Analysing the impact of Broadcast allocations on Instrument Landing Systems

October 19, 2021

Customer: DICT, Seychelles Broadcasting Authority
Location: Seychelles


Understanding the impact of interference between different spectrum allocations is essential for safety-critical systems at airports. This study looks at the impact of interference from different broadcast allocations and ILS and their impact.


ATDI prepared a study for DICT, the spectrum regulator in Seychelles. The study looked at the compatibility between Band II broadcast allocations and instrument landing system (ILS) localiser. The system used a VHF omnidirectional radio range (VOR) and VHF communications (COM) equipment, operating in the 108 to 137 MHz band. 


The project comprised of four stages, including: 

  1. The inspection of sites, measurement and monitoring;
  2. Determining the extent of the problems caused by unwanted emissions;
  3. Recommendations for remedial measures;
  4. Advice and training for staff. 

The project included site visits to ascertain emissions from a number of the FM broadcast sites. 

SM. 1009 calculation display showing virtual test points and the transmitters. Used to identify potential interferers
Fig. 1 SM. 1009 calculation display showing virtual test points and the transmitters involved


The study concluded there was no evidence of intermodulation or harmonic interference to aeronautical navigation systems across the Island. However, ATDI recommended additional filtering at some shared FM broadcast sites. They also recommended that frequencies should be calculated for each FM site to ensure new entrants do not create interference.

Lastly, the DICT team were presented with an overview of the study results. They received training on spectrum planning and how to identify potential issues in the future. This study and training recognise that spectrum management is a collaborative effort between the regulator and the licensees. 

Proposed spectrum mask for VHF FM transmitters
Fig. 2 Spectrum mask for VHF FM transmitters
Antenna radiation pattern
Fig. 3 DIP15 radiation pattern

HTZ Communications and HTZ Warfare are fully compliant with the International Civil Aviation Authority (ICAO). This sets standards and recommended practices for international air navigation across the globe. 

For more information about DICT, visit here.

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