The technical principle of a weather radar
Figure 1: Weather picture
The technical principle of a weather radar
The technical principle of weather radar is very similar to that of Primary Surveillance Radar (PSR) and it is also subject to many similar problems.
The most important difference is that in radar for air surveillance a target is only detected (target present?: yes/no), only the coordinates of the target location are measured. With weather radar, the echo signal is also measured in amplitude. These data finally provide information about the intensity and consistency of reflective objects in the observation room.
But there are also more major differences. This is often caused by the fact that the object searched for has a different shape. Cloud fields have much larger dimensions than a missile but are semi-transparent for some frequencies. Best results in the sense of a weather radar achieve here multi-frequency radars. (Which have a different meaning in weather radar than in a multi-frequency surveillance radar.)
Figure 2: Radar principle
The well-known radar graphic shows how a strong, powerful transmitted pulse is reflected by an object and is reflected in the direction of the antenna as an echo. Although the transmitted signal has exceptionally high power, the re-received echo is usually very, very weak, and therefore requires an extremely sensitive receiver to identify and interpret the echoes.
With primary surveillance radar, echoes from airplanes or other flying objects are expected. For this primary radar, echoes from weather phenomena are an unwanted disturbance that must be filtered out. In contrast, in weather radar, the echoes from airplanes are a cause of interference. However, both radars must take appropriate measures against interference from fixed targets.
Surveillance Radar vs. Weather Radar
Comparison of characteristics of both types of radars is shown in the table below.
|Frequency||L, S-band||S,C & X-band (+L-band)|
|Scanning||azimuth or Elevation||azimuth and Elevation|
|Processing||Complex & real-time||Very complex, not time-critical|
|Polarization||Linear and Circular||Dual (vertical and horizontal)|
|Peak Power||Various (kW - Mw)||Various (kW - Mw)|
|Processing||I (in-phase) & Q (quadrature)|
|“Picture” Update||6 - 12 seconds||5 - 15 minutes|
|Clutter Processing||Yes (but weather is clutter)||Yes (but aircraft are clutter)|
|Antenna Size||Larger (longer wavelength)||smaller (shorter wavelength)|
Table 1: comparison weather radar vs. surveillance radar
Air traffic control and air defense radar systems operate in the L-, S- band (mainly in the L- band). Weather radar normally transmits in S-, C-, X- band (less often in L- band). The L-band is particularly suitable for long-range reconnaissance radar, as it is least affected by weather conditions. This makes the S-, C-, X- bands more suitable for weather radar because of their shorter wavelength.
Doppler frequency processing
Doppler frequency processing for weather radars has become the standard since about the 1990s. Practically all commercially available weather radar systems are equipped with Doppler frequency processing.
Weather Radars work with a “Pencil Beam Antenna” which can be pivoted in Azimuth and Elevation (usually one revolution is scanned with a fixed elevation angle).
The main limitation when working with an additional weather channel in a primary radar is that the antennas are not highly focused vertically (15° … 30°), to be able to locate flight targets at any altitude with each rotation of the antenna. Therefore weather radar images from such a configuration are inaccurate and of insufficient quality.
Weather radars use an antenna, which is highly focused even in elevation. It picks up only a narrow elevation angle per rotation and then composes the 3-dimensional weather image from the individual scans. This takes several times more time than with surveillance radar and so the weather image can be updated after 5 minutes at the earliest. Faster is not necessary, because the weather situation is more constant than a current aircraft position.
Radar Signal Processing
The target processing in a surveillance radar passes through many functions and filters from the antenna to the display. This form of processing can be described as “complex”. These functions and filters are also used by the weather radar. But there is a special data processing, which compares the received echoes with values stored in tables and which also composes a 3-dimensional image from individual elevation angle scans. Therefore the signal processing in weather radar is described as “very complex”.
The surveillance radar works with either linear or circular polarization. The aim of the selection is to obtain a radar image without interference from weather phenomena. If strong target signals are coming from clouds, the radar switches to circular polarization to reduce the effect of these interfering signals.
However, the weather radar will not use circular polarization for this very reason. It will compare the echo from the linear vertical polarization with the echo from the linear horizontal polarization and thus obtain additional information about different weather phenomena. (more…)
The power level of each type of system will vary, depending on the microwave source and the technical characteristics which each frequency requires. However, every system varies and the general characteristics of both systems could foresee peak power levels between 200 MkW and about 1 500 MW. Modern air surveillance radars use solid-state transmitters with intra-pulse modulation. Thus their pulse peak power is much lower. This procedure is in principle also possible with weather radar. However, the time side lobes are a cause for inaccuracy. Therefore, a high power amplifier tube as like a klystron is preferred in the transmitter of weather radar units.
In principle, both systems work digitally with I & Q processing techniques. The dynamic range of a receiver has a slightly greater significance in weather radar than in surveillance radar. The data processing selects the receiver that offers the best signal to noise ratio without being saturated.
PSR systems of air traffic control require all data of the flight attitude for each revolution (that means about every 6 to 12 seconds). This is also necessary due to the dynamics of flight movements. In the case of a surveillance radar with a fan shaped or a cosecant squared pattern, one rotation of the antenna is sufficient for a complete scan, since all altitude ranges are covered by the antenna pattern.
The weather radar composes its complex weather image only after several rotations. In the narrow pencil-diagram of a weather radar, only a small elevation angle range is covered. Several scans in different elevation angles follow each other until a complete volume scan is obtained. Thus the weather image is updated every few minutes.
Both systems use extensive methods of interference echo suppression. Which method is used depends strongly on the type of radar. But in principle, the Doppler frequency is always used. The main difference is that different useful echoes are selected from the mixture of the different echo signals.
The size of the antenna depends on the operating frequency and the requirement for accuracy of beam focusing. In the long-wavelength L-band, the operating frequency range of air traffic control and air defense radars, antennas of respectable size are required. The antenna of the weather radar in the X-band fits into the nose of an aircraft with the same resolution.