that's their signal
( that ( 's ( their signal ) ) )
(ROOT (SBAR (WHNP (WDT that)) (S (VP (VBZ 's) (NP (PRP$ their) (NN signal))))))
That's their signal, a great bright light in the sky. 
( That ( ( 's ( ( ( their signal ) , ) ( ( a ( great ( bright light ) ) ) ( in ( the sky ) ) ) ) ) . ) )
(ROOT (S (NP (DT That)) (VP (VBZ 's) (NP (NP (PRP$ their) (NN signal)) (, ,) (NP (NP (DT a) (JJ great) (JJ bright) (NN light)) (PP (IN in) (NP (DT the) (NN sky)))))) (. .)))