# What is the differences of ADS-B and geometric height?

Why does ADS-B and geometric height (calculated height) differ from each other?

If the geometric heights are calculated at night where the temperature is lower, then the result should be different during the day, right?

Is ADS-B with night or day calculated height smaller?

What is the more accurate result to be referenced?

• I am having a really hard time trying to understand what you mean, could you please consider improving the wording of your question and making it clearer? Commented Jun 8, 2015 at 7:56
• ADS-B is a system, not a measurement. This is like asking why a car and speed are different from each other. I suspect you would like to discuss ADS-B reported altitude and ADS-B reported geometric height or otherwise measured geometric height, but please state so. Commented Jun 8, 2015 at 12:37

ADS-B, and the older Transponder Mode C, report pressure altitude. That is static pressure where the plane is expressed as altitude at which that pressure occurs in international standard atmosphere.

In low pressure area, the same pressure density corresponds occurs at lower geometric altitude with similar difference at all altitudes. In lower temperature, the air is denser and the isobars closer together, so the same pressure altitude occurs at lower geometric altitude and the difference increases with altitude.

However for purpose of traffic separation, pressure altitude is always used. Due to physical properties of air, the pressure always decreases with altitude and at the same altitude changes only slowly with position, so we can always rely on close aircraft observing sufficiently different pressure altitude being at sufficiently different geometrical altitude.

The reason is that while barometric altimeter is simple, accurate to less than 100 ft and every aircraft has it since the early days of aviation, there was no way to measure geometric altitude before selective availability was turned off in GPS in 2000.

Geometrical altitude is altitude above the reference geoid (usually WGS84), a.k.a the sea level. It is only used internally in EGPWS where it is provided by GPS.

In contrast height is in aviation always used to mean height above terrain. It may be measured directly using radar altimeter or it can be estimated from pressure altitude by correcting for sea-level pressure at the nearest airfield.

Radar altitude is used in GPWS as a backup datum and in older such systems from before GPS was available (those systems could warn if you were too low, but not if you were approaching high ground, because the radar altimeter only measures down while the new GPS-based systems have a terrain map and compare the GPS position with it) and is shown to pilot for reference, but it is not broadcast anywhere.

Approaches are normally flown in reference to the barometric altitude that is corrected for pressure at airport (the altimeter has setting for reference sea-level pressure, QNH), but not temperature. So the barometric altimeter shows true altitude at the airport, but when it's cold, it shows more than true altitude above. That's why some approaches have minimum temperature.

However, both Mode C and ADS-B broadcast pressure altitude, i.e. as calculated for standard pressure of 29.92 inHg/1013 hPa.

In aviation altitude usually refers to distance between MSL(mean see level) and aircraft and height to the distance between aircraft and terrain. Barometric altimeters are used to measure altitude. But they have a lot of errors. To get correct altitude you have to set correct pressure and correct for other errors like temperature. Temperature corrections are only used if it's cold and you are in danger of hitting obstacles. So you will never have exact altitude not to speak about height because altimeter knows nothing about terrain. Transponder just reports number based on standard pressure setting. So this number can differ by hundred of meters from real altitude.

ADS-B Airborne Position squitter carries altitude which might be uncorrrected pressure altitude or GNSS HAE. There are different format type codes for each altitude type. You might check Technical Provisions for mode s Extended Squitter document for this.