Perform accurate radar sensor testing
Rapid developments in autonomous vehicles (AV) and the demand for greater safety features fuel the need for sensitive and accurate automotive radar technology. Radar is one of the sensing technologies driving today’s market for advanced driver-assisted systems (ADAS) and AVs. Manufacturers must put radar sensors through rigorous performance tests while it is still in the manufacturing facility. To achieve optimal accuracy of sensors’ field of view (FoV) testing, manufacturers must align and calibrate the over-the-air (OTA) test setup.
OTA radar target simulator setup in a lab or manufacturing environment requires radar cross-section (RCS) accuracy. Several uncertainties pose a challenge, and path loss calibration can be complex with costly equipment. Alternatively, manufacturers can choose to use a known and stable-calibrated radar sensor, commonly known as the “golden DUT” as the reference. However, this requires constant documentation and maintenance to ensure stability and repeatability as a reference unit.
This blog provides best practices of OTA test setup alignment and calibration to achieve optimal measurement accuracy for radar sensor FoV testing.
How to align the angle of arrival
The azimuth angle accuracy of automotive radar could go as precise as 0.1 degrees at a far range. This is why it is crucial to set up the OTA test in a controlled environment and to ensure the radar sensor under test meets its specifications.
The figure below illustrates FoV performance testing as it shows an OTA setup, simulating a radar object with a radar target simulator (RTS). While RTS is basically re-transmitting the signal emitted by the radar sensor, the angle of arrival (AoA) is defined from the RTS remote front-end horn antenna to the radar sensor. The alignment of the horn antenna needs to be precisely perpendicular to the radar sensor (as shown in the black dotted line in Figure 1). Any drift of the angle of arrival (Ɵ) >0.1° may cause significant FoV testing inaccuracy.
Keysight E8718A Radar Target Simulator offers a remote front end equipped with an alignment laser at a tight tolerance of ±5mm at a 1m distance. Manufacturers may utilize the built-in laser source for mechanical alignments, where the remote front end remains at the chamber top. Meanwhile, a test jig with a mirror is mounted on a turn table positioner. The mirror test jig reflects a laser beam from the remote front end once the engineers turn on the laser source, which helps validate the alignment setup between the horn antenna and the positioner. Therefore, Ɵ can be narrowed down to <0.1° by aligning the positioner until the reflected laser overlaps with the laser beaming source, as shown on the right chamber in Figure 2. Once the manufacturer mechanically aligns the setup, they may start performing the FOV testing.
How to calibrate the OTA path loss
Another test parameter — radar cross section (RCS) — is crucial to ensure the radar sees the desired target area. Referring to the radar equation, a target RCS may be simulated based on the power reflected from the target to the radar receiver. One of the challenges in the RTS OTA setup is to keep the setup consistent with the radar equation formula. Deficient setups can be due to the flatness of the horn antenna gain, the co-polarization angle between the radar and horn antenna, or potential mismatch of the conductive path between the horn antenna and the RTS. OTA path loss calibration can be incorporated to account for all these uncertainties. Still, it typically requires a complex and costly setup with an analog signal generator and an E-band upconverter to generate a precise source from 76-81GHz.
The Keysight U9361M RCal Receiver Calibrator is a single device that enables millimeter wave generation up to 110GHz and eliminates the need for complex and expensive test equipment for OTA path calibration. The RCal, with a characterized standard gain horn (SGH) antenna for transmission of known output source, should be secured on the turn table positioner. The engineers should position the SGH antenna surface at the rotation axis with the same polarization as the radar sensor to be tested.
The built-in calibration utility in RTS helps to execute and store all path loss calibration data with a handheld signal analyzer. The Keysight N9914B FieldFox Handheld RF Analyzer streamlines the process of OTA path loss calibration for an accurate RCS simulation in accordance with chamber setup.
Achieve lower costs when performing OTA test
Test cost is a key factor when manufacturing a radar sensor. Manufacturers encounter many challenges in keeping the performance of the chamber to be consistent from one to the other. To add on, the downtime incurred due to set up process and the resources required also translates to higher costs.
To achieve lower test costs and improve time to market, manufacturers need an efficient set up of the lab or manufacturing line to perform the OTA test. Only then, they will be able to increase their competitiveness in the automotive industry.