barometers in the sky…

The altimeter is certainly one of the most important safety components in skydiving – alongside the canopy and harness.
Nevertheless, you should not blindly rely on the altimeter, as it is only a compromise when trying to measure altitude accurately.

Many skydivers assume that the altitude displayed on an expensive instrument must always be correct, and make important decisions – not only in emergencies – based on these instruments.

The fact is, however, that we have hardly any means of determining our exact height above ground.

An overview of methods for measuring altitude, advantages and disadvantages and a comparison of different systems

a classic: mechanical barometric altimeter

Actually a good compromise, because it is not particularly accurate, shows with clear colors when it is time to pull and does not even give the impression of being more accurate than it is.
Technically, it contains a partially vacuumed tin can, which is compressed more by the air pressure on the ground than at altitude. This difference is measured mechanically with a lever mechanism and converted into a pointer rotation.

the digital barometric altimeter

contains a MEMS chip (micro-electromechanical system) in which a micrometer-sized pressure chamber is also compressed, thereby detuning a resistance measuring bridge that can be read out electrically.
As is usual in electronics, this technology has made huge progress in recent years, so that extremely accurate measuring instruments can now be produced in a small and cost-effective way.
In fact, these sensors, which are built into every cell phone, can read changes in the height of a single step from the change in pressure, making it possible to locate the exact floor inside a building. That’s great!
More on what this means for skydivers later.

digital GPS altimeter

The satellite navigation system, formerly GPS, today GNSS (Global Navigation Satellite System – there are more than 5 systems operating in parallel) not only allows the position on the map to be determined, it also provides real 3D data and thus also the altitude.
GNSS is a very complex system, which in principle can be very accurate, but is also subject to a wide variety of interfering factors, so that absolute accuracy can never be specified. Instead, a probability is given for all three coordinates that the value corresponds to the exact position. Realistically, the position can be determined to an accuracy of approx. 10m without additional aids (e.g. correction signals transmitted on an extra channel).

This system would actually be perfect for an altimeter if it weren’t for the problem that GPS (used here as a synonym for all services offered) indicates the altitude either relative to mean sea level or to the geoid. The geoid is a mathematically defined, simplified earth datum line that does not necessarily correspond to the actual terrain surface at the landing site. In concrete terms, this means that the GPS reading on the ground is practically never zero. However, we expect our altimeter to show zero on the ground – which is why the local elevation would have to be subtracted as a correction value for a GPS altimeter. Not ideal either, of course.

From technology to practice

Apart from the GPS measurement, none of the devices used as altimeters measure altitude directly. Instead, the current barometric pressure is measured – in simple terms: the weight of the air column above the measuring point.

This pressure depends on numerous, constantly changing parameters: the general atmospheric pressure, which is influenced by the weather conditions (which is why the altimeter is set to zero at ground level on the day of the jump), as well as the temperature and humidity, which can also change during the ascent.

The general barometric formula is used to calculate the altitude based on the pressure difference between ground and altitude pressure. This is based on the ICAO standard atmosphere (15 °C at sea level at a pressure of 1013.25 hPa). All altimeter manufacturers and the aviation industry use this formula – in the knowledge that the underlying standard values can rarely be met in reality. As a result, the airplanes at least all fly at the same “wrong” altitude, so that the distance between them is correct.

However, our actual aim is to determine the remaining distance downwards precisely – which is not possible in this way. The display errors here tend to be more in the region of 100 meters rather than meters.

Every skydiver should be aware of this when planning flight maneuvers based on the altimeter display, especially in lower altitudes.

Precision

With a mechanical altimeter, nobody would have thought of assuming this precision; the 2, 3, 4 digits of the digital devices, however, give the illusion of an accuracy that unfortunately does not correspond to reality. And this is not a criticism of the devices, they are very accurate, but the air around us that we measure is not.
Why does the airlogOne even display 4 or 5 digits (ft)? Because we can – we have enough space on the large display and show the correct number without a conversion factor. However, the internal measurement is actually rounded to 5m steps before being displayed.

dynamic aspects

We have sometimes been asked why, for example, the readings from the airlogOne and the XXXX differ by 10-20m during the climb phase.
This is certainly not a question of accuracy (we use the latest generation of baro sensors, which have improved considerably in terms of absolute accuracy over the last 10 years).

Rather, the dynamics of the display and the filtering of the measured values play a role. If, for example, we have a continuously decreasing air pressure during the climb, then some devices follow this change more quickly and others more slowly.

If the situation becomes really dynamic, such as in freefall or radical flight maneuvers, then this influence becomes even clearer: If I follow too quickly, I run the risk of “overshooting the target”, i.e. the display briefly shows a clearly incorrect altitude, but then settles at the correct value, another device follows more slowly and reaches the correct value stably – this is what were aiming for with the airlogOne.
Every altimeter has its own characteristic, but it is not advertised. There is also no real right or wrong. However, anyone flying dynamic maneuvers should gain knowledge about this character of their device in order to make the right decisions.

Conclusion

Differences in the display of a few meters are quite normal. And: you should fly the last 50 meters by sight and not attempt an instrument landing.