International Civil Aviation Organization
ACP-WGF24/IP-07
Paris, France 17 – 21 March 2011
Agenda Item 8: Any other business
Bandwidth Requirements for Radio Altimeters
(Presented by John Mettrop, UK )
SUMMARY
This paper provides information on Radio Altimeter bandwidth requirements based on the required range resolution requirements necessary to support automated landing.
ACTION


Review the Paper for Technical Accuracy
Support the development of an ITU-R Recommendation on radio altimeter characteristics

Support the development of an ITU-R Recommendation on radio altimeter protection criteria and how sharing studies should be carried out
1.
INTRODUCTION
1.1
Radio altimeters provide an absolute measure of the distance between the ground and the aircraft 1 . They are an integral part of automated landings providing information on where the plane should flare out. They are also used as part of an aircraft’s ground proximity warning system being especially useful for helicopters flying over sea to oil rigs.
1.2
Whilst radio altimeters are permanently on their effective operational range is less than
762 metres (2,500 ft) above ground level but need to be accurate to within 0.9 metres (3 ft). Aircraft are
1 Barometric altimeters provide a distance over a pre-determined datum.
(4 pages)
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ACP-WGW01/WP-01 - 2 - normally fitted with 3 radio altimeters to ensure that the accuracy and reliability of the overall radio altimeter system meets the requirements for automated blind landing.
2.
DISCUSSION
2.1
As the apparent appetite for mobile communication systems continues to grow so the demand from the mobile community for access to increasing amount of spectrum continues to put pressure on existing spectrum users to justify the spectrum they use. At least two administrations have aspirations to identify at 500 MHz of government spectrum, including aeronautical, that can be released to the marker to satiate this demand.
2.2
Currently the spectrum being targeted is that below and around 5 GHz as this is seen as providing the best compromise between range, cost and reliability. Early indications are at the forthcoming World Radiocommunication Conference there will be a push to secure at least one agenda item for the subsequent World Radiocommunications Conference to address the perceived needs of the mobile industry for additional broadband systems.
2.3
Aviation operates a majority of its current systems in spectrum allocated below or around
5 GHz and hence it would be reasonable to assume that this spectrum is likely to be targeted. One band in particular that is likely to be targeted if early work in both the USA and the UK is any indication is the frequency band 4 200 – 4 400 MHz which is currently used for radio altimeters. The band itself is already shared on a secondary basis by passive sensors operating in the earth exploration-space and space research services. Use by the standard frequency and time signal-satellite service on 4 202 MHz ±2 MHz in the space to earth direction is also permitted.
2.4
Modern civil radio altimeters use Frequency Modulated Carrier Wave (FMCW) signals in order to achieve the measurement accuracy required to support automated blind landings. The following analysis addresses the minimum spectral bandwidth required to achieve the measurement accuracy and reliability required allowing for frequency drift due to variations in temperature.
3.0 ANALYSIS
3.1 Radio altimeters designed for use in automated landing systems are required 2 to achieve an absolute range resolution of less than 0.9 metres (3 feet). For an FMCW the range resolution is proportional to the bandwidth of the signal in accordance with the following formula:-
Or
For a range resolution this would result in a requirement for a 3 dB bandwidth of 166.5
MHz
2 Eurocae ED 30 or Arinc 707

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3.2 This assumes an ideal oscillator that has no drift. Oscillators that are being used in the latest designs have a frequency stability of 10 parts per million per degree C (ppm/C) although older designs typically had frequency stabilities of between 25 & 50 ppm/C. Given that a radio altimeter has to operate over a temperature range that varies by 90 ºC and that the centre frequency is nominally 4 300
MHz this would result in a maximum frequency drift of between 3.9 and 11.6 MHz. This would result in a required 3dB bandwidth of between 170.4 and 178.1 MHz.
3.3 Additionally for automated blind landings 3 radio altimeters are required. In order to avoid mutual interference their centre frequencies are normally offset by approximately 5 MHz resulting in nominal centre frequencies of 4 295, 4 300 & 4 305 MHz. Taking this into account the minimum 3dB bandwidth required is between 180.4 and 188.1 MHz
3.4 The Radio Regulations however require that at the edge of the allocated band the signal is at least 40 dB down on the primary signal. Recommendation ITU-R SM.1451 specifies for a FMCW signal that the 40 dB bandwidth is:-
Where:
3.5 Assuming that 2 B d
equates to the 3 dB bandwidth then the 40 dB bandwidth can be calculated which equals between 181.7 and 189.4 MHz leaving between 9.85 and 5.3 MHz at either end of the band. Taking account of the standard frequency and time space-satellite signal then there would be no spectrum gain at the lower edge of the band and only at best 9.85 MHz available at the upper edge although this is only a theoretical value for a radio altimeter working with the minimum level of accuracy.
4. CONCLUSION
4.1 Based on a radio altimeter that designed to meet the minimum required range accuracy, allowing for frequency drift due to variations in temperature and the need to have 3 radio altimeters the theoretical minimum bandwidth requirement is between 181.7 and 189.4 MHz.
4.2 Allowing for practical designs whose performance is slightly greater than the minimum required it can therefore be concluded that the entire frequency band 4 200 – 4 400 MHz is required and that there is no capacity to release spectrum for other services.
3.
3.1
ACTION BY THE MEETING
The ACP WG-F is invited to:

Review the paper for technical accuracy

Use the analysis provided as necessary in national meeting Support the development of an
ITU-R Recommendation on radio altimeter characteristics

Support the development of an ITU-R Recommendation on radio altimeter protection criteria and how sharing studies should be carried out

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