The GPS Toolkit

The GPS position solution can be improved by using a better satellite ephemeris. The US National Geospatial-Intelligence Agency (NGA) generates and makes publicly available a number of precise ephemerides, which are more accurate satellite orbits. Satellite orbits described by the broadcast navigation message have an error on the order of meters, and the precise ephemeris has decimeter accuracy. The International GPS Service (IGS) is a global civil cooperative effort that also provides free precise ephemeris products. Global networks of tracking stations produce the observations that make generation of the precise ephemerides possible.

GPS observation data from many tracking stations are available freely on the Internet. Many such stations contribute their data to the IGS. In addition, many networks of stations also post their data to the Internet, such as the Australian Regional GPS Network (ARGN) and global cooperatives including NASA's Crust Dynamics Data Information System (CDIS).

Typically, GPS observations are recorded in a standardized format developed by and for researchers. Fundamental to this format is the idea that the data should be independent of the type of receiver that collected it. For this reason, the format is called receiver independent exchange, or RINEX. Another format associated with GPS is SP-3, which records the precise ephemeris. The GPSTk supports both RINEX and SP-3 formats.

GPS receivers have become less expensive and more capable over the years, in particular handheld and mobile GPS receivers. The receivers have many features in common. All of the receivers output a position solution every few seconds. All receivers store a list of positions, called waypoints. Many can display maps that can be uploaded. Many can communicate with a PC or handheld to store information or provide position estimates to plotting software.

Typically, communication with a PC and other system follows a standard provided by the National Marine Electronics Association, called NMEA-0183. NMEA-0183 defines an ASCII-based format for communication of position solutions, waypoints and a variety of receiver diagnostics. Here is an example of a line of NMEA data, or sentence:

$GPGLL,5133.81,N,00042.25,W*75

The data here is a latitude, longitude fix at 51° 33.81 min North, 0° 42.25 min West. The last part is a checksum.

As a public standard, the NMEA-0183 format has given the user of GPS freedom of choice. NMEA-0183 is the format most typically used by open-source applications that use receiver-generated positions.

Closed standards also are common. SiRF is a proprietary protocol licensed to receiver manufacturers. Many receiver manufacturers implement their own binary protocols. Although some of these protocols have been opened to the public, some have been reverse engineered. GPSBabel is an open-source project to communicate with consumer-grade receivers. The Sharc Project is a similar project to provide communication with survey-grade receivers.

A number of interesting open-source applications are available that utilize consumer-grade receivers. With one, you can use open-source applications to navigate in your car. The GPS Drive Project helps you do that, using a graphical map. GPS Drive also can be linked to the Festival application to get driving directions in the form of speech output. Internet sites such as WiGLE.net have lists of the geographic coordinates of open wireless LANs; you can use your GPS unit to find these.

Traditionally, DGPS is accomplished with two or more receivers that communicate position information with radio waves. You can do DGPS over IP now, using open-source applications. The open-source project called gpsd essentially broadcasts NMEA-0183 sentences over TCP/IP. The gps3d Project, which allows you to visualize your position and the configuration of GPS in 3-D, also can utilize a gps3d server.

All of these applications are based on standard positioning. To move your positioning capability to the next level, you have to work directly with the observations made by the receiver. Only a few open-source or freely available programs exist that give the user this freedom. OpenSourceGPS is a project to create a GPS receiver based on the Zarlink chipset. teqc from UNAVCO performs quality assurance and processes raw data from receivers to generate RINEX, but it is closed source. In contrast, the purpose of the GPSTk is to give the user the ability to manipulate not only GPS observations but also to improve the processing algorithms.