Technical aspects of the VHF radar
The radar’s specifications
The Davis VHF radar consists of a 144-antenna main array, a secondary five-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||12x12 grid of 3-element Yagi antennas combined into groups of four||Driven element is a folded dipole|
|Effective area||1800 square metres (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||
20 kW – Initial installation
120 kW – Final configuration
|Upgrade to final configuration planned for late 2004|
|Average transmitted power||6 kW||5% duty cycle|
|Power Aperture Product||Approx 10^7Wm^2||At full power|
5x 2-element Yagi antennas for reception
1 crossed Yagi for transmission
|Transmit circular polarisation, receive linear polarisation|
Near 55 MHz, the atmospheric refractive index is related to the humidity, temperature and electron density of the atmosphere, and variations in this refractive index allow radar backscatter to occur. In the troposphere and stratosphere, the radar echo strength is related to temperature and humidity. These parameters in turn allow for good data rates throughout the day and night. Mesospheric reflections are related to electron density. Decreases in mesospheric electron density through the night preclude data collection at these heights and times without the aid of meteor trails. However, 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 extremely weak. With the knowledge that the atmosphere does not change much on time scales of a half a second or so, and the fact that it is generally possible to transmit quite a few radar pulses in this time, this problem can be overcome. 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 also of a type known as "Doppler receivers". The frequency reference that is used to create the transmitted radar pulse is also used to obtain the strength and "phase" of the returned signal. If the frequency of the returned signal is different to that transmitted, then this frequency change can be detected. This is important because the movement of the atmosphere can cause these frequency shifts. Known as the "Doppler shift", it is related to the rate at which the reflecting structures are moving toward or away from the radar. This is central to one the methods used in measuring atmospheric wind speeds.
The radar beam
In order to measure the wind speed using the Doppler shift, it is necessary to be able to direct the radar pulse in a narrow beam similar to that of a searchlight. It is this requirement for a narrow beam that has made the construction of an array of 144 antennas necessary.
If a single wire is used as an antenna, it is not possible to know where the radio pulse that is 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 is now possible to say that the pulse came from overhead (if the pulses are detected together) or away from the vertical (pulses arrive at slightly different times). 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 seven degrees.
For similar reasons, 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).