dd = 
            vel: [58x4x52 double]
             sw: [58x4x52 double]
            amp: [58x4x52 double]
             pg: [58x4x52 double]
         status: [58x4x52 double]
        heading: [52x1 double]
          pitch: [52x1 double]
           roll: [52x1 double]
    temperature: [52x1 double]
            HiV: [52x1 double]
        XmtAmps: [52x1 double]
             bt: [1x1 struct]
           dday: [52x1 double]
        ens_num: [52x1 double]
         config: [1x1 struct]
          error: []
dd.bt = 
      vel: [4x52 double]
    range: [4x52 double]
       pg: [4x52 double]

dd.config = 
          yearbase: 2000
            nbytes: 889
                 n: [1x1 struct]
               ofs: [1x1 struct]
             ndata: 5
       nbeams_used: 4
            ncells: 58
              cell: 8
             blank: 4
             pulse: 8
         sysconfig: [1x1 struct]
    vel_multiplier: 0.0025

dd.config.n =
    leader: 63
       vel: 348
        sw: 232
       amp: 232
        pg: 0
    status: 0

dd.config.ofs =
    leader: 14
       vel: 77
        sw: 425
       amp: 657
        pg: 889
    status: 889

dd.config.sysconfig =
    frequency: 150
      hirange: 1
     earthref: 0
           up: 4
      concave: 8
-----------------------------------------------------------------------------

need to add something like this to get depth
d.depth = config.tr_depth + config.bin1distance + (0:(NC-1)) * config.cell;

 
----------------------------------------------------------------------
- ping count wraps at 2^16
- dday is monotonic (at roughly 1s/s) but offset by an arbitrary amount

-----------------------------------------------------------------------------

% .aux (non GGA lines)                                                     .hdg            
%
% 972075544 pingtime                                                      lnx timestamp(s) 1
%                                        212 iping                        iping            2
%          -1 amp_offset 
%          -1 sw_offset
%                                       1782     raw_offset               raw_offset       3     
%                                       493939.0 att_time (GPS seconds)   att_time (GPS seconds)   
%                                       -174.31  gyro heading             gyro heading     5        
%                                        1.34    ash-gyro                 ash-gyro                 
%                                        -0.69   att.pitch                att.pitch        6        
%                                        -2.81   att.roll                 att.roll         7        
%                                        0       att.reacq                att.reacq        8        
%                                        0.040   refu                     refu             9        
%                                        -0.027  refv                     refv            10         
%                                        0.028   refw                     refw            11         
%                                        -0.014  refe                     refe            12         
%                                         10     numbins                  numbins         13         
%                                                
%                                                
%                                                
% .mat
%     gps: [5x5730 double]
%    j_time: 1
%     j_lon: 2
%     j_lat: 3
%    j_nsat: 4
%    j_hdop: 5