[Tom Robertson]

In The Sixto Report: D11 Sat Status Discussion thread we've had problems deciding what is a TLE. Since that became too OT for that thread, it's moved into this thread (that I had anticipated needing last night at 1:31am...)

Sixto's description from post #2 of the thread:

Two Line Element (TLE) Format:

Standard format of numbers for describing in detail a spacecraft's trajectory around the earth. Calculated and published by the U.S. government regularly

http://celestrak.com/columns/v04n03/

http://www.space-track.org/tle_format.html)​

Here are the rest of the What are TLE posts from the thread.

Enjoy,
Tom

 

[LameLefty]

Okay, just a reminder, any "real-time tracking" type site like n2yo is NOT "real-time!" They are using the same NORAD/Space Command two-line orbital element sets that the rest of us are and propagating them into the present - the "current" public TLEs are about 6 days and counting out-of-date, so the tracking sites are NOT a reliable source unless the element sets are current.

 

[jefbal99]

Okay, just a reminder, any "real-time tracking" type site like n2yo is NOT "real-time!" They are using the same NORAD/Space Command two-line orbital element sets that the rest of us are and propagating them into the present - the "current" public TLEs are about 6 days and counting out-of-date, so the tracking sites are NOT a reliable source unless the element sets are current.

Hey Lefty...

Question about TLEs

I've read on here both sides that TLEs are a snapshot of exactly where a satellite is at the timestamp of the TLE or that a TLE is a projection on where satellite is headed.

Yer the resident rocket scientist here, whats the official answer?

I always thought it as just a snapshot, kinda like the speedometer in your car. The instant you look at it, its valid, but after that it could be moved and you'd never know.

 

[LameLefty]

Hey Lefty...

Question about TLEs

I've read on here both sides that TLEs are a snapshot of exactly where a satellite is at the timestamp of the TLE or that a TLE is a projection on where satellite is headed.

Yer the resident rocket scientist here, whats the official answer?

I always thought it as just a snapshot, kinda like the speedometer in your car. The instant you look at it, its valid, but after that it could be moved and you'd never know.

It's a snapshot of the orbit but unlike a picture, the numbers can be used to predict the future position of the satellite at any other time. However, the TLEs are only sure to be valid at the instant they are generated. If the vehicle is in the process of maneuvering at the time the TLE is generated, it won't be valid for any other time. If the TLE is too old (especially for a low-orbit satellite subject to a lot of orbital drag) it won't be very accurate for more than a day or two.

GEO sats are more stable and thus TLEs are accurate for longer periods, but D11 is pretty likely to have moved in the last 6-plus days, given past history with D10 and the fact of the Boeing press release, signals on the 99(c) meter, etc. So it's almost a certainty that the "current" TLE is obsolete.

 

[DarinC]

It is a snapshot, but the current trajectory is included in the snapshot, so it is an indication of where the satellite is headed ASSUMING it doesn't change course. Obviously though, extrapolating projected satellite positions based on trajectories of a satellite that they are actively moving into a different orbit are not going to be accurate.

EDIT: or what Lefty said.

 

[harsh]

Speaking of Harsh, where is the little ray of sunshine?

Like the last four or five pages of posts, I didn't have anything to add.

I dispute Lame Lefty's and Sixto's interpretation of a TLE. A TLE is a model, it is not a snapshot. If the model is any good, it should be able to synthesize snapshots at any point in time.

NORAD maintains general perturbation element sets on all resident space objects. These element sets are periodically refined so as to maintain a reasonable prediction capability on all space objects. In turn, these element sets are provided to users.

The key phrase here is "reasonable prediction" which is not a record of the actual position at any point in time.

The lesson learned here is that TLEs are like junebugs: everything is fine until you don't hear anything and then it is time to duck.

It is notable that D10's handover was never formally announced by Boeing.

 

[Sixto]

Like the last four or five pages of posts, I didn't have anything to add.

I dispute Lame Lefty's and Sixto's interpretation of a TLE. A TLE is a model, it is not a snapshot. If the model is any good, it should be able to synthesize snapshots at any point in time.The key phrase here is "reasonable prediction" which is not a record of the actual position at any point in time.

The lesson learned here is that TLEs are like junebugs: everything is fine until you don't hear anything and then it is time to duck.

It is notable that D10's handover was never formally announced by Boeing.

There's layman's terms and there's the more technical view.

For the average person, it's a snapshot.

For the more technically inclined: http://celestrak.com/columns/v04n03/ and http://www.space-track.org/tle_format.html

 

[man_rob]

Like the last four or five pages of posts, I didn't have anything to add.

I dispute Lame Lefty's and Sixto's interpretation of a TLE. A TLE is a model, it is not a snapshot. If the model is any good, it should be able to synthesize snapshots at any point in time.The key phrase here is "reasonable prediction" which is not a record of the actual position at any point in time.

The lesson learned here is that TLEs are like junebugs: everything is fine until you don't hear anything and then it is time to duck.

It is notable that D10's handover was never formally announced by Boeing.

Do you ever tire of being wrong?

 

[LameLefty]

Hey if Harsh wants to get pedantic, I can put my aerospace and mechanical engineering degree up against his (along with my posting history here) any time.

Of course it's not a literal snapshot, but then again, neither is it a "model." It is an actual mathematical depiction of an orbit at a specific moment in time (e.g., a "snapshot.")

The "model" itself is something called the SGP4/SDP4 model, which uses this mathematical depiction. But I'm sure Harsh already knows that.

 

[hdtvfan0001]

Hey if Harsh wants to get pedantic, I can put my aerospace and mechanical engineering degree up against his (along with my posting history here) any time.

My money's on you.

 

[harsh]

For the average person, it's a snapshot.

For anyone with the wherewithal to make use of a TLE, it is a model; a predictor of where the satellite will be at any point in time.

 

[harsh]

Hey if Harsh wants to get pedantic, I can put my aerospace and mechanical engineering degree up against his (along with my posting history here) any time.

I've had my share of undergraduate engineering and mathematical simulation and approximation classes. No need to get out the measuring stick.

Of course it's not a literal snapshot, but then again, neither is it a "model."

Poppycock. A TLE is precisely the parameters in a time based mathematical model. The TLE tells you where it should be at any particular time until there is a new TLE. Knowing where a satellite was is of little use in the here and now.

The key to this being a model is the Ballistic Coefficient and Second Derivative of Mean Motion. Ballistic Coefficient is a first derivative and, obviously, the Second Derivative is also a derivative. Snapshots don't have time factors in them and the result of a TLE is obviously dependent on time.

You say that it is a snapshot and then you explain that it isn't. Then you say that it isn't a model but that the information is fed to a model which forecasts the position. It either is a snapshot or it isn't. It either is a model or it isn't.

The epoch date is the time at which the TLE in question became the official predictor. If you look at the epoch dates of most DIRECTV satellites, you'll find that they update them every few days even though the satellites are essentially geosynchonous.

 

[hdtvfan0001]

I've had my share of undergraduate engineering and mathematical simulation and approximation classes. No need to get out the measuring stick. Poppycock. A TLE is precisely the parameters in a time based mathematical model.

TLE at the OK Coral.

This is the point where Belushi usually yells "Foodfight"....

All I know my guy is packing a Sixto. :lol:

 

[man_rob]

TLE at the OK Coral.

This is the point where Belushi usually yells "Foodfight"....

All I know my guy is packing a Sixto. :lol:

Sixto, and Lame Lefty have proven their knowledge. They have been right over and over, and thus have earned the respect they receive.

 

[hdtvfan0001]

Sixto, and Lame Lefty have proven their knowledge. They have been right over and over, and thus have earned the respect they receive.

Yes indeed.

Onward with the D11 and TLE talk.....

 

[cartrivision]

For anyone with the wherewithal to make use of a TLE, it is a model; a predictor of where the satellite will be at any point in time.

Not exactly. It is as others have said a snapshot of the satellite's trajectory at a single point in time. You can use the data from that snapshot to predict where it will be for some time into the future, but not "at any point in time". As soon as the satellite's thrusters are fired, the previous snapshot of the trajectory is no longer a valid predictor of where the satellite will be, and you need a new snapshot of its trajectory.

That's why you incorrectly said yesterday that TLE showed that D11 wasn't close to its final parking spot. You were erroneously assuming that the TLE data that you were basing your statement on was still a valid predictor of the current position of D11, which it obviously wasn't.

 

[hdtvfan0001]

Not exactly. It is as others have said a snapshot of the satellite's trajectory at a single point in time. You can use the data from that snapshot to predict where it will be for some time into the future, but not "at any point in time". As soon as the satellite's thrusters are fired, the previous snapshot of the trajectory is no longer a valid predictor of where the satellite will be, and you need a new snapshot of its trajectory.

So in my layterm understanding...its a recorded bit information that, coupled with additional data or another comparible earlier bit of data....can be used to forecast a future location. I have this picture of a series of dots on a chart (TLE's)...which can then be used with math to calculate velocity and location. Am I getting it?

 

[bakers12]

...can be used to forecast a future location.

Assuming the orbit doesn't change by firing a thruster, non-uniformities in the earth's density, the pull of the moon, etc.

As we've seen with D10, no TLEs were issued for a long while. The speculation was that it made no sense while the orbit was going through lots of changes.

One TLE doesn't really relate to another, even for the same satellite, because something made its orbit change.

 

[harsh]

It is as others have said a snapshot of the satellite's trajectory at a single point in time.

As I pointed out earlier, snapshots don't have built-in time factors. Every TLE has a first and second derivative with respect to time (dt).

It is certainly easier to "dumb it down" for the general public who asked what a TLE is at least once a week in this thread, but that doesn't make it the truth.

Note to those who are speculating about dependency on previous TLEs: each TLE stands completely by itself and is entirely independent of any previous TLE or snapshot thereof. Note also that running the numbers on a TLE requires you to pick a time. For the purposes of his reporting, Sixto chose to use the epoch date.

 

[cartrivision]

Like the last four or five pages of posts, I didn't have anything to add.

I dispute Lame Lefty's and Sixto's interpretation of a TLE. A TLE is a model, it is not a snapshot.

As usual, you are incorrect. A TLE is indeed a snapshot of the satellite's trajectory at a specific point in time. It can be used to predict or model where the satellite will be at some point after the snapshot was taken by building a model using the snapshot of the last known trajectory (the TLE data), and the laws of physics, but the TLE itself is not a model.