In general, flagging consists of three parts: instrument flags (on/off), telemetry flags (preliminary flagging and editing) and data quality flags (geophysical processing flags).

  • Telemetry flags are initially based on altimeter modes and concern checking of telemetry data quality. Only severely corrupted data are not processed. The flag setting used is designed to get a maximum of data into the Sensor Data Records. Science data are processed only when the altimeter is in a tracking mode.
  • Quality flags involve residuals from smoothing or fits from the data themselves. Flag setting checks for gaps, exceeded limits and excessive changes. Among these flags, some have a general meaning, describing the measurement environment.
  • Other flags indicate what type of surface is beneath the satellite (ocean, ice, solid earth, etc).
  • Instrument flags tell us which instruments are on and/or which frequencies are used (mostly for Topex/Poseidon).

fig 1. The CryoSat L2 Intermediate flag indicating the success or failure of the retracking process from 13/7/17 to 11/8/17. (Credits UCL)