Implications (part 6): Orbital Mechanics 101

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All satellite orbits are full of compromises, the Near Polar Orbits (NOAA-19) are described herein.

Firstly, why near and not polar, well if a true polar orbit is used then the Poles are over flown repeatedly so a near polar orbit can reduce this redundancy. Now in order to get a good view of the poles we need to know how close (far away) we need to fly. This introduces the first and probably the most important compromise we must make.

To understand this, we must establish the ‘Swathe’ that the instruments aboard can observe. A combine harvester cuts a swathe through a corn field, the speed the combine harvester travels at gives a value for its footprint and from this the farmer can work out how much fuel is needed and how long it will take to harvest his crop.

If a low orbit is chosen then the swathe is quite small and we will need many orbits to get a full picture. If a really high orbit is chosen then the swathe is much wider but we will have problems associated with the earths curvature as we are overflying a near spherical object!

Then we have to satisfy the requirements of the people who are paying us to launch the satellite the scientists who want weather and climate data. They want regular data over long time periods. The end result is a near Polar orbit canted at about 9 degrees west from true north and with an orbit time of around 102 minutes. This is about as good as it can get!

Fourteen orbits every 24 hours, with each orbit half in daylight and in darkness. Yes, some places get looked at, at 6am and 6pm, while others get looked at, at 4am and 4pm. The Swathe is about 2500km wide, the data recorded for each orbit can be visualized as a 40075km long toilet roll some 2500km wide, so Lots of data to be downloaded twice a day as the satellite overflies the base station.

Why is the data sprinkled with ‘Approximately’, well, the earth is not a true sphere but the orbit is a true circle around the earths center of mass, so the distance to the ground varies as does the swathe.

Now do your math, the earths circumference is 40075km or thereabouts which when divided by 14 gives 2865.2km so the swathe is not quite wide enough! When they say ‘this is satellite data it cannot lie’, they are blowing smoke!

If you think that is of little importance then consider that data. The swathe will eventually overlap the data from the previous strip of toilet paper.

How and who determines what parts of that data is used and or discarded, there are grubby paw prints and computer algorithms all over what we are told is Satellite data that cannot lie!

The data about NOAA-20 is not as complete as that for previous satellites.

If you need more or better explanations, I can produce many versions of orbital mechanics 102-109. How do I know all this stuff? Well in 1962 I worked on the ‘Black Knight’ project in the UK and programmed the launch computer that was fitted to the vehicle.

Michael Logician


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Comments (5)

  • Avatar

    jerry krause

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    Hi Michael,

    Thank you for coming back to this issue because I believe I now understand the near polar orbit of the NASA-NOAA satellites better than you because I believe observation is more important than logic.

    First, you previously, and frequently, referred the portion of these satellite orbits over the tropics instead of that over the Arctic Circle (AC). About which I could have cared less because my focus was upon what is being observed from the satellite while over the AC. I have finally understood that the principal purpose of these satellites was to quantitatively measure to various radiances at the equator and the lower latitudes and not those over the AC. For obviously the portion of the earth’s surface of the tropics and the lower latitudes is far, far greater than the areas of a polar region.

    The definition of radiance contains a critically required factor—a per unit solid angle [steradian,sr] in a given direction by a unit area of a source. (radiance—the flux of radiation emitted [in the case the incident solar radiation reflected or scattered or both] per unit solid angle [steradian,sr] in a given direction by a unit area of a source.) (https://principia-scientific.com/how-prehistoric-glaciers-could-have-been-formed-part-3/) In your comments to the first and second parts of this essay (https://principia-scientific.com/how-prehistoric-glaciers-could-have-been-formed-part-1/) ; (https://principia-scientific.com/how-prehistoric-glaciers-could-have-been-formed-part-2/) you began to teach me and any PSI readers about the ‘black hole’ of Image 1 and its need for the reasons you have again stated in this, your, essay.

    The specific radiance measured and reported in Image 1(which cannot not be shown in this comment) had to be measured in a near downward direction and at a constant distance above the earth’s surface in an effort to keep unit area of the flux’s source as constant as possible.

    You ask: “Why is the data sprinkled with ‘Approximately’?” And your answer was: “Well, the earth is not a true sphere but the orbit is a true circle around the earths center of mass, so the distance to the ground varies as does the swathe.”

    In my most recent essay (https://principia-scientific.com/how-prehistoric-glaciers-could-have-been-formed-part-4/) I have written: ” I finally asked myself: What am I assuming that might not be the truth (correct)? Which question, given my recent blatant wrong assumption, should always be asked. I had been assuming each of the two circumferences was of a sphere. Which obviously, given the two different circumferences, the earth is not. The earth is an ellipsoid; or as I discovered, a spheroid. A sphere is based on a circle, while a spheroid (or ellipsoid) is based on an ellipse. A spheroid, or ellipsoid, is a sphere flattened at the poles.” (Wikipedia) Another way this could be stated is: that the curvature of the earth’s surface is greater at the equator than it is at its poles.”

    Hence, even if the orbit of the satellite is kept in an orbit which ‘is a true circle around the earths center of mass’, the distance to the ground varies as does the swathe. Now the four words—as does the swathe—becomes a critical factor. Here, the swathe needs to be kept narrow as practically possible to limit the word—’approximately’. Hence, the instrument measuring the radiance of Image 1, which will be used to calculate the albedo the earth’s surface and atmosphere (including any foreign particles, like cloud, other than gases) will nearly, as it swings side to side (swathe) will be pointing nearly directly downward as much as practically possible. The consequence of this must be the instrument can never ‘see’ much of the direct solar radiation being reflected from a smooth liquid water surface at the same reflected angle as the solar radiation’s incident angle to the surface, unless the sun is nearly directly above the satellite very near the time of midday. And, of course, this condition limits such an observation of solar reflection to be near midday at a certain latitude within the tropical region.

    So, thanks again for again you have helped tremendously to help me better understand and to claim that most all quantitative measurements made from satellite are likely meaningless.

    Have a good day, Jerry

    Reply

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      Matt Holl

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      Hi Jerry,
      I still keep learning “stuff” from you.
      The earth being a ball-oid, less curve at the poles, would be a function of less centrifugal force at the poles, I would presume.
      With centrifugal force, do we weigh more at the poles than we do at the equator? Is gravity of the same quantum of force at the poles than at the equator? One would intuitively think the earth would have more mass at 30 degrees North and 30 degrees South than at the poles.

      Have a happy Sunday. Matt

      Reply

  • Avatar

    jerry krause

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    Hi Matt,

    As I ponder I try to answer one question before I move on to another question. Very important to first ask a question, any question. But it is important to recognized that some scientific questions do not have answers. But I believe your question, “do we weigh more at the poles than we do at the equator?”, does.

    However, when we use the word ‘weight’ we must clearly define two words, weight and mass, and describe (define) how we would measure each.

    So I will play Michael Clarke’s reasoning game with you and ask you to tell me, him, and other PSI readers the answer (or answers) to your simple question (s). I ask you to do this because of your past history, during our conversations, I am confident you are able to answer this question for yourself. So, I will be looking forward to your answers to support my confidence in your abilities..

    Have a good day, Jerry

    Reply

    • Avatar

      Matt

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      Hi Jerry.
      You have held up a mirror which makes one pause for reflection.
      Have a Ponderous day. Matt

      Reply

  • Avatar

    Michael Clarke

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    Hi Guys,
    Two things, a few years ago Scientist in the UK was able to establish that his students weighed more, (May have been less will have to look it up), at the top of the local mountain than at school in the valley.
    The Near Polar Orbit of satellites are a perfect circle about the Earth’s center of mass which is why their height is given as a range, min at the equator, max at their closest NORTH pole height.
    Michael Logican

    Reply

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