One of the specifications that you won’t find for your aerial drone is its altimeter accuracy. In fact, if you call the manufacturer’s support staff they won’t be able to provide you with that spec either. Technology behind the common altimeter is consistent across many of the latest airborne vehicles, including drones. In most cases, altimeter inaccuracies are minor, so these instruments serve their purpose well.
However, aerial drones operate much closer to ground than most other aviation vehicles. This is where we as drone pilots might notice a drift in reported altitude throughout our drone’s flight. We’ll explain why this drift occurs and what can be done to compensate for it.
How Does an Altimeter Work?
What we know is that as one goes up in altitude, the air pressure goes down. The relationship between air pressure and altitude is quite predictable, but subject to minor variables such as the gas content and temperature. So, whether the readout is an analog meter or a more sophisticated digital instrument, the altitude is easily converted from the air pressure. Click here for a detailed explanation on how altimeters do this.
Which of These “Minor Variables” Affect My Drone’s Altimeter?
Over the time span of a drone flight, the atmosphere’s gas content, that is Nitrogen, Oxygen, CO2, water vapor, etc., can be treated as fixed. Surface-level barometric pressure changes can also be treated as fixed. That leaves variances in temperature as the most significant contributor to altimeter inaccuracy.
Think about where the barometer is located in your drone. It’s a small sensor mounted on or near the motherboard in a relatively confined space. From the beginning of a flight to its end, heat from the drone’s control circuitry builds up in this space, affecting the barometer. Whether it’s a change in air pressure or temperature, the result is the same – a change in reported altitude.
Low Altitude Operations
This is where the thermal effect on reported altitude is more noticeable. You may have noticed when coming in for a landing that your drone is reporting an altitude of some 15 feet or so when it’s actually 3 feet off the ground. If you were to simply take off and hover, you would find that the indicated altitude slowly increases while the drone is actually in a stationary position. These are thermal effects.
Temperature drift in your drone’s altitude reading is hardly noticeable when flying high, and may only be a minor nuisance when flying low. But, where it can cause trouble is when using a programmed mode that instructs the drone to fly at a certain altitude. If that happens to be 10 feet or so, then the program will try to drive the drone at that altitude, but in reality is flying it toward the ground. Even at higher altitudes, you can lose your margin when programmed flight decreases your drone’s actual altitude above objects such as buildings and trees.
What to do When Altitude Accuracy is Important
I recently had a client that required drone photography at several specific altitudes above ground level. This job had to do with precision surveying in advance of a construction project. Since my drone’s altimeter would not meet their requirements, they set up a surveying instrument to measure the drone’s altitude (see picture). You may also consider other workarounds, such as:
- At the time of your critical photos/videos, record the drone’s altitude, then land and record altitude again. Take the difference for true altitude.
- Allow the drone to warm up for several minutes before lift-off. At the end of flight, use the reported landing altitude to estimate in-flight altitudes. (e.g. interpolate the number of feet per minute of drift.)
- Use an independent measurement device, such as a laser rangefinder.