X-37B OTV repeating ground track history

From: Ted Molczan via Seesat-l <seesat-l_at_satobs.org>
Date: Mon, 23 Apr 2018 10:58:06 -0400
As has been noted before, the X-37B OTV flights have typically operated in orbits with ground tracks that repeated after
a number of days and revolutions. Prior to OTV 5, we had seen one or more examples of the following:

Days Revs
 2    31
 3    46
 3    47
 4    61
 6    91
 7   108

The number of revs after which the ground track repeated has often not been an exact integer, but typically within about
0.05 rev, seldom within greater than about 0.1 rev.

OTV 5 initially had about a 31:2 repeating ground track. After its recent manoeuvre, it is almost exactly 78:5, which is
a first for an OTV.

The following spreadsheet calculates the repeating ground track information for OTV and several other satellites of


The values required to be entered for the calculations are in red font. The key calculated result is the number of nodal
revolutions per nodal day, in column 8, which is a strong function of mean motion, and to a lesser degree, of
inclination and eccentricity. This is multiplied by the number of nodal days after which the ground track repeats, in
col 9, to arrive at the number of revolutions after which the ground track repeats, in col 10. I determine the value in
col 9 by guess, trial and error (this could be automated, but I did not find it necessary for this spreadsheet).

The second sheet covers several other satellites of interest. For example, all five FIA Radar satellites, have almost
exactly 27:2 repeating ground tracks. 

Repeating ground tracks are very common, especially for spacecraft that observe the Earth. That said, I do not know why
OTV has repeating ground tracks.

Observations during 2018 Apr 22-23 yield the following OTV 5 elements:

OTV 5                                                    311 X 323 km
1 42932U 17052A   18113.16415005  .00000000  00000-0  00000-0 0    04
2 42932  54.5163  87.2246 0009271 168.4625 191.6632 15.84769442    04
Arc 20180422.86-0423.18 WRMS resid 0.050 totl 0.013 xtrk

Ted Molczan

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Received on Mon Apr 23 2018 - 09:59:07 UTC

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