Altimeters used in Aviation

Altimeters used in Aviation

For any flying vehicle, no matter it is an aircraft, spacecraft or a missile, altitude is the one of the basic parameters used during different phases of the flight.
Basically, there are three types of altimeters using different references to measure altitude:
- Barometric altimeters that are based on atmospheric conditions and give MSL (Mean Sea Level) altitude,
- Laser altimeters that make use of optical technologies and determine altitude above ground level,
- Radio or radar altimeters that are based on electromagnetic wave propagation and provide altitude above ground level.
Another way of measuring altitude is to use GPS (Global Positioning System), which is mainly used to locate one's position accurately. However, GPS altitude is only used for complementary purposes.

All these different altitude sources find extensive applications in areas such as military, navigational aids, land surveying, remote sensing, recreations, and sports. Let us examine these systems in more detail to find out their usage in aviation.
1. Barometric Altimeter
The primary source of altitude for aircraft operations is normally altitude provided by barometric altimeter which measures air pressure and not altitude directly. Barometric altimeters are designed to output height above the mean sea level relative to the pressure difference. It measures changes in atmospheric pressure and converts these changes to an altitude. This conversion process assumes standard atmospheric conditions based on ISA (International Standard Atmosphere) model. According to the model the greater the pressure the lower the altitude. However, when the atmospheric conditions deviate from the standard model, errors will result in determining the altitude. Therefore, the barometric altimeter should be corrected according to local sea level data to maintain accuracy.

So, one drawback is, this sensor has to be calibrated with the known local sea level conditions when experiencing a change in location or weather.

Another drawback of pressure-based altimeters is that they don't take account of dangerous obstacles such as buildings and power lines, they do not give any information about the objects below the air platform.

2. Laser Altimeter
One of the way of measuring altitude above ground level involves emitting laser beams of infrared light down from a plane, helicopter, or satellite and calculating the time to return.

Laser altimeters operate in a similar way to radio altimeters, which uses RF pulses instead of light pulses. They generally have a smaller footprint, and thus higher spatial resolution than radar altimeters.

For the aircrafts flying not higher than 1000-1500 ft, laser altimeter usage will be beneficial but its operation is limited to clear atmospheric conditions.

3. GPS
The GPS (Global Positioning System) allows land, sea and airborne users to accurately estimate their location, speed and time continuously, under all weather conditions, anywhere on earth and with a relatively low-cost.

Using GPS is one of the ways of measuring altitude, but they're not widely used as the primary source of altitude on air platforms as the overall accuracy of GPS altitude is not high enough to be used directly. However, GPS altitude can be used in combination with other sources to provide a reliable estimate altitude.

4. Radar Altimeter
A radar altimeter is a low power radar system that measures the distance from the platform to the terrain directly beneath it and provides AGL (Above Ground Level) height. Like other radar devices they transmit electromagnetic waves and measure the round-trip time delay to earth’s surface to determine distance (i.e. height) from the air or spaceborne sensor.

Typically, radio altimeters have a measurement range from -6 meters to 6000 meters (-20 feet to 19685 feet). However, there are exceptions where some altimeters have a measurement range greater than 15000 meters (49.213 feet).

The frequency band 4.2 to 4.4 GHz in the microwave C-band is assigned to the aircraft radar altimeters. This frequency band is high enough to result in a reasonably small-sized antenna able to produce a 40° to 50° beam but is sufficiently low so that rain attenuation and backscatter have no significant range limiting effects. The radio altimeters that operate in the 4.2 to 4.4 GHz band are either pulse or FMCW (Frequency Modulated Continuous Wave) systems.

FMCW radio altimeters operate by a Tx/Rx working in conjunction with separate transmit/receive antennas. Operation requires a signal from the transmit antenna to be directed to the ground. When the signal hits the ground it is reflected back to the receive antenna. The system then performs a time calculation to determine the distance between the aircraft and ground, as the altitude of the aircraft is proportional to the time required for the transmitted signal to make the round trip.

Similar to FMCW, pulse radar altimeters provide the aircraft with accurate, independent and absolute measurement of the minimum distance to the earth surface below that aircraft. The operation of a pulse radar altimeter is based on the principle of reflecting short pulses of electromagnetic radiation from the earth’s surface.

In aviation pulse altimeters are more popular for altitudes above about 5000 feet whereas FMCW radar altimeters have been widely used for short range measurements benefiting from their high ranging resolution.

Radar Altimeter Operational Scenarios

Radar altimeters are frequently used by commercial aircraft for approach and landing, especially in low-visibility conditions and automatic landings, allowing the autopilot to know when to begin the flare maneuver. Radar altimeters give data to the autothrottle which is a part of the flight computer.

A radar altimeter also functions as part of terrain avoidance warning system providing predictive forward looking capability on the flight deck, and if necessary a warning, when an aircraft descends beneath a certain altitude or too close to the ground.

Another usage of radar altimeter technology is terrain aided navigation, which is an alternative to GPS that could be used for INS (Inertial Navigation System) alignment or simply to provide redundancy as a backup system. It is based on estimating the position of a moving platform by utilizing the terrain height profile under the platform and a stored terrain elevation map. The radar altimeter measurements of heights above ground and the barometric altimeter measurement above mean sea level are used to construct a ground profile of the platform throughout the flight. Then, the constructed terrain profile is compared with a stored map data to achieve a profile fit, thus the aim is to decide on the position of the vehicle and to correct the INS.

Radar altimeters are also used in military air platforms to fly quite low over the land and the sea to avoid radar detection and targeting by anti-aircraft guns or surface-to-air missiles.

Meteksan Defence Radar Altimeters

Meteksan Defence gained a great deal of knowledge and experience after starting to study this technology for the first time to meet the needs of Turkish Defence Industry. Since then, we designed and developed two different types of radar altimeters which can measure precisely up to 2500/5000 feet altitude and provide sensitive data on missiles with high speed and high maneuverability.

Both products are low probability of intercept (LPI) sensors with low output RF power, high bandwidth and power management characteristics. The products features start up, initiated and periodic built-in self-test capabilities and designed to used in cruise missiles as navigation aids for keeping the missile on a prescribed flight path and at certain flight altitudes. High performance of these advanced equipments has successfully proven in low level terrain tracking and sea skinning missiles.

Main specifications:
- Frequency: 4.2 GHz to 4.4 GHz
- Accuracy: 2 ft or %2
- Output: Command and Response
- Communication: RS422 or RS485
- System Weight: <450g (Radar Unit) <850g (All System)
- Platform Horizontal Velocity: 300 m/s

With our high valued radar altimeter solutions, our company is the only national supplier for Turkish Armed Forces, supplier for several Asian countries and also in competition with major companies in the world market.