This page lists both onsite and offsite resources for programmers trying to understand GPS technology.

In the descriptions below, the adjective "evil" applied to the distribution terms of a standard or technical specification means that (a) it is not available for free download, and (b) it cannot legally be redistributed. The phrase "extremely evil" means that not only are the distribution terms evil, but that attack lawyers for the document publisher have been known to threaten people who quote it on the Web.

This is a selection of the files available at GPSD's Google Code document repository

GPS standards

NMEA 0183: This is the official standard from the National Maritime Electronics Association describing how GPSes are supposed to report to computers over a serial or USB link. The distribution terms are extremely evil. Consequently, nobody on the GPSD project has ever looked at it.
NMEA sentences: Because the NMEA 0183 standard itself is extremely evil, the GPSD project has collated comprehensive information on the prescriptions of that standard from various Web resources written by people who have read it and/or studied the behavior of GPSes claiming to conform to it. This is the result.
NMEA data: This is one of the sources for the above. Includes some information on vendor-specific quirks that we don't.
Navstar Global Positioning Interface Specification: This is the official specification for interpreting radio transmission from GPS satellites. You do not need to read this unless you are trying to make sense of the raw 50BPS subframe data. Be warned: though this specification is not evil, it is complex and nasty.
Standard Positioning Service Performance Specification (2008): This document defines the levels of performance the U.S. Government makes available to civil users through the GPS Standard Positioning Service (SPS). The 2001 version is here
World Geodetic System: Technical and historical summary of WGS84.
WAAS System Specification: This is the official specification for the FAA's Wide Area Augmentation System. The original document is here.
RTCM Recommended Standards for Differential GNSS: This is the ordering page for the official RTCM SC-104 standards, both the obsolescent 2.x and current 3.x versions. The distribution terms are evil. At least one of the GPSD project members has purchased official copies, so we do have full details of the message format. Copies of some of these have leaked onto the Web and may be found by searching for "RTCM 10402.3" or "RTCM 10403.1".
ITU-R M.823-2: This specification documents some of the murky depths of rtcm-104, the specification used for broadcasting differential-gps corrections. Unfortunately, its distribution terms are also evil.
Galileo Open Service SIS ICD: This is draft 0 of the Galileo Open Service Signal In Space Interface Control Document. The latest version of the official standard can be requested here.
Ntrip home page: NTRIP is a protocol for shipping DGPS corrections that uses HTTP as a transport layer. The distribution terms for the NTRIP standard are evil, but a stripped down version of the document is freely available on the home page. Version 1.0 of the standard is poorly written, confusing and leaves too much open to the interpretation of the implementor. With the help of examples in the official standard (missing in the online version) and the C and Perl reference implementations, it is however possible to develop a working Ntrip client.
AIVDM/AIVDO protocol decoding: This document describes AIVDM, a standard layered on top of NMEA 0183 and used for the marine Automatic Identification System (AIS). The core AIVDM standard is no longer evil (see below), but complete information on AIS and its addenda remains otherwise difficult to collect from scattered and rather impenetrable standards documents.
M.1371 : Technical characteristics for an automatic identification system using time-division multiple access in the VHF maritime mobile band
GPX Exchange Format: GPX (the GPS Exchange Format) is a light-weight XML data format for the interchange of GPS data (waypoints, routes, and tracks) between applications and Web services on the Internet. The GPSD suite includes a client, gpxlogger, that makes logs in this format.

GPS operation

NAVSTAR Global Positioning and Surveying: US Army Corps of Engineers, Engineer Manual 1110-1-1003 "provides technical specifications and procedural guidance for surveying and mapping with the NAVSTAR Global Positioning System (GPS). ... Procedural and quality control standards are defined to establish... uniformity in the use of GPS..." This document offers significant insight into error modelling; anyone who dares to ask "so how accurate is my GPS?" should be gently encouraged to study it.
Worldwide Beacon DGPS Status: This paper dates from 1999 and some of the information in it is therefore probably out of date. Nevertheless, it's the best public documentation on the history and architecture of the worldwide DGPS beacon system we've found.
Inside assisted GPS: helping GPS help you: Good article on techniques for cutting GPS startup latency and increasing precision, mostly focused on assistance via cellphone networks.
Worldwide list of NTRIP Casters: Master list of sites that provide RTCM correction info over the Internet.

Vendor protocols

Due to the inadequacies of NMEA, GPS chipset vendors have invented NMEA extensions or their own more tightly-designed reporting formats. This is a collection of descriptions of them. Many of these have already been used in the implementation of gpsd; others may be used in the future.

If you have access to a GPS vendor manual we don't already carry, please send it to us. If it's live on the web, mailing the URL will be sufficient. The names in this SHA1 checksums file should be a good clue which ones we already have.

Good manuals generate customer interest and loyalty. Judging by the legal notices in them, either (a) many GPS vendors are run by people who think that making their documentation hard to get and illegal to redistribute doesn't damage their sales prospects, or (b) many GPS vendors unthinkingly slap in a boilerplate notice that makes them look like they are run by the aforementioned sort of idiot. We are blithely ignoring all this nonsense here. If you are a GPS vendor representative, be advised that if you so request, we will (a) remove your manuals, and then (b) laugh at and publicly mock you for having been so stupid as to demand this.

SiRF and SiRF variants

SiRF Binary Protocol Reference Manual (2.4, November 2008): The binary protocol for the chip used by over 80% of consumer-grade GPS mice. For comparsion we also have older versions: 2.2 (December 2007), 1.7 (October 2005) and 1.6 (April 2005) .
SiRF NMEA Protocol Reference Manual (2.2, November 2008): SiRF NMEA Protocol Reference Manual (2.1, December 2007)
SiRF GPS Protocol Reference Manual (revision 1.30, undated): This what SiRF gives OEMs as a reference for SiRF's NMEA 0183 extensions. Notably, it describes the commands for switching to SiRF binary mode.
TIM GPS Receiver Macro-Component (offsite link) (version E1, April 2003): Describes the SiRFstarII chipset used by uBlox and some others. gpsd supports it. While obsolete, this file documents certain aspects of the protocol that SiRF has removed from current versions of the protocol documentation.
u-blox extension to SiRF binary protocol (March 2002): u-blox was an OEM-enhanced firmware variant of the SiRF-1 chip. gpsd supports it. It's been rendered obsolete by the SiRF 2 and 3 chips.

Fastrax

NMEA Protocol Specification for iTrax02 Evaluation Kit (Rev 1.11, June 2003): Describes the NMEA extensions for the iTrax chipset used by Fastrax GPSes. Supported by gpsd.
iTALK protocol specification (Rev 1.5, March 2002): Describes the binary protocol for the iTrax chipset used by Fastrax GPSes. This documents version 2; version 3, which is not actually backward compatible, is documented at the iSuite developer website, iTrax2 is not supported by gpsd, iTrax3 is somewhat functional.

Garmin

GPS 16/17 Series Technical Specification (offsite link) (Revision C, October 2005): Describes the extended NMEA, with GARMIN proprietary sentences PGRMB, PGRME, PGRMF, PGRMM, PGRMT, and PGRMV (and optional GPALM sentence), emitted by the Garmin GPS16 and 17. For programming purposes, effectively identical to their earlier reference on the Garmin 15H and 15L.
Garmin Simple Text Protocol: Some digital cameras are alleged to use this very simple textual report format.

Novatel

Novatel Superstar II User Manual: Describes the OEM board, include some good general overview material on GPS technology. Doesn't describe control or reporting commands.
FV25 user manual (January 2005): Describes UBX binary protocol and NMEA extensions supported by the San Jose Navigation FV-25 (uBlox ANTARIS) GPS chipset. UBX is now supported by gpsd.

GPS standards

GPS operation

Vendor protocols

SiRF and SiRF variants

Fastrax

Garmin

Novatel

San Jose Navigation

Other Current GPSes

Obsolete products

Other