We operate our aircraft in a two-altitude world; one altitude is based on a calculation using the air pressure surrounding our aircraft and another is based on the 3D position solution from our GPS.
The first uses our pressure altitude, the altitude based on a sea level pressure of 29.92 inches, temperature of 15 degrees Celisus, and on a 2 degree Celesius lapse rate. This is the International Standard Atmosphere (ISA) to model the air below us. That solution must be corrected (in model calculations and on our altimeter or PFD) by the local altimeter setting to estimate our altitude using the atmospheric pressure at our aircraft.
The other altitude references our 3D GPS position solution to the WGS 84 datum, an ellipsoidal surface that roughly follows the earth’s MSL surface. This is the HAE altitude (height above the ellipsoid). The HAE altitude of airports with GPS approaches is stored in the GPS database. The actual MSL surface is the earth’s geoid (a surface of constant gravitational pull, and matching the mean surface of the seas), so to convert HAE altitude to MSL you need to add or subtract the local value of the earth’s geoid height (distance between those surfaces). In the US, the MSL surface varies between 8 and 53 meters below the WGS 84 surface. This geoid height information is stored in the GPS and is applied locally to your GPS 3D solution.
Each of these altitude systems has uses in our navigation endeavors. Neither one is absolutely accurate, but the GPS estimate is the gold standard with altitude errors (with WAAS) in the 5-10 foot range, while barometric altitudes can be off 1000 feet or more at higher altitudes. There is understandable confusion in mixing these two systems in flight, so let’s review how each of them is used.
My most recent articles here (and on my website www.avionicswest.com) gave the physics of how density altitude and barometric altitude are determined, while an earlier article gave details on GPS altitude. While GPS altitude is far more accurate than the barometric altitude displayed on your altimeter or PFD, we nonetheless fly at assigned altitudes based on the latter. We shoot approaches to minimums based on the altitudes shown on our altimeter or PFD.
Barometric Altitude
What I hope you took away from the article on altimetry is that you can accurately calculate your altitude from the barometric equation (that relates atmospheric pressure and altitude) if you have the baro-correction (altimeter setting) from a station directly below you, and if you know the temperature profile of the air between that station and your aircraft. But that temperature profile data does not exist. Even though you can find the temperature at an airport below you and know the temperature where you are, you don’t know how it varies between these two points. Is it a straight-line drop, or is there a temperature inversion between you and the ground or some other profile? Even if you had that temperature profile, the solution to the equation for your altitude is very complex and not something you could do on board. So barometric altitude, using the ISA model atmosphere, has inevitable errors that grow with altitude.
We don’t rely on our altimeter to avoid terrain at altitude, but we do fly assigned altitudes based on pressure readings – knowing there are large errors at altitude. However, since everyone in our vicinity has the same errors, this works out for traffic separation – but not for avoiding rocks in the sky. Near the ground, shooting an approach to minimums, we insist on having only a small error. It is, in fact, small if you input the local altimeter setting before starting an approach. As you descend toward the airport, there is little atmosphere between you and the ground so the temperature profile doesn’t matter much. Setting the altimeter does; on the ground the error should then be zero. On approaches where you use the altimeter setting from a nearby airport, the minimums will be raised because now the error is not zero (but likely small).
How about traffic advisories from ATC, who sees you and a target; how does ATC know the altitude of your traffic? What if that pilot forgot to enter the current altimeter setting? Is he where ATC thinks? Here, we assume ATC is monitoring our altitude-reporting transponders and not our ADS-B outputs. Our transponders are sending our pressure altitude, based on a 29.92-inch setting on the altimeter, not the current altimeter setting. ATC computers store the current local altimeter setting and apply it to both of you, so he knows your altitude difference based on the ISA model. It doesn’t matter if the other pilot sets his altimeter, or that altitude is in error. It only matters that your altitudes are close so that the errors from using the ISA model in that altitude difference are small. They are zero if you’re at the same altitude. You may have a TCAS or TAS system that interrogates other aircraft and if they have altitude reporting transponders you will know the difference in altitude based again on barometric altitudes.
On a hot or cold day (relative to the ISA temperature) are you higher or lower than shown by the altimeter? The answer depends not just on the temperature at your altitude, but also on the entire temperature profile below you. For example, if the pressure at your aircraft is 25 inches and the local altimeter is 29.92 inches, the altitude based on the ISA model (15 degrees at sea level, dropping 2 degrees per 1000 feet) is 4,858 feet, and that’s what your altimeter should show. But if the sea level temperature is 25 degrees, and the lapse rate is still 2 degrees per thousand, the temperature at your aircraft would be 15 degrees, or 10 degrees above ISA (+10 ISA). Your altitude calculated from the barometric equation for these assumptions is now 5,022 feet so you are 164 feet higher than is shown on your altimeter.
Again, the actual error depends on the entire temperature profile; it could be hotter or cooler on the ground and also have a lower or higher lapse rate. The temperature may not decrease in a linear fashion (constant lapse rate) but have a variable lapse rate with altitude. Nonetheless, if the temperature at the aircraft is higher than standard you are probably higher than shown on the altimeter. If it is below standard, you are probably lower.
If you have a GPS on board you don’t need to guess. You can read your actual altitude on the GPS (with an error of 5-10 feet). In Figure 1, on one corner of the map page of a Garmin GTN 750 we put the GSL data field, which is Garmin-speak for your GPS altitude. It’s easy to monitor the difference between that and your altimeter. Try it sometime, and you may be surprised.
GPS Altitude
If the FAA ever mandates a change to use GPS altitude for operating in US airspace, we could swap our altimeters for a small, dedicated digital device on the panel reading our GPS altitude. When flying GPS altitude, we would fairly accurately maintain those actual altitudes. Will this happen? Maybe not in our lifetime, but in this lifetime GPS altitudes already give us critical information. We now use them on GPS vertical approaches; glidepaths are defined by GPS altitudes (glideslopes are defined by ILS antenna beams). This is in contrast with baro-VNAV approaches that have been around since the 70s and are based on computer-generated slopes starting at barometric altitude waypoints. On all approaches, FAF altitudes and minimums on the approach are still barometric altitudes.
With a terrain database in your GPS, and using your GPS altitude, you know your actual altitude relative to the surrounding terrain. This is the basis of TAWS (Terrain Avoidance and Warning System). A common TAWS display colors terrain on the GPS map or on your MFD according to the space between you and the ground. When you are within 100 feet of the ground it’s red, and terrain up to 1000 feet lower than that is yellow. To see this display on your screen you generally have to make a selection. On the GTN 750, you choose Terrain on the Map Menu.
Another use of GPS altitude is for ADS-B traffic. In Figure 2 the Traffic page on the 750 shows targets with relative altitudes in hundreds of feet. The one at 4 o’clock is aimed at you but will pass behind you. But how do you know he is 800 feet above your barometric altitude? His ADS-B Out reports his GPS altitudes to ground stations and directly to you if you receive his frequency (978 or 1090 MHz).
Figure 2. Traffic targets on a GTN 750, with relative altitude in hundreds of feet. Here, we chose relative motion (rather than absolute) from the menu so the green lines show his track relative to yours (his actual track is in the direction of the chevron). Some of these show altitudes based on GPS altitude (ADS) and some based on barometric altitude (TCAS).
If you’re at 5,500 feet (barometric), how do you know he is at 6,300 feet? Simple. While you’re unlikely to be at 5,500 feet per your altimeter, the incoming target GPS altitude is compared with your GPS altitude to display the difference, regardless of the likely error in your barometric altitude. The difference, accurately, is 800 feet. By the way, your display (depending on equipment) may blend ADS-B targets and barometric targets and show you both. But if a given target is transmitting both, having an altitude reporting transponder and an ADS-B OUT unit, those blending units (such as a GTX 345) will decide which of these to use to represent that target, according to some algorithm devised by the manufacturer.
Summary
Both barometric and GPS altitudes are widely used today. Barometric altitudes are rough estimates based on the ISA model, but are the altitudes used for routes in the system. GPS altitudes are very accurate and are used for terrain avoidance. Both are used (differently) for approaches and for traffic alerts. Understanding each of them, and the limitations of barometrically determined altitudes, is critical for safe flight.
Dr. Thomassen has a PhD from Stanford and had a career in teaching (MIT, Stanford, UC Berkeley) and research in fusion energy (National Labs at Los Alamos and Livermore). He has been flying for nearly 60 years, has the Wright Brothers Master Pilot Award, and is a current CFII.
