1. SPECIFICS ON FM BROADCAST INTERFERENCE AND LATEST WORK
International Civil Aviation Organization Regional Preparatory Group (RPG) Meeting for World Radiocommunication Conference 2007 (WRC-2007), ACP Working Group B and F and NSP SSG Meetings Bangkok, Thailand, February RPG/ACP/NSP pres4
Agenda Item 5
SPECIFICS ON FM BROADCAST INTERFERENCE AND LATEST WORK
presentation by
Steve Mitchell

2. A LITTLE BIT OF HISTORY Changes to ITU Radio Regulations
recognition of potential interference to aeronautical systems
studies within the ITU
initial studies
further enhanced studies
final ITU recommendations
ITU World Administrative Radio Conference (WARC) agreed to extend the allocation for broadcasting in the frequency band MHz. The allocation was intended for FM sound broadcasting applications and it was recognised that this new allocation had the potential to interfere with aeronautical systems (ILS, VOR and Com.) operating in the frequency band MHz.
ITU Regional Administrative Conference for the Planning of VHF Sound Broadcasting (Region 1 and part of Region 3) took into account some initial compatibility with aeronautical systems. The Final Acts of this planning conference are generally known as the Geneva 84 agreement.
Work continued within the ITU to study compatibility between FM broadcasting and aeronautical systems with further refined models to those used under the Geneva 84 plan. This work resulted in 2 new ITU recommendations now numbered as ITU-R SM.1009 & SM.1140.

3. GENERAL TYPES OF INTERFERENCE
Four types of interference to consider
Type A A1 A2
Type B B1
2 signal
3signal B2
Type A interference - This is generated within the FM broadcast station.
A1 can occur when a number of broadcast frequencies are being transmitted from the same antenna or closely located antennas and an intermodulation product is produced on an aeronautical frequency, and transmitted from the broadcast site.
A2 can occur when a broadcast frequency is operating very close to the band edge of 108 MHz and some of the energy from it’s transmission “splashes” over into the aeronautical band.
Type B interference - This is generated within the aeronautical receiver.
B1 can occur when a two or more broadcast signals are received by the aeronautical receiver and generate an intermodulation product within the receiver on the the same frequency as that selected on the receiver.
B2 occurs when a high level broadcast signal is received by the aeronautical receiver causing it to saturate.

4. DIFFICULTIES WITH ILS/VOR/COM
Extensive testing
ILS/VOR receivers
3rd order intermodulation
saturation
Com receivers
ICAO Annex 10 SARPs requirements
3rd order 2 signal
Desensitisation
Testing on ILS/VOR receivers and discussion with avionics manufacturers showed how susceptible these receivers were to Type A and Type B interference. Due to planning constraints within the FM broadcast band itself, it has been considered only necessary to address 3rd order intermodulation for both Type A1 & B1.
With respect to communication receivers operating above MHz, with the “mix” of frequencies that can be generated by FM broadcasting, it is only possible to generate intermodulation products up to approximately 122 MHz. Similar given the frequency separation of the communications band from 108 MHz, Type B2 interference has been found not to be a problem. Therefore, FM immunity is not considered as an issue with respect to communication receivers.
The references in Annex 10 for FM Immunity requirements can be found at: Volume 1 Section for ILS, Volume 1 Section for VOR and Volume 3 Section for voice communications.

5. DIFFICULTIES WITH GBAS AND VDL 4
Testing of GBAS and VDL 4 receivers
same mechanisms apply as for ILS/VOR/COM
avionics manufacturers tests above 112 MHz
split in desensitisation requirements
similar A1and B1 criteria as ILS/VOR
ICAO Annex 10 SARPs requirements
3rd order 2 signal
Desensitisation
Bench testing of both GBAS and VDL 4 receivers have shown that the same Type A and Type B interference mechanisms that are valid for ILS/VOR receivers are also valid for GBAS and VDL 4. This is because the Type B1 and B2 interference are generated within the RF stage of the aeronautical receiver and this stage is similar for all VHF aeronautical receivers.
Tests on desensitisation showed that it was possible to produce “tighter” requirements for aeronautical operation above 112 MHz and this is being reflected in the SARPs FM immunity requirements for GBAS and VDL 4.

6. Co-ordination Why co-ordination? Between FM broadcasters
mutual sharing
Between FM broadcasters and aeronautical
avoid interference to aeronautical services
Between aeronautical and FM broadcasters
avoid potential problems
As should happen with any legal user of spectrum, co-ordination needs to take place between broadcasters in order to avoid mutual interference between their services. In general this is carried out through the radio administration in a State who is then able to keep an up to date database of FM broadcast stations. Given that a database exists for aeronautical services, it is then possible to plan in most cases either FM broadcast frequencies to avoid interference to aeronautical systems or for aeronautical frequencies to be selected that avoid the interference mechanisms highlighted on the previous slides.
In Europe, given the relatively small size of some States, co-ordination is important and an international agreement known as LEGBAC has been signed by a number of States. This agreement is based upon the material contained in ITU-R Recommendation SM.1009 and is important in allowing both services to live successfully side by side.

7. Example of co-ordination process
Example of an approach to co-ordination
European example
inside a State
databases
computer tools
agreement mechanism
between States
LEGBAC/EBU
Internally, the approach taken in different European States varies. In all States however it is the radio administration that carries the necessary international co-ordination relating to FM compatibility.
In the UK as an example, the FM broadcast planners, the CAA and the radio administration all have access to common database information regarding FM broadcast and aeronautical sites but all three use independently developed software tools based upon the ITU-R Recommendation SM.1009.
From an aeronautical perspective, if a new FM broadcast station is required an assessment would be undertaken by the FM planners to check its compatibility with existing ILS/VOR. If no problem is identified then the proposal is submitted to both the CAA and the radio administration to confirm all is OK. If both these agree then the proposal goes forward for international co-ordination. If a problem is identified by the FM planner then the proposal can still be forwarded to the respective parties and the CAA will then have to make a decision on what it requires the broadcaster to do to avoid interference or whether it is acceptable operationally.

8. TYPE A1 CALCULATION

9. TYPE A2 CALCULATION

10. TYPE B1 CALCULATION 2 SIGNAL

11. TYPE B1 CALCULATION 3 SIGNAL

12. TYPE B2 CALCULATION

13. TRIGGER AND CUT-OFF CALCULATIONS

14. ILS PROTECTION
Fixed ILS test points are at set heights (max 600m) and angle from the localiser within the diagram shown. Additional test points are generated directly above a broadcast station when it falls within the ILS coverage diagram.
It needs to be remembered when making calculations on potential interference that it will not only be broadcast transmitters close to the ILS service but may also include stations some distance away e.g. 100 km if these distant stations are above the cut-off threshold.

15. VOR PROTECTION
Additional test points will be generated even if the broadcast transmitter falls outside of the DOC.
If the broadcast transmitter is outside the DOC but within 3km of it’s edge, the information on the slide applies.
If the broadcast transmitter is outside the DOC and greater than 3km from it’s edge then either the line of sight rule (B1 cut-off) applies or 125 km (A1 and B2) depending on the calculation being made.
If the VOR is used say as an approach aid then additional test points can be added to check in a certain geographic location.

16. CURRENT STATUS OF WORK ITU ICAO Working Party 8B NSP SSG
Development of new recommendation
Information on interference to FM receivers
ICAO
NSP SSG
ICAO input to ITU
Interference from broadcast systems
At the last meeting of Working Party 8B the UK introduced a document that is intended to be the first step towards developing a new recommendation for the FM broadcast compatibility with GBAS. This document is now available as a WP 8B TEMP document and can be found as 8B/TEMP/34 and has been made available to this meeting.
Within ICAO, the NSP SSG at it’s last meeting in October 2004 reviewed the ITU TEMP document and have a number of proposed corrections and changes to make. Once these proposals have been co-ordinated by the NSP SSG they will be input into the ITU as a working document. Comments from participants to this meeting on the ITU TEMP document are very welcome.
Interference to broadcast systems
Both within the ITU and ICAO a presentation of studies undertaken by Germany has been made which indicates that providing the new aeronautical services are more than 2MHz away from the broadcast frequency, then interference will not be experienced by the broadcast receiver.

17. FURTHER WORK WITHIN ICAO
ILS
GBAS
No compatibility shown
VDL 4
For both GBAS and VDL 4 no compatibility work has been undertaken within ICAO. This would need to be completed before any consideration could be given to sharing MHz between ILS and any new system. This does not however preclude the introduction of new aeronautical systems in this frequency band providing no ILS are being operated in that geographical area.