FMCW base module and SAR module - How do they work together?

SkyRadar's FMCW and SAR Product work together. They help to demonstrate underlying theory and principles of FMCW and SAR and introduce into a targeted application of the radar concepts in practice.

Concept of FMCW

The FMCW radar (Frequency Modulated Continuous Wave Radar) is a Continuous Wave radar where the transmission signal is modulated in frequency. The FMCW based module operates with high resolution radar transceiver, 3 receiving channels and a phase-locked loop transmitter.

The wave-form is generated through a Voltage Controlled Oscillator. Several frequency modulated continuous wave forms are made possible (including triangle, saw-tooth, square and FSK).

Cooperation of FMCW and SAR system

The FreeScopes software allows for comfortable control of the system.

The software also provides calculation and visualization for the Synthetic Aperture Radar extension. 

In fact, the basis of every Synthetic Aperture Radar (SAR) is an FMCW radar. An FMCW radar is moved horizontally along the target. It shoots radar images in same-sized increments. Then it aggregates the results into a high resolution image. The minimum step-size (increments) is < 1mm due to a high precision step motor. So maximum displacement (stop position) is around 2.00 m. Logically the maximum displacement is also the maximum step size.

The vertical distance resolution of the system is appox. 83 cm.

The SAR Sequence

The SAR signal is produced as follows:

• The SkyRadar FMCW base module is mounted on a linear axis and moved horizontally along the target (plug and play).
  
  
• After every defined displacement the FMCW shoots a radar image
  
  
• In a continuous approach, the SAR analysis software integrates (based the SAR algorithm) the images into a high resolution image.
  
  
• During the SAR process, the images are updated permanently - the image improves visibly. 

Eliminating Clutter

The SAR system includes a means of sophisticated clutter elimination: 

Step 1: the location is scanned without the target

Step 2: the location is filmed with the target

Step 3: the image of the location without the target is deducted from the image including the target. The same principle is used with operational radars, where electronic maps with the pre-scanned clutter (e.g. buildings) are deducted from the image with target.

The final image of the above shown experiment will look similar to this one:

Exercises

SAR can make use of all features available in the FMCW screen. So the typical image optimization can be done, such as

• Range Resolution
• Sensitivity Time Control / STC Filter
• Effects of Noise
• Threshold Function
• PPI-Screen and Limiter
• A-Scope and I-Data (Doppler Effect)

Read more in the page: Exercises for FMCW.

The SAR module is dedicated to show the mathematically basics for a synthetic aperture in practice. It uses a primary antenna with a real aperture and relatively poor horizontal directivity. After the processing the SAR shows the surroundings with noticeable improvement in the resolution capability. The higher number of measurements (i.e. the pulse repetition rate) and a broader antenna pattern improve the resolution of the radar image.

Learning Outcomes

• Learns will be able to conduct a measurement with the Synthetic Aperture Radar and calibrate the scopes to achieve a reasonably good measurement result.
• Learners will be able to describe and explain the advantages of the Application of SAR.
• Learners will be able to take a synthetic aperture radar image of a reflecting object.
• Learners will be able to understand and interpret the results indicated in the different scopes

In addition, clutter elimination through deduction of the empty location image is part of the SAR sequence.

Many additional SAR specific exercises can be added.
E.g.,

• what is the minimum distance of 2 objects (vertically and horizontally in order not to be seen as one object)
• how does the image improve, when the step size is reduced? Is there a linear relation?