Orbital Focus - International Spaceflight Facts and Figures
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Tyneside, UK
2024 May 20
Monday, Day 141

Curated by:

X-37B And The KH-11 Spysats

In the days before planned launch in May 2020, formal notices were published giving details of danger areas off the US east coast and the times they were to be active. They covered the fall back to Earth of Atlas V launch vehicle components. The times were particularly useful because they pointed to the planned launch-window on May 17 and further windows on May 18 and May 19..

Unusually, the day to day change in the times was neither logical at first glance nor did it seem to follow a pattern.

It is is actually the norm for OTV/X-37B launches. Some analysis was done nearly eight years ago. The possibility of KH-11 involvement was raised at the time of the OTV-2 mission in 2012 in the Seesat-L discussion group. There is also a page devoted to the subject on the Zarya website.

An earlier version of this page speculated on the possibility of OTV-6 having an interaction with both the polar-orbiting KH-11 satellites. A subsequent review showed an error in interpretation of the pre-launch Navigation Warnings so there is only one KH-11 involved as there has been for all previous X-37B missions.

Publication Of The Launch Windows

Below is the Maritime Navigation Warning:

A. 28-36-51N 080-35-57W, 28-41-00N 080-26-00W, 28-36-00N 080-23-00W, 28-31-36N 080-33-34W.
B. 32-28-00N 075-12-00W, 33-50-00N 072-51-00W, 33-08-00N 072-17-00W, 31-45-00N 074-41-00W.
C. 38-43-00N 062-38-00W, 40-23-00N 058-26-00W,39-18-00N 057-47-00W, 37-34-00N 061-56-00W.
2. CANCEL THIS MSG 181534Z MAY 20.

Areas A, B and C define the drop zones for Atlas V components including the fairing and the first stage. A further warning was issued for the Indian Ocean and re-entry of the Centaur stage. Together they were used to calculate the expected orbit inclination near 44°.6.

Line 1 gives the times in abbreviated format. For May 16, the window covers 12:24 - 14:53 UTC (149 minutes). For May 17 it is 13:14 - 15:32 UTC (138 minutes) and for May 18 it is 13:54 - 14:34 UTC (40 minutes).

Earlier OTV launches had similarly varying windows and they turned out to hide sets of up to three distinct short windows of ten minutes. This was picked up from conversations on the Launch Director's communication net during live brodcast of lift-off.

The new launch launch is no different. The period of validity for the Navigation Warnings covers A ten minute launch window, then a wait, then a second ten minute window. A further thirty minutes is then tacked on for the post launch hubbub to die down.

Window 1
open (UTC)
DurationWindow 2
open (UTC)
2020 May 1612:2410 minutes14:1310 minutes
2020 May 1713:1410 minutes14:5210 minutes
2020 May 1813:5410 minutes

Earlier Missions

With the earlier launches, each day's set of windows involved a wait in between them of about about 87 minutes. That made an interval of about 97 minutes between the opening times of successive windows. Speculation was that it related to the orbital period of another satellite already in space. A prime candidate was a KH-11 imaging spy satellite in sun-synchronous polar orbit. They operate with orbital periods just over 97 minutes. The reason for the recuring 97/98 minute pattern was thought to allow set up the X-37B and KH-11 orbits so that the two satellites reached the intersection at the same time. It would allow photographic inspection of the X-37B's thermal insulation for potential damage caused during the launch.

In order to meet the KH-11, launch had to occur at a time that would put the X-37B in the right place to be seen by the KH-11 camera. If an opportunity was missed then a wait ensued for the KH-11 to come round again on its next orbit. On three successive passes, sunlight on the X-37B orbit would have been right for photography then it would become necessary to wait until the next day for opportunities to come round again.

OTV-6 Mission

In order for the KH-11 to photograph the X-37B, they need to pass near each other. As their orbit planes are at a very steep angle to each other, photography is only possible near the points at which they cross. There are two, one in the northern hemisphere and one in the southern hemisphere. With the OTV-6 launch and both of the operational KH-11s, the northern intersection is in sunlight and the southern one is in the Earth's shadow.

The two KH-11 satellites are USA 224 (37348/2011-002A) and USA 245 (39232U/2013-043A).

The northern hemisphere orbit crossing point for a launches aimed at either satellite is around 30°-35° north latitude with the KH-11 heading north-south and the X-37B east-west.

With USA 224 the Local Solar Time in the days following the OTV-6 launch is about 10:00. For USA 245, Local Solar time is around 13:00. Illumination and viewing angles are therefore similar with both encounters occurring about 1-2 hours away from noon.


In 2020 May, both KH-11s were in orbits with matched orbital periods of 97.65 minutes, perigees near 260 kilometres and apogees 1020 kilometres. USA 224 would be climbing to apogee as it intersected the OTV-6 orbit and be about 600 km above it. USA 224 would meet it at about 245 kilometers altitude and potentially be below the X-37B.

The USA 245 encounter would be at a higher angular rate and it would be illuminated from the side, leading to deep shadows. It could be very much closer than that of USA 224.

Both have their pros and cons so it is difficult to say for certain which of the two would be involved but the higher USA 224 would require less precision interms of pointing but 600 kilometres is a long way in terms of photography.

2020 May 16 - Special Case?

All five previous OTV missions had intervals of 97-98 minutes between opening of successive windows on a particular day. The difference between windows on one day and those on the following day are an exact multiple of 97/98 minutes.

Similar is true for OTV-6 with the exception of May 16 where the interval is 109 minutes. On this particular day there must have been something about the exact circumstances of a KH-11 encounter when launched into the first window that did not apply to any subsequent window over the three days

In Conclusion

This page builds on a piece of work from eight years ago that seems to have stood the test of time. The original web page from 2012 covers missions OTV-1, OTV-2 and OTV-3. A check of Missions OTV-4 and OTV-5 reveals the same 97 minute periodicity occurs with them too. OTV-5 used a Falcon where all others were Atlas launched, ruling out that the pattern results from a quirk of the launch vehicle. It is definitely mission-related.

Page Date: 2020 May 17

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