Drones, precision agriculture and autonomous vehicles are among the applications that require higher accuracy than standard GNSS can provide. One option is the L-Band, which is home to data services that augment GNSS signals to maximize granularity down to the centimeter level.
Also known as precise point positioning (PPP), these L-Band services provide a low-cost alternative to real-time kinematic (RTK) correction services, making them a good fit for applications that require a delicate balance of accuracy and affordability.
It’s important to note that this L-Band runs from 1525 MHz to 1560 MHz and is not the same as GNSS signals that share the same letter. For example, GPS’s L1 signal is at 1575.74 MHz, which is outside the L-Band. The GLONASS G2 signal often is referred to as L2, but it’s located at 1246 MHz.
L-Band or RTK?
L-Band and RTK services share the same goal: Achieving precision that’s often lost due to factors such as signal delays, satellite clock drift, and orbit deviations.
One key difference is that RTK services require an internet connection to receive the correction data, which is calculated by one or more terrestrial base stations in the vicinity of where the drone, AGV, or other device is operating. That requirement can be a problem for applications in rural and remote areas — such as mining and agriculture — where 4G/5G cellular coverage is spotty or simply unavailable.
By comparison, L-Band services use satellites to provide the correction data. This eliminates the need for — and expense of — equipping each autonomous guided vehicle (AGV), drone, or other equipment with a cellular module and service. Plus, many GNSS receiver modules now include L-Band support.
The downside is that the correction data is broadcast by satellite. As a result, their 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. Another potential challenge is that the device is installed in, or travels to, a location that lacks L-Band coverage, such as the poles.
L-Band services don’t offer the same level of precision as RTK services, which can achieve sub-centimeter accuracy. But that granularity comes at a premium. For an example, see u-blox, whose L-Band correction plans can be cheaper than using RTK services.
Ultimately the cost aspect comes down to what the application requires and what its users are willing to pay. Take the example of precision agriculture, where some applications benefit from accuracy in the range of 10 centimeters. During harvests, pinpointing the areas with the highest and lowest yields enables more efficient application of fertilizer and seeds the following season. Over the course of thousands or tens of thousands of acres, this precision application can save a significant amount of money and thus justify the expense of L-Band hardware and service.
Design Considerations and Tips
Obviously the GNSS module and antenna both must support the L-Band. If either or both do not, then the device will need to be redesigned to add L-Band support. The cost and lead time for redesigning and testing can affect the device’s price point, competitiveness, and time to market.
One example is the Taoglas Magma AL100, an active, external magnetic mount antenna designed to minimize time to first fix thanks to a low noise figure to preserve signal quality. The AL100 supports only the L-Band, making it a convenient way to add that capability to a device whose receiver already supports it. The AL100 also has high out-of-band rejection to mitigate those signals from overdriving or damaging its LNAs.
For embedded applications, the Taoglas ALPDF254 active patch antenna uses LNAs and front-end SAW filters to reduce out-of-band noise from sources such as nearby cellular transceivers. Like the AL100, the ALPDF254 is an L-Band-only product. When paired with a high-precision GNSS receiver, the ALPDF254 can achieve centimeter-level accuracy.
Another option is to choose an antenna that supports both the L-Band and the frequencies used by one or more GNSS constellations. One example is the Taoglas Magma AA.212, a ceramic patch, magnetic mount external antenna with optimized gain for GPS L1/L2, Galileo, GLONASS, and BeiDou, along with the L-Band.
A patch antenna is ideal for taking full advantage of the L-Band’s benefits because it’s Right-Hand Circularly Polarized (RHCP) and thus matches the signal’s polarization. Extensive tests by Taoglas also show that a patch antenna even matches linear polarized antenna performance when not facing the sky. That’s a major plus for applications where the antenna may not always have a view of the sky.
The bottom line is that L-Band correction services are an attractive option for location applications that need to balance precision with price. To learn more about how to select the right GNSS antenna, speak to Taoglas’ Engineering team by clicking on the button below.
