SKA Memorandum #2                                                          

A Radio Protection Zone to protect the SKA telescope from the electro-magnetic environment can be set up two ways. 
 

1. The simplest option would be the establishment by the national and local governments of a zone with a fixed-size within which the other spectrum users are restricted in their operations. The zone could then be called a “quiet zone”, where all transmission operations are forbidden, or a “coordination zone”, where all active operations need to be coordinated with the Observatory. Because the SKA telescope does not require all the spectrum all the time, the operational conditions may vary from band to band and planned “time-sharing” between the Observatory and active service operators of certain bands may be a realistic option. 

Examples of quiet zones exist around a large number of the world’s radio observatories. In particular, an effective quiet zone of 160 km square protects the radio telescopes of the National Radio Astronomy Observatory in Green Bank (West Virginia, USA) from mostly terrestrial operations. Similarly, a coordination zone comprising the area of the island of Puerto Rico and surrounding islands serves to protect the Arecibo Observatory (Puerto Rico, USA).

2. Alternatively the  impact on the radio telescope may be used as a criterion for defining the quiet zone. This would imply that the size of the quiet/coordination zone be kept variable depending on the power levels of the operations in the surrounding areas. In this manner the effective power levels resulting from the transmissions at the location of the telescope itself may be used to coordinate (or reject) operations at a certain distance. The zone would be thus larger for more powerful transmitting stations. 

A radio-quiet zone/reserve would thus refer to the quietness imposed on other nearby spectrum users and does not necessarily mean that it is always electro-magnetically quiet at the site of the telescope. 

The implementation of these procedures depends in part on the operating rules for active services that have been set by the national and local governments. A zone with fixed coordination procedures would be easier to administer, but depending on the size of the zone and the local population density the manpower needed for administering the zone could be significant. The degree of freedom of licensing under the national legislation will play a significant role in implementing the zones.

The role of the national and local governments is crucial for initiating and supporting actions at the ITU and at other international organizations leading towards an internationally recognized radio-quiet zone/reserve. Particularly for the case of service operations that cross national boundaries, the relevant regulations need to have a well-accepted multi-national or international basis.

The protection criteria for Quiet Zones

The International Telecommunications Union (ITU) has documented the thresholds for the (spectral) power flux densities in specific bands allocated to the Radio Astronomy Service (RAS), which define the onset of data loss for the radio astronomy service. These detrimental levels have been given in Recommendation ITU-R RA.769. In addition, specific power flux densities have also been attached to a number of active service bands in order to protect nearby RAS operations. Because radio astronomy uses very sensitive receiving systems for reception of "signals" from the Universe, these detrimental interference levels are significantly lower than those required for protecting active radio communications services from each other.

Figure A1. The desired spectral power flux density (SPFD) levels for the 0–25 GHz range based on the detrimental SPFD levels given by ITU-R RA.769 for continuum observations in the bands allocated to the Radio Astronomy Service.

Figure A1 presents a curve of threshold values obtained from interpolation of the values for the RAS bands from Recommendation ITU-R RA.769 across the whole frequency range up to 25 GHz. This figure represents the measure of radio-quietness across the entire operating range of SKA, which includes all active service bands. In order to ensure effective operation of SKA at the desired level of sensitivity, the effective SPFD levels across electro-magnetic spectrum should be at or below these values for a significant part of the spectrum and for a significant part of the time. In addition, the expected levels of interference need to be predictable in time and spectrum.

A Quiet Zone for Terrestrial Operations

The purpose of the Radio Quiet Zones for SKA is to achieve an effective electro-magnetic environment for all the SKA stations. Since the SKA telescope will be configured as an array, the SKA will benefit from interferometric decorrelation on any baseline, which eliminates a fraction of the interference signals from the data. As a result, the protection requirements at each of the SKA stations will depend on the distance to its nearest neighbours. At the same time, it is also desirable that none of the SKA stations has nearby transmitters that could saturate the sensitive electronics of the receiver systems. A certain “coordination distance” is required to reduce the intended signals of active service operators to an workable level at the location of the SKA stations. This coordination distance will depend on the power of the transmitted signal, on the distance to the nearest neighbour and on the intervening terrain. 

The objective of SKA is to access as much of the spectrum as possible, which includes many of the active service bands. In order to allow operation of all SKA stations, the incoming power levels should be maintained below a certain threshold at all times or for a predictable part of the time. For instance, the highest priority would be to protect all stations in the whole central 100 km of the SKA configuration to the fullest extent. This would require coordination with stronger transmitters at much larger distances than the 50 km from the centre. Similarly, each of the outlying stations should have protection zones with a size that decreases with their distance to the centre. However, there needs to be a minimum size for all SKA stations. In order to reach the same protection levels in all bands, the coordination distances should depend on the usage and transmitted power in each active service band. It would be administratively easier for local and national administrations to administer a quiet zone by imposing licensing and operational limitations in a fixed-size zone that is sufficiently large. 

In order to achieve sufficient protection for the whole SKA telescope, specific actions should be requested from the national and local governments. 

• The local government should then be requested to initiate and establish procedures and legislation for terrestrial operations in order to prevent signal strength at all SKA stations up to certain agreed levels based on the ITU-R RA.769 levels. 
• Legislative actions should be enacted by the national government in a timely manner in order to adopt spectrum-planning procedures that are optimised to protect the SKA stations. 
• In the case of the SKA configuration passing national border, the national government should initiate the establishment of a protection zone across the border with similar standards. 

In addition, interference mitigation schemes to be implemented in the SKA data processing flow may permit higher levels of interference to be tolerated in active bands. Consequently the levels of interference that will be acceptable for SKA are the effective levels following the application of an appropriate mitigation scheme. The advantages gained with certain RFI mitigation methods should only be taken into account in frequency bands that are shared with other spectrum users and bands that are not already exclusively for the use of passive services. 

Quiet Zones for Space, Aeronautical, and HAPS Operations

Active service operations from space and aeronautical platforms can be particularly harmful to terrestrial passive operations of the Radio Astronomy Service. Besides the spreading losses resulting from the distance to the transmitter, there are no terrain shielding losses and the curvature of the Earth to be taken into account. Furthermore, interferometric decorrelation may be less help to reduce the effect of the interference. The coordination distances for downlinks from space platforms and aeronautical stations will need to be much larger than for terrestrial operations. A number of aspects need to be considered for space, aeronautical, and High Altitude Platform Station (HAPS) operations:
 

1. Space-to-Earth operation for the satellite services use a footprint that is often confined to the shape of the continent or landmass that it seeks to cover. The footprints of space operations may decrease with frequency but are generally not less than 1000 km at frequencies below 2 GHz. Aeronautical operations and HAPS operations have coordination distances that reach up to 600-800 km.
• Downlinks may need to be turned off during passage over the SKA site or in the direction of the SKA site. The ground area over which this procedure needs to be implemented could be on the order of 1000 km in radius. 
• Time-sharing of certain active bands may be considered as an option.

2. Space-to-Earth operations of the satellite services have generally been inadequate in protecting adjacent band operations and particular those of the passive services. Since a large fraction of the radio astronomy bands have nearby downlink operations, the ability to protect passive services from spurious and out-of-band emissions should be required for any satellite downlink operations near the SKA telescope site.
• Licensing procedures for these three services need to include pre-launch demonstrations that adjacent and nearby passive bands will be protected from unwanted emission to the levels prescribed in ITU-R RA.769. For the SKA Quiet Zone all spurious emissions should fall below the interpolated RA.769 limits. Such requirements need to be enforced by the national government and need to be supported by ITU regulations.

3. The operational conditions for space, aeronautical and HAPS operations are governed by ITU and national regulations. The national government plays an important role in enforcing their adherence.
• The national government needs to be willing to initiate coordination procedures with all “notifying administrations” of all satellite systems operating over its national territory in order to ensure adherence to the guidelines for the SKA Quiet Zone. 
• The national government needs to be requested to coordinate with any neighbouring countries to ensure that the same operational guidelines exist across the border and within the established radius of the Space Quiet Zone.
• Agreements need to be made in order to establish guidelines for operations of safety-of-life systems from these types of platforms.