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Once satellites were developed and put into service to transmit ephemerides for the reception of any land based receiver, a new generation of usefulness emerged. With this new capability comes GPS for example, a utility that users today are accustomed to so much that they can hardly imagine a world without it. Many drivers of vehicles like Uber or Lyft cars rely on their GPS for every part of their route, and when a passenger suggests a better way of reaching the destination more quickly or with less obstruction due to traffic or road conditions, the driver generally doesn't believe his passenger!
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Here is a video that records a series of presentations that was originally composed using photographic film, made in the early 1970's. It describes the classical system of celestial navigation that uses positions of stars (night) and the sun (day) for determining the latitude and longitude of a vessel or airplane even when the navigator has no idea where he is. (Navigators generally have some idea where they are approximately, but the point is there are ways of establishing precise location without recourse to any estimated position.)
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Historical developments such as precursors to the sextant are described, as well as the advents of the telegraph, the transatlantic cable, and radio are told, along with how an increased accuracy of man's knowledge of the shape of the earth was learned. The precise overland distances between points in different countries and continents was gained, as well as was more precise knowledge of differences in elevation based on a theoretical ellipsoid and its more real counterpart, the geoid.
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In the last half hour of this video the entire topic is the worldwide network of ground stations, stations that communicate with each other for the tracking of artificial satellites in orbit around the earth. But these satellites were nothing like the ones we use today! The satellites described in this video were simply reflective metallic shapes that contained no transmitting equipment. Their purpose was to move around the earth in orbit as cameras on earth could take their picture against a backdrop of known stars in their respective locations. By today's standard they were extremely LOW TECH satellites! By taking these pictures from three positions on earth separated by hundreds of miles in a large triangle, three different views of where a given satellite was located at a precise moment of time (within one ten-thousandth of a second) was provided for study and analysis. Keep in mind that all this fancy comparison against the position of stars could only be done at NIGHT when the stars were visible, and that only during FAIR WEATHER when there was no cloud cover! What about daytime? What about during overcast skies or a storm? Can you imagine being unable to use your GPS unless it was during nighttime and clear skies?
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To be clear, today our GPS systems work by ground based receivers interpreting data from radio transmissions of artificial satellites which carry atomic clocks on board, and the information they transmit includes their own location and orbital data which they receive from tracking stations on the ground. Whereas in the early days, described in the video below, all the information was kept on earth, in the charts of data received from observation stations and mostly in the MINDS of the men who analyzed them. The reports they produced could be in error, and were subject to constant revision, in order to arrive at the truth of reality they were attempting to observe. In the early days, the satellites had no sophisticated equipment on board, and they did not relay any radio messages. All they did was move about their orbit reflecting the light of the sun so that photographs of them could be taken. Those ground based stations had to be solidly fixed on terra firma, just like a theodolite or builder's level must be kept reliably motionless. In other words, satellites in their first phase of development were of NO USE to vessels or planes or automobiles with GPS, because they are in motion. In fact, there was no such thing as GPS in those days.
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Effectively, without our modern luxury of GPS or similar systems, in order to make use of satellites, users would have to be A) MOTIONLESS, and could only obtain information about their location B) AT NIGHT WITH CLEAR SKIES. Even then, the user would have to wait for someone to interpret the data produced unless the person was a celestial navigator himself.
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Those were the days when a lot of information was gathered in regards to one position of one artificial satellite at one moment of time, which was then studied intensely for perhaps days and later referred to off and on, for years to come. We have come a long way.
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Those were the days when the artificial satellite did not transmit any coded information (ephemeris) but rather moved in its mute manner along a predicted orbit in a fairly reliable way so that ground based observation stations could take pictures of them using photographic plates which had to be chemically developed in a darkroom by hand.
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In contrast, today there would be no photographic plates nor development in a darkroom, even if by machine. Today digital photography would have taken the place of photographic emulsion film. But never mind the pictures because today we don't rely on pictures anymore. Today, our GPS systems do not rely on any observation of satellites nor their relative position in the sky compared to stars or even the sun. Today, the position of each one of 4 artificial satellites at a given moment of time as transmitted by each of those 4 satellites is combined by a portable receiver (such as your car's GPS or even your cell phone's) to determine your latitude and longitude; even your elevation above (or below) the ellipsoid is found by your portable receiver.
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The receiver does not (necessarily) transmit this information to any outside entity. That is to say, the operation of the GPS system does not inherently involve the transmission of a receiver's own computed location to any other device located elsewhere. However, in the cases of lost persons or lost cell phones, these can be found when their signal is transmitted, such as when placing a cell phone call, and this can be done even when no call is being placed. Therefore, the location of a cell phone can be established by a third party provided that the phone has power (the battery is not removed or discharged), and perhaps even when the phone is turned OFF.
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Topic: Global Navigation Satellite Systems -- tutorial (Read 14182 times)