Why use a retrograde orbit?

Question

I've been looking around and saw on What is the highest inclination orbit ever attained from a Cape Canaveral launch? a question about launching into retrograde orbit. Of course, due to range safety requirements you can't launch into a retrograde orbit from Canaveral, but in one of the comments, Matt Large stated that NROL-25 was launched into a retrograde orbit from Vandenberg.

Simply, why would you want a retrograde orbit? For a reconnaissance satellite, especially, I would think that a polar orbit would be the obvious choice. A retrograde orbit means that you're moving over the surface quite fast, which would probably induce some motion blur in photos and reduce the window of time for which the satellite is over a given target. A polar orbit, while it has significant surface speed, allows you to surveil any point on the planet. Realistically, however, the reconnaissance folks are unlikely to be checking out the penguins' nuclear missile testing (you heard nothing and if you did the penguins will get you!) and could thus be put in an orbit that excludes the poles and allows for more frequent visitation of points of interest.

A retrograde satellite still doesn't make much sense. If the concern is range safety, launch from the East coast instead into a prograde orbit. Prograde orbit also allows for a small reduction in delta-V required to get to orbit, thus saving money on the launch.

For scientific or communications satellites, retrograde still doesn't seem to make sense. Communications satellites are usually in geostationary orbits, and scientific satellites are put in whatever orbit is needed for their particular experiments.

In addition, retrograde orbits vastly increase the potential for space debris - any possible collision now occurs at much higher velocities with larger spreads of debris.

So, I have two questions:

1) Why was NROL-25 launched into a retrograde orbit?

2) What are some uses for a retrograde orbit?

Answer 1

Retrograde orbits have multiple use-cases. First of all you should note that "retrograde" doesn't mean 180° inclination - everything > 90° is considered retrograde. This places all sun-synchronous satellites which operate at about 98° inclination in retrograde orbits. The usefulness of sun-synchronous orbits should be obvious.

Retrograde orbits of course pass over the ground track faster and thus provide higher frequency of updates. But I somewhat doubt this is the reason.

The biggest advantage of retrograde inclinations seems to come from the use of synthetic aperture radar (SAR) techniques. SAR offers better spatial resolution then conventional beam-scanning techniques. The distance the SAR device travels over a target in the time taken for the radar pulses to return to the antenna creates a large synthetic antenna aperture and thus increases the synthetic size of the antenna. With higher aperture comes higher resolution - no matter if you actually have a large antenna or just move the antenna. A retrograde orbit increases the distance traveled and thus increases the spatial resolution of such satellites.

122° / 123° inclination orbits are quite popular for remote sensing (especially radar) satellites.

Some countries, most notably Israel, only have an West coast and can not launch eastwards, mandating retrograde orbits for satellites launched from their soil (unless you have no problems launching over foreign territory, which is usually not possible).

References:

• https://en.wikipedia.org/wiki/Artificial_satellites_in_retrograde_orbit
• https://en.wikipedia.org/wiki/Synthetic-aperture_radar
• https://en.wikipedia.org/wiki/Sun-synchronous_orbit

Answer 2

My lightly-edited question The strange orbit of Ofeq 11 - how does it (actually) do this? has become at least a supplemental answer to @Polygnome's.

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 - (The Spaceflight 101 article Israel Launches Advanced Optical Reconnaissance Satellite) 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. (emphasis added)

So in addition to six daylight passes per day for advanced optical reconnaissance, it's the only choice for a sensitive government payload of a country with no east coast.

By the way, all of those questions are answered in @PearsonArtPhoto's excellent answer!

Answer 3

A retrograde orbit means that you're moving over the surface quite fast, which would probably induce some motion blur in photos and reduce the window of time for which the satellite is over a given target.

Note that (for low Earth orbit), orbital speeds are typically somewhat above 7000 m/s relative to the center of the Earth. At the equator (which is moving fastest) the speed of the ground is about 470 m/s relative to the center of the Earth. Thus, the difference in ground speed between a prograde equatorial orbit and a retrograde equatorial orbit is "only" about 900 m/s which is a difference of a bit over 10%. So, if your imaging technology is able to deal with the relative motion for a prograde orbit it isn't much harder to deal with it for a retrograde orbit. Compared with other considerations the ground speed probably isn't really a significant factor in deciding on the orbit for a reconnaissance satellite.