Technical aspects of the MFSA radar

Antenna layout for the Davis MFSA radar
Antenna layout for the Davis MFSA radar
Signal-to-noise ratio as a function of height and time

The Davis MFSA radar operates at a frequency of 1.94 MHz, which corresponds to a wavelength of approximately 150m. 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 10m. The three receiving antennas hang from 7m at their centre to 2m 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, 42, 44… up to 108km. 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 receiving antennas, of which there are three, are crossed dipoles. The two 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 three of these crossed dipoles are arranged in an equilateral triangle of side dimension 180m. These sample the returned electric field pattern in such a way that the motion of the pattern can be measured. The speed with which this pattern moves is twice the wind speed, and in this way, the wind speed at a given range can be inferred. The analysis method used to calculate these winds is based on the work of the late Dr Basil Briggs and is known as the “Full Correlation Analysis”.

In order to measure the pattern movement, a time series of the complex echo amplitudes is required. This typically has 280 points at each height and takes 112 seconds to obtain. The remainder of a two minute time block is spent transferring data to the analysis computer, changing the polarisation (if required) and checking the transmitter status. The analysis routine subjects the data to some acceptance criteria. If these were met at all times, then a new velocity data point would be available every 2 minutes, 24 hours a day at all the heights that are sampled. Even though this is not the case, the data acceptance rate for the Davis system is often around 70% at 84–86km.

The power of the echoes from the mesosphere and lower thermosphere is related to the square of the amplitude of the signal that is detected by the radar. However, the power can sometimes contain a component due to noise sources unrelated to the radar. Another parameter, the signal-to-noise ratio is able to identify and quantify these noise sources. Both the signal power and the signal-to-noise ratio for the last day are presented in the image below.

The signal-to-noise ratio is an important parameter in deciding if a wind determination is possible. During the summer, when the sun is illuminating the sampled height range throughout the day, there is little variation in the signal-to-noise ratio. This is less true in winter and during periods of high solar activity.

The design of the radar hardware had some aspects associated with its operation in 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 had to be protected from the wind and the snow. Ground runs of coaxial cable were fed through 50mm irrigation pipe to protect them and prevent 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 such that they could not flex in the wind and crack due to the cold temperatures. This necessitated the inclusion of a total of eight masts in the array construction. (One in each corner and one on each side.) The four side masts support the coaxial cable run and have a balun box at their head.

The computer that is used for configuring the radar and for analysis of the data allows remote access. Thus, 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).