Here are best practices for OTA test setup alignment and calibration to achieve optimal measurement accuracy for radar sensor FoV testing.
Radar is one of the sensing technologies that is driving today’s market for advanced driver assisted system and autonomous vehicles. While road safety is always the upmost priority, manufacturers first must put the sensor through rigorous performance tests while it is still in the manufacturing facility. This article provides best practices of over-the-air (OTA) test setup alignment and calibration to achieve optimal measurement accuracy for radar sensor field of view (FoV) testing.
Aligning Angle of Arrival
It is notable that some automotive radar azimuth angle accuracy could go as precise as 0.1 degree at far range distance. Therefore, it is important to assure the OTA test is setup in a controlled environment and ensure the radar sensor under test is meeting its specification. Figure 1 illustrates an OTA setup simulating radar object with Radar Target Simulator (RTS) for field of view performance testing. While RTS is basically re-transmitting the signal emitted by radar sensor, angle of arrival (AoA) is defined from horn antenna of RTS remote front end to the radar sensor. The alignment of horn antenna needs to be precisely perpendicular to the radar sensor—as shown in blue dotted line of Figure 1, any drift of Ɵ >0.1° may cause significant inaccuracy of FoV testing.
Figure 1 OTA test setup with miss-aligned turn table positioner inside anechoic chamber.
The Keysight E8718A RTS offers a remote front end that is equipped with an alignment laser at a tight tolerance of ±5mm at 1m distance, see Figure 2.
Figure 2 E8718A Radar Target Simulator Bi-Static Antenna Remote Front End with Alignment Laser.
A manufacturer 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 designed to be mounted on a turn table positioner. The mirror test jig reflects a laser beam from the remote front end once the laser source is turned on; which helps to validate the alignment setup between the horn antenna to positioner. Therefore, Ɵ can be narrowed down to <0.1° by aligning the positioner until the reflected laser is overlapping with the laser beaming source—see Figure 3 for further illustration.
Figure 3 Before and after alignment from picture of the left and right.
Calibrating OTA Path Loss
Once the setup is aligned mechanically, a manufacturer may start performing the FOV test. Also, 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. Some examples of deficient setups are: horn antenna gain flatness, the co-polarization angle between the radar and horn antenna, and potential mismatching of the conductive path between the horn antenna to RTS. OTA path loss calibration can be incorporated to account in all these uncertainties, but it typically requires a complex and costly setup with an analog signal generator, coupled with 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 SGH antenna surface should be positioned at the rotation axis with same polarization as the radar sensor to be tested—see Figure 4 for further illustration. The built-in calibration utility in RTS helps to execute and store all path loss calibration data with a handheld signal analyzer, streamlining the process of OTA path loss calibration for an accurate RCS simulation – in accordance to chamber setup.
Figure 4 OTA path loss calibration setup for radar target simulation.
Cost of test is a key factor in the total cost of manufacturing a radar sensor. Manufacturers often encounter challenges to keep their chamber setup performance consistent from one to another, setup down-time, and manage resources—which eventually translate into higher cost of test. The recommended tools and methodologies described, help to increase manufacturers’ competitiveness in the automotive industry with the efficient setup of their OTA test in a lab or a manufacturing line, to achieve lower cost of test and improve time to market.
About the Authors
Teng Kee Lok is a R&D manager with Keysight’s Automotive and Energy solutions team, leading the automotive radar test solution program planning and definition for manufacturing users. He was a technical marketing engineer and R&D project manager, spending a total of 9 years at Keysight working in the automotive test industry which enabled in-depth automotive market insights.
Beng Hooi Tan is a R&D mechanical engineer with Keysight’s Automotive and Energy solutions team. He has over two decades of mechanical engineering experience, with 15 years at Keysight in various roles, delivering solutions to manufacturing test challenges such as: custom RF switch matrix products, high performance functional test system, ICT products, PXI/PXIe, and the radar target simulator.