So you just purchased a Deep and Steep altimeter and your previous visual or audible altimeter is made by another company? Please read the sections below for important information about altitudes and mixing different manufacturer’s altimeters.
Accuracy and Differing Altitudes Between Brands #
Sometimes we get asked a very valid question: is my altimeter accurate? The answer is yes, it is as accurate as a pressure sensor altimeter can be. The sensors we use have a precision of 1 meter or less. However the true accuracy of an altimeter has to do with a lot more than how precise the sensors are.
You may notice that your Deep and Steep altimeter doesn’t always report the exact same altitude as another altimeter on the airplane. Let’s explore why this can happen.
Software Differences #
Every digital altimeter is essentially a computer that takes pressure readings continuously to calculate an altitude above ground level (aka altitude AGL). The most direct way to perform this calculation is to save the pressure when you take off in the aircraft and then calculate a difference between that pressure and the current air pressure as you move through your skydive.
However there are other tricks that manufacturers will use to adjust the altitude based on your speed, if the altimeter thinks you are in a burble, etc… Because of this the altitudes shown can differ because of the software manipulating the altitude before it is shown to the user. The difference is generally less than 50 feet, but in extreme cases in freefall the altitudes can differ by as much as 200 feet.
Saved Pressure on Takeoff #
Another way altimeters on the same airplane can display different readings is how the “ground” is calculated on takeoff. Because the air pressure changes throughout the day, each altimeter regardless of manufacturer will update the “ground” air pressure every few minutes.
Upon takeoff this air pressure value is saved as the ground reference for the skydive. Different altimeters may save the pressure slightly before takeoff, or while the airplane is accelerating down the runway. Because of this each altimeter, even from the same manufacturer, may record a “ground” pressure that is 20 or 40 feet different.
So Which Altimeter is “Correct”? #
Unfortunately the answer is “none of them are correct”. How do we know? Because we make altimeters that also use GPS, and the GPS data is highly accurate and unaffected by burbles and other problems that affect pressure sensors. Through lots of testing with our GPS data and 3rd party data from high end GPS units (those found in aircraft) it is easy to see that pressure altitudes are at best a rough indication of your altitude above the ground. We tested all of the major brands of skydiving altimeters to confirm this is true of any altimeter on the market today.
None of the altimeters you see on the plane are telling you a true distance from the ground. They are all an approximation that changes with altitude, speed, burbles, air temperature, atmospheric disturbances (weather), etc… Pressure based altimeters can be close to the truth, within 20 or 30 feet, but they can also be off by many hundreds of feet. This is especially true at high altitudes.
Safely Transitioning to a New Altimeter #
When you change altimeter models or manufacturers it is always a good idea to be conservative with your canopy flight, especially if you do any sort of “swooping” or aggressive canopy inputs close to the ground that dive the canopy and increase speed. It is possible that your new altimeter will report somewhere between 20 and 50 feet different than you old altimeter or audible, either higher or lower.
Always increase your setup altitudes slightly under canopy and play it safe. Use the same process of evaluation and sight picture that you used to learn your previous swoop altitude or turn from base to final altitude. After a couple of skydives you’ll have the confidence needed to trust your new equipment and know what altitude is correct for your canopy when you switch to a new altimeter.
