| EM datagrams on *.kmall format Reg.no. 410224 rev G

Reference points and offsets

### Time

The datagram time stamp for multibeam datagrams is the time of the midpoint of the first transmitted pulse of the ping. All delays are referred to this time if not specifically stated.
For sensor datagrams, the timestamp in the datagram header is timestamped when received in PU.

### Transducer orientation

Angular offsets (installation angles) for TX and RX are defined the angular offset between the array coordinate system and the vessel coordinate system. For the EM 2040 echosounder, the default orientation of the transducers is with the cable plug facing the port side (TX) and forward (RX) (i.e. if standing on the boats deck, face in the boat's forward direction, and looking downwards).

If the orientation of the EM 2040 TX is with the cable plug to the starboard (or aft for RX), the installation heading is 180 degrees in the #IIP datagram. This misalignment must be indicated in the K-Controller, under 'Installation', 'Angular offset' menu, by choosing the right configuration (figure TX Transducer: Orientation). The Offset angles (deg.) for TX and RX in the 'Angular offset' menu in the K-Controller are then relative to the orientation indicated in the figures in the menu. However, when processing in the PU, the heading angles are 180 + heading offset.

### Transducer subarrays

For most EM type echosounders, all elements in the TX and RX transducer arrays are used for all beams, giving a common x,y,z location for each transducer. But for the EM 2040 this is different. The TX transducer has 3 separate arrays, used for port, centre and starboard sectors, and the RX array also has an along-ship offset.
The transducer subarray offsets x,y,z are taken into account by the PU when calculating the #MRZ datagram. If it is needed to recalculate XYZ data in post-processing, these subarray offsets should also be taken into account.
For the EM 2040, the transducer installation coordinates refers to the center of the transducer faces. The installation coordinates are given in the Installation parameters.
The figure below illustrates the offsets of the 3 subarrays for EM2040. The TX transducer is seen from behind (default orientation). Subarray 0 = port subarray, 1 = centre subarray, 2 = starboard subarray. Here port/starboard refer to the array coordinate system. E.g. if the TX is installed with a heading offset of 180 degrees, the port subarray (ref. array, subarray 0) will be on the vessels starboard side.

### Vessel reference point

Sensor data as well as the installation offsets (location offsets and angular offsets) of the sensors, are all referred to the vessel coordinate system. Sensor data that are described as corrected have been adjusted to compensate for the offset between the sensor and the vessel reference point. The offsets to be used for correction are set by the operator in K-Controller, Input Setup, Position/Attitude system, Location offset and Angular offset (attitude only). If data are described as raw, the offsets and delays have not been applied, and data in the output datagrams are as received from input sensors.
The beam data (x, y and z) are referred to the vessel reference point, and in the surface coordinate system (SCS). Heave, roll, pitch, sound speed at the transducer depth and ray bending through the water column have been applied. The distance from the vessel reference point to the water lever (z_waterLevelReRefPoint_m) should be subtracted from the beam depths to derive the depths re the water line. See also Coordinate systems