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What is WAAS

نوشته شده در سه‌شنبه ۳٠ مهر ۱۳۸٧ ساعت ۱۱:٥٦ ‎ب.ظ توسط محمد زراعت پیشه

You've heard the term WAAS, seen it on packaging and ads for Garmin® products, and 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

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How accurate is GPS

نوشته شده در سه‌شنبه ۳٠ مهر ۱۳۸٧ ساعت ۱۱:٥٢ ‎ب.ظ توسط محمد زراعت پیشه

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

Sources of GPS signal errors

Factors that can degrade the GPS signal and thus affect accuracy include the following:

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 tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits ephemeris data showing the orbital information for that satellite and for every other satellite in the system.

Almanac 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

  • 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
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    What is GPS

    نوشته شده در سه‌شنبه ۳٠ مهر ۱۳۸٧ ساعت ۱۱:٥٠ ‎ب.ظ توسط محمد زراعت پیشه

    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

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    Why Use GIS

    نوشته شده در سه‌شنبه ۳٠ مهر ۱۳۸٧ ساعت ۱۱:٤٢ ‎ب.ظ توسط محمد زراعت پیشه

    Your organization has new and legacy data stored in a variety of formats in many locations. You need a way to integrate your data so that you can analyze it as a whole and leverage it to make critical business and planning decisions

    GIS can integrate and relate any data with a spatial component, regardless of the source of the data. For example, you can combine the location of mobile workers, located in real-time by GPS devices, in relation to customers' homes, located by address and derived from your customer database. GIS maps this data, giving dispatchers a visual tool to plan the best routes for mobile staff or send the closest worker to a customer. This saves tremendous time and money

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    Three Views of a GIS

    نوشته شده در سه‌شنبه ۳٠ مهر ۱۳۸٧ ساعت ۱۱:۳۸ ‎ب.ظ توسط محمد زراعت پیشه

    A GIS is most often associated with a map. A map, however, is only one way you can work with geographic data in a GIS, and only one type of product generated by a GIS. A GIS can provide a great deal more problem-solving capabilities than using a simple mapping program or adding data to an online mapping tool (creating a "mash-up").

    A GIS can be viewed in three ways

    The Database View: A GIS is a unique kind of database of the world—a geographic database (geodatabase). It is an "Information System for Geography." Fundamentally, a GIS is based on a structured database that describes the world in geographic terms

    The Map View: A GIS is a set of intelligent maps and other views that show features and feature relationships on the earth's surface. Maps of the underlying geographic information can be constructed and used as "windows into the database" to support queries, analysis, and editing of the information

    The Model View: A GIS is a set of information transformation tools that derive new geographic datasets from existing datasets. These geoprocessing functions take information from existing datasets, apply analytic functions, and write results into new derived datasets.

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    What is GIS

    نوشته شده در سه‌شنبه ۳٠ مهر ۱۳۸٧ ساعت ۱۱:۳٠ ‎ب.ظ توسط محمد زراعت پیشه

    A geographic information system (GIS) integrates hardware, software, and data for capturing, managing, analyzing, and displaying all forms of geographically referenced information

    GIS allows us to view, understand, question, interpret, and visualize data in many ways that reveal relationships, patterns, and trends in the form of maps, globes, reports, and charts.

    A GIS helps you answer questions and solve problems by looking at your data in a way that is quickly understood and easily shared.

    GIS technology can be integrated into any enterprise information system framework.

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    The Elements of Geodesy: The Horizontal Datum

    نوشته شده در یکشنبه ٢۱ مهر ۱۳۸٧ ساعت ٢:٤۸ ‎ب.ظ توسط محمد زراعت پیشه

    At its most basic level of definition, the horizontal datum is a collection of specific points on the Earth that have been identified according to their precise northerly or southerly location (latitude) and easterly or westerly location (longitude) (National Geodetic Survey, 1986).

    To create the horizontal datum, or network of horizontal positions, surveyors marked each of the positions they had identified, typically with a brass, bronze, or aluminum disk or monument. These markers were placed so that surveyors could see one marked position from another. To maximize the line-of-sight between monuments, they were usually set on mountaintops or at high elevations. When monuments were set on flat land, towers were built above them to aid surveyors in locating them.

    To "connect" the horizontal monuments into a unified network, or datum, surveyors have used a variety of methods, including triangulation. As technology has improved, surveyors now rely almost exclusively on the Global Positioning System (GPS) to identify locations on the Earth and incorporate them into existing datums.


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    The Elements of Geodesy: Datums

    نوشته شده در یکشنبه ٢۱ مهر ۱۳۸٧ ساعت ٢:٤٦ ‎ب.ظ توسط محمد زراعت پیشه

    Datums (sets of data) are the basis for all geodetic survey work. They act as reference points in the same way that starting points do when you give someone directions. For instance, when you want to tell someone how to get to your house, you give them a starting point that they know, like a road or a building. Geodesists and surveyors use datums as starting or reference points when they create maps, mark off property boundaries, and plan, design and build roads, bridges, and other structures.

    Another way to think about a datum is as a set of information that acts as a foundation for other data. For example, when a skyscraper is about to be built, the construction team must first pour the foundation. Without this element, the skyscraper would be unstable and unsafe. This is the same concept as a datum. While a datum is a mathematical and geometric concept, it acts like the concrete foundation of a skyscraper. Once the foundation is set, the construction workers can build on top of it, creating the building's structure. After the building is complete, offices or apartments can be created inside the building. If the structure is an apartment building, its tenants can bring in furniture and decorate as they please. Although the foundation of the building probably isn't the first thing on the minds of the tenants, without it, the building would not be a safe place to live.

    In geodesy, two main datums create the foundation for navigation and transportation in the United States. These datums -- called the horizontal and vertical datums -- make up the National Spatial Reference System (NSRS). Geodesists, surveyors, and people interested in precise positioning use the NSRS as their foundation for reference.

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    The Elements of Geodesy: The Figure of the Earth

    نوشته شده در یکشنبه ٢۱ مهر ۱۳۸٧ ساعت ٢:۳٩ ‎ب.ظ توسط محمد زراعت پیشه

    The Earth's shape is nearly spherical, with a radius of about 3,963 miles (6,378 km), and its surface is very irregular. Mountains and valleys make actually measuring this surface impossible because an infinite amount of data would be needed. For example, if you wanted to find the actual surface area of the Grand Canyon, you would have to cover every inch of land. It would take you many lifetimes to measure every crevice, valley, and rise. You could never complete the project because it would take too long.

    To measure the Earth and avoid the problems that places like the Grand Canyon present, geodesists use a theoretical mathematical surface called the ellipsoid. Because the ellipsoid exists only in theory and not in real life, it can be completely smooth and does not take any irregularities - such as mountains or valleys -- into account. The ellipsoid is created by rotating an ellipse around its shorter axis. This matches the real Earth's shape, because the earth is slightly flattened at the poles and bulges at the equator.

    While the ellipsoid gives a common reference to geodesists, it is still only a mathematical concept. Geodesists often need to account for the reality of the Earth's surface. To meet this need, the geoid, a shape that refers to global mean sea level, was created. If the geoid really existed, the surface of the Earth would be equal to a level in between the high-tide and low-tide marks.

    Although a geoid may seem to be a smooth, regular shape, it isn't. The Earth's mass is unevenly distributed, meaning that certain areas of the planet experience more gravitational "pull" than others. Because of these variations in gravitational force, the "height" of different parts of the geoid is always changing, moving up and down in response to gravity. The geoidal surface is an irregular shape with a wavy appearance; there are rises in some areas and dips in others (Geodesy for the Layman, 1984).

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    The History of Geodesy

    نوشته شده در یکشنبه ٢۱ مهر ۱۳۸٧ ساعت ٢:۳۱ ‎ب.ظ توسط محمد زراعت پیشه

    Throughout history, the shape of the Earth has been debated by scientists and philosophers. By 500 B.C. most scholars thought the Earth was completely spherical. The Greek philosopher Aristotle (384-322 B.C.) is credited as the first person to try and calculate the size of the Earth by determining its circumference (the length around the equator) He estimated this distance to be 400,000 stades (a stadia is a Greek measurement equaling about 600 feet). With one mile equal to 5,280 feet, Aristotle calculated the distance around the Earth to be about 45,500 miles (Smith, 1988).

    Around 250 B.C., another Greek philosopher, Eratosthenes, measured the circumference of the Earth using the following equation:

    (360° ÷ θ) x (s)

    In this calculation, (s) is the distance between two points that lie north and south of each other on the surface of the Earth. If you were to draw a line from each of these points to the center of the Earth, the angle formed between them would be θ.

    Obviously, Eratosthenes could not go to the center of the Earth, so he got the angle measurement using the rays of the sun. At noon on the longest day of the year, the summer solstice, the sun shone directly into a deep well at Syene (which is now Aswan, Egypt), casting no shadow.


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    What is Geodesy

    نوشته شده در یکشنبه ٢۱ مهر ۱۳۸٧ ساعت ٢:٢٦ ‎ب.ظ توسط محمد زراعت پیشه

    Geodesy is the science of measuring and monitoring the size and shape of the Earth. Geodesists basically assign addresses to points all over the Earth. If you were to stick pins in a model of the Earth and then give each of those pins an address, then you would be doing what a geodesist does. By looking at the height, angles, and distances between these locations, geodesists create a spatial reference system that everyone can use.

    Building roads and bridges, conducting land surveys, and making maps are some of the important activities that depend on a spatial reference system. For example, if you build a bridge, you need to know where to start on both sides of the river. If you don't, your bridge may not meet in the middle.

    As positioning and navigation have become fundamental to the functions of society, geodesy has become increasingly important.

     

    Geodesy helps the transportation industry ensure safety and reliability, while reducing costs. Without geodesy, planes might land next to -- rather than at -- airports, and ships could crash onto land. Geodesy also helps shipping companies save time and money by shortening their ships' and airplanes' routes and reducing fuel consumption.

    Geologists, oceanographers, meteorologists, and even paleontologists use geodesy to understand physical processes on, above, and within the Earth. Because geodesy makes extremely accurate measurements (to the centimeter level), scientists can use its results to determine exactly how much the Earth's surface has changed over very short and very long periods of time (Careers in Geodesy, 1986).

    The Earth's surface changes for many reasons. For instance, its surface rises and falls about 30 centimeters (about 1 foot) every day due to the gravitational influences of the moon and the sun. The Earth's outermost layer, the crust, is made up of a dozen or more "plates" that ride atop a sea of molten rock, called magma, which flows beneath the surface of the Earth.

    Plate tectonics is the scientific discipline that looks at how these plates shift and interact, especially in relation to earthquakes and volcanoes. Although these phenomena are violent and usually affect large areas of land, even smaller events, such as erosion and storms, have an impact on shaping the Earth's surface. Geodesy helps us determine exactly where and how much the Earth's surface is changing.


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    What is the geoid

    نوشته شده در یکشنبه ٢۱ مهر ۱۳۸٧ ساعت ۱٢:٤۱ ‎ب.ظ توسط محمد زراعت پیشه

    DEFINITION

    There have been many definitions of the "geoid" over 150 years or so. Here is the one currently adopted at NGS:

    geoid: The equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level

    Even though we adopt a definition, that does not mean we are perfect in the realization of that definition. For example, altimetry is often used to define "mean sea level" in the oceans, but altimetry is not global (missing the near polar regions). As such, the fit between "global" mean sea level and the geoid is not entirely confirmable. Also, there may be non-periodic changes in sea level (like a persistent rise in sea level, for example). If so, then "mean sea level" changes in time, and therefore the geoid should also change in time. These are just a few examples of the difficulty in defining "the geoid".


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    عید فطر مبارک

    نوشته شده در سه‌شنبه ٩ مهر ۱۳۸٧ ساعت ۱۱:٠٥ ‎ب.ظ توسط محمد زراعت پیشه

    کم کم غروب ماه خدا دیده میشود 

    صد حیف زین بساط که بر چیده میشود

    در این بهار رحمت و غفران ومغفرت

    خوشبخت آنکسی است که بخشیده میشود

    حلول عید سعید فطر مبارک والتماس دعا

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