11
$\begingroup$

I assume the retrograde orbit ($\theta_{inc}=142°$) is a side-effect of geography - ocean overfly access only lies West of the secured launch site - this article in Spaceflight 101 makes several points that I don't quite understand. I think if one can actually understand this particular orbit, all of these questions can be answered at the same time, so I'm grouping them together here as a 'what the heck is going on here' type question.

The Ofeq satellites are operated by the Israeli Ministry of Defence and built by Israel Aerospace Industries and Elbit Systems. Their primary purpose is to collect data from territories relevant to Israel.

Therefore, the satellites typically operate from equatorial orbits inclined 142 degrees covering Israel and surrounding territories.

While that orbit prevents the satellites from collecting intelligence on a global scale, it does provide six daylight passes over the Middle East for the first one-and-a-half years of their missions, delivering rapid response imagery of targets of interest to the Israeli intelligence community.

The number of favorable passes per day decreases with mission duration due to orbital mechanics, but the Ofeq satellites can still achieve more relevant passes over the area of interest than a polar orbiting spacecraft that would only have one or two daily passes with favorable look angles.

1) "While that orbit prevents the satellites from collecting intelligence on a global scale..." Is it the inclination, or the sun-synchrony, or both? Is this a repeat ground track orbit?

2) "...six daylight passes over the Middle East..." per day?? How does that work??

3) "...Ofeq satellites can still achieve more relevant passes over the area of interest than a polar orbiting spacecraft that would only have one or two daily passes with favorable look angles." I think I understand the part about polar orbit, the earth's surface at 32N latitude moves about 2100 km in 90 minutes, so at say 700 km altitude you could possibly make two passes looking at $+/- 56°$ degrees, or one pass more vertical and one more oblique?

4) "The number of favorable passes per day decreases with mission duration due to orbital mechanics..." some kind of precession?

$\endgroup$
  • 1
    $\begingroup$ +1 for finding an interesting orbit. I don't have enough info for an answer, but if you go to the Heavens-Above website, enter your location as somewhere in Israel, and search the satellite database for "Ofeq*" you'll be able to view passes. Around the date of its launch, there were indeed 5-6 daylight passes (though the viewing angle above the horizon varied a great deal). $\endgroup$ – Andy Sep 20 '16 at 13:23
  • $\begingroup$ @Andy I noticed that the sat maps on the internet have TLEs - for example the page n2yo.com/satellite/?s=41759 shows "1 41759U 16056A 16262.10113449 0.00138035 00000-0 24500-2 0 00 2 41759 142.5282 325.3519 0184795 50.1757 311.5128 15.32477128 00" (screen shot: i.stack.imgur.com/DJfY9.png) which is the morning of Sept. 18th. But when I try space-track or nssdc.gsfc.nasa.gov/nmc/masterCatalog.do?sc=2016-056A (n2yo's supposed source) there's nothing! So from what data source are these sat maps calculating? $\endgroup$ – uhoh Sep 20 '16 at 14:51
  • $\begingroup$ Good question - the nasa page doesn't seem to know reference 2016-056A (I think.) I can see a few other Ofeq satellites on there, for example Ofeq 10 seems to be "2014-019A". So possibly it's not up to date (...or maybe they don't list current military satellites?) $\endgroup$ – Andy Sep 20 '16 at 15:03
  • $\begingroup$ I have now asked a follow-up question about the this. $\endgroup$ – uhoh Sep 20 '16 at 16:42
  • $\begingroup$ For the first, the orbit inclination of $142$, which is the same as $38$ degrees but retrograde, means the satellite never reaches further North or South than latitude $38$. There is a lot of area beyond those limits, but they are saying they don't care about it. $\endgroup$ – Ross Millikan Sep 20 '16 at 22:00
11
$\begingroup$

Okay, first of all, let's collect all of the information we need to do anything with this:

  • The orbit is 600 km.
  • The inclination is 142 degrees. (-38)
  • The Middle East's latitude is about 38 N at the highest point.
  • The orbital period is about 96 minutes.

Okay, so how could you put all of that together to get 6 daylight passes per day? The orbit is somewhat sinusoidal, moving from across the sky per day. It will tend to be at the northern portion of that orbit for most of the time. An object at, say, 30 N would likely be visible for much of that portion of the day, as the satellite passes over roughly 38 degrees at it's highest point. Given the orbital period, and a day of 12 hours, there are 7.5 possible orbits to cover the daylight portion of the day, which is greater than the 6 stated. However, there will be several marginal orbits in that period of time.

This is further compounded by the fact that the orbit is retrograde. The Earth is actually moving the opposite direction, effectively giving over that 12 hour period of time an extra half-orbit, giving the possible orbits up to 8, which means that the 6 number is more likely to work well.

All orbits precess unless you try very hard to make it not so, by using a specific inclination. This is not one of those orbits. The orbital path will eventually shift such that the daylight portion of the orbit is in the southern hemisphere, and then return to the northern hemisphere. As the satellite depends at least in part on daylight, it's effectiveness will diminish with time, although it will eventually return to it's optimal usage.

$\endgroup$
  • 2
    $\begingroup$ Aha! OK I understand much better now! So the retrograde actually helps a bit, as does being not-too-far from the equator as well. If you lived on the equator a retrograde zero inclination orbit satellite would pass overhead almost hourly! $\endgroup$ – uhoh Sep 20 '16 at 13:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.