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).
