# Performance Characterization of GNSS/IMU/DVL Integration under Real Maritime Jamming Conditions
## 5 Results
### Results of the Maritime Jamming Campaign
#### 5.2.2 Tightly and Loosely Coupled UKF Results
Here, the positioning error of the tightly coupled filter clearly stays below 20 m, while the loosely coupled filter results in positioning errors up to 50 m. The advantage of the tightly coupled filter is clearly visible. In the loosely coupled filter regime, the uncertainty in state estimate is growing during the time without GPS position updates; therefore, the new GPS positions, containing the large errors, will lead to a relatively high weight in the Kalman gain. In contrast to that, for the tightly coupled filter for almost all times, pseudorange measurements from at least two satellites are available and limit the uncertainty in the filter state. Therefore, the tightly coupled filter performs better in this challenging scenario. For the fourth passing of the jammer, the performance of both filters is roughly comparable with slight advantages of the tightly coupled configuration. The linear drift of the tightly coupled UKF and loosely coupled UKF for the starboard and midship antenna (see Figures 11 and 12), when no GPS measurements are available, seem to be mainly induced by a rather constant course over ground (COG) error to the starboard side. Estimating the COG error, which causes an 18 m drift over a time period of approximately 4 min, assuming a velocity of approximately 5 m/s, results in \(\Delta COG\approx 0.8^{\circ}\). Possible root causes for this COG error are random noise measurement errors in the GNSS compass baseline vector determination, misalignment of the GNSS compass, measurement errors of the Doppler velocity log (DVL), and modeling errors and possible misalignment of the DVL. Because of the observed rather constant COG error, the contributions from the misalignment, of both the GNSS compass and the DVL with respect to the ship body frame, require further investigation. For a GNSS compass with a relatively short baseline of 1.2 m, as used in the described setup, an error of the surveyed position of the GNSS antennas of 2 cm could potentially lead to the observed \(0.8^{\circ}\) error. For the DVL, the alignment error directly translates into a COG error. Note that, to display the actual vessel speed (the main application of the DVL), a misalignment of \(1^{\circ}\) is fully acceptable. Nevertheless, it could have a significant impact on the positioning solution in a hybrid navigation system. Although the constant COG error can be removed by a manual misalignment adjustment, further options could include a simultaneous estimation of the misalignment parameters along with the kinematic state of the ship. Although conceptually beautiful, this approach is not guaranteed to converge, especially bearing in mind fairly limited dynamics of the vessel in typical application scenarios. Nevertheless, the achieved position performance of the tightly coupled UKF in the presented measurement campaign, where the PPD jammer was passed several times in varying distances, with \(\mathrm{HPE}\leq 30\) m can be considered sufficient for basic maritime applications.