The greater the number of satellites you GPS can “see”, the more accurate it will determine your position. The number of satellites that your GPS is able to acquire depends on the time of day and your location. Thunderstorms, tall buildings, and dense overhangs can block the signals from the satellites. Also keep in mind, that due to military restrictions , GPS receivers for recreational or commercial purposes are only accurate to within approximately 2-10 meters.
A GPS receiver simply measures the travel time of the signals transmitted from the satellites, then multiplies them by the speed of light to determine exactly how far the unit is from every satellite it’s sampling. By locking onto the signals from a minimum of three different satellites, the GPS receiver can calculate a 2D (two-dimensional) positional fix, consisting of your latitude and longitude. Looking onto a fourth satellite allows the GPS receiver to calculate a 3D (three-dimensional) fix, calculating your altitude as well as your latitude/longitude position.
NMEA is a specification published by the National Marine Electronics Association that covers marine related devices
including GPS receivers. The NMEA-0183 standard has been universally adopted by GPS manufacturers and virtually
every GPS product on the market supports the protocol.
Datum is a mathematical model describing the shape of an ellipsoid. Since the earth isn’t uniform, there are more
than 100 datums for different area of the earth. Most of the GPS Receiver (like Holux GPS) use WGS 84 as default
datum. This is very common setting to the GPS receiver due to it considered a worldwide standard.
To make GPS even more accurate, the U.S. Coast Guard created a radio beacon system based upon differential GPS
techniques. This system is a series of more than 60 ground-based stations that transmit a correction or differential
signal on marine radio beacon frequencies. This signal can make a GPS receiver accurate from 1-3 meters by
applying additional ‘corrections’ to the existing GPS signal.
The GPS receiver will receive the signal and calculate your position with the standard method. At the same time, the
ground station receives the same signal at a known spot. The difference between what is received and the actual
known spot is calculated and broadcast as the DGPS signal. The properly equipped GPS receiver will make the
necessary calculations to correct itself. All this takes place in seconds.
Currently the DGPS signal is available on the Coasts of the U.S., Canada and Australia and around the Great Lakes.
These reference stations do broadcast to a large area that extends far inland.
Standard GPS methods produce a position accurate to about 10 meters (32.8 feet). The US Federal Aviation
Administration desired to increase this accuracy and allow aviators to use GPS for more precise guidance. As a result,
they developed a system similar to DGPS (Differential Global Positioning System) but on a much larger scale. This is
the WAAS (Wide Area Augmentation System). Although still in its early roll-out stage, this system can improve a GPS
receivers accuracy to about 3 Meters (9.8 feet) or less.
The WAAS system uses base stations at known reference points to calculate the accuracy of the GPS signal. This is
accomplished at each of the 25 ground reference stations (Currently only in the US) receiving a standard GPS signal. A
set of correction data deetermined from the difference between the GPS calculated position and the known position is
transferred to one of two ground control stations that then uplink the data to the WAAS satellite. The WAAS (InMarSat)
satellites then transmit this information back down to the GPS user using a GPS-like signal complete with the
correction information. The GPS receiver then decodes this information and applies it to its calculated position to
significantly improve the accuracy.
Currently the system is only accurate in North America and primarily in the United States. However, the signal can be
received over half of the world on the Inmarsat AOR-W and POR satellites. This means that in parts of the world not
covered by the base station corrections, you will get a WAAS signal, but the corrections will place you well off your mark
in Australia, South America and Europe, for example. This is why it is best to turn off the WAAS reception outside North
If you live in Europe the implementation of WAAS is known as EGNOS (European Geostationary Navigation Overlay
Service ). EGNOS is currently in test mode with live operation planned from April 2004. EGNOS could potentially refine
your position to within 1 to 2 meters.
Number of Channels
Number of channels is how many satellites it can monitor simultaneously.
Assisted GPS. It mainly uses a cellular phones network to enhance the start up of a satellite base GPS.