The president is unlikely to see the balloon as he made his journey by helicopter from the USA ambassador's residence in Regent's Park to watch a military display at Sandhurst, the famous academy for the training of military officers.
Khan has been an outspoken opponent of Trump, saying in January that Londoners "have made it clear that Donald Trump is not welcome here", after the president testily canceled a trip to the United Kingdom.
It turns out there are no specific rules when it comes to meeting Queen Elizabeth II. You don't see, like, anything embarrassing.
During the meeting, which was aired live on TV, Donald and Melania walked to meet the Queen where they shook hands before engaging in small talk.
Opponents of Donald Trump flew a six-meter blimp of "Baby Trump" on Friday to protest against the controversial visit of the US President to the UK.
"She is a tremendous woman".
All migrant kids under 5 to be back with parents by Thursday
The Trump administration is missing its court-imposed deadline to reunite 102 young immigrant children with their parents. The government has until July 26 to reunite almost 2,800 other children, ages five through 17, now in its custody.
As you may be aware by now, their visit to Windsor Palace is not official state business but more of a casual encounter between the two heads of state.
With that prospect now in greater doubt, her 104-page proposal tabled earlier this week appears increasingly shaky.
The Queen has received three other US Presidents at Windsor Castle since the 1980s - Barack Obama in 2016, Mr Bush in 2008 and Ronald Reagan in 1982.
Trump has previously spoken about Prince William's wife Kate and his late mother Diana.
During an interview with The Sun, Trump personally blamed Mr Khan for failing to stand up to terrorists.
"Given Trump's appalling actions and rhetoric, he doesn't even deserve to look at a statue of Churchill let alone sit in his seat".
"I'm also quite clear that although there has been an increase in violent crime across England and Wales, including London, over the last four years, I don't think the rise in crime is because of immigration".