closer than the satellites that guide your GPS [telecom]

Scott Pelle, on the CBS Evening News tv show, just stated that an asteroid will pass about 18,000 miles from earth next week, which he said is "Closer than the satellites that ... guide your GPS".

I understand that "news" programs are now expected to make a profit, and that some networks consider them to be entertainment, but this is sad. It's an afront to viewers of the once-proud CBS News, and a sad reminder of how far CBS has fallen from Murrow's and Cronkite's heydeys.

Academically, the United States ranks near the bottom on the list of math and science test scores when compared to other nations. I think careless and casual "reports" such as this one contribute to that dismal record.

Bill

Reply to
Bill Horne
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But even Murrow and Cronkite couldn't hold a candle to [the late] I.F. Stone and his beloved Weekly. :-{

Cheers,

-- tlvp

Reply to
tlvp

What's the problem with this report? GPS satellites are in geosynchronous orbits, about 22,000 miles above sea level.

Here's part of a report about the asteroid at investorplace.com:

Specifically, it will hit perigee - closest approach - at 11:24 a.m. pacific standard time on the 15th. At that point it will be 17,200 miles from the surface and 5,000 miles inside the orbits for our Global Positioning System satellites. It will still be thousands of miles above the atmosphere and the International Space Station, which orbits at about 240 miles above the surface.

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Is your complaint that they have to use GPS as a reference point in order to "dumb it down" for US audiences? Or maybe that he referred to

17,200 as "about 18,000"?
Reply to
Barry Margolin

Meh. Possibly they confused miles for kilometers - GPS satellites orbit at about 20,000km. Asteroid 2012DA14 will come within about 21,000mi of th earth. Easy mistake for anyone; even rocket scientists do it sometimes ...

Alternatively, the confused GPS for "geosyncronous". There is some concern that the asteroid's path will take it right through the geostationary belt and a satellite will get nailed.

In any event, it's the closest encounter with an earth-crossing asteroid in many years. A little excitement is warranted!

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Interesting read for a contrary opinion on the state of science education in the US. *

Reply to
PV

It appears that someone has "kilometers" confused with "miles".

According to the government's own GPS website at

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sats are at 20,200 KILOMETERS, which would be approx. 12,550 miles, ie; less than 17,200 miles of the asteroid.

Reply to
Retired

My complaint is that CBS didn't check the facts, and therefore is guilty of sloppy reportage. Although I am often amazed at the impact television has on the public's knowledge base, I'll concede that TV /does/ have power to affect public perception, and therefore, an obligation to get the story straight.

Let me say, right up front, that I'm not a rocket scientist. What little knowledge I have of orbital mechanics comes from my experience using OSCAR (Orbiting Satellite Carrying Amateur Radio) Satellites as a ham radio operator. When I was working on SS7 at Verizon, another engineer in our group was buying GPS timing sources to replace "Stratum I" clocks, so I assume that I gained some background on GPS from him, but I don't hold myself out as any sort of expert.

GPS satellites are not in geosynchronous orbit. They do /not/ share the Clarke Belt with communications relay satellites: in fact, the satellites that make up the GPS "Constallation" orbit at a much lower altitude.

This is from Wikipedia:

The space segment (SS) is composed of the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs, eight each in three approximately circular orbits,[44] but this was modified to six orbital planes with four satellites each.[45] The orbits are centered on the Earth, not rotating with the Earth, but instead fixed with respect to the distant stars.[46] The six orbit planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection).[47] The orbital period is one-half a sidereal day, i.e., 11 hours and 58 minutes.[48] The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface.[49] The result of this objective is that the four satellites are not evenly spaced (90 degrees) apart within each orbit. In general terms, the angular difference between satellites in each orbit is 30, 105, 120, and 105 degrees apart which, of course, sum to 360 degrees.

Orbiting at an altitude of approximately 20,200 km (12,600 mi); orbital radius of approximately 26,600 km (16,500 mi), each SV makes two complete orbits each sidereal day, repeating the same ground track each day.[50] This was very helpful during development because even with only four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones.

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And, from a Powerpoint slide file I found at NOAA:

  • GPS Nominal Constellation * 28 Satellites in 6 Orbital Planes * 4 Satellites in each plane * 20,200 km Altitudes, 55 Degree Inclination

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Frankly, when I heard the CBS report, I was surprised to find that the GPS Constellation isn't in *LOW* Earth orbit - I had assumed it was, just like the OSCAR birds - but rather in *MID* Earth orbit. In other words, I had my own preconceived notion of how high the GPS satellites are, but I /knew/ they weren't in the Clarke Belt, so I looked it up. I think CBS' viewers are entitled to a higher standard of research than what little I did.

The fact that investorplace.com echoed CBS' error only bolsters my argument: I don't know which cart got in front of which horse, but a national news organization is responsible for paying attention to the basics.

Bill

Reply to
Bill Horne

According to Wikipedia, that font of infallible knowledge, GPS satellites orbit at about 12,600 miles. After all, they have no need to remain at a fixed point in the sky. A better comparison would have been to the satellites that retransmit TV signals, or that take weather photographs.

I see that report also assumes that GPS satellites are in geosynchronous orbit, but I see no reason why a website called investorplace should have special expertise here. NASA, on the other hand, doesn't mention GPS satellites in its FAQ on the asteroid, but does distinguish between two groups of satellites:

[Asteroid DA14] will pass between the outer constellation of satellites located in geosynchronous orbit (22,245 miles/35,800 kilometers) and the large concentration of satellites orbiting much closer to Earth.

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Lee

Reply to
Lee Choquette

More likely, they assumed GPS satellites were in geosynchronous orbits (22,000 miles), like most communication satellites. I was actually surprised to see that they aren't.

Reply to
Barry Margolin

Then they wouldn't be usable at high latitudes, which would be a problem. (Remember where the DEW line was.)

There's enough GPS satellites that no matter where you are, there are always several visible overhead so your receiver can triangulate among them.

Reply to
John Levine

Err, would not a higher altitude make for better polar coverage?

GPS birds are lower because their transmissions are weak, and have to be heard by substandard receivers/antennas.

Reply to
David Lesher

If they were in equatorial orbits, yes, higher is better.

They're not.

Insignificant (about 4dB, vs geosync), inconsequential, and irrelevant.

GPS, and other forms of 'triangulation', is more precise when the angular difference between stations is larger.

Having the line of sight well above the horizon is preferable. (geostationary is actually below the horizon _at_ the Poles, at ground level)

Non-geosync orbits ensure a bird is not permanently blocked by a stationary ground-based obstacle.

'High in the sky' is less likely to be blocked temporarily by a stationary obstacle.

Solving 'all of the above' for 'everywhere on Earth, all the time' is non trivial.

GPS birds have to be in stable/predictable orbits, you can't maneuver them at will.

The vast majority of 'space junk' is in LEO. A good reason to avoid that region for long-life birds.

Orbital velocity is lower in higher orbits, reducing Doppler-shift effects.

Selecting the orbits (height, azimuth, inclination, eccentricity) for the GPS constellation was a _complex_ set of trade-offs.

Reply to
Robert Bonomi

You would think that they would be able to run the story by someone in their employ who was scientifically minded, who might actually know the answer, or at least would be curious enough to check it. Perhaps a meteorologist would fit that bill, or just a plain old science consultant.

Or maybe they *did* run the story by such a person, and that person got it wrong.

Jon

Reply to
Jon Danniken

Not if they're over the equator. Even if they were infinitely far away, at the pole your antenna would still be horizontal, and any feature higher than you would block it.

They're not that much lower.

Iridium satellites really are in low orbit for that reason, but GPS is higher than LEO, but not as high as geosync.

R's, John

Reply to
John Levine

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