( ( ( ( ( ( a pair ) ( of ( drift meters ) ) ) ( -LRB- ( ( ( HDM and ) VDM ) -RRB- ) ) ) ( to ( ( measure ( arrival angles ) ) ( of ( the ( horizontal ( and ( vertical ( drift velocities ) ) ) ) ) ) ) ) ) ( in ( the ( instrument plane ) ) ) ) ; )
(ROOT (NP (NP (DT a) (NN pair)) (PP (IN of) (NP (NN drift) (NNS meters))) (PRN (-LRB- -LRB-) (NP (NNP HDM) (CC and) (NNP VDM)) (-RRB- -RRB-)) (PP (TO to) (NP (NP (NN measure) (NN arrival) (NNS angles)) (PP (IN of) (NP (DT the) (JJ horizontal) (CC and) (JJ vertical) (NN drift) (NNS velocities))))) (PP (IN in) (NP (DT the) (NN instrument) (NN plane))) (: ;)))
a pair of drift meters (HDM and VDM) to measure arrival angles of the horizontal and vertical drift velocities in the instrument plane;
(ROOT (S (NP (NP (NNP Instruments)) (SBAR (WHNP (WDT that)) (S (VP (VB measure) (NP (NP (DT the) (NN angle)) (PP (IN of) (NP (NP (DT the) (NN slope)) (PP (IN of) (NP (DT a) (NN volcano)))))))))) (VP (VBP are) (VP (VBN called) (NP (NN tilt) (NNS meters)))) (. .)))