GPS receiver module is a device that can determine your latitude, longitude and altitude. The GPS receiver listens for radio signals from an array of satellites in orbit and receiver can then determine how long it took to receive the initial signals and by analyzing the received signal strength, it can compute (triangulate) how far it is from the relative satellites in orbit. This is possible as the satellites are in geostationary orbit i.e. their locations are always the same in relation to the surface of the earth. Most GPS Modules have a serial interface which makes it very easy to interface with microcontrollers by simply connecting them to the UART of microcontroller. Most receivers will start sending data as soon as power is applied. The data is typically in ‘NMEA’ format. This can vary depending on the GPS receiver. Documentation on the NMEA protocol can be found at: http://www.madeeasykits.com/docs/SiRF_NMEA_Protocol.pdf
When considering embedding GPS application in your projects or products whether as GPS tracking device, as navigating device, in robotics, speed monitoring or control device, in precision timing operation and so on. A good understanding of some of the vital features of GPS receiver is important as this will avert west of time, money and energy. The followings are criteria to think about when shopping for GPS module;
Accuracy
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The accuracy is a function of receiver module, time of the day, clarity of reception, etc. The good news is you can usually find out where you are, anywhere in the world, within 30 seconds, down to +/- 10m. Most modules can get it down to +/-3m, but if you need sub meter or centimeter accuracy, it gets really expensive.
Update Rate
This is about the rate at which GPS receiver updates location data. Considering a car navigation system, 1Hz (or once per second) update rate should be sufficient. A typical car simply doesn’t move fast enough that we need to know where you are on the globe more than once per second. There are some applications, such as planes and Unmanned Aerial Vehicles (UAVs), where you need greater update rates. 5Hz and even 10Hz are becoming available and cheap. You can always configure a GPS receiver to slow down, and update less often (1Hz) if your microcontroller or application can’t handle all the NMEA data.
Number of Channels
This is ability to track multiple satellites at the same time and it’s a great marketing strategy by the manufacturers. You’ll see GPS modules that have 50 channels of tracking while we all know that there are 24 GPS satellites, and it’s impossible to stand anywhere and view more than 12 at a time. All modules designed since 2008 have more than enough channel tracking. Don’t bother about the number of channels.
Voltage levels
It is also important to consider the supply voltage to the module while you are making your choice. Interface voltage can be of problem at times because you can have 5V power supply module with 3.3V for the interface communication. In such a case, you have to design a simple signal level shifter.
Antennas
Each antenna is finely trimmed to pick up the GPS L1 frequency of 1.57542 GHz. Many modules come with a precisely made chunk of ceramic on them which are antennas. There are some other GPS antenna technologies (chip and helical) which are not as common, a bit more expensive.
Since the satellites are in the sky, 12,552 miles above us, so be sure to point the ceramic towards the sky and you can certainly get GPS signal indoors, but it’s hit-or-miss.
Size
These modules are getting smaller every day but figure out what your application is. As a general rule, the smaller you make the module, the more likely you are to have antenna problems (longer lock time, less accuracy, etc.). Also consider the interface circuit connection as some interfaces are surface mount contact while others are wired.
Power
GPS units are taking in large amounts of timing data from the satellites and crunching it down. The current average power consumption is around 30mA at 3.3V. 30mA may not sound like a lot, but it’s a lot. Note that antennas use power because they often use an amplifier which can account for 20-30mA of current. If a module has really low power specs, it’s probably because the module doesn’t have an antenna built in.
I have worked with EM406 GPS receiver module and it worked fine. It’s very compact (includes antenna), status LED, 5V Power, and it has 1Hz Update rate. But the interface communication signal level is 3.3 volt which I constructed a signal level shifter. Its picture is as shown above.