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FAQ: Frequently Asked Questions

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

Read on, or use the links above to navigate to a particular question.

What is This Site?

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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?

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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?

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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?

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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?

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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.

Diagram of orbital elements
Orbital elements (image credit: braeunig.us/space)

The Keplerian Elements Defined

Orbit Size and Shape

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

Orbital Plane Orientation

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

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:

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:

Table of Keplerian Elements

Generic Name Geocentric Name Notation Symbol Definition Units
Eccentricity " e Lowercase Latin e Measure of ellipse circularity Unitless; varies from 0 (perfect circle) to 1 (parabola) and greater (hyperbola)
Semimajor axis " a Lowercase Latin a Half the major axis of the ellipse kilometers
Inclination " i Lowercase Latin i Angle between orbital plane and reference plane degrees
Longitude of the Ascending Node Right Ascension of the Ascending Node (RAAN) Ω Uppercase Greek omega Angle from reference plane origin to ascending node degrees
Argument of Periapsis Argument of Perigee ω Lowercase Greek omega Angle from ascending node to point of periapsis/perigee degrees
True Anomaly " ν Lowercase Greek nu Angle from periapsis/perigee to object location on orbit degrees

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

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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

This TLE is broken down line-by-line below:


Title Line

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

Column 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69
Data I S S ( Z A R Y A )
Field 1

Field Columns Content Data
1 01-24 Satellite name
ISS (ZARYA)


Line 1

Column 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69
Data 1 2 5 5 4 4 U 9 8 0 6 7 A 0 8 2 6 4 . 5 1 7 8 2 5 2 8 - . 0 0 0 0 2 1 8 2 0 0 0 0 0 - 0 - 1 1 6 0 6 - 4 0 2 9 2 7
Field 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Field Columns Content Data
1 01-01 Line number
1
2 03-07 Satellite Catalog Number
25544
3 08-08 Classification (U=Unclassified, C=Confidential, S=Secret)
U
4 10-11 International designator (last two digits of launch year)
98
5 12-14 International designator (launch of the year)
067
6 15-17 International designator (piece of the launch)
A
7 19-20 Epoch year (last two digits of year)
08
8 21-32 Epoch (fractional day of year)
264.51782528
9 34-43 First derivative of mean motion (Ballistic coefficient)
-.00002182
10 45-52 Second derivative of mean motion (decimal point assumed)
00000-0
11 54-61 Drag term (radiation pressure coefficient or BSTAR value) (decimal point assumed)
-11606-4
12 63-63 Ephemeris type (always 0 in public data)
0
13 65-68 Element set number; incremented with each new TLE
292
14 69-69 Checksum (modulo 10)
7


Line 2

Column 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69
Data 2 2 5 5 4 4 5 1 . 6 4 1 6 2 4 7 . 4 6 2 7 0 0 0 6 7 0 3 1 3 0 . 5 3 6 0 3 2 5 . 0 2 8 8 1 5 . 7 2 1 2 5 3 9 1 5 6 3 5 3 7
Field 1 2 3 4 5 6 7 8 9 10

Field Columns Content Data
1 01-01 Line number
2
2 03-07 Satellite catalog number
25544
3 09-16 Inclination (degrees)
51.6416
4 18-25 Right ascension of the ascending node (RAAN) (degrees)
247.4627
5 27-33 Eccentricity (decimal point assumed)
0006703
6 35-42 Argument of perigee (degrees)
130.5360
7 44-51 Mean anomaly (degrees)
325.0288
8 53-63 Mean motion (revolutions/day)
15.72125391
9 64-68 Revolution number (at epoch time)
56353
10 69-69 Checksum (modulo 10)
7

What Can I Use This For?

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