More than 700,000 people ride in fully autonomous robotaxis each week. Meanwhile, fully autonomous tractor trailers shuttle freight in and between Dallas, Houston, and other major cities.
Artificial intelligence plays a fundamental role in enabling autonomous vehicles applications, but AI is only as good as the data it has to work with. That’s where GNSS comes in. The GPS, Galileo, GLONASS, and BeiDou constellations provide the positioning, navigation, and timing (PNT) information that AI needs to maximize safety and efficiency.
GNSS is equally important when a human is behind the wheel. In trucks, buses, and passenger vehicles, PNT data enables a host of applications including turn-by-turn navigation, route optimization, geofencing, asset tracking, and fleet management.
The common denominator in these and every other GNSS-powered transportation application is the antenna. Read on to learn what features and specs to consider when choosing an antenna and how to integrate it effectively.
Close Enough isn’t Good Enough
Standard GNSS is accurate to within 16-32 feet (5-10 meters), but a vehicle going 70 mph covers that much ground in a less than a second. That’s one example why many existing and future transportation applications — such as truck platoons — require GNSS correction services to achieve centimeter-level accuracy.
As their name implies, correction services mitigate a variety of factors that undermine GNSS accuracy. For example, GNSS signals are weak by the time they reach the Earth, so they’re vulnerable to atmospheric interference. Closer to the ground, dense foliage and tall buildings further attenuate them, leaving the receiver with even less to work with.
One category of correction services is Differential GNSS (DGNSS), where terrestrial reference stations broadcast location data to augment what’s coming from the satellites to achieve centimeter-level accuracy. An example is Real-Time Kinematic (RTK), which require a subscription and a cellular connection to receive the correction data. (For a deeper dive, read “The Role of GNSS-RTN in Transportation Applications” and watch the on-demand webinar “High Precision GNSS and RTK Positioning.”)
For price-sensitive transportation applications, an alternative is L-Band services, which use satellites to broadcast the correction data. This eliminates the need for — and expense of — a cellular module and service plan. Plus, many GNSS receiver modules now support L-Band, also known as precise point positioning (PPP). One downside is that the L-Band signals may be susceptible to the same errors and challenges as the GNSS signals, such as attenuation from heavy foliage and tall buildings. (For more information, see “How to Leverage the L-Band to Balance Accuracy and Affordability for GNSS Applications.”)
Antenna Types and Key Considerations
Weak signals highlight the importance of choosing the right GNSS antenna system to meet each application’s requirements for availability, reliability, resilience, and performance. The antenna’s design, as well as its integration with the receiver and the rest of the device, directly affect:
- Signal Strength and Sensitivity: High-gain antennas improve reception in challenging environments.
- Time to First Fix (TTFF): The faster the receiver can acquire GNSS signals, the faster it can provide PNT data to the application, such as for turn-by-turn navigation.
- Multipath Mitigation: Reduces errors caused by signal reflections in urban canyons.
- Interference Rejection: Filters out unwanted signals from adjacent bands like 5G.
GNSS antennas come in a variety of designs:
- Patches are common in transportation applications because they’re ideal for installations where they’ll always be facing up with a clear view of the sky, such as on the vehicle’s roof. For optimal performance, they require a ground plane, which often is provided by the vehicle’s roof.
- Quad Helixes are directional. They’re a good choice for use cases where they will be facing up most of the time, but with the possibility of some variability. They also have excellent multipath rejection, which is a plus for urban applications such as transit buses and delivery vehicles. They are best used without a ground plane which makes them perfect for Drone applications.
- Active antennas include a Low Noise Amplifier (LNA) to boost weak signals. They’re more common in transportation than passive models, which leave amplification up to the receiver. (For more information, see “Active vs. Passive GNSS Antennas: Choosing the Right Solution.”)
Strength in Numbers
Some transportation applications benefit from having access to multiple GNSS constellations. For example, if the vehicles travel into latitudes above 55 degrees — such as hauling equipment to arctic mines — GPS accuracy begins to suffer because its satellites rise only 45 degrees above the horizon. This limits visibility of its satellites. A module and antenna system that supports GLONASS and/or Galileo enables the application to take advantage of constellations whose orbits provide better coverage in higher latitudes.
Multi-constellation support also is valuable for mission-critical applications such as military vehicles that will be deployed across multiple continents. It also mitigates jamming and spoofing by enabling devices to switch to another constellation.
Support for multiple bands can be another asset. For example, mission-critical and other demanding applications will benefit from antennas that cover L1, L2, and L5 for improved accuracy and resilience. (For a closer look at the band options, see “How to Navigate the L1, L2, L5, E5a, E5b, and G2 Alphabet Soup of GNSS Constellations and Signals.”)
Finally, if the application will use RTK, look for antennas with phase center stability. (For more information, see “Six Key Parameters to Consider When Comparing GNSS Antennas.”)
What Makes an Antenna Transportation Grade?
Vehicles spend a decade or more enduring temperature extremes, the sun’s heat and UV rays, rain, and jarring potholes. That means their antennas must be equally resilient. Focus on models that have IP ratings and other specs for resistance to vibration, weather, water, and dust. If the antenna will be external, make sure the cable and its connectors also are ruggedized. (For a deeper dive, see “How IP and IK Ratings Measure Real-World Durability” and “What’s Inside is What Counts: Understanding Antenna Coaxial Cables.”)
