on a vast array of other equipment to work properly, and their interaction, or lack of it, affects their accuracy. A GPS needs to recieve 3 satellites to get a position fix, but that is not enough to get a good accurate altitude. For that they need at at least four and more is better to produce better accuracy. I'm not saying GPS can't do that, it can, but it's reliability may be flimsy at times. Do you want to bet your life on that? ATC doesn't.

ADS-B out is required for airplanes to use all of the NAS system. Not all airplane have it. If they don't their flight area is somewhat restricted. Also, ADS-B out reports GPS position and barometric altitude to ATC. ATC does not receive GPS altitude at all. GPS vs Barometric altitude accuracy is irrelevant in general aviation.
 
William Pete Hodges said:
The mechanical altimeter in your airplane is a stand-alone device. It uses static air pressure. I have seen hikers use a similar device that looks a lot like a pocket watch, another stand alone device. GPS, is definitely NOT a stand alone device. It relies on a vast array of other equipment to work properly, and their interaction, or lack of it, affects their accuracy. A GPS needs to recieve 3 satellites to get a position fix, but that is not enough to get a good accurate altitude. For that they need at at least four and more is better to produce better accuracy. I'm not saying GPS can't do that, it can, but it's reliability may be flimsy at times. Do you want to bet your life on that? ATC doesn't.

ADS-B out is required for airplanes to use all of the NAS system. Not all airplane have it. If they don't their flight area is somewhat restricted. Also, ADS-B out reports GPS position and barometric altitude to ATC. ATC does not receive GPS altitude at all. GPS vs Barometric altitude accuracy is irrelevant in general aviation.
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Probably shouldn't post this because it doesn't matter, but gps altitude is in the ADSB message in addition to the barometric altitude. I don't know why it is, because of all the things you said, but it is.
 
Might be a back up if the altitude encoder goes inop.
 
Salty said:
Probably shouldn't post this because it doesn't matter, but gps altitude is in the ADSB message in addition to the barometric altitude. I don't know why it is, because of all the things you said, but it is.
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A form of GPS altitude is broadcast by ADS-B Out, but it is using a spheroid model of the earth's sea level surface. It is called geometric altitude and is based on the WGS-84 model. It does not take into consideration the differences in the local gravity. In the US, this can be as much as 200 feet in some areas. At my airport, the difference is 110 feet between ADS-B reported altitude and what my GTN GPS reports as GSL altitude. The latter GSL corrects for differences in the local gravity, so matches field elevation usually within 20 feet.
 
Dana said:
Pressure altitude will always be more accurate than GPS... with the caveat, as others have pointed out, that pressure altitude may or may not correspond with the actual altitude unless the altimeter setting is correctly entered.
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GPS altitude is normally a much more accurate indication of true altitude verses a Baro altimeter and is what is used for terrain avoidance. GPS altitude is unaffected by non standard pressure lapse rates and temperature. A Baro altimeter set to the appropriate altimeter setting will indicate field elevation within +/- 75 feet. The GPS GSL altitude will typically be within +/- 25 feet. As the temperature rises, the Baro altimeter will indicate lower than you are and at altitudes above the airport where the altimeter setting is determined, it can indicate up to 500 feet lower than reality whereas the GPS GSL altitude will still be +/- 25 feet. Cold temperatures are the opposite and the Baro altimeter will indicate above your true altitude. It is for this reason cold temperature correction is needed for some approaches to remain safe. Temperature plus non standard pressure lapse rates, particularly over mountains can be problematic and we use +1000 feet above the highest obstacle in the lowlands and +2000 feet in Mountainous areas when flying IFR. Notice that Class E generally starts at 1200 feet above the surface, which accounts for unknown obstacles up to 200 feet above the terrain plus 1000 feet. However, since a GPS is not required nor even a radio or a transponder in much of the airspace, GPS doesn't do a good job vertically separating traffic. That is where the Baro altimeter shines, because below 18000 feet, we all use the same measuring stick, that is MSL Baro altitude ideally corrected to an altimeter source within 100 NM. Above 18000 and over the ocean, using MSL isn't practical, both because of the speeds involved and the availability of Baro measuring stations, so we switch to everyone using 29.92 as the setting which is pressure altitude.

So GPS altitude is more accurate and if you use it to avoid obstacles and terrain, that is good. To avoid hitting other traffic, you really want to use either Baro MSL altitude or Pressure altitude depending on using MSL for below 18000 or FL above 18000 or oceanic. ADS-B, transponders, TAS, and TCAS systems all use pressure alt because it is common and doesn't need to be corrected. For LPV approaches, the Baro Altimeter is used for MSL altitude minimums and the DA/MDA, but the vertical guidance to 200 feet above the TDZE uses WAAS corrected GPS geometric altitude. Temperature still plays a role with Baro Altimeters and needs to be considered on approaches in very cold conditions.
 
It's kind of meaningless to ask which kind of altitude is more "accurate" as they measure different things and are used for different purposes. Barometric altitudes are used to vertically separate traffic using pressure altitudes in some version (different above and below 18,000 feet), and are used to establish minimums for instrument approaches. It doesn't matter if it is accurate, as long as it is relatively precise for all users operating within a designated area. Anyone with a (required) altimeter can use this measure of altitude. GPS altitudes measure height above a specific theoretical datum, and are under the hood for establishing things like LPV vertical guidance profiles, but are only available for those with appropriate GPS units.
 
Makes no difference which is more accurate. After decades of barometric pressure use, I think it’s safe to say that it has proven adequate. The main thing is consistency between aircraft and that is being xpaccomplished.
 
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