a = 
            vel: [75x4x0 double]
            cor: [75x4x0 double]
            amp: [75x4x0 double]
             pg: [75x4x0 double]
        heading: [0x1 double]
          pitch: [0x1 double]
           roll: [0x1 double]
     soundspeed: [0x1 double]
    temperature: [0x1 double]
             bt: [1x1 struct]
            nav: [1x1 struct]
           dday: [0x1 double]
        ens_num: [0x1 double]
      num_pings: [0x1 double]
         config: [1x1 struct]
          depth: [1x75 double]
          error: []


a.bt = 
      vel: [4x0 double]
    range: [4x0 double]
      cor: [4x0 double]
      amp: [4x0 double]
     rssi: [4x0 double]

a.nav =
   sec_pc_minus_utc: [0x1 double]
                txy1: [3x0 double]
                txy2: [3x0 double]

a.config = 
    yearbase: 2000
          nbytes: 1786
           ndata: 10
         offsets: [10x1 double]
             ids: [10x1 double]
     nbeams_used: 4
          ncells: 75
            cell: 16
           blank: 16
            mode: 10
       code_reps: 0
              EZ: 81
    bin1distance: 31.9700
           pulse: 16
             lag: 2.9100
      head_align: 0
       head_bias: 0
       sysconfig: [1x1 struct]
        tr_depth: 5
              bt: [1x1 struct]


a.config.sysconfig = 
    frequency: 38
           up: 0
       convex: 1
        angle: 30


 a.config.bt =
    BP: NaN
    BC: NaN
    BA: NaN
    BX: NaN