Doppler radar systems are instantly able to detect whether targets are getting closer to the scanner, or moving away, by measuring the minute shift in frequency of the echoes returning to the radar scanner. It uses this information to colorize these targets red (inbound) or green (outbound) to call extra attention to them. In busy waterways the Doppler colorization can reduce the workload on the radar operator, making it easier to determine which radar targets present a hazard and which do not.