Lindgren, T., Stenberg, G., Larsmark, M., Borg, J. and Johansson, J.
Luleå University of Technology, Luleå, Sweden
This presentation will give an overview of possible techniques for the intra-array timing in the planned EISCAT_3D incoherent scatter radar. The challenges related to using GNSS as the timing solution is discussed in more detail with particular focus on modelling the multipath environment.
With the EISCAT_3D radar it will be possible to obtain instantaneous three-dimensional radar measurements spanning the entire vertical extent of the ionosphere. In order to achieve this, the system will include multiple receive antenna arrays situated 90-280 km from the main transmit/receive site. Each site, including the main site, will consist of an antenna array with 2000 to 16000 elements. One of the main challenges when designing the arrays is the timing between the antenna elements. The current requirement for this is that the total timing error must not exceed 150 ps. Several solutions to this are being studied, these are:
Cable solution: A common high-end oscillator can be used to distribute a clock pulse to each element in the array. To compensate for temperature effects in the cables this solution requires that the total propagation time to each antenna element is measured continuously.
External radio source: A signal can be transmitted from e.g. a tower to calibrate the elements in the array. This has a clear advantage since the same signal can be used both to calibrate for timing and amplitude of the antenna elements. Substantial infrastructure will be needed for this solution and there is also a risk that the tower structure inteferes with the scientific measurements.
GNSS timing: Signals from GNSS satellites can be used to time the antenna elements individually or in groups. This solution is slightly more complicated than the ones mentioned above but has the advantage of being robust to changes in temperature. It also also gives absolute timing to UTC which enables synchronization between the sites.
GNSS receivers can be used for timing purposes with very high accuracy requirements. For receivers at short distances almost all external errors on the GNSS signals (i.e. errors affecting the signal before it reaches the GNSS receiver) can be assumed to be identical and hence the accuracy is increased further. The only remaining significant error source is the multipath effects arising when the signal is scattered of structures and the terrain. A tool for modelling the multipath environment in a large antenna array has been developed and will be presented.