QA/QC procedure

Automated procedure for telemetry data

The QA/QC procedures below are used to automatically process the wave data from Datawell Directional Waverider buoys. The aim of the procedures is to produce good valid data that has passed through a series of algorithms that are easily understood and well documented. The algorithms may not remove all unwanted data but user feedback suggested that people wanted all the data with a relatively "light touch" in terms of QC. Post deployment data will undergo a fuller set of QA/QC procedures before release.

Directional Waverider via ARGOS and ORBCOMM satellite telemetry

a) Validation of data email from ARGOS ADS service

Each Directional Waverider buoy with ARGOS telemetry transmits the most recent wave data every 90 seconds throughout the deployment. In general, one of the ARGOS satellites passes over a buoy location about every 90 minutes. Therefore, under ideal circumstances wave data should be available every hour and a half.

However, the ARGOS telemetry system used by the Waverider buoys does not implement handshaking or error checking. Therefore there is no guarantee that the data is uncorrupted, error-free and valid.

The Waverider Data Import software checks the validity of each data email received from the ARGOS Automated Distribution Service (ADS). To obtain the full set of wave parameters from a given email message, there must be at least one duplicate transmission of all 16 data elements. In some cases, the email message may not pass this criterion but does contain duplicate data elements 0, 14 and 15. These data are sufficient to determine Hsig, Tz and sea surface temperature only (but not Tpeak, wave direction and spread).

b) Data email from ORBCOMM satellite telemetry

Transmission of data from Directional Waveriders using the ORBCOMM telemetry system is far more reliable than ARGOS. The rate can be set as required with the maximum frequency of data every 30 minutes. The ORBCOMM system uses handshaking and error checking to ensure that data corruption is extremely rare. ORBCOMM coverage is not continuous and therefore there may still be the occasional time delay for the data to reach the Cefas server for processing.

c) Calculation of wave parameters

For valid data messages, wave parameters are calculated using algorithms provided by Datawell to decode ARGOS telemetry data in the 'Compressed Spectrum' format. These parameters are stored in the database for display in the tables, graphs and plots.

d) Range checking of data

The data are then checked against the following maximum and minimum values:

Data falling outside the ranges are stored to the database and flagged, but not displayed in the tables, graphs or plots.

e) Removal of spikes

The automated de-spiking of the data checks if the absolute change from the previous data point exceeds a maximum 'rate of change' for assigned to the parameter. The following values are used:

Data falling outside the 'rate of change' criteria are stored to the database and flagged, but not displayed in the tables, graphs or plots.

Third Party Wave data via Global Telecommunications System (GTS)

a) Calculation of wave parameters

The UK Met Office routinely loads files containing third party data in SHIP code format onto a dedicated FTP server at Cefas. These data have already undergone basic QA prior to transfer. The GTS Data Import software regularly processes the files and decodes wave parameters, which are then stored in the database for display in the tables, graphs and plots.

b) Range checking of data

The data are then checked against the following maximum and minimum values:

Data falling outside the ranges are stored to the database and flagged, but not displayed in the tables, graphs or plots.

c) Removal of spikes

The automated de-spiking of the data checks if the absolute change from the previous data point exceeds a maximum 'rate of change' for assigned to the parameter. The following values are used:

Data falling outside the 'rate of change' criteria are stored to the database and flagged, but not displayed in the tables, graphs or plots.

(* Data from UK Met Office Sevenstones and Channel Lightships have the higher 'rate of change values' applied.)