The radar’s specifications
The Davis VHF radar consists of a 144-antenna main array, a secondary 5-antenna meteor detection array and the transmitter, receiver and antenna switching and control electronics. The main parameters of the radar are described below.
|Frequency||55.0 MHz||Wavelength is 5.45 m|
|Main array||12 x 12 grid of 3-element Yagi antennas combined into groups of four||Driven element is a folded dipole|
|Effective area||1,800 m² (side dimension of 42.427 m)||Antenna spacing of 0.7 x wavelength|
|Approx one-way beam width||7 degrees||Using whole array|
|Peak transmitted power||Approximately 70 kW|
|Average transmitted power||6 kW||5% duty cycle|
|Power Aperture Product||Approximately 10^7Wm^2||At full power|
5 x 2-element Yagi antennas for reception
1 crossed Yagi for transmission
|Transmit circular polarisation, receive linear polarisation|
The atmospheric refractive index changes with frequency. The refractive index at a frequency around 55 MHz is related to the humidity, temperature and electron density of the atmosphere. Variations in this refractive index allow radar backscatter to occur. In the troposphere and stratosphere, temperature and humidity allow for good data rates throughout the day and night.
Mesospheric reflections are related to electron density. During the night, the mesospheric electron density decreases. This means that night time data can only be collected with the help of meteor trails. In the polar regions, the phenomenon known as Polar Mesosphere Summer Echoes (PMSE) produces a greatly enhanced echo strength during the summer months.
The rate and height range of data collection is also affected by radar power and sensitivity. The physical size of the radar and the power contained in the transmitted radar pulse are critical in determining the data acceptance rate. These factors are described in a parameter known as the Power-Aperture product (PA). This parameter takes the product of the area of the antenna array (A) and the average power of the transmitter (P) to produce a performance figure for VHF radars.
The VHF radar reflection mechanism is very weak, but this problem can be overcome. The atmosphere doesn’t change much on time scales of a half a second or so, and it’s generally possible to transmit multiple radar pulses in this time.
The samples of the returned signal (with its component of noise) can be combined using a technique known as 'signal averaging'. The amplitude of the noisy part has an average of zero but the atmospheric reflection component does not. This technique allows the atmospheric echo to be extracted from the noisy radar returns.
The radar receivers are of a type known as 'Doppler receivers'. They detect the rate at which the reflecting structures are moving towards or away from the radar. This effect is called the 'Doppler shift'. The movement of the atmosphere can cause the frequency shifts. This is central to measuring atmospheric wind speeds.
The radar beam
In order to measure wind speed using the Doppler shift, the radar pulse needs to be directed in a narrow beam. This is similar to a searchlight. It is this requirement for a narrow beam that made the construction of an array of 144 antennas necessary.
If a single wire is used as an antenna, we can’t identify where a radio pulse picked up on that wire has come from. By having another antenna next to the first, and by measuring when the radio pulses are detected at each antenna, it becomes possible to identify the direction of the pulse. If pulses are detected together, they came from overhead. If pulses arrive at slightly different times, they came from an angle. This principle is extended to the 36 groups of four antennas that make up the VHF radar array. The result is a beam that has a width of 7 degrees.
The meteor array consists of just five receive antennas. This is to allow it to detect meteor trail reflections from as much of the sky as possible, while still being able to calculate their direction.