text: In a column in the National newspaper, he said he has "little doubt" the "next test is coming".
  He said that his "guess" on timing is that "it is likely to be the autumn of 2018".
  Opposition parties pointed out that the SNP previously said the independence referendum was "once in a generation".
  Mr Salmond resigned as SNP leader after losing the 2014 referendum, which was held two years ago on Sunday.
  Writing in the National, he said: "If I have to guess a date then I'd say that it is likely to be the autumn of 2018. And even on that two-year timescale there is much to be done.
  "In the first vote, what began as a Scottish government initiative transformed into a popular campaign. We should never forget that inspiration.
  "However, indyref2 can't just be the same. As Leicester seem to be finding out in the Premier League, much to the regret of this romantic football fan.
  "You can't roll out the same tactics and trust them to be as effective as before."
  Mr Salmond made similar comments during an interview with Russia Today.
  Independence supporters are holding a rally in Glasgow's George Square to campaign for a second referendum. A second rally is due to be held on Sunday.
  Mr Salmond's successor, Nicola Sturgeon, has said a referendum re-run is "highly likely" following the UK's vote to leave the EU.
  However, the first minister is also exploring "options" to maintain Scotland's links with the EU, as part of the UK's Brexit negotiations.
  Writing in The Times, Scottish Labour leader Kezia Dugdale said "there should be a renewed focus on the day job from the SNP".
  "Two years on, the divisions caused by that historic contest have not healed," she said.
  "On social media and in the real word, the scars that are Alex Salmond's political legacy remain painfully obvious.
  "Mr Salmond and Nicola Sturgeon promised voters that the result would stand for a 'generation'.
  "I believe that when politicians promise something, the voters have a right to expect them to honour that.
  "Yet here we are again, two years later, and Nicola Sturgeon is agitating for another referendum."
  The Scottish Conservatives have been holding street stalls across Scotland to campaign against a second independence referendum.
  Leader Ruth Davidson said: "Scottish Conservatives are out in towns and cities across Scotland today to stand with all those who don't want our country dragged back to a second referendum.
  "Two years ago, we settled this issue - we decided to remain within the United Kingdom. This anniversary should be the moment when we move on, and we will stand with all those people across the country who want to do just that.
  "We call on the SNP today to respect the decision we made.
  "There is no justification to drag us back to an argument that belongs in the past. But the truth is that, far from listening to Scotland, it is the Scottish Nationalist Party that is proving itself to be deaf to Scotland's interests."
target: Former first minister Alex Salmond has suggested a second referendum on Scottish independence could be held in two years time.