Do I need an external antenna for my GPS unit while operating it in my vehicle?
Whether or not you need an external antenna, depends on where you have located the
GPSr in the vehicle. Many owners find that the existing unit's GPS antenna is adequate, however, we do get a lot of
standard vehicle & 4wd users who have some of vehicle roof obscuring the GPS
from the sky. You need clear line of sight to track the signal - any metal between you and the satellite will block
GPS signals. In these cases we recommend using an external antenna.
What does it do for me?
The external antenna, when mounted in a location that will offer as much of a clear, unrestricted, 360 degree view of the sky will improve the performance of your GPS receiver and the position output it provides.
Inside the vehicle, the antenna can only 'see' the satellites that are visible through the front and side windows, and/or any sunroof you may have. If the receiver does not have a direct line-of-sight to each of the visible satellites, then it will either not be able to track it, or will track a reflection of the satellite signal resulting in multipath error and potentially increased error in your position output.
Usually there are between 8 and 12 satellites visible at any time from your location. The GPS receiver uses up to 12 visible satellites and their transmitted data to calculate your position. These satellites themselves are constantly moving, orbiting the earth once every 12 hours.
Most GPS receivers have separate electronics, called 'channels', to individually track all 12 satellites. Each 'channel' is set to track a specific satellite as it 'rises', then follows it through its path in the sky, and then, as it 'sets' (ie disappears under the horizon), is re-assigned another satellite to track, if one is visible.
GPS satellites transmit a signal that is very weak - in fact, the actual transmitted power of each satellite provides a signal at ground level that is just barely above the ambient 'signal noise' level on earth. The tracking ability of each channel is determined by its ability to 'sniff' out the specific satellite's signal from all the other electronic 'noise' surrounding it. The more visible the satellite, the stronger the signal, and hence, the quicker the channel will re-acquire and lock onto the satellite.
When a channel loses lock on the satellite it is tracking, the channel goes into a 're-acquisition' mode and begins a time-based search of the sky for the signature of the satellite. If it cannot find the satellite within a few seconds, it 'opens up its bandwidth' to search over an even wider bandwidth of signal, and keeps doing so until it re-acquires the satellite, or is re-assigned another satellite to track.
The GPS receiver itself calculates its position from the data from all the visible satellites. This calculation is often referred to as a 'filter', with data from all the satellites constantly being updated in it. As satellites are lost and then re-acquired and then lost and then re-acquired, etc. (typical performance as a vehicle travels along a route which is rarely ever
straight for a period of time, especially in urban areas), the filter jumps around and produces position calculations with varying degrees of accuracy. The more satellites that are lost and re-acquired, the more the position will jump around. Ideally, the receiver would like to be able to constantly see as many satellites as possible with as little re-acquisition work as possible - this is where the external antenna provides its benefits.
A further benefit to users is the improvement in geometry provided by the external antenna which can see much more of the sky. By way of explanation, consider the following: You are standing still, holding the GPS receiver and noting the position output. Now the precision of the output you see is greatly enhanced when the satellites you are tracking are evenly spaced about you, that is, evenly spaced in a circle around you in the north, south, east, and west, and ideally at an elevation of 45 degrees from the horizon. On the flip side, precision is reduced when, say, all the satellites you are tracking are concentrated or mostly located in the northwest, or south. which is a typical situation when your GPS receiver is inside your vehicle and sees outside the front and side windows, effectively giving it just a wedge shaped portion of the sky to see. Again, an external antenna will remove most of this effect.