How you use the unit, atmospheric conditions, where satellites are positioned relative to each other, and your surroundings can all affect the accuracy of your GPS at any given time. Understanding how these “conditions” affect the ability of a GPS receiver to calculate its location helps explain why there can be differences from day to day.
How you use the GPS receiver: Simple things like the position in which you hold a receiver can affect how accurately it calculates a location. If the receiver has an internal antenna, how you hold it can change the antenna’s orientation and impact how well it receives GPS signals and calculates a position. Since different manufacturers use different equipment, be sure to read the owner’s manual to learn how best to orient the receiver.
Atmospheric conditions: Electrical charges in the ionosphere can delay GPS signals and reduce GPS receiver accuracy. The Federal Aviation Administration’s Wide Area Augmentation System (WAAS) uses a network of ground base stations throughout North America to calculate ionospheric signal delays and broadcast corrections to WAAS satellites that relay the corrections to WAAS-enabled GPS receivers. If your receiver is not WAAS enabled or has WAAS turned off, then ionospheric conditions can affect GPS accuracy. Water vapor in the troposphere also delays GPS signals and can reduce calculated locations. Such conditions tend to be localized and can change relatively quickly; they are also difficult to account for.
Satellite geometry: Because GPS satellites orbit the Earth twice daily, their positions relative to each other and your GPS receiver are constantly changing. Ideally, you want to have 12 satellites visible (most GPS receivers can track 12 satellites), and you want them to be evenly distributed. However, landscape features, buildings, and other things can block satellite signals. Also, satellites may be close together, which will result in lower GPS accuracies. Loss of accuracy due to satellite positions is referred to as Geometric Dilution of Precision (DOP). Free software is available from Trimble that lets you calculate DOP effects at an time and location.
Your surroundings: Finally, take a look at your surroundings. Reflective surfaces such as tall buildings, canyons, rock faces, etc. can reflect GPS signals causing what is called multipath error. Multipath error occurs when the same GPS satellite signal arrives at your receiver at two different times (directly from the satellite and slightly later when it bounces off a reflective surface). More expensive receivers have the ability to detect and reject some multipath signals.
The bottom line is that there are a number of conditions that affect GPS accuracy, and these conditions vary over time.
