20
INSTRUMENTATION — CHAPTER 20 · NOTES
Radio Altimeter
by Ghost Aviator
Table of Contents
1. Introduction and Functions
The Radio Altimeter measures height above ground level (AGL) with high accuracy. It has three important functions:
- Flight deck display of height AGL
- Automatic flight system data — supplies height data for automatic landings when used with ILS/MLS
- GPWS — furnishes height information and rate of change of height to the Ground Proximity Warning System
2. FMCW Principle of Operation
The radio altimeter uses Frequency Modulated Continuous Wave (FMCW) transmission in an elliptical pattern vertically below the aircraft.
How it measures height:
- Transmitter continuously sweeps frequency from start level ± 50 MHz, 300 times per second
- Radio wave travels to ground and back — during this travel time the transmitted frequency has changed
- Equipment compares transmitted vs. received frequency — the difference = time of flight
- From time of flight → height is calculated
graph LR A["Tx frequency
continuously swept
±50 MHz / 300 Hz"] --> B["Wave travels to
ground and back"] B --> C["Rx frequency
(older, reflected signal)"] A --> D["Beat frequency
= Tx − Rx"] C --> D D --> E["= Time of flight
= Height AGL"]
Handling the frequency reversal problem: When the transmitter reverses its sweep direction, a frequency breakdown occurs. This is overcome by averaging the beat frequency over a short sampling period — the changeover points are ignored.
3. Frequencies and Antenna
| Band | Frequency Range | Status |
|---|---|---|
| SHF | 4200 MHz to 4400 MHz | Currently used |
| UHF | 1600 MHz to 1660 MHz | Previously used — obsolete |
Antenna characteristics:
- Flush-mounted horn antenna — transmits downward
- Conical/elliptical beam — wide enough to allow some portion to travel vertically even at pitch angles of ±30° and roll angles of ±60°
- Height determined from the shortest path to ground — always the vertical path
- Transmission is continuous → separate transmit and receive antennas are required (to avoid interference)
- Radiated power: ~1 watt
4. Basic Indicator
Height Scale
The scale is logarithmic: expanded from 0–500 ft, then non-linear from 500–2500 ft.
Mask
The height pointer disappears behind a mask when:
- Altitude exceeds 2500 ft
- Any fault in the transmitted signal
- Altimeter is switched off
Failure Warning Flag
Appears when:
- Too much radio noise (corrupts returning signal)
- Local reflections received from airframe
- Loss of power to the equipment
Press-to-Test Button / Height Selector (dual purpose)
- Pressed: height pointer swings to a known pre-set altitude (confidence check)
- Turned: selects the reference Low Altitude Index (decision height marker)
Low Height Warning
Warning light illuminates if the aircraft flies below the pre-selected height. An alert tone sounds with increasing loudness from approximately 100 ft above the decision height setting, then ceases suddenly at the DH.
5. EFIS Indicators
Boeing Style — EFIS Display
Boeing digital/analogue display behaviour:
- Above 1000 ft: digital read-out only (white)
- Below 1000 ft: pictorial altimeter dial drawn alongside digital read-out; icon shrinks in size as height decreases
- As DH is approached: colour changes from white → flashing amber
Airbus Style — EFIS Display
Airbus (A300 style):
- Height indication at the base of the EADI/PFD attitude display
- Above DH: green numbers
- Below DH: amber, slightly bigger numbers
- Enhanced by a voice warning system giving aural height callouts at significant heights (as agreed by manufacturer and airline)
6. Range and Accuracy
Radio Altimeter Performance:
- Range: 0 to 2500 ft AGL
- Overall accuracy: ± 3% of indicated height OR ± 1 ft — whichever is the greater
- Error contributors: Doppler shift, step error in digital counting circuits, height lag
7. Aircraft Installation Delay (Cable Length Compensation)
The radio altimeter must indicate zero height AGL as the main wheels touch down. In practice, the design target is ±1 ft accuracy.
Two sources of installation offset that must be compensated:
- Residual Height — height difference between the antennas on the fuselage and the bottom of the trailing main wheels on the approach to touchdown. Varies with aircraft weight and oleo compression.
- Cable run length — distance between avionics bay and antenna on the underside of the fuselage. On a B747 this may be up to 100 ft of cable; on a small corporate jet as little as 6 ft. Compensation is applied × 2 (Tx to antenna AND Rx from antenna).
Why the RA may read negative on the ground: The equipment is calibrated to read zero when the main wheels first touch the runway (trailing wheel attitude). When parked level on the ground, the antenna is below its calibrated position relative to the landing attitude — so the RA may show a small negative altitude. The B747, with its multi-wheel trailing assembly, typically reads −8 ft on the ground.
8. Key Facts Summary
| Parameter | Value |
|---|---|
| Modulation type | FMCW (Frequency Modulated Continuous Wave) |
| Frequency (current) | 4200–4400 MHz (SHF band) |
| Frequency sweep | ±50 MHz, 300 sweeps/second |
| Transmission power | ~1 watt |
| Operating range | 0–2500 ft AGL |
| Accuracy | ±3% or ±1 ft (whichever greater) |
| Beam coverage | ±30° pitch, ±60° roll |
| Mask activation | Above 2500 ft, fault, or switched off |
| Scale type | Logarithmic (0–500 ft expanded, 500–2500 ft non-linear) |
| B747 on-ground reading | Typically −8 ft |
| Key system it supports | GPWS, Autoland (ILS/MLS), EFIS |
Important connection: The Radio Altimeter is a major component of GPWS. It provides the crucial height AGL and rate-of-change-of-height inputs that GPWS needs to generate terrain proximity warnings.
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