What is GNSS multipath mitigation?
GNSS multipath mitigation, also often called GPS multipath mitigation, is the process of minimizing or correcting multipath errors. These errors occur when satellite signals are diffracted or reflected from objects like buildings around the receiver rather than the signals being received directly from the satellites (line of sight).
What is multipath error?
Multipath error is also often referred to as GPS multipath, GPS error, GNSS multipath, multipath effect, and multipath interference. It happens when satellite signals arrive at the receiver from different directions following different paths, which results in an error in pseudorange measurements that affects positioning accuracy. All satellite constellations are effected by it. A clear example of this is when one satellite signal takes a longer path due to reflection than another satellite signal takes via a direct path with no reflection, which results in a correlation discrepancy in position.
Why don’t satellite signals take a direct path to the GNSS receiver?
Multipath errors occur much less often in open-sky rural environments, where there is almost no reflection of signals, compared to urban environments, where signals are often reflected.
Multipath has the greatest impact in dense urban environments, referred to as “urban canyons.” When the receiver is surrounded by buildings, satellite signals can diffract, sometimes multiple times, before finally reaching the receiver. Where the position accuracy of a product in a rural environment is 2 m, when placed in an urban area, this drops to even 30 m due to the significant potential for error caused by multipath signals.
The table below presents the results of a typical car drive test comparing position accuracy of L1 vs. L1/L5 band GNSS receivers:
| Position accuracy CEP68 | Expected multipath mitigation effect | L1 single band GNSS receiver | L1/L5 dual band GNSS receiver |
|---|
| Rural area | Minor | < 2 m | < 2 m |
| Deep urban area | High | < 30 m | < 4 m |
How can multipath be mitigated?
There are several approaches to multipath mitigation, including tracking multiple satellite constellations to increase the number of satellites that are within line of sight. However, this approach does not effectively mitigate the multipath effects caused by signals bouncing off surrounding structures and objects.
The GNSS receiver does try to detect multipath signals and avoids using them for navigation. To effectively mitigate multipath within urban canyons, dual band technology is required.
Significantly reducing multipath error with dual band
Dual band GPS/GNSS technology mitigates multipath effects from urban canyon interference by tracking signals in frequency bands that each take different paths to reach the receiver. Knowing that L5 signals are much more resilient to multipath effects, the GNSS firmware algorithm uses more L5 signals for navigation than L1 when it detects being in a multipath environment.
Dual band GPS/GNSS use cases:
Dual band technology provides highly effective multipath mitigation for wearables such as fitness trackers, micromobility, vehicles, and delivery robots.
