GPS Input

SoarPilot can process GPS data using the PalmPilot’s serial, infrared (IR), bluetooth (BT) or USB 1) interface. The GPS must be capable of outputting data which is compatible with the NMEA-0183 version 1.5 or 2.0+ (auto-select) standards.

Minimum Required NMEA Sentences

The minimum sentences required by SP are:

  • GPRMB & GPRMC - For NMEA version 1.5. If using this version, the Garmin PGRMZ sentence is needed to get GPS altitude. If this is unavailable, pressure altitude from an attached source of pressure altitude can be used.
  • GPGGA & GPRMC - For NMEA version 2.0 and above. This version is used by all but the oldest GPS’ today. GPS altitude is taken from the GPGGA sentence.
  • In addition to the above, it is highly suggested that the GPGSA and GPGSV sentences be sent from the GPS as well if available. These sentences provide the information needed for the GPS Status Screen to function properly.

What is NMEA 0183?

The NMEA (National Marine Electronics Association) 0183 Standard for Interfacing Marine Electronics Devices is a voluntary industry standard, first released in March of 1983. The NMEA 0183 Standard defines electrical signal requirements, data transmission protocol, timing and specific sentence formats for a 4800 baud serial data bus.

Overview

NMEA has become a standard protocol for interfacing navigational devices, e.g. GPS and DGPS receivers. It is based on the RS232 interface. NMEA settings for the RS232 are:

Baudrate 4800
Data bits 8 2)
Stop bits 1 or 2
Parity none
Handshake none

NMEA informations are transmitted from a ‘talker’ device to a ‘listener’ device in ‘sentences’ with a maximum length of 80 characters. You may have read that the maximum length of a GPGSV sentence is 210 characters. Don’t panic, GPGSV information is broken down into a whole sequence of GPGSV sentences. Each individual sentence is no longer than 80 characters, and every GPGSV sentence contains the total number of GPGSV sentences to be transmitted in this sequence. Each NMEA sentence starts with ‘$’ and ends with [CR][LF]. Example:

$GPRMC,154232,A,2758.612,N,08210.515,W,085.4,084.4,230394,003.1,W*43[CR][LF]

The first 5 characters following the ‘$’ are called the address field. The rest of the line consists of the comma-delimited data fields. The first 2 characters of the address are the so called Talker-ID, in our example the sender identifies as a GPS device (GP = GPS device). The Talker-ID is followed by 3 characters describing the type of the sentence (type RMC = Minimum Navigation Information in our example). Data fields which are undefined at send time are left empty (two commas with nothing in between). An optional checksum can be added to the sentence. This checksum must be preceeded by a ‘*’. It is the XOR-combination (in hexadecimal notation) of all characters between ‘$’ and ‘*’. Most devices send only a small subset of NMEA sentences. Their sequence and frequency depends on device type and device status.

This NMEA FAQ by Peter Bennett can be consulted if you want to know more about NMEA 183 protocol. This FAQ also dissects a few sample sentences.

Related Pages

1) Special option for the USB port on a Tungsten E using a Kirrio cradle/cable GPS to USB interface
2) Bit 7 set to 0
 
soarpilot/gpsinput.txt · Last modified: 2008/11/13 02:26
 
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