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FAQ: Frequently Asked Questions

If you're reading this, you're probably wondering something like:

• What is This Site?
• What is US Space Command?
• What is the Satellite Catalog?
• How are Satellites Tracked?
• What are Orbital Elements?
• What is a Two-Line Element Set (TLE or ELSET)?
• What Can I Use This For?

What is This Site?

This is satcat.net, a free, online, searchable version of US Space Command's satellite catalog. This site is NOT affiliated with USSPACECOM, the US Space Force, the US Department of Defense, or any other branch, agency, or organization of the United States Government.

What is US Space Command?

USSPACECOM is a Unified Combatant Command of the United States Armed Forces. It is a geographic combatant command whose area of responsibility (AOR) begins at 100 km above sea level. USSPACECOM is responsible for military operations in outer space, including the operational employment of space forces provided by the USSF (such as GPS and military satellite communications) as well as strategic missile defense and more.

What is the Satellite Catalog?

The satellite catalog (also called the "spacetrack catalog") is a list of the most up-to-date TLEs published under the authority of US Space Command and maintained by the US Space Force's 18th Space Control Squadron. You can view the official 18 SPCS catalog site here, but you need to make an account to use all its features. This site exists to provide a simpler way to get the same information.

How are Satellites Tracked?

Detailed technical discussion of radars, telescopes, and metric observations is beyond the scope of this FAQ. For more in-depth reading on this and related subjects, see the advanced topics page.

Several countries including the United States, Russia, and China have robust national space infrastructure, portions of which are dedicated to tracking objects in Earth orbit for both civil and military purposes.

In the United States, the Space Surveillance Network performs the majority of this function. (Some commercial entities possess their own means for tracking their satellites.) The SSN evolved out of the missile warning network of the early Cold War and now includes sensors dedicated to spacetrack such as the Ground-Based Electro-Optical Deep Space Surveillance sites, as well as sensors that primarily serve the missile warning network such as PAVE PAWS and PARCS. The output from the SSN is fused by the 18th Space Control Squadron into TLEs, which are then published in the catalog.

What are Orbital Elements?

Detailed technical discussion of orbits, orbital elements, and their calculation is beyond the scope of this FAQ. For more in-depth reading on this and related subjects, see the advanced topics page.

Orbital elements are the parameters used to describe an orbit, such as that of a satellite around the Earth. An orbiting object has six degrees of freedom (meaning it is free to move in three directions and may have a certain velocity in each of those directions), so six independent parameters are required to describe its orbit. There are various ways to define these six parameters.

State Vectors

One way is to use six rectilinear parameters: three position values and three velocity values (each corresponding to one of the three spatial dimensions). These six parameters, along with the moment in time when they are valid, make up a state vector. State vectors have the advantage of being mathematically straightforward, but they have drawbacks as well:

1. They're not very human-readable. In other words, you likely can't visualize an orbit if you are given only position and velocity values for a single point in time!
2. They change constantly with time. This means knowing the state vector at one moment won't, by itself, tell you where the object will be or what the orbit will look like an hour, a day, or a week from now.
3. They're computationally expensive. If you know a satellite's state vector at 12:00, computing the satellite's position at 12:30 will require you to integrate the equations of motion for the moments in between. The more accuracy you need, the more intermediate computational steps you need to perform.

Keplerian Elements

Most of the time, the better way is to use the classical orbital elements (sometimes called Keplerian elements after the astronomer Johannes Kepler.) Five of these elements are constant over time. Two describe the size and shape of the orbital ellipse. Two more describe the orientation of the plane containing the orbital ellipse. A fifth element describes the orientation of this ellipse within its orbital plane. Lastly, the sixth and final element describes the position of the orbiting object on its ellipse. This is the only classical or Keplerian element that changes over time. This makes Keplerian elements a much simpler way to represent an orbit and to predict the future position of an orbiting object.

Keplerian elements are part of an idealized orbital model that neglects certain real-world perturbations such as gravitational effects of other bodies, solar radiation pressure, atmospheric drag, and special and general relativity. The longer you simulate an orbit, the more the Keplerian model will diverge from the real world. Most propagators, such as SGP4/SDP4, accept Keplerian elements as input and account for the effects of some perturbations in their output. This provides a much more accurate representation of orbits over time than the pure Keplerian model, while retaining the features that make Keplerian elements more attractive than state vectors.

The Keplerian Elements Defined

Orbit Size and Shape

Two elements define the size and shape of the orbital ellipse:

• Eccentricity (e): A measure of how elliptical the orbit is. An eccentricity of 0 is a perfect circle; an eccentricity of 1 is a parabola. All Earth-orbiting objects have an eccentricity that lies between 0 and 1.
• Semimajor axis (a): Half the longest axis of the ellipse. This is also half the sum of the apoapsis and periapsis radii. It is the actual distance from the orbit barycenter for circular orbits and the average distance from the center for elliptical orbits.

Orbital Plane Orientation

The orbital plane (the plane in which the orbital ellipse actually lies) is described with two elements:

• Inclination (i): The angle between the orbital plane and a reference plane. For Earth orbits, the reference plane is the equatorial plane.
• Longitude of the Ascending Node (Ω): The angle between the ascending node (the place where the orbit crosses upward through the reference plane) and the reference plane's origin of longitude. In the Earth-centered coordinate system, the origin of longitude points to the First Point of Aries, which is the location of the vernal equinox. In this frame, Ω is called the Right Ascension of the Ascending Node (RAAN). It is measured counterclockwise from the vernal equinox.

Orbit Orientation within Plane

Since the orbital plane is now fully defined, the orientation of the orbit's ellipse within its plane may be described by a single element:

• Argument of periapsis (ω): The angle between the ascending node to periapsis (the point at which the orbiting object is closest to the primary object.) For Earth-centered orbits, periapsis is called perigee and ω is called the argument of perigee.

Position of Object on its Orbit

The first five Keplerian orbits describe the size, shape, and orientation of the orbit itself. The sixth and final element is an angular parameter (called an anomaly for historical reasons) that describes the object's actual position on its orbit. There are three anomalies that can be used to describe this:

• True anomaly (ν): The position of the object on its orbit at a specified time called the epoch. For Earth-orbiting objects, ν is an angle measured counterclockwise from perigee.
• Mean anomaly (M): True anomaly does not vary linearly with time for non-circular orbits. The mean anomaly is a convenient abstraction for simplifying this problem. It is the angular position that the object would have if its orbit were perfectly circular with the same orbital period.
• Eccentric anomaly (E): An intermediary value used to compute the true anomaly ν from the mean anomaly M. Draw a right triangle with one leg located on the major axis and the hypotenuse connecting the center of the ellipse to the object's location at the specified time. E is the angle between the leg on the major axis and the hypotenuse.

Table of Keplerian Elements

What is a Two-Line Element Set (TLE or ELSET)?

For more explanation of the TLE format, as well as more technical explanations of other FAQ subjects, see the advanced topics page.

A two-line element set (TLE or ELSET) is a standardized format for encoding the orbital elements of an Earth-orbiting satellite as measured at a certain point in time, known as the epoch. The United States Space Force tracks all detectable objects in Earth orbit and produces a TLE for each one. These TLEs are maintained in a satellite catalog which is kept up-to-date by the 18th Space Control Squadron, whose members process the 24/7 stream of input from radars and telescopes dispersed all over the globe in the US Space Surveillance Network.

The TLE Format Explained

Here is an example TLE for the International Space Station:

ISS (ZARYA)
1 25544U 98067A   08264.51782528 -.00002182  00000-0 -11606-4 0  2927
2 25544  51.6416 247.4627 0006703 130.5360 325.0288 15.72125391563537

Title Line

This line is optional. When included, it gives the common name for the satellite.

ISS (ZARYA)

Line 1

1

25544

U

98

067

A

08

264.51782528

-.00002182

00000-0

-11606-4

0

292

7

Line 2

2

25544

51.6416

247.4627

0006703

130.5360

325.0288

15.72125391

56353

7

What Can I Use This For?

Detailed technical discussion of orbital perturbations and satellite orbit prediction models is beyond the scope of this FAQ. For more in-depth reading on this and related subjects, see the advanced topics page.