Where do I get TLE Information, TLE Supplemental Data, and Other TLE Resources?

What is a TLE?
A two-line element set (TLE) is a data format encoding a list of orbital elements of an Earth-orbiting object for a given point in time. The state of the object at any point in the past or future can be estimated to a limited degree of accuracy. You can use TLE data in the SGP4 propagator in STK. The format uses two lines of 80-column ASCII text to store the data. For more information, refer to the STK Desktop help topic TLE File Format.
 
How do you add a satellite using TLE data within STK?
There are two methods for defining a satellite object with TLE data in STK.

1. The Standard Object Database (SOD) in STK includes all satellites defined by their publicly available TLEs and enables you to conduct searches for satellites of interest. These SOD entries include descriptive data about each satellite — Satellite Owner, Launch Date, Common Names, Mission, etc. — as well as its orbit via TLE information. The SOD is accessible with all STK licenses. To learn how to use the Standard Object Database, refer to the STK Desktop help topic Insert Spacecraft Using the Standard Object Database.

2. You can add a satellite directly from a TLE file.
   1. In the Insert menu, click New...
   2. In the Insert STK Objects dialog box, select Satellite in the Scenario Objects section and From TLE File in the Select A Method section, and then click Insert...
   3. In the dialog box that appears, browse to the file the you want to use and then click Open.
   4. In the dialog box that appears, select which satellites that you want to create from the TLE data in the file.

What is the source of the TLE database?
The main source of TLE data comes from the STK Database from the Combined Space Operations Center (CSpOC) under the United States Space Command 's Combined Force Space Component Command. CSpOC is a multinational space operations center that provides command and control of space forces. CSpOC currently keeps active track of more than 17,000 space objects using the United States Space Surveillance Network (SSN). These tracked objects constitute the space object catalog. While most of the catalog is made available to the public, some information is restricted. AGI provides the publicly released information from this catalog for use with STK in the form of satellite database files and two-line mean element files (TLEs).

How often is the TLE database updated?
The AGI Satellite database is updated three times a day at approximately 0100, 0900, and 1700 ET. 
 
How accurate is a TLE?
The TLE sets provided by CSpOC have different degrees of accuracy. All TLEs provided to the public have been derived from radar and optical observations of the U.S. Space Surveillance Network (SSN). This process can cause problems, particularly for deep-space objects (orbital periods greater than 225 minutes), which rely on optical observations. Limited geographic distribution of these optical sites, together with unpredictable weather conditions, can result in long intervals between observations. This results in issues such as delays in cataloging objects, old and inaccurate data, and even difficulty in recovering objects. There are additional accuracy issues with respect to operational satellites that have maneuvering capability; the TLE determination process for an object can take days to recover its solution after a maneuver has occurred.

Therefore, in general, the accuracy of a TLE ranges from 1km to 200km depending on many factors, including the satellite’s orbit, the number of SSN tracking sites in view of it, the weather at SSN’s optical sites, the satellite’s maneuverability, and potentially other unknown factors. To get a consistent and accurate orbit solution, you will need to find additional sources of ephemeris data (discussed next in this article).

The accuracy of the TLE will also degrade as you propagate away from the epoch time of the TLE. Generally, you should only propagate TLEs for a few days to a couple of weeks from the epoch date. 
 
Is there a more accurate ephemeris data source than the TLE?
The one major source of ephemeris data for all satellites is the Commercial Space Operations Center (COMSPOC™). COMSPOC is a space situational awareness (SSA) facility that fuses satellite tracking measurements from a continually growing global network of commercial sensors. COMSPOC generates High Definition Ephemeris (HiDEph™) and other highly accurate SSA data products for resident space object (RSO) characterization.

Uncertainty for objects in the COMSPOC’s catalog is far below 1km and is maintained throughout the entire orbit. The COMSPOC uncertainty of all orbits is about 50-200 meters for GEO and about 40-80 meters for LEO. For more information, visit COMSPOC.com.
 
Where can I get supplemental TLE data / ephemeris data?  
Besides the high-accuracy, comprehensive HiDEph dataset offered by COMSPOC, there are additional sources of supplemental ephemeris data collected from owner-operators. The accuracy of these sources is better than the TLE, but they are only available for a limited number of operational satellites, making these data sets incomplete. For a limited number of additional satellites, Celestrak offers supplemental two-line element sets (TLEs) derived directly from data supplied by owner-operators. While an uncooperative approach to tracking space objects is required for a large portion of the current satellite catalog, many of these objects are operational payloads that are routinely tracked by their owners or operators. Some of this orbital data, such as for the GPS constellation, is publicly available via the Internet. For Planet, orbits are derived from two-way UHF ranging data and are typically accurate to better than 2 km within 24 hours of epoch. This owner-operator provides better TLE data than JSpOC because their ranging data is typically accurate to ~1 km and always positively identifies the satellite, since there is a hardware ID included in packets.  This data is for the Global Navigation Satellite Systems (GNSS), in RINEX format. 
 
How do I download the AGI Satellite Database?
AGI has two methods to download the AGI Satellite Databases.

1. Use the STK/ODTK internal Data Update Utility to download the latest TLE files. To use this utility, refer to the AGI FAQ article Updating STK Databases.

2. Use the AGI Satellite Database’s Archive. This method will enable you to download any TLE file from the archived database. To download from the archived database, visit AGI's Satellite Database resource web page. For more information about the AGI Satellite Database, refer to the STK Desktop help topic Insert Spacecraft Using the Standard Object Database.

Additional resources for this article:
Using STK to verify the results in AIAA 2006-6753 Revisiting Spacetrack Report #3