GPSparser is a Python GPS NMEA string parsing module.

This module provides GPS NMEA string parsing through the GPSString class. A GPSstring is an object whose I{msg} is any ASCII text string containing a NMEA GPS string somewhere within it. NMEA strings characteristicly start with the leading "$" and end with the *hh where hh is the two character checksum. All of the following examples are fine:

$GPGGA,154809.00,4305.52462642,N,07051.89568468,W,1,3,4.1,48.971,M,-32.985,M,,*54
RTK1_GPS         DATA    2008-11-21T15:48:10.510017      $GPGGA,154809.00,4305.52462642,N,07051.89568468,W,1,3,4.1,48.971,M,-32.985,M,,*5
09/12/2003,04:01:46.666,$GPGGA,040145.871,7117.3458,N,15700.3788,W,1,06,1.4,014.6,M,-000.4,M,,*50
posnav  2008:226:18:31:34.1365  $INGGA,183133.898,7120.91996,N,15651.72629,W,1,11,0.8,-1.06,M,,,,*19

GPSparser provides methods for extracting the NMEA string from the larger set of ASCII characters, verifying its checksum and parsing the string's fields. The fields may then be accessed as attributes of GPSString (i.e. C{GPSString.latitude}). The complete set of attributes available after a string has been parsed can be found in the attribute list C{GPSString.fieldnames}. Some fields are not returned, such as those that provide units which, by standard or convention, never seem to change.

GPSparser is designed for follow-on processing and plotting of values, and therefore every effort is made to convert all fields to meaningful, numeric values (objects of the Decimal Class). For example, Latitude and Longitude are converted to decimal degrees to 10 digits past the decimal point. Similarly, GPS fix type in RMC strings are indicated by an "A" when the GPS has a fix and a "V" when it does not. These values are reported by GPSParser as 1 and 0, respectively.

Time in GPS strings are converted to datetime objects (see the datetime module). Many NMEA strings contain time-of-day, but not date (GGA for example). GPSparser will, by default, convert these strings to datetime.time objects. However if the GPSString.date attribute is set with a datetime.date object prior to calling the parse() method, a full datetime.datetime object will be returned combining the supplied date and time parsed from the string. This is a handy way to produce full date-time stamps for each string.

A common thing to overlook, however, when parsing a file of strings from a parent script, in which the times rotate over a UTC day, one must be sure to also rotate the value used to set the GPSString.date attribute.

When the module is called as a script, it will parse a file for a single string type, which must be specified on the command-line (see C{gpsparser.py -h}). The fields are written in tab-delimited format to standard-out. Date-time stamps are written as tab-delimited vectors (C{YYYY MM DD HH MM SS}). This format makes reading parsed data files into Octave or MATLAB trivial ( C{load('datafile')} ), with the notable exception of GSV strings which have variable numbers of fields depending on the number of satellites tracked.