WP3: Evaluation of the options for the active element
February 12, 2010
A major part of the cost of the new EISCAT_3D facility will be invested in the production of the high power transmission capability. The relative benefits of different approaches were evaluated as part of this Work Package and used to identify the optimum solution in terms of scientific return, performance, and ease of manufacture.
In summary, the conclusions from this Work Package are:
- As specified already in the EISCAT_3D Performance Specification Document, produced in Work Package 2, only a full-fledged phased array system can reach or approach the active element performance demanded by the scientific user community. This is therefore the system of choice.
The active element array shall be a filled circular aperture with the array elements laid out on an equilateral triangular grid. An element-element distance of 0.7 λ will provide essentially grating-lobe free performance out to 40° zenith angle.
- The individual array element will comprise a radiator, a dual 300+300 Watt linear RF power amplifier, a high performance direct-digitising receiver, a digital signal processing system and support electronics. The preferred radiator is a crossed Yagi antenna with a minimum directivity of about 7 dBi, e.g. the so-called “Renkwitz Yagi”.
- The array will be physically subdivided into hexagonal groups of 49 elements. For practical reasons, all electronics for the group will be housed in a common equipment container and the radiators connected to the container by low-loss coax cable.
- The target is a 16,000-element, 120-m diameter array. This will have a half-power beamwidth of about 0.75° , i.e. comparable to that of the present EISCAT UHF. Its power-aperture product will be about 100 GW m², i.e. about one order of magnitude greater than that of the present EISCAT VHF when it is operated in single-beam, dual klystron mode.
- To get started with limited funding, already a 5000-element, 70-m diameter array would exceed the performance of the current VHF system, providing a 1.3° half-power beam-width, a power-aperture product of about 10 GW m², and full steerability. This array could then be expanded as additional funding became available. Provisions for expansion should be built in from the beginning.
In either case, a basic set of receive-only outlier arrays for interferometry should be put in place from the start to meet the horizontal resolution performance requirement.
- The extreme joint time/height resolution requirements laid down in the EISCAT_3D Performance Specification Document are unrealistic; even a 36,000-element array configuration would fail to meet them at all altitudes by a factor of 25 to 350!
- Relaxing the height resolution by a factor of 4 to 10 (from 100 m to 1 km at 150-km altitude and from 1 km to 4 km at 300-km altitude), will put the 16,000-element array in a position to meet the PSD time resolution requirements.
- Agreement on whether incoherent-scatter altitude resolutions better than 1 km are really scientifically meaningful and required at altitudes above 150 km must be reached before the EISCAT_3D project proceeds to its next phase.
The work in this Work Package was performed by EISCAT Scientific Association and Swedish Institute of Space Physics.