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GPS Explained

What is GPS?
How it works

How accurate is GPS?
The GPS satellite system
What's the signal?
Sources of GPS signal errors
GPS Language - NMEA and SiRF
GPS Receiver "states"

Note: I hadn't originally planned to include a page like this since the site was mainly focused on "GPS aware" people. However questions I've received made me think it could be useful to have a few explanations on what GPS is all about, what language it uses, how accurate it is, etc...Instead of recreating the wheel on this I used existing write-ups on the subject (except for the GPS language paragraph) and mainly those found here on Garmin's site

Here's a more technical resource: How GPS Works

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? (see also WAAS)

Today's GPS receivers are extremely accurate, thanks to their parallel multi-channel design. most GPS'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. Most GPS receivers are accurate to within 15 meters on average.

Newer 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 traveling 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):

bulletThe first GPS satellite was launched in 1978.
bulletA full constellation of 24 satellites was achieved in 1994.
bulletEach satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.
bulletA GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.
bulletTransmitter 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.

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:

bulletIonosphere 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.
bulletSignal 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.
bulletReceiver 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.
bulletOrbital errors Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.
bulletNumber 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.
bulletSatellite 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.
bulletIntentional 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.

GPS Language - NMEA and SiRF

While GPS receivers use a variety of languages/protocols, such as NMEA, Sirf, Garmin, Delorme, etc...we'll focus on the two main languages used in Pocket PC GPS receivers

bullet While all GPS receivers output the NMEA, only receivers using the Sirf chipset can output to Sirf. Besides almost all GPS receivers based on this chipset can be switched between the NMEA or Sirf output, the Navman sleeve being the exception. The utilities to switch your GPS can be found here

Should this  language issue matter to you ?
Not really, because most mapping programs use the NMEA protocol, some can use both (TomTom GPS, although Sirf seems to be broken in v1.36 of their driver). One program however, Destinator, currently one of the better solutions out there, performs poorly when fed NMEA information, read more about this issue here|



Here's a write-up  by Sven from PPCPassion's board:
"NMEA is the National Marine Electronics Association. They wrote some standards. One is NMEA 0183 which "defines electrical signal requirements, data transmission protocol and time, and specific sentence formats for a 4800-baud serial data bus." Of course RS232 only goes up to 20K too, according to the standard. One of the groups of folks that have adopted the NMEA standard is the GPS community. People can choose to ignore specifics of any standard or selectively support portions of the standard, and so, sure, you can run NMEA 183 at a higher data rate. Bit if either the transmitter or receiver does not support the aberration, you are normally limited to the 'standard'. My Garmin eTrex can output data in a number of 'standards'. My TOPO! program can accept several of them, including the Garmin proprietary one, and the NMEA standard. PocketStreets only accepts NMEA 0183. My Delorme Tripmate only outputs NMEA.

You can read some about SiRF at
http://www.sirf.com/index2.html. They are a company touting another standard (theirs). May be better, but just as WMA may be better than MP3, the latter is more widely implemented, and you would just be well advised to ensure the additional purchases you may make will support the one you adopt, or better still, both (...)Nothing wrong with Sirf, but look to see how many commercial mapping programs support it. (Today anyway). "


You've heard the term WAAS, seen it on packaging and ads for GARMIN products, maybe even know it stands for Wide Area Augmentation System. Okay, so what the heck is it? Basically, it's a system of satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy. How much better? Try an average of up to five times better. A WAAS-capable receiver can give you a position accuracy of better than three meters, 95 percent of the time. And you don't have to purchase additional receiving equipment or pay service fees to utilize WAAS.

What is WAAS?

The Federal Aviation Administration (FAA) and the Department of Transportation are developing the WAAS program for use in precision flight approaches. Currently, GPS alone does not meet the FAA's navigation requirements for accuracy, integrity and availability. WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing and satellite orbit errors and provides vital integrity information regarding the health of each GPS satellite. Although WAAS has not yet been approved for aviation, the system is available for civilian use, such as for boaters and recreational GPS users.

How it Works

WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.

Who benefits from WAAS?

Currently, WAAS satellite coverage is only available in North America. There are no ground reference stations in South America, so even though GPS users there can receive WAAS, the signal has not been corrected and thus would not improve the accuracy of their unit. For some users in the U.S., the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is ideal for open land and marine applications. WAAS provides extended coverage both inland and offshore compared to the land-based DGPS (differential GPS) system. Another benefit of WAAS is that it does not require additional receiving equipment while DGPS does.

Other governments are developing similar satellite-based differential systems. In Asia, it's the Japanese Multi-Functional Satellite Augmentation System (MSAS), while Europe has the Euro Geostationary Navigation Overlay Service (EGNOS). Eventually, GPS users around the world will have access to precise position data using these and other compatible systems.

It just keeps getting better

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.

Summary Table of GPS "states"

bulletWe've all noticed (and cursed) that our GPS receivers sometimes took longer than seemed necessary to acquire a position. Below is a table summarizing the various states a GPS receiver can be in while acquiring a position. While this applies specifically to PocketPC GPS receivers, it should also be true of other receivers, including standalone receivers.
For a GPS receiver based on the SiRF II chip, such states can be "forced" using Leadtek's CE Monitor
# State Impact Real Life Equivalent
1 Factory
bulletGPS has no idea where it is, reset to original protocol/bitrate
bulletYou've just unpacked your GPS ;-)
2 Cold
bulletGPS has no idea where it is
bulletYou haven't used your GPS for ages and the internal battery has run out.
3 Warm
bulletGPS knows where it last was and which satellites were around but needs to get data from the satellites again (elevation, azimuth)
bulletYou turned off your GPS or your PocketPC
4 Hot
bulletGPS knows where it last was and will reuse the satellite data
bulletYou've driven under a bridge/tunnel  and lost reception




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