Plot extractor
An assembly in a radar device is called a plot extractor , in which a digital target symbol for display on the radar screen is formed from the analog echo signals . The name is not uniform. Sometimes the entire upper module for target data processing is called a plot extractor, sometimes only the sub-module in it, which generates a standardized data format from the echo signals of the local radar device. In the first case, this subassembly is then called the hit processor .
The modules of the plot extractor form the end of the radar signal processing. Here, a data set is generated from each (analog or mostly digital) video signal. Additional information such as course and speed can also be processed. The practical system solutions range from simple position determination through to the various filters and signal channels for fixed target suppression through to complex spectrum comparison analyzes using databases.
history
In the past with manual target character processing, the targets were drawn on a planchette with a pen suitable for writing on glass (a so-called "grease pen") . The draftsman was jokingly referred to as a plotter and his work was consequently referred to as a plot (drawing of a plotter). Thus, in the western world, which is influenced by the English language, the term plot has also established itself for the display of radar data.
The first plot extractors were developed in the early to mid-1970s, when the first TTL logic modules were available in large numbers and were also fast enough to process the amount of data. Initially, these devices only worked two-dimensionally with only the calculation of X and Y coordinates. It was only with the advent of 3D radars in the 1980s that height was also processed as a third dimension (Z).
For the conversion of the analog data and the assignment of range cells in the radar data processor, the system clock of the radar device was initially used. By limiting the speed of contemporary TTL logic modules, the sampling rate was limited to 10 to 20 MHz. Modern plot extractors work with a clock in the gigahertz range and use analog-digital converters with sampling rates of several 100 MHz. With this technology, entire databases can be saved on different samples of spectra and compared with currently received echo signals.
functionality
A radar device observes the surroundings and delivers analogue impulses from the detected objects at the receiver output . However, because the radar antenna rotates slowly measured against the pulse repetition frequency , the radar often receives a large number of echo signals from a reflecting object . However, since the radar antenna continues to rotate, each pulse has a different azimuth assignment. A data record (called a “report”) is now formed from these individual pulses in the plot extractor. Several methods are possible here:
- Monopulse method
- Sliding window
- Center of Mass
With the monopulse method , a so-called Off-Boresight Angle (OBA) is measured from each echo signal. This angle is the deviation of the real direction of the detected object from the current mechanical antenna position. The correct side angle information is calculated from the sum of both angles. The coordinates can be measured from just one echo signal.
The sliding window method creates this information from a group of input pulses. In practice, a mean value is formed here from a fixed number of pulses each with different side angle information.
The “Center of Mass” is a mathematically better method than the simple sliding window. It consists of calculating a central point in a set of coordinates generated by the Hit Processor. These hits (in German: "hits") are grouped first in the distance and then in the lateral angle and result in a statistical average. This is then the center in the frequency distribution and the coordinates of this point are then defined as plot coordinates. This method makes better use of the radar data and enables a more precise position determination. This method can also be used to calculate the total energy (the sum of all impulses combined in a report) and to transmit it to the plot processor with the report. This energy parameter is an important indicator for suppressing false targets .
Block diagram
From the non-uniform designation mentioned at the beginning, the following block diagram is partly referred to as a plot extractor, partly only the one subassembly. If the name plot extractor is used for the entire system, then the subassembly is named Hit Processor.
After the receiver output, the echo signal is still available as an intermediate frequency (IF). This signal contains both amplitude and phase information. With an I&Q phase detector , both pieces of information are processed into bipolar videos (video signals that constantly change polarization), from which a unipolar video is then calculated. Only signals that exceed a certain threshold are processed in the detector. This threshold value defines important parameters such as the probability of detection and the false alarm rate .
The sub-assembly plot extractor (hit processor) processes the reports (data records still without side angle information) to form the plots (data records with side angle information). The sensor tracker creates the track from several plots. Its data set also contains information such as flight direction (course) and flight speed.
Data forwarding
In order to be able to display this information at workstations that are far away, the data formats are standardized (for example the All Purpose Structured Eurocontrol Surveillance Information Exchange from Eurocontrol ). This radar data is correlated in a further assembly, called a combiner, with the data from the local secondary radar and from other primary radar devices. Correlating means that information that is obviously the same and possibly slightly different is linked to form a data set. These differences arise, for example, because the radars do not rotate synchronously and thus up to 30 seconds of flight time can pass before the second radar detects the same aircraft.
Since the coordinates are no longer available in real time due to the processing time and a constant processing time can no longer be assumed, a time stamp must be used from now on at the latest. Each data set therefore contains the exact time at which the echo signal was received by the radar device.
The connection of different radar devices and the display of their data on a common display device is organized via a network called RADNET , of which there is also a military variant, "milRADNET". There is also the RAMOS network especially for querying Mode-S .
Web link
- www.radartutorial.eu Chapter radar signal processing