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I'm writing a C# library to make working with orbits easier.

Given I plan on being able to input data from space-track elsats, which uses TEME SGP4 orbital elements, it's looking increasingly likely that I'm going to need to use their SGP4 lib for reference frame conversions.

It's not clear to me under what license the SGP4 lib has been released, so it's not entirely clear to me whether or not I can package this as part of my lib.

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  • $\begingroup$ I'm not familiar with a library distributed by Space-Track, but the origial SGP4, etc code are public domain. The community maintained versions, also public domain, are available at the URL below. You can also generally find code with a permissive license in pretty much any programming language by searching for it. celestrak.org/software/vallado-sw.php $\endgroup$ Jan 7, 2023 at 22:07
  • $\begingroup$ @GregMiller - This is the library I'm talking about space-track.org/documentation#/sgp4 $\endgroup$ Jan 7, 2023 at 22:08

2 Answers 2

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You can redistribute and even sell the software based on the terms of the "AIR FORCE SPACE COMMAND (AFSPC) OPEN SOURCE AGREEMENT for SGP4" as quoted in Ryan C's incorrect answer. Ryan was kind enough to quote the whole license here, but I'll quote the relevant portions:

  1. GRANT OF RIGHTS

A. Under Non-Patent Rights: Subject to the terms and conditions of this Agreement, each Contributor, with respect to its own contribution to the Subject Software, hereby grants to each Recipient a non-exclusive, world-wide, royalty-free license to engage in the following activities pertaining to the Subject Software:

  1. Use
  2. Distribution
  3. Reproduction
  4. Redistribution
  5. Display

B. Under Patent Rights: Subject to the terms and conditions of this Agreement, each Contributor, with respect to its own contribution to the Subject Software, hereby grants to each Recipient under Covered Patents a non-exclusive, world-wide, royalty-free license to engage in the following activities pertaining to the Subject Software:

  1. Use
  2. Distribution
  3. Reproduction
  4. Sale
  5. Offer for Sale

C. Redistribution and use of binary forms, are permitted provided that the following conditions are met:

  1. Redistributions in binary form must reproduce the above copyright notice,

Check the license from whatever SGP4 version you're using, because you're required to preserve the copyrights when you redistribute it and contributors may add their own copyright notices.

this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

  1. Neither the name of the Air Force Space Command nor the names of its contributors may be used to endorse or promote products derived> from this software without specific prior written permission.
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You can freely make and distribute your own code which calls the SGP4 libraries, but you may not distribute those libraries themselves. Everyone who wants to use whatever you create will need to make their own (free) account on space-track.org, and will have to download their own individual copy of the US Government libraries. The full license is included as a text file in the zip distribution, in several different places. It reads:

AIR FORCE SPACE COMMAND (AFSPC) OPEN SOURCE AGREEMENT for SGP4 - (Simplified General Perturbations #4)

THIS OPEN SOURCE AGREEMENT ("AGREEMENT") DEFINES THE RIGHTS OF USE, REPRODUCTION, DISTRIBUTION, MODIFICATION AND REDISTRIBUTION OF CERTAIN COMPUTER SOFTWARE ORIGINALLY RELEASED BY THE UNITED STATES GOVERNMENT AS REPRESENTED BY THE GOVERNMENT AGENCY LISTED BELOW ("GOVERNMENT AGENCY"). THE UNITED STATES GOVERNMENT, AS REPRESENTED BY GOVERNMENT AGENCY, IS AN INTENDED THIRD-PARTY BENEFICIARY OF ALL SUBSEQUENT DISTRIBUTIONS OR REDISTRIBUTIONS OF THE SUBJECT SOFTWARE. ANYONE WHO USES, REPRODUCES, DISTRIBUTES, MODIFIES OR REDISTRIBUTES THE SUBJECT SOFTWARE, AS DEFINED HEREIN, OR ANY PART THEREOF, IS, BY THAT ACTION, ACCEPTING IN FULL THE RESPONSIBILITIES AND OBLIGATIONS CONTAINED IN THIS AGREEMENT.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

The intended purpose of the SGP4 propagator is to properly propagate an object's Two Line Element Set (TLE) supplied by the USSTRATCOM'S Joint Functional Component Command for SPACE Joint Space Operations Center (JSpOC) such that the positions calculated for that object agree with those calculated by the JSpOC. SGP4 propagator is to be used solely for the purpose of propagating JSpOC created TLEs. TLEs created and supplied by the JSpOC are created in such a way as to be compatible with the SGP4 propagator. In this way, compatibility/interoperability issues between the JSpOC and the user of JSpOC SGP4 TLEs are addressed. Examples of what SGP4 should not be used for, included but are not limited to, propagating JSpOC created state vectors which come from any of the various other messages the JSpOC creates, such as Vector Covariance Messages, Orbital Conjunction Messages, Conjunction Data Messages, etc.

Government Agency: Air Force Space Command Government Agency Original Software Designation and Software Title: SGP4

User Registration Requested. Please Visit: Government Agency Point of Contact for Original Software:

  1. DEFINITIONS

A. "Contributor" means Government Agency, as the developer of the Original Software, and any entity that makes a Modification.

B. "Covered Patents" mean patent claims licensable by a Contributor that are necessarily infringed by the use or sale of its Modification alone or when combined with the Subject Software.

C. "Display" means the showing of a copy of the Subject Software, either directly or by means of an image, or any other device.

D. "Distribution" means conveyance or transfer of the Subject Software, regardless of means, to another.

E. "Larger Work" means computer software that combines Subject Software, or portions thereof, with software separate from the Subject Software that is not governed by the terms of this Agreement.

F. "Original Software" means any or all of the computer software first released under this Agreement by Government Agency with Government Agency designation SGP4, with title identified herein, including object code and accompanying documentation, if any.

G. "Recipient" means anyone who acquires the Subject Software under this Agreement, including all Contributors.

H. "Redistribution" means Distribution of the Subject Software to another party.

I. "Reproduction" means the making of a counterpart, image or copy of the Subject Software.

J. "Sale" means the exchange of the Subject Software for money or equivalent value.

K. "Subject Software" means the Original Software, or any respective parts thereof.

L. "Use" means the application or employment of the Subject Software for any purpose.

  1. GRANT OF RIGHTS

A. Under Non-Patent Rights: Subject to the terms and conditions of this Agreement, each Contributor, with respect to its own contribution to the Subject Software, hereby grants to each Recipient a non-exclusive, world-wide, royalty-free license to engage in the following activities pertaining to the Subject Software:

  1. Use
  2. Distribution
  3. Reproduction
  4. Redistribution
  5. Display

B. Under Patent Rights: Subject to the terms and conditions of this Agreement, each Contributor, with respect to its own contribution to the Subject Software, hereby grants to each Recipient under Covered Patents a non-exclusive, world-wide, royalty-free license to engage in the following activities pertaining to the Subject Software:

  1. Use
  2. Distribution
  3. Reproduction
  4. Sale
  5. Offer for Sale

C. Redistribution and use of binary forms,are permitted provided that the following conditions are met:

  1. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

  2. Neither the name of the Air Force Space Command nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

D. The rights granted in Paragraphs A. and B. allow the Recipient to sublicense those same rights. Such sublicense must be under the same terms and conditions of this Agreement.

  1. OBLIGATIONS OF RECIPIENT

A. Distribution or Redistribution of the Subject Software must be made under this Agreement except for additions covered under paragraph 3H.

  1. Whenever a Recipient distributes or redistributes the Subject Software, a copy of this Agreement must be included with each copy of the Subject Software; and

B. Each Recipient must ensure that the following copyright notice appears prominently in the Subject Software:

Copyright 2015 United States Government as represented by the Administrator of Air Force Space Command. All Rights Reserved.

C. A Contributor may add its own copyright notice to the Subject Software. Once a copyright notice has been added to the Subject Software, a Recipient may not remove it without the express permission of the Contributor who added the notice.

D. A Recipient may not make any representation in the Subject Software or in any promotional, advertising or other material that may be construed as an endorsement by Government Agency or by any prior Recipient of any product or service provided by Recipient, or that may seek to obtain commercial advantage by the fact of Government Agency's or a prior Recipient's participation in this Agreement.

E. In an effort to track usage and maintain accurate records of the Subject Software, each Recipient, upon receipt of the Subject Software, is requested but not obligated to register with Government Agency by visiting the following website: astrodynamicStandards.org. Recipient's name and personal information shall be used for statistical purposes only.

F. A Recipient may choose to offer, and to charge a fee for, warranty, support, indemnity and/or liability obligations to one or more other Recipients of the Subject Software. A Recipient may do so, however, only on its own behalf and not on behalf of Government Agency or any other Recipient. Such a Recipient must make it absolutely clear that any such warranty, support, indemnity and/or liability obligation is offered by that Recipient alone. Further, such Recipient agrees to indemnify Government Agency and every other Recipient for any liability incurred by them as a result of warranty, support, indemnity and/or liability offered by such Recipient.

G. A Recipient may create a Larger Work by combining Subject Software with separate software not governed by the terms of this agreement and distribute the Larger Work as a single product. In such case, the Recipient must make sure Subject Software, or portions thereof, included in the Larger Work is subject to this Agreement.

H. Notwithstanding any provisions contained herein, Recipient is hereby put on notice that export of any goods or technical data from the United States may require some form of export license from the U.S. Government. Failure to obtain necessary export licenses may result in criminal liability under U.S. laws. Government Agency neither represents that a license shall not be required nor that, if required, it shall be issued. Nothing granted herein provides any such export license.

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    $\begingroup$ What part says it can't be redistributed? Section 2C seems to explicitly allow redistribution. $\endgroup$ Jan 8, 2023 at 2:48

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