Yes... you will travel North into Barca before travelling South to Salou. A taxi to Sants will be expensive especially if it is anywhere near rush hour.
Why are you trying to find GPS co-ordinates of Sants? Get the train from El Prat airport to Sants. At Sants change to the train of your choice to Salou.
The slower trains (circa 90 minutes) will be cheaper and hug the coast for a great journey BUT they tend not to have buffet facilities so maybe stock up at Sants with some refreshments

I just don't want to get lost or lose time I am only there a short time thanks for all your info!