The Lighthouse Trilogy [1] is a set of three books by Irish author Adrian McKinty.
In this book, Jamie O'Neill and his friend Ramsay have been back in Ireland for a year since their previous adventures on Altair.
When a signal from Altair arrives on Earth and Ramsay decodes it, they learn it is a call for help that was sent from a hundred years in the past.
The story revolves around Jamie and Ramsay trying to save their companions and adjusting to how the world has changed since their previous adventures.
Hunted by a rogue CIA agent, Jamie O'Neill, Wishaway, Ramsay MacDonald and Thaddeus Harper go to Altair for a final confrontation with the Alkhavans.