What is GPS?
The Global Positioning System
(GPS) is a satellite-based navigation
system made up of a network of 24
satellites placed into orbit by the
U.S. Department of Defense. GPS
was originally intended for military
applications, but in the 1980s, the government made the system
available for civilian use. GPS works in any weather conditions,
anywhere in the world, 24 hours a day. There are no subscription fees or
setup charges to use GPS.
How it works
GPS satellites circle the earth twice a day in a very precise orbit and
transmit signal information to earth. GPS receivers take this information
and use triangulation to calculate the user's exact location. Essentially,
the GPS receiver compares the time a signal was transmitted by a
satellite with the time it was received. The time difference tells the GPS
receiver how far away the satellite is. Now, with distance measurements
from a few more satellites, the receiver can determine the user's position
and display it on the unit's electronic map.
A GPS receiver must be locked on to the signal of at least three satellites
to calculate a 2D position (latitude and longitude) and track movement.
With four or more satellites in view, the receiver can determine the
user's 3D position (latitude, longitude and altitude). Once the user's
position has been determined, the GPS unit can calculate other
information, such as speed, bearing, track, trip distance, distance to
destination, sunrise and sunset time and more.
How accurate is GPS?
Today's GPS receivers are extremely accurate, thanks to their parallel
multi-channel design. Garmin's 12 parallel channel receivers are quick to
lock onto satellites when first turned on and they maintain strong locks,
even in dense foliage or urban settings with tall buildings. Certain
atmospheric factors and other sources of error can affect the accuracy of
GPS receivers. Garmin GPS receivers are accurate to within 15 meters
on average.
Newer Garmin GPS receivers with WAAS (Wide Area Augmentation
System) capability can improve accuracy to less than three meters on
average. No additional equipment or fees are required to take advantage
of WAAS. Users can also get better accuracy with Differential GPS
(DGPS), which corrects GPS signals to within an average of three to five
meters. The U.S. Coast Guard operates the most common DGPS
correction service. This system consists of a network of towers that
receive GPS signals and transmit a corrected signal by beacon
transmitters. In order to get the corrected signal, users must have a
differential beacon receiver and beacon antenna in addition to their GPS.
The GPS satellite system
The 24 satellites that make up the GPS
space segment are orbiting the earth
about 12,000 miles above us. They are
constantly moving, making two complete
orbits in less than 24 hours. These
satellites are travelling at speeds of
roughly 7,000 miles an hour.
GPS satellites are powered by solar energy. They have backup batteries
onboard to keep them running in the event of a solar eclipse, when
there's no solar power. Small rocket boosters on each satellite keep them
flying in the correct path.
Here are some other interesting facts about the GPS satellites (also
called NAVSTAR, the official U.S. Department of Defense name for
GPS):
The first GPS satellite was launched in 1978.
A full constellation of 24 satellites was achieved in 1994.
Each satellite is built to last about 10 years. Replacements are
constantly being built and launched into orbit.
A GPS satellite weighs approximately 2,000 pounds and is
about 17 feet across with the solar panels extended.
Transmitter power is only 50 watts or less.
What's the signal?
GPS satellites transmit two low power radio signals, designated L1 and
L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF
band. The signals travel by line of sight, meaning they will pass through
clouds, glass and plastic but will not go through most solid objects such
as buildings and mountains.
A GPS signal contains three different bits of information - a
pseudorandom code, ephemeris data and almanac data. The
pseudorandom code is simply an I.D. code that identifies which satellite
is transmitting information. You can view this number on your Garmin
GPS unit's satellite page, as it identifies which satellites it's receiving.
Ephemeris data, which is constantly transmitted by each satellite,
contains important information about the status of the satellite (healthy
or unhealthy), current date and time. This part of the signal is essential
for determining a position.
The almanac data tells the GPS receiver where each GPS satellite should
be at any time throughout the day. Each satellite transmits almanac data
showing the orbital information for that satellite and for every other
satellite in the system.
Sources of GPS signal errors
Factors that can degrade the GPS signal and thus affect accuracy include
the following:
Ionosphere and troposphere delays - The satellite signal slows
as it passes through the atmosphere. The GPS system uses a
built-in model that calculates an average amount of delay to
partially correct for this type of error.
Signal multipath - This occurs when the GPS signal is reflected
off objects such as tall buildings or large rock surfaces before it
reaches the receiver. This increases the travel time of the signal,
thereby causing errors.
Receiver clock errors - A receiver's built-in clock is not as
accurate as the atomic clocks onboard the GPS satellites.
Therefore, it may have very slight timing errors.
Orbital errors - Also known as ephemeris errors, these are
inaccuracies of the satellite's reported location.
Number of satellites visible - The more satellites a GPS receiver
can "see," the better the accuracy. Buildings, terrain, electronic
interference, or sometimes even dense foliage can block signal
reception, causing position errors or possibly no position
reading at all. GPS units typically will not work indoors,
underwater or underground.
Satellite geometry/shading - This refers to the relative position
of the satellites at any given time. Ideal satellite geometry exists
when the satellites are located at wide angles relative to each
other. Poor geometry results when the satellites are located in a
line or in a tight grouping.
Intentional degradation of the satellite signal - Selective
Availability (SA) is an intentional degradation of the signal
once imposed by the U.S. Department of Defense. SA was
intended to prevent military adversaries from using the highly
accurate GPS signals. The government turned off SA in May
2000, which significantly improved the accuracy of civilian
GPS receivers
Direct - Broadcast Satellite:
Direct broadcast satellite (DBS) is a term
used to refer to satellite television broadcasts intended for home
reception.
A designation broader than DBS would be direct-to-home signals, or
DTH. This has initially distinguished the transmissions directly intended
for home viewers from cable television distribution services that
sometimes carried on the same satellite. The term DTH predates DBS
and is often used in reference to services carried by lower power
satellites which required larger dishes (1.7m diameter or greater) for
reception.
In Europe, prior to the launch of Astra 1A in 1988, the term DBS was
commonly used to describe the nationally-commissioned satellites
planned and launched to provide TV broadcasts to the home within
several European countries (e.g. BSB in the UK, TV-Sat in Germany).
These services were to use the D-Mac and D2-Macformat and BSS
frequencies with circular polarization from orbital positions allocated to
each country. Before these DBS satellites, home satellite television in
Europe was limited to a few channels, really intended for cable
distribution, and requiring dishes typically of 1.2m. SES launched the
Astra 1A satellite to provide services to homes across Europe receivable
on dishes of just 60 cm-80 cm and, although these mostly used PAL
video format and FSS frequencies with linear polarization, the DBS
name slowly came to applied to all Astra satellites and services too.
MSAT :
MSAT, short for Mobile Satellite, is a satellite-based mobile
telephony service developed by the National Research Council of
Canada. Supported by a number of companies in the US and Canada,
MSAT hosts a number of services, including the broadcast of CDGPS
signals.
The MSAT satellites were built by Hughes (now owned by Boeing) with
a 3 kilowatt solar array power capacity and sufficient fuel for a design
life of twelve years.
TMI of Canada referred to its MSAT satellite as MSAT-1, while
American Mobile Satellite Consortium (now SkyTerra) referred to its
MSAT as AMSC-1, with each satellite providing backup for the other.
VSAT :
A very small aperture terminal (VSAT), is a two-way
satellite ground station or a stabilized maritime Vsat antenna with a dish
antenna that is smaller than 3 meters.
The majority of VSAT antennas range from 75 cm to 1.2 m. Data rates
typically range from 56 kbit/s up to 4 Mbit/s. VSATs access satellite(s)
in geosynchronous orbit to relay data from small remote earth stations
(terminals) to other terminals (in mesh topology) or master earth station
"hubs" (in star topology).
VSATs are most commonly used to transmit narrowband data (point of
sale transactions such as credit card, polling orRFID data; or SCADA),
or broadband data (for the provision of satellite Internet access to remote
locations, VoIP or video).
VSATs are also used for transportable, on-the-move (utilizing phased
array antennas) or mobile maritime communications.
Home - TV System (Satellites):
Satellite television is television programming
delivered by the means of communications satellite and received by an
outdoor antenna, usually a parabolic reflector generally referred to as a
satellite dish, and as far as household usage is concerned, a satellite
receiver either in the form of an external set-top box or a satellite tuner
module built into a TV set.
Satellite TV tuners are also available as a card or a USB peripheral to be
attached to a personal computer. In many areas of the world satellite
television provides a wide range of channels and services, often to areas
that are not serviced by terrestrial or cable providers.
Direct-broadcast satellite television comes to the general public in two
distinct flavors - analog and digital. This necessitates either having an
analog satellite receiver or a digital satellite receiver. Analog satellite
television is being replaced by digital satellite television and the latter is
becoming available in a better quality known as high-definition
television.
