ADAS positioning accuracy can be impaired by signal attenuation caused by overhead foliage or RF noise. Designers must find ways to maximise receiver sensitivity.
Have you ever noticed your satnav playing up when you’re driving through woodland? If so, you’ve experienced one of the technical challenges facing designers of advanced driver-assist systems (ADAS), and one of the engineering roadblocks on the journey to mass-market hands-free driving.
The issue is known as signal attenuation, and it occurs when already-weak signals from satellites in space are further weakened by mild obstructions between the signal and the GPS or GNSS receiver.
In 2011, researchers at Inha University in South Korea showed that overhead foliage can weaken GNSS signals without blocking them, affecting the ability of the receiver to compute an accurate position. While that may be a fleeting annoyance for a manual driver, it’s a bigger concern for developers of ADAS and autonomous vehicle positioning systems.
A faint signal made even fainter: The challenge of signal attenuation
The problem is that GNSS signals are only as powerful as a 45-watt lightbulb, so any obstruction can make it harder for a receiver to pick them up. Buildings can block them completely, just like the glow from a dim light can be blocked by a blackout curtain. With foliage they can still pass through, but the effect is like a thin curtain making a dim light even dimmer.
Signal attenuation is also an issue when the radio frequency (RF) environment around the receiver is particularly noisy. RF noise can be much stronger than even normal-strength signals, whether it’s noise from other components near the receiver, 4G or 5G cell towers by the roadside, or deliberate interference from GPS signal jammers. Imagine trying to make out a dim light through a thin curtain when you’re in a brightly-lit room, and you get an idea of the challenge.
Receiver sensitivity is a critical consideration for ADAS developers
The problem of signal attenuation must be addressed if hands-free driving is to become safe for use everywhere. Designers and developers of ADAS systems must ensure that the GNSS receiver has the requisite levels of sensitivity, measured in decibel-milliwatts (dBm), to distinguish faint signals.
If the receiver does not have the requisite sensitivity, the positioning system may not always meet the parameters set out in the system’s performance specification, including:
Time to first fix (TTFF): The GNSS receiver in the ADAS system needs to establish where on the Earth’s surface it is, both when the vehicle is first started and when it is exiting from an area with no GNSS coverage, like a garage or underground car park. If there is overhead foliage or noise in the environment, the time to first fix may be longer than set out in the performance specification.
Tracking sensitivity: For hands-free driving, the GNSS receiver needs to maintain a continuous lock on to GNSS signals, even in areas where the signal is attenuated. If the receiver loses lock due to the signals being too faint, the vehicle risks being unable to establish an absolute position until the signal is reacquired.
Position accuracy: In real-world conditions there is often a sliding scale of effects when the vehicle encounters areas of weak signal. Rather than suddenly losing lock and being unable to estimate a position, the weak signals may first cause the receiver to estimate an erroneous position. Such a scenario creates obvious risks that need to be mitigated for hands-free driving.
S-GNSS Auto®: An affordable way to boost GNSS receiver sensitivity
Increasing the sensitivity of the GNSS receiver is one way to address the issue, but depending on the approach, it can be an expensive undertaking. In today’s autonomous positioning systems, the solution is usually a specialist adaptive antenna system capable of physically narrowing its beam to home in on the GNSS signal and block out other frequencies. But that’s a costly setup, requiring both advanced signal processing algorithms and bulky antenna hardware.
If hands-free driving is going to reach the mass market, OEMs need a better solution, and that’s what our S-GNSS Auto software offers. It uses our Supercorrelation™ signal processing algorithms to create a software-based synthetic aperture antenna, effectively bringing the functionality of a specialist adaptive antenna into the firmware. The receiver gains the equivalent of a 10dB sensitivity boost that helps it acquire weaker signals faster and maintain lock for longer.
As S-GNSS Auto runs within the GNSS chipset or receiver firmware, it’s an affordable way for OEMs and Tier 1 suppliers to address not just the issues of signal attenuation and RF noise, but also the positioning issues that can be caused by multipath interference and GNSS signal spoofing.