Sensors for driverless car navigation
FMCW Radar
Radar systems measure the time of flight from when a transmitted signal leaves the radar, hits a reflecting target and returns to the radar.
Frequency Modulated Continuous Wave (FMCW) radar is a form of radar where the frequency of the transmitted signal is continuously varied at a known rate over a defined time period. The reflected frequency signal is received by the radar and compared. The difference between the transmitted and reflected signals is directly proportional to the time of flight, which in turn is therefore proportional to the range.
The FMCW class of radar offers the next level in robust sensing to autonomous vehicles. Due to its high range resolution and accuracy, the Navtech sensor produces an extremely rich set of data which is vital for improving safety in the harshest of conditions.
FMCW Radar
- Superior range resolution
- Measures the target range and velocity simultaneously
- Robust data produced
- Low power consumption
Sawtooth frequency modulation
Various forms of frequency modulation can be applied. Sawtooth modulation, used by the Navtech sensors, is often used in FMCW radars where range information is needed for object detection.
When using this technique, range information is mixed with the doppler velocity.
Modulation can be turned off on alternate scans to identify velocity using unmodulated carrier frequency shift. This allows range and velocity to be found with just one radar set.
Radar versus Lidar
A radar beam is not a pencil beam like a laser, it has a 3D profile. Since the radar returns a value from every range measurement, it is able to see through noisy objects, something that most time of flight sensors are unable to do.
By splitting each azimuth into an array of radially spaced intensity values the radar provides an extra dimension that Lidar does not. It is this functionality that provides the ability for the radar to build a top-down, photo-like image, something that a Lidar unit is unable to do without implementing multiple channels.
All-weather operation
Another crucial difference between the Navtech radar sensor and other sensors is its ability to work through adverse weather conditions.
The longer wavelength of the radar signal is unaffected by rain, fog, dust or dirt so the sensor can be used in all scenarios giving the radar a very large operating window. As the radar does not have a lens, it does not require cleaning which means no unnecessary downtime.
Detection Factors
Which factors determine how well we can see an object?
The main factor that determines how well we can see an object is the amount of power that is reflected back to the receiver. This depends on several factors and, fortunately for us, there is an equation that describes these factors.
The reflected power Pr returning to the receiving antenna is given by the radar equation. This will depend on the amount of transmitted power, target range, and the reflecting characteristics of the target (described as the radar cross-section σ). As we know the sensitivity of the radar receiver, the radar equation gives us the theoretical maximum range, R.
Our FMCW sensors are engineered to deliver maximum power from a compact, rugged, 360° radar designed to withstand the most challenging environments.
