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GPS FAQ
  - by Tong Zhang, Senior Editor

1. What is GPS?
GPS (Global Positioning System) is a navigation technology that provides precise time and location anywhere, anytime and under any atmospheric conditions, by using the NAVSTAR satellites.

2. How does GPS work?
GPS is a satellite-based navigation system that works by receiving navigation messages fro satellites and calculating locations. GPS receivers locate the satellites transmitting the incoming signals and use CDMA (Code Division Multi Access) method to identify individual codes. This means GPS system is able to identify each satellite's unique ID to calculate precise location and navigational data. Here are the steps:
All satellites have clocks set to exactly the same time
All satellites know their exact position from data sent to them from the systems controllers
Each satellite transmits its position and a time signal
The signals travel to the receiver delayed by distance traveled
The differences in distance traveled mark each satellite appear to have a different time
The receiver calculates its own position.

3. What affects GPS accuracy?
Many factors affect how accurate your GPS is. The atmosphere, the ionosphere and the position of your receiver could all affect GPS accuracy. Any buildings, natural structures or heavy foliage that obstruct the GPS' view of the sky may decrease the position accuracy. Your GPS accuracy will also depend on your level of clearance with the US DOD. There are two available radio signals that receivers can use: the Standard Positioning Service (SPS) for civilians and the Precise Positioning Service (PPS) for military and authorized personnel. The DOD occasionally jams the GPS signals for civilians on a short-term basis.

4. What is NAVSTAR?
NAVSTAR is an acronym for Navigation Satellite Timing and Ranging, a name given to the GPS satellite system by the US Government.

5. What is Acquisition Time?
The time it takes for a GPS receiver to acquire satellite signals and determine the initial position.

6. What is an Atomic Clock?
It's a very precise clock carried by each of the GPS satellites. These clocks are accurate to within 1 second in every million years.

7. What is NMEA 0183?
NMEA stands for National Marine Electronics Association, a US standards committee that defines data message structure, contents and protocols to allow the GPS receiver to communicate with other pieces of electronic equipment. NMEA 0183 is a standard data communication protocol used by GPS receivers.

8. How many GPS Satellites are there?
There are nearly 30 navigational satellites orbit the Earth and more might be added. Each satellite makes one Earth orbits every 12 hours. The satellite orbits repeat almost the same ground track (as the earth turns beneath them) once each day. The orbit altitude is such that the satellites repeat the same track and configuration over any point approximately each 24 hours (4 minutes earlier each day). There are six orbital planes (with nominally four Space Vehicles in each), equally spaced (60 degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane. This constellation provides the user with between five and eight Space Vehicles visible from any point on the earth.

9. What is WAAS? Why is a GPS receiver that utilizes WAAS better than the others?
With more and more GPS receivers coming out with WAAS, it might help to understand what WAAS is and how it works better for your GPS. WAAS stands for Wide Area Augmentation System. The FAA states that WAAS is based on a network of approximately 25 ground reference stations that covers a very large service area. Each of these precisely surveyed reference stations receive GPS signals and determine if any errors exist. Each wide area ground reference station in the network relays the data to the wide area master station (WMS) where correction information is computed. The WMS calculates correction algorithms and assesses the integrity of the system. A correction message is prepared and uplinked to a geosynchronous satellite via a ground uplink system (GUS). The message is then broadcast from the satellite on the same frequency as GPS (L1, 1575.42MHz) to GPS receivers, which are within the broadcast coverage area of the WAAS.

In a simpler term, WAAS-capable GPS receivers give you better position accuracy by correcting signal errors caused by ionospheric disturbances, satellite orbit errors and other spatial or technical data degradation factors. These GPS receivers will use some of their channels to track WAAS satellites and use WAAS satellite in their position solution. Here is a data accuracy chart published by Garmin:

100 meters: Accuracy of the original GPS system, which was subject to accuracy degradation under the government-imposed Selective Availability (SA) program.
15 meters: Typical GPS position accuracy without SA.
3-5 meters: Typical differential GPS (DGPS) position accuracy.
< 3 meters: Typical WAAS position accuracy.

At the present time, there are two geo-stationary satellites serving the WAAS area, which is only in North American. One covers the Pacific Ocean Region (POR), the other covers the Atlantic Ocean Region-West (AOR-W). (If you wish to see a map of WAAS coverage, follow this link.) Europe and Asia will eventually be served by EGNOS and MSAS. Hopefully soon, everyone on earth will have access to precise position data using SBAS (Satellite Based Augmentation System).

 

 

 

 

 

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