Every GPS device that you buy today already has a GPS antenna in it. These antenna help your GPS receiver pick up the signals being sent out by the Global Positioning Satellites, giving your navigation or GPS tracking device more accurate information to make their position calculation with. In some circumstances, GPS devices can benefit from having an external GPS antenna in addition to the one already included in the device. As far as I am aware, there are three main type of external antenna to choose from. They are active, passive, and reradiating antenna.
Active GPS antenna
An antenna is consider active when it uses something that is called a Low Noise Amplifier (LNA). LNA is useful because it can help an antenna capture very weak signals by giving them a “boost” in sending them along to your GPS receiver. There are a couple different ways to visualize how this works, but I think that this one might work for a lot of people:
Imagine for a second that the signals sent from GPS satellites are messages in a bottle that get dropped into a very large swimming pool. In the pool is a man sitting in the shallow end who is there to collect these messages and read them aloud. This man represents your GPS device (navigation or tracking). Strong GPS signals are represented by bottles that get dropped within arms reach of the man – they are easily read by him. Weak signals are dropped off into the deep end and almost never make their way to the man in the shallow end of the pool. They are lost.
Now we need to add the active GPS antenna to our analogy. For this, lets imagine a wave machine represents what an active, LNA antenna does to those weak signals that get dropped in the deep end. Through the sheer force of wave power, the LNA antenna pushes the weaker signals along toward the GPS so that it can gather up the information contained within the bottles.
While this is just an analogy, it might really help some people understand just how an active GPS antenna works. It will also help illustrate the fact that this type of antenna also requires some power to work properly. You need some power to move the signal along the antenna just like you need some power (in the form of waves) to move the bottles from the deep end to the shallow end.
As you can imagine, having to use power to work properly can have some downsides if battery life is a concern for you. There is no use in having a stronger signal if your device has been drained of all its power by your antenna. However, if your device is plugged into your car or some other power supply that is essentially limitless, having this type of antenna can really help improve your signal strength and the accuracy of your GPS device.
Passive GPS antenna
This type of antenna, on the other hand, does not carry an active charge and does not require any power to improve your GPS’ performance. The improvement that can be provided by passive GPS antenna is significantly less than that provided by an LNA GPS antenna. The main reason for this is that the length of the antenna is shortened by the absence of power since there is no “wave” to push the signals gathered at the end of the antenna toward the device.
To continue the use of our analogy, you still have all the pool stuff from above but instead of having a wave machine the deep end (the active GPS antenna) you have the man in the shallow end using a pool cleaning net to pick up bottles (GPS signals) that are just outside of his reach. So he can gather up more signals than before, but he just can’t get all the ones that he might with an LNA antenna.
Reradiating GPS Antenna
These differ from both the active and passive versions in that they make use of two different antenna in order to give the GPS receiver and extra boost. Also, a reradiating GPS antenna is good for those devices that do not have an input to plug in an external antenna. The way that these antenna work is that they collect the GPS signals at one point and then transmit the information to any GPS device within a certain radius of the it. It is this process of rebroadcasting the signals that the antenna receives that puts the ‘re’ in reradiate.
If we stick with our pool analogy we could see a reradiating GPS antenna as a little helper who is treading water in the deep end. As the message bottles land around them they place them inside a floating basket, which they push toward the man in the shallow end.
Like the active antenna, the reradiating antenna is going to need some type of power supply in order to rebroadcast the signals that it receives. They are also pretty hard to find, but there are a few different guides on how to build your own if you have some time and electric know how. Here is one that is for a portable GPS from Garmin called the eTrex: http://www.gartrip.de/antenna.htm
GPS Antenna and GPS Tracking
There is obvious applications for the use of GPS antenna in GPS trackers of all varieties. Fleet tracking could make use of these antenna to improve GPS signal strength in lots of different circumstances. If your fleet is often traveling off road in thick forests or among the urban canyons created by sky scrappers then you could use an antenna for your fleet to ensure a strong, consistent signal. You could probably employ any one of the three varieties mentioned above (active, passive, reradiating) and be perfectly fine. I would generally recommend getting an active or reradiating antenna and hard wire them to the vehicle so that they would have ample power supply and get a good boost to GPS signal
You could also use these in GPS car tracking as well. Again, all three types of external GPS antennas could be used. If you are not comfortable hard wiring the antenna to a power supply yourself, I would probably go with a passive antenna so you don’t have to worry about it. And depending what you are using this GPS car tracking for (monitor a teen driver, improve your driving habits, or as a safety feature on your vehicle) this type of antenna should do just fine.
One final note, the GPS receivers of today generally don’t need much modification to get a great position fix. More advanced GPS chips combined with the advent of A-GPS has made the use of GPS antenna relatively unnecessary. But if you still suffer from weak signal in some of the places you drive or you require very accurate GPS locations then you still might want to consider purchasing a GPS antenna for your GPS tracker or navigation device.