Understanding Radar Blind Speed: Definition, Formula, and Mitigation

The Doppler shift is a change in the frequency of a wave, like a radar signal, caused by the movement of the object reflecting the signal. Radar systems use the Doppler effect to measure the speed of objects (aircraft, vehicles, etc.) by analyzing the frequency shift between the transmitted and received radar signals.

What is Blind Speed?

Blind speed refers to a situation where the Doppler shift of a radar signal is unable to accurately detect the speed or velocity of a target. Essentially, the radar “blinds” itself to the true speed.

This happens when the Doppler shift is equal to, or very close to, zero. It makes it difficult for the radar system to determine the target’s speed accurately.

This situation usually occurs when a target is moving directly toward or directly away from the radar antenna. In these scenarios, there’s minimal frequency shift in the reflected signal. Because of this, the radar may not be able to distinguish between stationary objects and those moving at that “blind speed”.

Overcoming the Blind Speed Problem

To overcome this challenge, radar systems often use a few different techniques:

• Multiple Radar Antennas: Using antennas with different orientations helps ensure that at least one antenna will receive a sufficient Doppler shift.
• PRF (Pulse Repetition Frequency) Modulation: Varying the PRF can shift the blind speeds, preventing a target from remaining hidden.
• Signal Processing Techniques: Advanced algorithms can help to extract speed information even with minimal Doppler shift.

These methods enable radar systems to detect and measure speeds of targets even when they’re moving at, or near, blind speeds.

Radar Blind Speed Formulas

There are a couple of formulas used to calculate blind speed:

Formula 1:

V_blind = { c / ( 2 * f * cos(θ)) }

Where:

• V_blind is the blind speed in meters per second (m/s).
• c is the speed of light, approximately 3 x 10^8 m/s.
• f is the operating frequency in Hertz (Hz).
• θ is the angle between the radar beam and the direction of motion of the target, measured in radians.

Formula 2:

This is another common formula, often used in blind speed calculators:

Radar Blind Speed Calculator and Example

Let’s say we have the following:

• Frequency (MHz): 2.5 GHz (2500 MHz)
• PRT: 20 µs

Using the appropriate formula (Formula 2), you would get:

• Blind Speed (m/s): 3000 m/s

Conclusion

The blind speed in radar can be calculated using formulas based on radar frequency, wavelength, and pulse repetition time (PRT). Understanding and mitigating blind speed is a crucial consideration in radar systems for achieving accurate target velocity measurements.