![]() ![]() ![]() The change in precession and nutation from the epoch of the TEMEįrame to the epoch of the transformation. Subsequent rotation matrices must therefore account for TEME of Epoch-In this approach, the epoch of the TEME frame is heldĬonstant. All transformations are found using the complete (1)), but this implementation enables comparison with the TEME Gone directly to PEF without the TOD frame (second equation inĮq. Standard transformation from TOD to ECF is computed. The transformation to ECEF is done by first finding theĬonversion from TEME to TOD (third equation in Eq. ![]() The same as the epoch of the associated ephemeris generation ![]() TEME of Date-With this option, the epoch of the TEME frame is always However, you should be aware of an additional nuance, specifically the The following is from " Revisiting Spacetrack Report #3". Because the reference time used in the TLEs, thus TEME, has never been officially confirmed. How do I determine the Greenwich sidereal time from a given TEME But I apologize in advance if my interpretations are wrong! The precession is a slow motion of the CEP with a perod of 25700 years. The "of date" term means that the precession is taken into account. 1, you have to take the intersection between the Ecliptic and the "Mean Equator" (Equator computed with precession equation, but ignoring nutation), then project it to the "True Equator". we can see that the "Mean Equinox", in the TEME sense, is on the "True Equator" plane, but it is not the intersection of this True Equator with the (true) Ecliptic (so not really an Equinox in its classical sense!). This is better explained in, page 4, just before Fig.1, which I also reproduce here. What does "mean Equinox" refer to? It means the Equinox located on the "mean Equator". The mean Equator is the plane perpendicular to an axis computed from the CEP from which the nutation terms are excluded. What does "true Equator" mean? According to my reading, it means the plane perpendicular to the CEP at a given specified time. Wiki ( ) gives a clear definition: "true" is when all variations are accounted for and "mean" is when oscillating variations are ignored (eg "small periodic oscillations of nutation"). With this in mind, we must understand first the difference between "true" and "mean" used in TEME. Yet it does! Wrt to the Celestial Sphere, Earth orbital trajectories have a nutation and a precession. That's good so far, provided that this Z direction doesn't move wrt to the stars (Celestial Sphere). įirst, we all know that TEME is in the class of ECI (non-rotating) reference systems, with the prime direction (X) pointing along the Vernal Equinox and the Z direction is the CEP (Celestial Ephemeris Pole). My answer will be based largely on this paper. Especially, when I discovered that "TEME" may have different definitions, interpretations, in various implementations of SGP4 (cf ). The reason is that I thought I understood the TEME concept until, trying to answer your question leads me to be more cautious. Sorry if some of this doesn't make sense, I am trying to learn and any help would be greatly appreciated. My question then is, how do I determine the Greenwich sidereal time from a given TEME reference time - it makes sense to me when we have a well defined J2000 epoch - but doesn't make sense to me if this epoch time is changing every time TLE data is published. Where I am getting confused now, is that apparently TLE (two-line element) data from satellites use a TEME (true equator, mean equinox) ECI reference frame, which a given epoch time with the TLE data. This Greenwich sidereal time, we use (what I assume to be) an empirical formula to determine the angle, with some time from the J2000 epoch. I followed this article here, where we determine my local sidereal time, by determining the Greenwich sidereal time, which from what I understand, is the conversion between the x-axis in the ECI reference frame, to the ECEF reference frame (with the prime meridian as the x-axis). From this, I can either translate my position on Earth to the ECI reference frame, or translate the satellites position from the ECI, to my ECEF (earth-centered, earth-fixed) reference frame. I understand the importance of defining orbits to an ECI (earth-centered inertial) reference frame. I have zero background in astrophysics and the such, but I have learned a ton so far. I am undertaking a project of developing a "surface station" for my desktop, which will track the position of different satellites as they fly overhead. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |