Global positioning system (GPS) instruments and GPS modules are radio-navigational devices that provide accurate position information about objects on Earth, based on information from orbiting satellites. Equipped with rubidium atomic clocks, 24 GPS satellites in high-orbital planes use coarse acquisition (C/A) codes to communicate with GPS ground stations. These Earth-based control segments monitor almanac and ephemeris signals from GPS satellites, and communicate the small corrections that become necessary when there are changes to either a satellite’s orbit or to Earth’s ionosphere. By using the information in the almanac and ephemeris signals, GPS receivers can determine the time required to transmit a signal. Because that time is proportional to the distance traveled, GPS modules and devices can determine the arc on which the GPS receiver lies and, therefore, the intersection point and position. Selecting GPS instruments and GPS modules requires an analysis of parameters for product type, performance, power, and connection and communication. Product types include GPS receivers, GPS transmitters, GPS antennas, and GPS data loggers. Performance specifications for GPS receiver modules include the number of channels, frequency range, sensitivity, position accuracy, and time to first fix. There are two main sensitivity parameters: tracking and acquisition. There are four variables for time to first fix: obscuration recovery, hot start, warm start, and cold. In terms of power, GPS modules and devices carry product specifications such as input voltage, current draw, antenna supply, and antenna current monitor. SMA connectors are used with some GPS antennas. Double rows of low-profile socket pins are used for data-power. In terms of communication, some GPS modules and devices come with integral GPS software and serial ports. GPS instruments and GPS modules differ in terms of features. For example, GPS antennas can be active or passive, have an antenna patch onboard, or come equipped with antenna status detection. Some GPS modules plug into motherboards, are designed for indoor use, and interpret GPS data. Others have low power consumption, a small footprint, and are designed to withstand varying levels of shock and vibration. Features such as data logging and dead reckoning are also available. With dead reckoning, GPS instruments and GPS modules use sensor signals to calculate the position of GPS antennas and GPS receivers when satellite signals are blocked. For greater positional accuracy, GPS products may support standards such as wide area augmentation service (WAAS) and the municipal state aid system (MSA).