# Tag Info

20

Here is a good tutorial on the navigation of deep space vehicles. The two main data types used are two-way Doppler (using an atomic clock reference at the DSN station, with the frequency locked to and sent back to Earth by the spacecraft), which gives the velocity component along the Earth-spacecraft line to better than 0.1 mm/s, and ranging (sending a ...

15

From russian wikipedia: Another instrument helped to the pilot to decide when to start manual operation to return to Earth - it was a small globe with a clock mechanism, which shows the (calculated, not measured) current location over Earth. It followed the craft position as calculated with the mechanical computer inside, following parameters initially ...

9

The distance is quite easy for something so far out there. You send a command to the satellite, and see how long it takes to respond. The time it takes to respond times the speed of light is the distance that it is away from the Earth. Furthermore, the velocity is likewise easy. If one day it is 17 light hours exactly, and the next day it is 17 hours and 10 ...

8

Yes it absolutely is! I'm writing this to make this clear that your question is squarely on-topic. Here is an example of a (very experienced) amateur, Scott Tilley, both finding and determining the orbit of a satellite, using (very nice) amateur equipment, (very nice) amateur software, and of course a software defined radio or SDR. You can see from ...

7

Since my question didn't yet receive an answer, I did some digging around of my own. This should be considered as a temporary answer, before more precise ones will be actually possible. The answer is, that for the time being, we don't know yet, because we don't yet know what orbit around 67P/Churyumov–Gerasimenko will be first attempted, or is indeed the ...

7

There are several aspects playing a role here: first of all, every measurement has a certain random inaccuracy. Ask ten people to measure the size of an object with the best precision possible and you'll likely get ten slightly different answers. The average of those measurements is usually a much better estimate for the size than any individual measurement. ...

7

Not without a lot of error. The elements in the TLEs are "mean elements", not "osculating elements". i.e. although they have the same names as the Keplerian elements, they are not really measuring the same things. What you can do is evaluate the TLE at epoch using SGP4, and take the resulting Cartesian state vector and transform that via the usual closed-...

6

Here is a good tutorial on the navigation of deep space vehicles. The two main data types used are two-way Doppler (using an atomic clock reference at the DSN station, with the frequency locked to and sent back to Earth by the spacecraft), which gives the velocity component along the Earth-spacecraft line to better than 0.1 mm/s, and ranging (sending a ...

5

Comparisons between predicted orbits (numerically integrated) and fitted orbits (from tracking data) are usually done in an inertial reference frame, such as ICRF or Mean of J2000. Within these frames, the orbit comparison can take several forms, such as point by point comparisons, or comparisons of the orbital elements

4

Propagating a state vector seems easier, you just account for the gravity, change the velocity and update the position. You described the symplectic Euler method (aka semi-implicit Euler method, Euler–Cromer, Newton–Størmer–Verlet, and other names). This is a rather lousy numerical integrator. It loses accuracy rather quickly. Extremely small time steps ...

4

Is there any such record of distinct tracking time required in missions? I was unable to find any mission specific examples of utilized tracking time following a TCM or otherwise. I believe this is simply because it is a highly technical aspect of the mission and is not generally publicized. That being said, there are a few common methods of orbit ...

3

Is it possible to update an existing TLE using new data? Of course it is. How do you think TLEs are updated? That said, it's not easy. You'll need to Compute the Jacobian of your new data with respect to the elements of a TLE you might want to modify. Come up with a weighting matrix that indicates which of those new data are better than others. For ...

3

Here's amateur listener Sven Grahn watching the launch of Chang'e-1 and boost to deep space in 2007, from Tracking Chang’E-1 from Sweden The map in the last one certainly doesn't look like Stockholm, but at that point in time Chang'e-1 is already in deep space so as long as it's in the listener's hemisphere it will be above the horizon. The author also ...

3

This is my tentative answer to my own question (based on suggestions in the comments). Obtaining three different geocentric position vectors (km) over 10 minutes from skyfield.api import EarthSatellite, Topos, load import numpy as np ts = load.timescale() minutes = np.linspace(0, 10, 3) t = ts.utc(2020, 11, 26, 0, minutes, 0) l1 = '1 25544U 98067A 20331....

3

The short answer is: its not clear**, however, if you have no independent measurements of your own then TLEs will have to do. If you have some comparable case where you have both your own measurements and the TLEs then you could make a meaningful comparison. Planet labs used to provide both on their website though I can't immediately find the page ++. ...

3

I sincerely hope that my answer contributes in a meaningful way. I found a very nice visualisation of the manouvers that the Rosetta will go through after catching up to 67P (see below). http://www.youtube.com/watch?v=fNBUep7mPdI It seems as if the short answer to your question may indeed be 'yes' but I guess anything may happen when the spacecraft ...

3

As I understand tone ranging is one of the technique used to determine satellite range from the earth station. In this case the position along the line of sight is being determined. How the position along the other 2 directions are determined ? By taking lots of readings, spread over time. The problem with angle measurements is that even the most ...

2

If you got the position along the line of sight, you can measure the elevation and direction angle of the line of sight too. You have one distance and two angles and using a coordinate transformation from polar coordinates to a cartesian system you can get three distances in x, y and z direction.

2

For a body in orbit in 2-body scenario, for Keplerian elements only a single variable (true anomaly) changes over time. For Carthesian the whole state vector is in constant flux. When working with orbital mechanics in Keplerian coordinates, e.g preparing maneuvers, you don't need to work on a body in motion - 2-degree differential equations of motion all ...

2

ECI doesn't have an actual reference time, it's just an arbitrary one that is useful. It works well for rockets and things with limited lifespans, but for satellites it is a bit trickier. As Celestrak states: What is the reference frame of the resulting coordinates? This question is a bit more technical than most we have covered. To be precise, the ...

2

As the answer to that question states: In general, an epoch time and six scalar values that completely represent the state at the epoch time are needed. The six values you can choose as you like, as long as they are independent variables. For example the position in x,y,z and the velocities in x,y,z directions would suffice. You can choose polar ...

2

I'm not an expert and this is not an expert answer, but these points may be helpful. Don't even think about using SGP4, per my comments below this question and Wikipedia it's a circa 1980's clever approximation to get approximate state vectors within a few weeks of any given TLE's epoch. It's an approximator based on evolution of orbital elements, not a ...

2

Many satellites do not have any position or velocity measurements. Time tag tables including satellite state, magnetic and sun vector are been uploaded from the ground and the satellite uses it for attitude control and orbit control maneuvers. LEO satellite can use GPS and OD features to obtain the satellite state without being dependent on ground updates. ...

2

The first question you need to ask yourself is the following: does the spacecraft need to know its own position? Many, if not most, spacecraft do not need that information. The ground team simply needs to know when to schedule maneuvers, which means that only the ground team needs to know the position of the spacecraft, not the spacecraft itself. If the ...

2

From Space-Track.Org FAQ TLEs can contain future epochs. About 20 satellites are categorized as "multi-day objects" because their period is so large. Consequently, our data provider propagates the epoch into the future based on perigee to enable better tracking by available sensors when the object finally comes back into view. An example is Object ...

2

I thought this question has been answered here before, and that the answer went into some detail about why this is done from time to time, but I can't find any trace of that now. The epoch could, technically, potentially, be in the past or the future by quite a lot, as long as the satellite isn't falling too quickly, since the SGP4 algorithm propagation is ...

2

Short answer, yes. Long answer: There are 6 keplerian orbital elements (but note that these parameters are not the only way to describe an orbit). Semi-major axis, eccentricity, inclination, argument of periapsis, right ascension, and true anomaly. Here is a surface level explanation of each: Semi-major axis describes the "size" of the orbit. In ...

1

(Ok, this is my attempt of answering my own question based on the suggestions in the comments, full credits to @uhoh and @David-Hammen) The main issue As pointed out by @uhoh, the main issue here is parsing of the positional vectors. This part: r1, r2, r3 = geocentric.position.km Parses r1 = [r1.x, r2.x, r3.x], r2 = [r1.y, r2.y, r3.y] and r3 = [r1.z, r2.z, ...

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