## 1 Introduction
### Related Work  
Different studies have been carried out to mitigate the errors in INS/GNSS system. The linear offset lever-arm can be determined indirectly by computing the difference between GNSS-derived positions of the antenna reference point and the camera projection centers issued from bundle block adjustment [9]. This offset can also be estimated as an additional parameter during the bundle block adjustment, however, the accuracy is limited due to its correlation with camera interior orientation parameters [10; 11; 12]. While, with classical methods (by a calliper or by photogrammetric approaches), the linear offset lever-arm can be determined, it is not possible for the boresight calibration to reach sufficient accuracy in the same way. Accordingly, several boresight calibration methods have been proposed [13; 14; 15; 16]. It can be performed either with \"one-step\" method (i.e., estimation within the BBA process) or with \"two-step\" method (i.e., comparison of the INS/GNSS-derived attitude with that obtained by BBA process). Moreover, taking into account the remaining temporal correlations within the navigation system can lead to a more realistic estimation [17]. Despite the possibilities of improving the INS/GNSS system accuracy, the INS system demands more effort to reduce errors and often needs accurate GNSS positions as constraints for error control. Given the limited UAV payload capability, this demand on accuracy and thus on high quality of INS/GNSS sensors can be difficult to meet. Therefore, for small UAV photogrammetric acquisitions, the position aerial control of GNSS systems outperforms the attitude aerial control of INS systems with its better accuracy, lower cost and lower payload requirements. The assisted bundle block adjustment with GNSS data, tie points and GCPs seems to be a more interesting approach. This approach is also referred to as Integrated Sensor Orientation (ISO). On the other hand, due to the substantial inconvenience and cost of GCP establishment, especially in areas with difficult access, the number of GCPs is to be reduced to a minimum. In 2002, a multi-site test investigating sensor orientation is carried out by the European Organization for Experimental Photogrammetric Research (OEEPE) [18] using the state-of-the-art GNSS/IMU technology of 1999. It shows that the direct sensor orientation can reach an accuracy of 5-10 cm in planimetry and 10-15 cm in altimetry, which is larger than the standard photogrammetric result by a factor of 2-3. The additional introduction of tie points into BBA procedure without GCPs improves in particular the accuracy in height and an accuracy of 5-10 cm is reached. If a minimum of GCPs is introduced, one can achieve an accuracy in object space very similar to that of conventional photogrammetry. Though with direct sensor orientation, the time and cost decrease significantly, integrated sensor orientation is preferable when very high accuracy is indispensable. In [19], an aerial acquisition of block configuration is performed on a 250 m \(\times\)300 m block with an embarked GNSS-inertial system integrated with a Sony a7R camera. Eight north-south strips and one cross strip are flown at 80 m, resulting in a GSD of 1 cm. With aerial control issued from the GNSS-inertial system and one GCP included in the BBA procedure, an accuracy of 3 cm is obtained in both horizontal axis and 11 cm in vertical component. For a corridor mapping, two flight strips are flown at 135 m [20]. Data of a high-grade dual-frequency GPS receiver are introduced in BBA procedure as aerial position control. With pre-calibrated camera and without GCPs, an accuracy of 12, 11 and 64 cm are reached in northing, easting and height components, respectively. With the aid of precise aerial position control together with a pre-calibrated camera, two case studies for accurate mapping are presented without GCPs [5]. The first one being block configuration,a self-calibration is performed within the BBA procedure without GCPs.