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        | att_time         |      
                         |            (GPS seconds)  |   (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