ardupilot/libraries/AP_GPS/AP_GPS_NMEA.h

161 lines
5.6 KiB
C++

// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: t -*-
//
// NMEA parser, adapted by Michael Smith from TinyGPS v9:
//
// TinyGPS - a small GPS library for Arduino providing basic NMEA parsing
// Copyright (C) 2008-9 Mikal Hart
// All rights reserved.
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
// Lesser General Public License for more details.
//
/// @file AP_GPS_NMEA.h
/// @brief NMEA protocol parser
///
/// This is a lightweight NMEA parser, derived originally from the
/// TinyGPS parser by Mikal Hart. It is frugal in its use of memory
/// and tries to avoid unnecessary arithmetic.
///
/// The parser handles GPGGA, GPRMC and GPVTG messages, and attempts to be
/// robust in the face of occasional corruption in the input stream. It
/// makes a basic effort to configure GPS' that are likely to be connected in
/// NMEA mode (SiRF, MediaTek and ublox) to emit the correct message
/// stream, but does not validate that the correct stream is being received.
/// In particular, a unit emitting just GPRMC will show as having a fix
/// even though no altitude data is being received.
///
/// GPVTG data is parsed, but as the message may not contain the the
/// qualifier field (this is common with e.g. older SiRF units) it is
/// not considered a source of fix-valid information.
///
#ifndef AP_GPS_NMEA_h
#define AP_GPS_NMEA_h
#include <GPS.h>
#include <avr/pgmspace.h>
/// NMEA parser
///
class AP_GPS_NMEA : public GPS
{
public:
/// Constructor
///
AP_GPS_NMEA(Stream *s);
/// Perform a (re)initialisation of the GPS; sends the
/// protocol configuration messages.
///
virtual void init();
/// Checks the serial receive buffer for characters,
/// attempts to parse NMEA data and updates internal state
/// accordingly.
///
virtual bool read();
private:
/// Coding for the GPS sentences that the parser handles
enum _sentence_types {
_GPS_SENTENCE_GPRMC = 10,
_GPS_SENTENCE_GPGGA = 20,
_GPS_SENTENCE_VTG = 30,
_GPS_SENTENCE_OTHER = 0
};
/// Update the decode state machine with a new character
///
/// @param c The next character in the NMEA input stream
/// @returns True if processing the character has resulted in
/// an update to the GPS state
///
bool _decode(char c);
/// Return the numeric value of an ascii hex character
///
/// @param a The character to be converted
/// @returns The value of the character as a hex digit
///
int _from_hex(char a);
/// Parses the current term as a NMEA-style decimal number with
/// up to two decimal digits.
///
/// @returns The value expressed by the string in _term,
/// multiplied by 100.
///
unsigned long _parse_decimal();
/// Parses the current term as a NMEA-style degrees + minutes
/// value with up to four decimal digits.
///
/// This gives a theoretical resolution limit of around 18cm.
///
/// @returns The value expressed by the string in _term,
/// multiplied by 10000.
///
unsigned long _parse_degrees();
/// Processes the current term when it has been deemed to be
/// complete.
///
/// Each GPS message is broken up into terms separated by commas.
/// Each term is then processed by this function as it is received.
///
/// @returns True if completing the term has resulted in
/// an update to the GPS state.
bool _term_complete();
uint8_t _parity; ///< NMEA message checksum accumulator
bool _is_checksum_term; ///< current term is the checksum
char _term[15]; ///< buffer for the current term within the current sentence
uint8_t _sentence_type; ///< the sentence type currently being processed
uint8_t _term_number; ///< term index within the current sentence
uint8_t _term_offset; ///< character offset with the term being received
bool _gps_data_good; ///< set when the sentence indicates data is good
// The result of parsing terms within a message is stored temporarily until
// the message is completely processed and the checksum validated.
// This avoids the need to buffer the entire message.
long _new_time; ///< time parsed from a term
long _new_date; ///< date parsed from a term
long _new_latitude; ///< latitude parsed from a term
long _new_longitude; ///< longitude parsed from a term
long _new_altitude; ///< altitude parsed from a term
long _new_speed; ///< speed parsed from a term
long _new_course; ///< course parsed from a term
int _new_hdop; ///< HDOP parsed from a term
uint8_t _new_satellite_count; ///< satellite count parsed from a term
/// @name Init strings
/// In ::init, an attempt is made to configure the GPS
/// unit to send just the messages that we are interested
/// in using these strings
//@{
static const prog_char _SiRF_init_string[]; ///< init string for SiRF units
static const prog_char _MTK_init_string[]; ///< init string for MediaTek units
static const prog_char _ublox_init_string[]; ///< init string for ublox units
//@}
/// @name GPS message identifier strings
//@{
static const prog_char _gprmc_string[];
static const prog_char _gpgga_string[];
static const prog_char _gpvtg_string[];
//@}
};
#endif