SoftGPS project

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Data files

Description of data file

The ADC sampling frequency is 12 Ms/s (MHz). The intermediate frequency is 3.563 MHz. Signal is sampled using one bit.
One bit in the file represents one sample (e.g. one byte in the file contains 8 samples).

The following PRN-s are present in the record: 01, 03, 07, 19, 20, 22, 24, 28, 31.

An example how to read the data file in Matlab:

[fid, message] = fopen(fileName, 'r', 'b');
data = fread(fid, numberOfSamplesToBeRead, 'ubit1')';
data = data * 2 - 1; %To convert values to +/- 1

Notes regarding SoftGPS and this data file

Please note that the fopen command for this data file differs from the one in SoftGPS. The difference is the 'b' parameter ('rb' is not the same as 'r', 'b'). Please add the 'b' option like this: fopen(settings.fileName, 'rb', 'b'). It is required because of the way this particular data file was written.

Please use the correct data type (ubit1). And then add the code to convert 0 and 1 levels to -1 and 1 as described above. There was a mistake on this page (the fread used an incorrect data type - bit1).

The tracking code must be modified in few places. I will not provide the code, but I will tell where to look for the problems.

The first place is the fseek command. This command here works fine, when one sample is one byte in a file. Now one sample is one bit in the file. Therefore some code instead fseek must be developed.

The second place - the computation of the absoluteSample - it is also based on a byte size samples. This must be fixed too.

Description of data file

This is a demo file made by the USB front-end. The sampling frequency is 16367600 Hz and the intermediate frequency is about 4130400 Hz (both are nominal values). One signed char is used per sample, but only two bits are exercised (two bit sampling at the front-end).

The softGPS software does not need any code modifications to read this file. Only sampling and intermediate frequencies must be modified in the settings.

Project is developed by the Danish GPS Center