02 / WHY IT WORKS

Shape, orientation and timing

Inclination tilts an orbital plane relative to the equator. Eccentricity describes departure from a circle. The ascending node and argument of perigee orient the orbit; mean anomaly describes phase within the model. Mean motion is measured in revolutions per day. Do not read fixed-width fields as space-separated free text.

Why old elements become less useful

Propagation extends a fitted model beyond its reference interval. Unmodeled forces, maneuvers and estimation errors can accumulate. Element age is a warning signal, not a universal conversion to kilometers of error. Read the data source and epoch together with the requested time.

03 / UNDER THE SURFACE

From intuition to a model

A separate two-body experiment

The ellipse animation solves Kepler’s equation at equal time increments, so the satellite moves faster near perigee and slower near apogee. Earth is placed at a focus, not at the ellipse center. This teaches ideal orbital geometry; it is not an implementation of the TLE propagation model.

M=E−esin⁡EM=E-e\sin Ex=a(cos⁡E−e)y=a1−e2sin⁡E\begin{aligned}x&=a(\cos E-e)\\y&=a\sqrt{1-e^2}\sin E\end{aligned}

M: mean anomaly; E: eccentric anomaly (radians); e: eccentricity; a: semi-major axis. Coordinates are relative to the focus. Ideal radius r = a(1 − e cos E).

Reading the sample fields

In the bundled GPS example, epoch 24247.05618026 means day 247.05618026 of 2024. Eccentricity 0083689 has an implied decimal point: 0.0083689. Mean motion 2.00561065 rev/day corresponds to an approximate period of 1440 / 2.00561065 minutes. Use a supported GP format and compatible library for real propagation.

T≈1440n  minT\approx\frac{1440}{n}\;\mathrm{min}

n: mean motion in revolutions per day; this period estimate is not a precision orbit reconstruction.

04 / A WORKED EXAMPLE

Change eccentricity without changing the semi-major axis

Start at e = 0: the orbit is circular and the focus coincides with the center. Set e = 0.40. The nearest and farthest radii become 0.60a and 1.40a. Pause at those points and compare the speed ratio readout. This deliberately exaggerated orbit makes the effect visible; it does not claim that the sample GPS TLE has e = 0.40.

Return to the experiment and try it ↑

05 / PUT IT TO WORK

Try it in GNSS View

  1. Open the observation data details and inspect source and epoch.
  2. Keep requested dates near the available element interval.
  3. Treat stale-data notices as a reason to qualify results, not to hide them.
Open the observatory ↗

06 / COMMON QUESTIONS

Is a TLE the same as broadcast ephemeris?

No. They use different representations and models for different purposes. Receivers use navigation messages and other corrections for positioning.

Why is Earth off-center?

In the ideal two-body model, the central body is at one focus. The center-to-focus distance is ae.

Can I predict far into the future?

A calculation may run, but that does not establish accuracy. Use element age, propagation limitations and fresh data to judge suitability.

07 / FURTHER READING

An original GNSS View explanation. These references document the models, terminology and system context.

CelesTrak · General Perturbations data ↗