My goal is to calibrate a simple SABR model.
I do have $tenor$, $expiry$, $forward$ and "market volatilities for strike spread" ranging from -150 to 150 bps.
I think the model can only be calibrated for strike spreads greater than 0.
Is this correct?
I believe this to be true because: $$ \log (f/K) $$ is only defined if $K \gt 0$ assuming $f \gt 0$
excerpt from Hagan et al (2002) paper link