UAV Navigation in depth: Altimeters
Altimeters are devices designed to calculate the height of an aircraft above the surface directly below it. This height may be Above Ground Level (AGL) or Above Sea Level (ASL).
Different types of altimeter use different technologies to calculate this height, including pressure-density to altitude relationship and the propagation and reflection of electromagnetic waves etc.
There are four main types of altimeter:
GNSS (GPS, Galileo, etc).
Altitude can be calculated by comparing the atmospheric pressure at the current height with the pressure at sea level. In general terms, the greater the altitude the lower the pressure.
However, air pressure is affected not only by altitude; air pressure may also fluctuate due to changes in the weather which may cause changes in both pressure and temperature. These variables must be taken into account in order to obtain an accurate reading from a barometric altimeter.
Barometric Altimeter Calculation.
To calculate altitude, a barometric altimeter uses the following equation:
c is a constant that depends on the acceleration of gravity and the molar mass of the air,
T is absolute temperature,
P is the pressure at altitude z,
and Po is the pressure at sea level.
The drawing above shows the pressure altimeter invented by Victor Carbonara of the Bendix Aviation Corporation (picture from US Patent #2,099,466: Altimeter, courtesy of US Patent and Trademark Office). As can be seen in the image, air pressure variations are detected by a sealed bellows device connected to a series of mechanical linkages which amplify the movement caused by pressure change. The final part of the mechanical linkage is displayed on a rotary dial by a mechanically driven pointer.
Radio altimeters are based on the principle of reflection of electromagnetic wave pulses by the surface of the earth or sea. These waves fall within the radio spectrum range.
Electromagnetic waves travel at the speed of light and thus the calculation of the distance is effectively immediate. Although they are affected by surface irregularities generating deviations in the radio signal, radio altimeters provide a reliable and accurate method of measuring height.
Radio Altimeter Calculation.
Altitude is calculated by measuring the time taken by the wavefront to travel from the aircraft to the surface and back again.
In order to understand how a radio altimeter works, consider a radio pulse emanating from a radio beacon to a flat surface.
Ref: the configuration when the trailing edge of the pulse arrives at the flat surface.http://topex.ucsd.edu/rs/altimetry.pdf
Radio altimeter calculations use the Pythagorean theorem:
rp is the radius of the leading edge of the pulse,
H is the altitude,
lp is the pulse length.
As squared lp is very small, it can be neglected so rp is:
tp is the time for the pulse of length.
Note that the time delay between transmission and reflected wave is too short to be able to use a single antenna for both functions. Two antennae are therefore required, which must be physically separated in order to avoid interference.
Another type of aviation radio altimeter is the frequency-modulated radio altimeter. This type takes advantage of the fact that the reflected signal will be received at a different frequency to the transmitted signal. The rate of change in signal frequency is constant, meaning that altitude can be calculated as it will be proportional to the frequency difference between transmitter and receiver.
Global Navigation Satellite System (GNSS) receivers can also determine altitude by trilateration with four or more satellites. To make this calculation, the time of flight of radio waves from a known point to another is again used.
Altitude calculated using GNSS is, however, not accurate or reliable enough to obviate the use of a backup system, such as a barometric altimeter - unless some method of augmentation is used. Errors in height calculation using GNSS are typically in the region of 5 meters.
Although useful for many Unmanned Aerial Vehicles (UAV) during flight (when a GNSS may provide sufficient accuracy for general navigation), GNSS is not accurate enough to provide height information for precision manoeuvres such as low altitude flight or landing.
This type of altimeter works by using electromagnetic waves within the visible range of the spectrum instead of radio waves.
Laser altimeters work in a similar way to radio altimeters. Again, the time taken for the emitted signal to travel from the transmitter to the surface and back again is measured.
Once reflected, the beam of light is received and collected using a series of mirrors and lenses which focus the beam onto a photocell detector which is sensitive to infrared light.
USE OF ALTIMETERS IN AVIATION
In aviation, altimeters are generally used for maintaining a constant altitude, whether during routine flight, reduced visibility maneuvers or for automatic actions. They may also be used during critical phases of flight, such as landing.
A Digital Elevation Model (DEM) is an altitude model which has been created at a specific time. It may contain significant errors, and for this reason it is not safe to rely exclusively on the information provided by a DEM for the landing manoeuvre; another source of information must be considered.
GNSS and barometric altimeters may be used as part of the sensor suite within an autopilot’s Attitude and Heading Refence System (AHRS). UAV Navigation’s autopilots use these sensors in order to estimate altitude as part of the estimation algorithm. Their use enhances the overall accuracy of the system and provides a level of robustness and redundancy that other systems simply do not have. However, when a precision landing capability is required, or low altitude manoeuvres must be provided for within the Flight Control System (FCS), then a precision altitude measuring device such as a laser or radio altimeter will also be required. Such precision altitude measuring devices are routinely integrated for customers with UAV Navigation’s autopilots for automatic landing and also advanced flight modes such as sea skimming. Communication between the autopilot and the device may be RS-232, RS-485 or CAN. A specific driver within the autopilot software for the payload will be required, and UAV Navigation has ample experience with integrating such devices.