Rotation of Velocities to East-North-Up Co-ordinate System
Velocity data acquired from RDI ADCPs can take on several forms. The original, unaltered velocity data, read from the raw files is presented in the MAT file product as adcp.velocity. Also in the MAT file product is the config struct that contains information on how the data was collected. Based on the configuration, the original data is processed to present the velocities relative to East-North-Up: this is the final data in the netCDF, MAT files and in the daily current plot PNG and PDF (see adcp.u, adcp.v, adcp.w in the MAT file and in the *MAT* file description below). u,v,w velocities are always relative to geographic North and local gravity (for Up).
The coordinate system parameter determine the type of data that is acquired and how it is processed to produce u,v,w velocities. In the MAT file product, see config.coordSys. If the co-ordinate system is set to 'Beam', the original data contains the radial (along-beam) velocities for each of the 4 or 5 beams, otherwise the velocities are rotated to an orthogonal co-ordinate system: 'Instr' means the original velocities have been rotated to a basis defined by horizontal plane of the instrument with an azimuth relative to the direction of beam 3, while 'Earth' is relative to the onboard magnetic compass or a fixed direction defined by config.heading_EH. Our default setting is 'Beam' so that the most raw form of the data is acquired. 'Ship' is not recognized/used. If either 'Earth' or 'Instr', the 4th row in the adcp.velocity matrix is the error velocity (the measurement error on the velocity at that bin and time).
Sensor source and sensor availability parameters determine if additional sensors are available to be used for rotation, such sensors include: magnetic compass, tilt sensor (for pitch and roll), depth, conductivity and temperature (the last three are used to determine the sound speed, however we normally fix that value, see config.soundspeed_EC). In the MAT file, see config.sensorSource_EZ and config.sensorAvail. If the magnetic compass is available, it is often used for Earth co-ordination rotation.
Magnetic compasses are not reliable when in close proximity to varying electromagnetic or magnetic fields (electric power cables or the steel instrument platform can cause the compass to be inaccurate). To improve the data, we normally supply a fixed heading from the site and device metadata. If the device mobile, deployed on a mooring, cabled profiler or glider, a mobile position sensor is normally supplied. Here is an example of a mobile ADCP site: http://dmas.uvic.ca/Sites?siteId=100206 and here is an example of fixed site: http://dmas.uvic.ca/Sites?siteId=1000359. If the device is mobile and is attached to a device that can supply better orientation information, such as an optical gyro, the data processing code will make use of that device when its sensors are assigned to the heading/pitch/roll of the ADCP's site (we currently do not yet have an example of such a scenario). If the onboard magnetic compass is the only source of heading data, a correction is applied for magnetic declination. For fixed sites, the pitch and roll of the ADCP will be obtained from the site information. If none is supplied, but a mobile sensor not supplied either, the onboard tilt sensor will be used for pitch and roll but averaged over the dataset, since we can assume that the device is not mobile. If the devices' onboard pitch/roll sensor are assigned as mobile sensors for that site, then the pitch/roll data is not averaged.
Rotation of the input velocities to product East-North-Up velocities (u,v,w) is performed accounting for the incoming co-ordinate system, types and sources of the data. This is somewhat complicated, therefore, all of the processing steps are documented in the MAT files, see adcp.processingComments. For instance, when the raw data has been collected relative to the 'Earth' co-ordinate system defined by the onboard magnetic compass and we have better heading data from a fixed measurement, the heading from the compass is used to back-out the rotation, then the better heading is applied (fixed or variable). In that case, adcp.processingComments would say:
Comments: adcp.velocity(1:3,:,:) contains the unaltered velocity data relative to the co-ordination system: Earth. acdp.velocity(4,:,:) is the RDI error velocity. adcp.u/v/w are processed velocities relative to true East/North/Up, respectively. Used a fixed heading of 228 degrees from meta data. Rotated adcp.u/v/w to true North from onboard Earth magnetic North, using a fixed or variable external source heading.