Core 4 Orbits and Resolution

By Kepler's second law, where in an elliptical orbit does a satellite move fastest, and where slowest?

Fastest at perigee (closest to Earth), slowest at apogee (farthest).

What is a Molniya orbit?

A highly elliptical orbit with about a 12-hour period whose high apogee sits over high northern latitudes.

What makes a geostationary orbit a special case of geosynchronous?

It is also circular (eccentricity 0) and equatorial (inclination 0°), so the satellite appears fixed over one point on the equator.

In a GOES image, why are pixels over Canada much larger and more distorted than near the equator below the satellite?

Canada is viewed far off nadir (far from the point directly below the satellite): a longer slant distance and oblique angle stretch each pixel.

What does off-nadir viewing do to pixels and to the signal?

Pixels get larger and more distorted, and light travels a longer path through the atmosphere (more attenuation).

IFOV vs FOV: which one sets pixel size and which sets swath width?

IFOV (one detector) sets pixel size. FOV (whole sensor) sets swath width.

Sun-synchronous chain, step 1: what feature of Earth makes an inclined satellite orbit's plane slowly rotate (precess)?

Earth's equatorial bulge (its oblateness, called the J2 effect).

Sun-synchronous chain, step 5: the orbit plane turns ~0.986°/day eastward, keeping pace with Earth's trip around the Sun. What is the result?

The plane keeps a fixed angle to the Sun, so the satellite crosses the equator and each latitude at the same local solar time every day.

What is the main drawback of a sun-synchronous orbit for climate studies?

It sees each place only at the same one or two local times, so it cannot sample the diurnal (daily) cycle.

Why did the A-Train satellites fly just minutes apart in the same orbit?

So their different instruments observed nearly the same scene at nearly the same time and could be combined (e.g. CloudSat radar + CALIPSO lidar + Aqua MODIS).

Why do the Jason altimeters avoid a sun-synchronous orbit?

A sun-synchronous orbit samples each spot at the same solar time, so the Sun-driven part of the tide would be caught at the same stage every pass and look like a fixed sea level offset (aliasing).

A day of satellite data shows nearly north-south swaths that converge toward the poles. What orbit made it?

A polar (typically sun-synchronous) LEO orbit, such as Aqua's.

What is the difference between orbital period and revisit period?

Orbital period is the time for one trip around Earth. Revisit period is how long until the sensor can image the same spot again, which can be days even with a 99-minute orbit.

A day of polar-orbiter data shows black diamond-shaped gaps between swaths near the equator but none at high latitudes. What causes them?

The swath is narrower than the ~2,750 km spacing between successive ground tracks at the equator, while the tracks converge toward the poles.

For a polar-orbiting satellite, which latitudes get the most frequent revisits?

High latitudes, where swaths overlap. The equator gets the fewest looks.

Why does MODIS revisit a spot every 1 to 2 days while a 30 m imager like Landsat needs 16 days?

MODIS's swath is about 2,330 km, Landsat's only about 185 km.

In a CALIPSO or CloudSat curtain image, how can you tell which way the satellite was moving?

The x-axis runs in time order, left to right. If latitude increases to the right, the satellite was moving north (ascending, the A-Train's daytime pass).

Why do geostationary satellites have better temporal but worse spatial resolution than sun-synchronous LEO satellites?

GEO stares at one region and images it every few minutes, but it is ~50 times farther away, so the same IFOV covers far more ground per pixel.

Why does a pushbroom scanner get better signal-to-noise than a whiskbroom?

Each detector stares at its own ground pixel for the whole time the satellite takes to advance one pixel, a much longer dwell time.

What image artifact comes from poorly cross-calibrated detectors in a pushbroom array?

Striping: lines running parallel to the flight direction.

Is pixel size (GSD) the same thing as spatial resolution?

Not always. Pixel size is the sampling grid, while true resolution can be coarser because of optical blur, diffraction or motion.

Can resampling a 1 km image onto 250 m pixels improve its spatial resolution?

No. Resampled smaller pixels add no new detail.

Rules of thumb: what is the smallest kind of object you can make out at about 30 m, 10 m and 1 m resolution?

30 m: fields and city blocks. 10 m: large buildings. 1 m: cars.

Which filter best removes salt-and-pepper (random black and white dot) noise?

A median filter.