Performance

            There are several performance characteristics of a GPS system.  Starting with power up, the first characteristic is how quickly the module can obtain a lock.  A lock is made when the module can receive the GPS signal from enough satellites to calculate position.  The amount of time required is referred to as the start time.  Other performance concerns are the positional accuracy of the module, and how well it gets and holds a lock in the presence of obstacles (urban settings, canyon areas, etc). 

            These performance characteristics depend on several technological differences.  The number of satellite channels the module can simultaneously receive is an important spec.  GPS modules today can receive up to 20 channels.  More channels mean more places for the module to look for a signal, corresponding to faster locking times and better locking in urban settings.  Onboard processing of the GPS signal is also an important factor.  Newer chipsets, such as the SiRF III provide faster and more robust locking than previous SiRF II based designs.  New features such as the wide area augmentation system (WAAS), or differential GPS (DGPS) provide better positional accuracy by correcting GPS signals using other sources (such as ground stations). 

Interface

            The great thing about all the GPS modules we sell is their ease of use.  All of them are NMEA 0831 compatible.  They can output standard NMEA messages every second reporting speed, 3D position, number of satellites in view, GPS time, and more.  NMEA is transmitted in ASCII format at 4800 Baud, 8N1.  On many of the modules NMEA output is the default, so all the user has to do is apply power and the module starts spitting out NMEA data.

There is often access to lower level information, using a variety of protocols.  This differs from module to module, so check the data sheet to see what commands and protocols are available. 

Batteries and Antennas

            A GPS system requires a battery and antenna.  A GPS antenna optimized for GPS signal reception must be used with the receiver.  Some modules have a built in antenna, which can be convenient.  Others have a connector for an external antenna.  We sell a variety of antennas with compatible connectors. 

            A battery is also needed to store last received GPS information when the unit is off.  This information, called an almanac, helps the module get a lock quickly.  A hot start is when the module starts up with help from the almanac, where in a cold start this information is not present.  Some of the modules contain a battery, while others require an external battery.   Usually it is a 3 volt lithium coin cell. 

 Power and Other Considerations

           Some of the GPS modules run on 3.3 volts while others require 5.  The current consumption also varies a bit, usually from around 30mA to 80mA depending on the module.  For instance, the Lassen iQ 12 channel module is one of the lowest power modules (as low as 33 mA), where the EM-406 20 channel device is more power hungry (more like 80mA).  

            Another thing to look at is our assortment of evaluation boards.  If you are just getting started they provide an easy way to get up and running in minutes.  Most of them have all the nessary connections and contain either USB or RS-232 connection to the PC.  

Overview of GPS modules