A Range altimeter or altitude radar calculates how high a drone is flying above the ground. It is more accurate than a barometric altimeter because a barometric altimeter provides the distance between the drone and the average sea level.
History of radars
Radars were conceptualized during the 1910s when Bell Labs was facing difficulty in understanding the signal reflections due to impedance in telephone lines, more specifically at points where equipment was connected to the wires.
Engineers took notice of the “humpy pattern” of the reflections. The issue would only be noticeable if the devices were positioned at specified positions in the line for any given signal frequency.
This gave rise to the concept of sending a test signal into the line, adjusting its frequency until noticeable echoes appeared, and then figuring out how far away that device was so it could be located and fixed.
While at Bell Labs, Lloyd Espenschied had the idea to use the same phenomenon to gauge wire lengths. His 1919 patent (issued in 1924) based on the idea of transmitting a signal into railroad tracks and determining the distance to discontinuities was one of his early innovations in this area.
Importance of drone altimeters
Altitude radars in drones are a very crucial module as it serves the following purposes:
- It relays real-time data to the pilot for mapping purposes and controlling the flight
- To comply with the law regarding safe altitude to fly the drone in that particular area
- To make sure that the drone is flying above the recommended Minimum Obstacle Clearance Altitude (MOCA) to avoid collision
- When flying over a restricted air space, the drone should not fly above 400 ft to avoid any interference with aircraft operations
Accuracy of altimeters
Drones may have more than one system of measuring altitude. That way it can switch between the optional systems according to the situation to yield high accuracy in its readings. These are the following systems:
- Infrared System: Typically, drones have a sensor called Vision Positioning System (VPS) that employs infrared. Infrared-based sensors give highly accurate results within the permissible range as long as it is not flying over a reflective surface, like water.
An infrared sensor gives accurate data even for longer flight times and doesn’t need recalibration. But the downside of infrared sensors is they generally phase out as soon as the drone reaches heights between 20 to 30 ft. Beyond 25 ft infrared signal gets weak.
- Barometric System: The predominant system of measuring altitude, the barometer uses air pressure to calculate height. The air pressure steadily drops as the drone keeps ascending. The drone’s barometer can measure readings both in AGL and MSL units. Unlike the Infrared system, the barometer needs to be calibrated and reset at sea level, where the pressure should be approximately 101.325 kPA (kilopascal).
The barometer, though being a ubiquitous system, might give imperfect measurements in case of a sudden variation in temperature or air pressure.
In such cases, the drone needs to be landed to reset. A good quality air pressure column should give readings within 10 feet of the present height the drone is at.
- GPS System: A drone’s inbuilt GPS receivers triangulate signals from many GPS satellites to establish its location. This technique is used to calculate a drone’s horizontal position. Applications for drones that use spatial data, such as mapping and 3D modeling, heavily rely on GPS data. GPS systems are highly susceptible to altitude drift and can be unreliable at really high altitudes. Mapping platforms are biased towards barometer-based data due to this.
Additionally, the radar altimeter is a very important component of TTF mode. In drones, the TTF radar helps the drone fly at low and steady AGL heights to avoid detection without losing velocity.
TTF in drones: Explained
TTF radar allows drones to fly close to the ground, by sending a signal to understand the contour of the surface and maneuvering the drone to a calculated path to avoid crashing into the ground.
TTF is especially useful in farming, mining, and search and rescue operations. True Terrain Following is achieved by the use of special sensors like LiDar, radar altimeter, and stereo cameras to generate a real-time 3-D map of the terrain.
It administers the use of pre-coded software which has algorithms to control the flight path of the drone. TTF mode helps in reducing the drag force on the drone all the while lowering air resistance by flying close to the ground, which in turn helps increase battery life. It has a higher accuracy of aerial mapping and surveying while flying over forests, urban canyons, and mountains avoiding trees, and power lines by using ground contour data to reduce the threat of accidents.
