Since the deployment of an XBT does not require the ship to slow down or otherwise interfere with normal operations, XBT's are often deployed from vessels of opportunity, such as cargo ships or ferries, rather than a dedicated research ship where a CTD would normally be used in preference.
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For a considerable time, these equations were relatively well-established, however in 2007 Gouretski and Koltermann showed a bias between XBT temperature measurements and CTD temperature measurements.