As testing on oil samples taken from the search site for Malaysia Airlines flight MH370 begin, preparations are being made for the fourth underwater search.
Unexpected depths and technical issues hampered earlier searches.
The autonomous vehicle has so far scoured approximately 90 square kilometres.
Nothing of interest was found on data retrieved in the first two missions and data from the latest search is being analysed.
Oil samples, collected earlier in the week from an oil slick about 5.5 kilometres downwind from where "pings" were recorded, have now arrived in Perth.
The samples will be subject to detailed testing and analysis.
The Bluefin-21 takes about two hours to descend and is capable of scanning parts of the search area for 16 hours before taking another two hours to return to the surface.
This would allow a five kilometre by eight kilometre area to be searched.
Analysing data from the device can take up to four hours.
Up to 10 military aircraft, two civil aircraft and 11 ships will assist in the search on Thursday across a visual search area totalling approximately 40,349 square kilometres.
The centre of the search areas lies approximately 2170 kilometres north west of Perth.
The weather forecast for today is isolated showers and southeasterly winds.
It has been 41 days since the flight, carrying 239 passengers disappeared.
