The main causes of noisy data are objects that reflect or intermittently obstruct the signals from one or more of the satellites in view. Such obstacles are usually trees or buildings. Do not place another GPS antenna close to the VBOX antenna, as GPS antennas radiate a small amount of noise at exactly the wrong frequency.
The measurement of the time taken for the satellite signal to reach the antenna is the important factor, so anything that may cause the signal to take a longer path (such as reflection) is undesirable. When the signal reaching the antenna is made up of the signals coming from the direct path plus a reflected path, it is known as Multipath, and it is detrimental to the accuracy of the measurements.
If an antenna is not mounted on a large enough ‘ground plane’ then the multipath reflections will also be from the ground beneath the antenna. If you are using the antenna on something without a large ground plane (such as a bike or carrying the unit by hand), then you can put a sheet of metal underneath the antenna (can be silver/copper foil), or use an antenna with strong multipath rejection properties (available from Racelogic). These kinds of antennas are much larger and more expensive than the standard antenna supplied with the VBOX.