From the TSIP Reference document (P/N 34462-00 rev. C, 4/99):

The Signal Level parameter (Byte 5-8) (an approximation of C/N0) 
is stated in antenna amplitude measurement units (AMUs), a 
Trimble-devised unit. An approximate correlation of AMU levels 
to C/N0 follows:

      5 AMUs ~ 41 dB Hz
     16 AMUs ~ 51 dB Hz
     26 AMUs ~ 55 dB Hz

Formula: C/N0(dBHz) @ 20 log(AMU) + 27 

Measured correlation (courtesy of Mark Sims):


  AMU    dBc                 AMU    dBc

  0.0    20.0                0.0    20.0
  0.1    20.4                1.0    24.0
  0.2    20.8                2.0    28.0
  0.3    21.2                3.0    31.6
  0.4    21.6                4.0    34.6
  0.5    22.0                5.0    36.6
  0.6    22.4                6.0    38.6
  0.7    22.8                7.0    40.2
  0.8    23.2                8.0    41.2
  0.9    23.6                9.0    42.2
                            10.0    43.2
  1.0    24.0               11.0    44.1
  1.1    24.4               12.0    45.1
  1.2    24.8               13.0    45.7
  1.3    25.2               15.0    47.0
  1.4    25.6               16.0    47.5
  1.5    26.0               17.0    48.0
  1.6    26.4               18.0    48.5
  1.7    26.8               19.0    49.0
  1.8    27.2               20.0    49.3
  1.9    27.6               21.0    49.6
                            22.0    49.9
  2.0    28.0               23.0    50.3
  2.1    28.4               24.0    50.6
  2.2    28.8               25.0    50.9
  2.3    29.2 
  2.4    29.6 
  2.5    30.0 
  2.6    30.3 
  2.7    30.6 
  2.8    30.9 
  2.9    31.2 

  3.0    31.6 
  3.1    31.9 
  3.2    32.2 
  3.3    32.5 
  3.4    32.8 
  3.5    33.1 
  3.6    33.4 
  3.7    33.7 
  3.8    34.0 
  3.9    34.3 

  4.0    34.6 
  4.1    34.8 
  4.2    35.0 
  4.3    35.2 
  4.4    35.4 
  4.5    35.6 
  4.6    35.8 
  4.7    36.0 
  4.8    36.2 
  4.9    36.4 

  5.0    36.6 
  5.1    36.8 
  5.2    37.0 
  5.3    37.2 
  5.4    37.4 
  5.5    37.6 
  5.6    37.8 
  5.7    38.0 
  5.8    38.2 
  5.9    38.4 

  6.0    38.6 
  6.1    38.8 
  6.2    39.0 
  6.3    39.2 
  6.4    39.4 
  6.5    39.6 
  6.6    39.8 
  6.7    39.9 
  6.8    40.0 
  6.9    40.1 

  7.0    40.2 
  7.1    40.3 
  7.2    40.4 
  7.3    40.5 
  7.4    40.6 
  7.5    40.7 
  7.6    40.8 
  7.7    40.9 
  7.8    41.0 
  7.9    41.1 

  8.0    41.2 
  8.1    41.3 
  8.2    41.4 
  8.3    41.5 
  8.4    41.6 
  8.5    41.7 
  8.6    41.8 
  8.7    41.9 
  8.8    42.0 
  8.9    42.1 

  9.0    42.2 
  9.1    42.3 
  9.2    42.4 
  9.3    42.5 
  9.4    42.6 
  9.5    42.7 
  9.6    42.8 
  9.7    42.9 
  9.8    43.0 
  9.9    43.1 

 10.0    43.2 
 10.1    43.3 
 10.2    43.4 
 10.3    43.5 
 10.4    43.6 
 10.5    43.7 
 10.6    43.7 
 10.7    43.8 
 10.8    43.9 
 10.9    44.0 

 11.0    44.1 
 11.1    44.2 
 11.2    44.3 
 11.3    44.4 
 11.4    44.5 
 11.5    44.6 
 11.6    44.7 
 11.7    44.8 
 11.8    44.9 
 11.9    45.0 

 12.0    45.1 
 12.1    45.2 
 12.2    45.3 
 12.3    45.4 
 12.4    45.4 
 12.5    45.5 
 12.6    45.5 
 12.7    45.6 
 12.8    45.6 
 12.9    45.7 

 13.0    45.7 
 13.1    45.8 
 13.2    45.8 
 13.3    45.9 
 13.4    46.0 
 13.5    46.0 
 13.6    46.1 
 13.7    46.1 
 13.8    46.2 
 13.9    46.2 

 14.0    46.3 
 14.1    46.3 
 14.2    46.4 
 14.3    46.5 
 14.4    46.6 
 14.5    46.7 
 14.6    46.8 
 14.7    46.8 
 14.8    46.9 
 14.9    46.9 

 15.0    47.0 
 15.1    47.0 
 15.2    47.1 
 15.3    47.1 
 15.4    47.2 
 15.5    47.2 
 15.6    47.3 
 15.7    47.3 
 15.8    47.4 
 15.9    47.4 

 16.0    47.5 
 16.1    47.5 
 16.2    47.6 
 16.3    47.6 
 16.4    47.7 
 16.5    47.7 
 16.6    47.8 
 16.7    47.8 
 16.8    47.9 
 16.9    47.9 

 17.0    48.0 
 17.1    48.0 
 17.2    48.1 
 17.3    48.1 
 17.4    48.2 
 17.5    48.2 
 17.6    48.3 
 17.7    48.3 
 17.8    48.4 
 17.9    48.4 

 18.0    48.5 
 18.1    48.5 
 18.2    48.6 
 18.3    48.6 
 18.4    48.7 
 18.5    48.7 
 18.6    48.8 
 18.7    48.8 
 18.8    48.9 
 18.9    48.9 

 19.0    49.0 
 19.1    49.0 
 19.2    49.0 
 19.3    49.0 
 19.4    49.1 
 19.5    49.1 
 19.6    49.1 
 19.7    49.2 
 19.8    49.2 
 19.9    49.2 

 20.0    49.3 
 20.1    49.3 
 20.2    49.3 
 20.3    49.4 
 20.4    49.4 
 20.5    49.4 
 20.6    49.5 
 20.7    49.5 
 20.8    49.5 
 20.9    49.6 

 21.0    49.6 
 21.1    49.6 
 21.2    49.7 
 21.3    49.7 
 21.4    49.7 
 21.5    49.8 
 21.6    49.8 
 21.7    49.8 
 21.8    49.9 
 21.9    49.9 

 22.0    49.9 
 22.1    50.0 
 22.2    50.0 
 22.3    50.0 
 22.4    50.1 
 22.5    50.1 
 22.6    50.1 
 22.7    50.2 
 22.8    50.2 
 22.9    50.2 

 23.0    50.3 
 23.1    50.3 
 23.2    50.3 
 23.3    50.4 
 23.4    50.4 
 23.5    50.4 
 23.6    50.5 
 23.7    50.5 
 23.8    50.5 
 23.9    50.6 

 24.0    50.6 
 24.1    50.6 
 24.2    50.7 
 24.3    50.7 
 24.4    50.7 
 24.5    50.8 
 24.6    50.8 
 24.7    50.8 
 24.8    50.9 
 24.9    50.9 

 25.0    50.9 
 25.1    51.0 
 25.2    51.0 
 25.3    51.0 
 25.4    51.1 
 25.5    51.1