EOSOC Observing Program – FAQs

Earth Orbiting Satellite Observing Program  Frequently Asked Questions

This page will be used as a collection point for common questions submitted to the EOSOC  Coordinator. If you have a question that is not answered here, please send it to the EOSOC Coordinator as shown on the main EOSOC page – you probably aren’t the only one with that question!


Q: What are the requirements of the EOSOC program?

A: The EOSOC task requirements are listed below:

  • Active Payloads (4) – Observe four different objects that are active, i.e. operational, payloads. Examples include the Hubble Space Telescope, GPS satellites, etc.
  • Rocket Bodies (4) – Observe four different objects that are discarded rocket bodies. These are often annotated with “r/b” in the name in the elset file.
  • Manned Spacecraft (2) – Observe three different manned spacecraft. For example, the International Space Station, a manned Soyuz spacecraft and a manned dragon spacecraft.
  • Multinational Spacecraft (4) – Observe five objects that belong to five different countries, other than the USA. Examples include objects belonging to Russia, India, China, Brazil, France, Japan etc.
  • Multipass (2) – Observe two different objects, recording observations of each satellite on two different passes in the same night. Note: this task will result in four different observations. For example, two passes for rocket body one, and two passes for payload two.
  • Formation (2) – Observe two pairs of objects flying in formation. For example, a resupply vehicle and the International Space Station just prior to docking (or just after undocking). Each pair of satellites should be visible at the same time. The satellites observed must be deliberately flying in formation, and not a coincidental alignment. Each formation recorded is considered one observation, and the satellite pair should be recorded on the same log sheet. Both satellites should be visible at the same time.
  • Aged Elsets (2) – Observe two different objects with current and aged element sets. The current elset observations should be made with elsets less than one week old, and the aged elset observations should be made with elsets more than three weeks old. Note: this task will result in four different observations.
  • Constellation (2) – Observe two different objects in the same constellation of satellites. A satellite constellation is a group of artificial satellites working together as a system to provide mission coverage that exceeds that of a single satellite (i.e. GPS), or provides data that exceeds what a single satellite is capable of (i.e. A-train). It is recommended to use constellations that use low earth orbits. Typical satellites in low-earth orbit constellations, to use for EOSOC, are; Starlink, Iridium, A-train, SPOT and others.
  • The requirement to observe Iridium flares has been deleted.


Q: What, exactly, is a “pass”?

A: A pass refers to when a satellite rises above the horizon, crosses some portion of the sky, and then sets. For the purposes of the Earth Orbiting Satellite Observers Club, a given pass of any satellite can only be used to satisfy one observing task. For example, if the International Space Station (ISS) makes a pass overhead, you can use that pass to satisfy one of the manned spacecraft tasks. If you wish to use ISS for another task (perhaps a formation task), you must wait for another pass.

Q: Can I reuse the same object on two different tasks?

A: Yes, provided two conditions are met: 1) you must not use the same pass for two different tasks, and 2) the task description doesn’t explicitly state that different objects must be used. For example, using the International Space Station (ISS) to satisfy a manned spacecraft task, and then on a separate pass, to satisfy a formation task is permitted. However, using the ISS to satisfy a manned mission in February, and then again to satisfy a manned mission in November is not allowed, the manned tasks are required to use two different spacecraft. However, a SpaceX Dragon, Starliner, or other manned spacecraft that is re-launched at a later date could be used a second time.

Q: Is a “dead” satellite considered an “active payload”?

A: No. An active payload is defined as an object that continues to serve its primary mission. If, for example, the Hubble Space Telescope were to die in orbit, it would no longer be usable as an active payload task. Similarly, spent rocket bodies are not considered active payloads (they no longer serve their primary mission). There are a few special cases to this rule – some objects are simply passive objects that are used for space surveillance or gravitational geopotential references. These often consist of an inert, shiny object, with no active on-board components. These would qualify as active payloads.

Q: Can I use observations of satellites that I recorded in the past?

A: Yes, provided you have the information specified on the EOSOC Observation Logs (observer position, elset date, etc.), and if necessary an “official” elset archive can be located that can be used to verify the observation.


Q: Can I use observations of satellites that I recorded in the past?

A: One way to perform this requirement is to download a current element set for the satellite you want to use. Satellite rocket bodies are recommended. Note: because the orbit of the ISS is frequently ‘adjusted’ up to prevent its decay as well as many other active/operational low earth satellites; these are not good satellites to use. Next observe this object within 2 or 3 days of the element set epoch date. Then wait 3 weeks. After 3 weeks, use what is now the same ‘old’ element set in your propagation software to predict where and when to observe it. Observe the satellite and record when the pass occurs. Its time should be noticeably different than predicted since the orbit is slowly ‘decaying’. Be sure to be in-place to observe the pass well before the predicted time.


Q: Can I use geosynchronous satellites to satisfy observing tasks?

A: Yes, but not recommended, you need to realize these satellites can only be viewed through a moderate size telescope as they are at most about 12th magnitude. Their magnitude brightness is a function of the sun-satellite-earth angle and so their brightness can vary significantly based on this ‘phase-angle’. Another issue is in how to plot the position of a geosync satellite on the observation forms. You can certainly observe the satellite over an extended period of time to generate a “pass” with respect to the stars (actually, the stars would be streaks and the satellite would be stationary). The most practical way to plot them would be to plot their positions against the stars about every 30 minutes over a 2 or 3-hour period. This would result in the apparent motion of the satellite, but in fact what you would be recording is the diurnal motion of the stars.

Q: Can I use two geosynchronous satellites to satisfy the formation flight tasks?

A: No. The intent of the formation flight tasks is to locate and track two (or more) objects that are deliberately flying in close proximity to each other. In the case of geosync satellites, the fact that any two are near each other is coincidental, as they do no rely on being near each other to complete their respective missions. Similarly, two satellites that happen to pass near each other (in different orbits) do not qualify for the formation tasks. The likely candidates for formation flight are ISS and space vehicles arriving just before docking (or just after separating), or payload/booster pairs just after staging.


Q: Can I use the SpaceX ‘Space Train’ Starlink satellites after a launch to meet this requirement?

A: No, the Starlink satellites are not technically in formation. After the launch they are slowly spacing themselves in equal distance around the earth in the same orbit plane. They are members of the same satellite constellation and can be used to meet the Constellation requirement.


Q: Will there be an advanced EOSOC award program?

A: Under consideration. An advance version would probably include tasks such as observing geosynchronous satellites, GPS satellites, Molniya satellites, initial orbit determination and photograph/CCD imaging. We will continue to monitor overall interest.


To go back to the EOSOC Observing Program main page, click here.

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