Update: Sussex Police assistant chief constable Steve Barry said the last confirmed sighting of a drone was at 10pm last night.
It comes as Ryanair and Aer Lingus will operate to and from other airports in London after a drone forced Gatwick Airport to shut down since Wednesday night.
British Transport Secretary Chris Grayling has speculated the drone could be part of "an environmental protest".
He said: "This isn't clear yet, we have not yet completed enquiries, police are following a number of leads there have been a number of pieces of speculation, it could be an environmental protest but we genuinely don't know."
The Cabinet minister told the BBC that whoever the perpetrator or perpetrators were, they needed to "go to jail for a long time".
Asked if it was possible the drone was being operated by an agent of a foreign government, Mr Grayling said: "I don't want to speculate on that, we genuinely don't know who it is or what the motivation was."
"I think it's unlikely to be, but at the moment I'm not ruling out anything", he added.
"The key is we've now got some options around the airport from a police perspective and that puts us in a much better position to respond to drone sightings," Assistant chief constable Steve Barry told BBC Breakfast.
"There are a range of options, some of them very sophisticated and some of them less sophisticated, but it has as I say put us in a much better position to respond to any of those drone sightings."
He went on: "The last few days has been unprecedented nationally I would say, I mean we have had minor incursions previously at Gatwick and we have responded to those really effectively.
"But like I say, this scenario that we're dealing with has been unprecedented so in terms of getting the level of options available to respond to it, to mitigate it, to detect it and respond to it, that has taken us some time, but I'm really confident now that we've got everything we possibly could have here at Gatwick."
Asked if the disruption was part of an environmental protest, he said: "Well it's certainly something that we would consider. Yes, I would agree that's a possibility.
"At this stage we're certainly not linking it to terrorism, but obviously we keep an open mind and I can understand the perception.
"It is, as I say, an unprecedented level of disruption to national infrastructure to the passengers themselves. It's really really significant. But in terms of the motivation as I say you know we're exploring lines of enquiry around that but at this stage we're not saying that it is terrorist related."
Earlier: Irish airlines put contingency plans into place as Gatwick reopens runway
Ryanair has announced that all of its flights scheduled to fly in and out of Gatwick Airport today will instead operate to and from London Stansted.
The airline says all affected customers have been notified.
Aer Lingus has said it is also putting contingency plans in place to minimise disruption.
It is increasing capacity to Heathrow and also accommodating guests at other London airports.
A drone which has forced the airport to shut down since Wednesday night hasn't been seen since late last night.
The travel plans of more than 120,000 people have been affected by the chaos - with dozens of Irish flights among those cancelled or diverted.
Gatwick says any passengers travelling today should check the status of their flight before going to the airport.
It comes as a number of airlines have indicated that the runway at Gatwick has reopened.
A spokesman from Gatwick Airport said: "Gatwick's runway is currently available and a limited number of aircraft are scheduled for departure and arrival.
"Gatwick continues to advise passengers to check the status of their flight with their airline before travelling to the airport as departures and arrivals will be subject to delays and cancellations."
In an update on its website, easyJet said: "Gatwick airport have confirmed that the runway is now open. We do expect that the number of departures and landings will be restricted to begin with, which means that we are likely to experience more disruption to the flying programme.
"We would like to request our passengers to continue to check the status of their flight on the easyJet Flight Tracker."
In response to a customer on Twitter, British Airways said: "We've received confirmation approx 30 minutes ago from London Gatwick, advising the airport is back open and the majority of flights operating as scheduled".
According to Flightradar24, a plane from East Midlands Airport landed at Gatwick at 5.58am.
Gatwick chief operating officer Chris Woodroofe said the airport has scheduled almost 700 departures for today.
Mr Woodroofe, speaking to BBC Radio 4's Today programme, said: "Our advice to our passengers is to check with their airline on each of those flights that they're intending to get, to establish whether it's one of the flights that's being operated or one of the flights that's being cancelled, before they come to the airport.
"I'd just like to apologise to all of those affected over the last 36 hours - 120,000 passengers who were due to fly to their destinations or arrive into Gatwick who have not travelled."
Mr Woodroofe was pressed on why the airport had decided to reschedule flights while the drone had not been found, he said: "We have been working overnight with the police, with a number Government agencies and with the military to put in place additional mitigating measures which have enabled me to reopen our airport."
Asked if the "mitigating measures" meant the drone would be shot down, he said: "You'll appreciate that there are certain things I can't talk about in detail."
The number of drone sightings at Gatwick Airport since Wednesday has been "unprecedented" but police now have "a number of options" around its perimeter to help prevent further disruption, Sussex Police assistant chief constable Steve Barry said.
