The Australian Communications and Media Authority (the ACMA) is progressively reviewing the technical frameworks of all spectrum licensed bands as they approach expiry. As part of this review the ACMA also assessed the appropriateness of the digital elevation model (DEM) that it uses in the spectrum licence environment.
All spectrum licence technical frameworks originally made use of the 9-second DEM called RadDEM in the relevant s.145 Determinations of Unacceptable Interference. The coordinate system referenced by RadDEM is the Australian Geodetic Datum of Australia 1966 (AGD66). Copies of the RadDEM are available from the ACMA.
Since 2000, all spatial data published by the Commonwealth and state surveying and mapping agencies – excepting unrevised historic data – has used the Geocentric Datum of Australia 1994 (GDA94). GDA94 is an earth-centred datum compatible with satellite-based navigation systems and other major international geographic systems, such as the World Geodetic System 1984 (WGS84).
In order to better align with the general shift in datum, the ACMA has made a policy decision to progressively move to GDA94 as the basis for coordinate references for spectrum licensing.
The GEODATA 9 Second Digital Elevation Model Version 3 (DEM-9S) is the latest 9-second DEM referenced in GDA94 to be released by Geoscience Australia and is the proposed DEM to be incorporated in all updated and new spectrum licence technical frameworks from 2012 onwards. DEM-9S is available from Geoscience Australia.
Licensees and accredited persons are advised to consult the relevant s.145 Determinations of Unacceptable Interference for each band to determine the appropriate DEM to use.
Interpretation of DEM-9S
The move to DEM-9S resulted in the need for the ACMA to release information clarifying the results it expects to receive from licensees and accredited persons when using it. This information will be made available on the ACMA website in the document Digital Elevation Model Interpretation (Word or PDF formats).
This document is intended to provide accredited persons and/or licensees the necessary information to achieve the same output from DEM-9S based on given latitude and longitude points, as well as a given latitude and longitude and associated 3x3 surrounding DEM cells as required in the calculation of the device boundary criterion of the relevant section 145 determination.
Retention of 9-second DEM
The ACMA has chosen to retain a 9-second DEM even though there are more dense DEMs available. The averaging already used in the device boundary process, for which the DEM is most required, means there is no need for the ACMA to adopt a denser DEM. This is because the averaging area would likely remain a similar size in order to maintain results like those already being achieved.
For a DEM to be successfully used for spectrum licensing, the ACMA also believes that it needs to be a product that can be maintained and easily obtained by licensees and other parties as well as being a proven and mature release. In contrast, the dense DEMs are typically only a first release and do not meet these criteria.