The Davis MFSA radar operates at a frequency of 1.94 MHz, which corresponds to a wavelength of approximately 150 m. The transmitting array is a square of half wave dipoles with opposite elements driven in parallel. By swapping the phase difference between the two pairs of sides, it is possible to change from RH to LH circular polarised transmission. The polar diagram of the transmitting array has a maximum overhead and a null toward the horizon.
The square transmitting array is suspended at a height of 10 m. The 3 receiving antennas hang from 7 m at their centre to 2 m at their extremities.
The transmitted pulses are partially reflected from the mesosphere, or in electron density terms, the D-region. The returned signal is sampled, after pulse transmission, at times that correspond to heights of 40 to 108 km. These samples are averaged coherently over 32 pulses to improve the signal-to-noise ratio. A pulse repetition frequency of 80 Hz means that an averaged sample is stored every 0.4 seconds.
The 3 receiving antennas are crossed dipoles. They are arranged in a triangle 180 m apart. The 2 elements of the cross are phased in the same manner as the transmitting array so that they are sensitive to the polarisation that is being transmitted.
The receiving antennas sample the returned electric field pattern. The speed with which this pattern moves is twice the wind speed. In this way, the wind speed at a given range can be inferred.
The method used to calculate these winds is called ‘Full Correlation Analysis’. It is based on the work of the late Dr Basil Briggs. In order to measure the pattern movement, a time series of the complex echo amplitudes is needed. This typically has 280 points at each height and takes 112 seconds to obtain. The data is then transferred to the analysis computer, the polarisation is changed (if required) and the transmitter status is checked. The whole process takes 2 minutes. The analysis routine subjects the data to some acceptance criteria. If the data met these criteria every time, then a new velocity data point would be available every 2 minutes. The data acceptance rate for the Davis system is often around 70% at 84 to 86 km.
Some of the radar hardware is specially designed for the cold weather. The baluns for the transmit and receive antennas are all sited at the centre point of the antennas. This has meant that an unbalanced line could be used to drive the transmit antennas. Coax cable was used, but it had to be protected from the wind and the snow. Ground runs of coaxial cable were fed through 50 mm irrigation pipe to protect them and stop them being moved by the strong winds that can blow at Davis. The coaxial feeds to the midpoints of each side of the square transmitting array had to be made so they didn’t flex in the wind and crack in the cold temperatures. This required the total of 8 masts in the array construction. The 4 side masts support the coaxial cable run and have a balun box at their head.
The computer that configures the radar and analyses the data allows remote access. The radar can be checked and re-configured from the Antarctic Division in Kingston (Tasmania) or from Atmospheric Radar Systems (ATRAD) in Adelaide (South Australia).