Field Control of the Hydrographic Attitude Sensors Print

Attitude determination of a moving platform, i.e. roll, pitch, and heading, is one of the essential issues in hydrographic surveying. Commonly, the attitude parameters are measured by motion reference units (MRUs) that are installed onboard the platform. Indeed, in order for bathymetry, the accuracy of the onboard MRU must be in agreement with the corresponding IHO standards. As an alternative method, on the other hand, the platform attitudes can reliably be determined via GPS phase observations. Therefore, GPS-based methods for determining the attitudes can be regarded as a suitable experimental tool for quality control of onboard MRUs. In this contribution, a new GPS-based method is presented for quality assessment of MRU. The proposed method can algorithmically be explained as follows: (i) Establishment of a GPS antenna at berth near to the field operation area. (ii) Installation of four GPS antennae onboard the vessel. (iii) Realization of the body frame (b-frame) with the origin at one of the onboard antennae, hereafter Ant. 1, and measurements of the b-frame coordinates of the other onboard antennae. (iv) Starting GPS observations at about 20 minutes before departure for ambiguity resolution, while the vessel is moored at the berth. (v) Sailing from minimum to maximum cruise speed while GPS and MRU observations are going on. (vi). Realization of a local level frame (ℓℓ-frame) at Ant. 1 as well as transformation of the GPS geocentric coordinates to the established ℓℓ-frame. (vii) Using GPS observations to compute the attitude parameters and the corresponding precisions based on the least squares. (viii) Comparing the GPS-derived attitude parameters with those derived by the onboard MRU. (ix) Estimation of the accuracy and bias of the MRU. As the case study, the presented method is applied to 27-meter long survey vessel of National Geographic Organization (NGO) based on four dual-frequency Javad GNSS receivers with choke-ring antennae in the surrounding waters of Kish harbor at different cruise speeds ranging from 5 to 12 knots. Numerical results asserted achievement of mean precisions of 28", 28" and 17" for the pitch, roll and heading, respectively, using carrier phase observations according to the proposed procedure. Moreover, the estimated accuracies for the pitch, roll and heading of the onboard MRU are 7.5', 7.7' and 32.6', respectively. Furthermore, the correction coefficients were computed for the pitch, roll and heading of the onboard MRU. The overall results approved the efficiency of the proposed method to reliably derive the attitudes of moving platforms.