The main reason land-imaging satellites are placed in sun-synchronous orbits is that

they pass over each latitude at the same local solar time, giving comparable illumination

they stay fixed over one point on the equator

they avoid the atmosphere altogether

they image every point of the Earth every hour

Compared with a whiskbroom scanner of the same pixel size, a pushbroom scanner

has a longer dwell time per pixel and therefore a better signal-to-noise ratio

needs a rotating mirror to sweep across the track

images only one pixel at a time

On a night-time thermal image of a lake and its surrounding dry land, the lake usually appears

warmer than the land, because water has high thermal inertia

cooler than the land, because water reflects moonlight

identical to the land, because both are at air temperature

invisible, because thermal sensors do not work at night

Which of the following are advantages of imaging radar over optical sensing?

It can image through cloud cover

Its backscatter is sensitive to surface roughness and moisture

It can image through cloud cover

Its backscatter is sensitive to surface roughness and moisture

Which of the following statements about platforms and sensors are correct?

A geostationary satellite has an orbital period of one sidereal day

A thermal infrared sensor is passive

A geostationary satellite has an orbital period of one sidereal day

A thermal infrared sensor is passive

A sun-synchronous orbit is an equatorial orbit

Which of the following are correctly matched in visual image interpretation?

Texture — the frequency of tonal change, such as the smooth look of a paddy field against a rough forest

Pattern — the spatial arrangement of objects, such as the regular rows of an orchard

Shadow — helps estimate the height of a tower

Texture — the frequency of tonal change, such as the smooth look of a paddy field against a rough forest

Pattern — the spatial arrangement of objects, such as the regular rows of an orchard

Shadow — helps estimate the height of a tower

Tone — the size of an object relative to known objects

Remote Sensing II: Platforms, Sensors, Data Products and Interpretation

The second half of Part A's Remote Sensing heading is “Platforms and Sensors, Remote Sensing Data Products — PAN, Multispectral, Microwave, Thermal, Hyperspectral data; Visual and digital interpretation methods”. This chapter follows that list. It sets out the platforms and the two orbits that matter (geostationary and sun-synchronous), the difference between passive and active sensing and between whiskbroom, pushbroom and framing sensors; then each data product in turn — panchromatic, multispectral, hyperspectral, thermal and microwave, with the radar geometry (slant- and ground-range resolution, real and synthetic aperture, foreshortening, layover and shadow) that makes microwave images look so different; and finally the elements of visual interpretation, colour composites and the step to digital interpretation. The numericals are radar resolutions, and they turn on a factor of two and a sine.

1. Platforms and orbits

Sensors ride on ground-based platforms (towers, vehicles, handheld spectroradiometers), airborne platforms (aircraft, drones, balloons) and spaceborne platforms. Aircraft and drones give fine detail and flexible timing over small areas; satellites give repetitive, synoptic coverage of large areas at a fixed schedule.

The two orbits
Property	Geostationary	Sun-synchronous (near-polar)
Altitude	about 35 786 km above the equator	typically 500–900 km
Period	one sidereal day; stays over one longitude	about 100 minutes, about 14 orbits a day
Inclination	0°	slightly retrograde, about 98°
Strength	continuous watch of one hemisphere — weather, cyclones	crosses each latitude at the same local solar time, so illumination is comparable from date to date — land imaging
Weakness	coarse pixels; poor view of high latitudes	revisit of days, limited by swath

2. Sensors: passive and active, whiskbroom, pushbroom and frame

A passive sensor records naturally available energy — reflected sunlight or emitted heat — so optical sensing needs daylight and clear skies, while thermal sensing works by night too. An active sensor supplies its own illumination and records the return: radar (microwave), LiDAR (laser) and sonar. Active sensors work day and night, and radar also through cloud.

How an optical image is built
Scanner	Mechanism	Consequence
Whiskbroom (across-track)	a rotating or oscillating mirror sweeps a few detectors across the track	short dwell time per pixel; moving parts; panoramic distortion at the swath edges
Pushbroom (along-track)	a linear array of detectors, one per ground cell, images a whole line; the platform's motion adds lines	longer dwell time and better signal-to-noise; no moving mirror; detector-to-detector striping must be calibrated
Frame camera	a two-dimensional array exposes the whole scene at once	central-perspective geometry — the basis of photogrammetry

🎯 Why pushbroom won

In a pushbroom each detector stares at its ground cell for the whole line time instead of a tiny fraction of it, so it collects far more photons. That extra signal is what allows narrow bands and small pixels together.

3. Data products: PAN, multispectral, hyperspectral, thermal and microwave

Panchromatic (PAN): one broad band across the visible (and often NIR); the wide band collects much energy, so PAN has the finest pixels — typically 2 to 4 times finer than the same satellite's multispectral bands. It is fused with multispectral data in pan-sharpening.
Multispectral: a few (about 3–15) broad, separated bands, e.g. blue, green, red, NIR, SWIR. The workhorse of land-cover mapping.
Hyperspectral (imaging spectrometry): hundreds of narrow (about 10 nm), contiguous bands, giving a near-continuous spectrum per pixel — the airborne AVIRIS records 224 bands from 0.4 to 2.5 µm. It identifies minerals and plant species, at the cost of huge data volume and highly correlated bands.
Thermal: emitted radiance in 8–14 µm (and 3–5 µm), converted to brightness temperature. Depends on emissivity and on thermal inertia — water, with high inertia, is cooler than land by day and warmer by night. Used for surface temperature, urban heat islands, fires and soil moisture.
Microwave: passive radiometers record weak emitted microwaves (coarse pixels; soil moisture, sea ice); active radar records backscatter. Radar bands in common use are X (about 3 cm), C (about 5.6 cm) and L (about 23 cm); longer wavelengths penetrate vegetation and dry soil further. Backscatter rises with surface roughness, moisture (dielectric constant) and corner-reflector geometry; polarisations HH, HV, VH, VV add information.

Side-looking radar geometry
Quantity	Formula	Note
Slant-range resolution	cτ/2 (τ = pulse length)	the 2 is the round trip
Ground-range resolution	cτ/(2 sin θ), θ = incidence angle	worse in near range, where θ is small
Azimuth resolution, real aperture	λR/L (R = range, L = antenna length)	kilometres from orbit
Azimuth resolution, synthetic aperture (SAR)	about L/2	independent of range and wavelength

Worked: a 0.1 µs pulse gives a slant-range resolution of 3 × 10⁸ × 10⁻⁷/2 = 15 m, and at 30° incidence a ground-range resolution of 15/sin 30° = 30 m. A 10 m antenna at λ = 5.6 cm and range 850 km would resolve only 0.056 × 850 000/10 = 4760 m in azimuth as a real aperture, but about 5 m as a SAR. Radar images slant range, so a slope facing the sensor is compressed (foreshortening); if its top is nearer the sensor than its foot, the top is imaged first (layover); a slope facing away steeper than the look angle returns nothing (radar shadow). Coherent illumination gives the grainy speckle that radar filters address.

4. Visual and digital interpretation

Visual interpretation uses the elements of image interpretation: tone or colour, size, shape, texture, pattern, shadow, and site and association. Tone and colour are the primary elements; texture and pattern come from the arrangement of tones; shadow reveals height and profile; association uses context (a school has a playground, a thermal power plant has a cooling tower and a coal yard). Colour composites assign three bands to the red, green and blue guns: a true colour composite puts red, green and blue bands in their own guns; the standard false colour composite (FCC) puts NIR in red, red in green and green in blue, so healthy vegetation appears red and water dark.

Digital interpretation treats the image as numbers: preprocessing (radiometric and geometric correction), enhancement, transformation, and classification by computer, supervised or unsupervised (Part B2 develops each). Visual interpretation uses context and shape that computers find hard; digital interpretation is repeatable, quantitative and uses every band at full radiometric precision. Most projects use both: digital classification checked and edited by an interpreter.

⚠️ Red in an FCC is not red on the ground

In a standard FCC the red channel shows NIR, so bright red means strong NIR reflectance — dense healthy vegetation — and says nothing about the object's colour on the ground. Reading an FCC as if it were true colour is the commonest interpretation error.

Key takeaways

Geostationary: about 35 786 km, fixed over one longitude, for weather; sun-synchronous: 500–900 km, about 98° inclination, same local time, for land imaging.
Passive sensors use sunlight or emitted heat; active sensors (radar, LiDAR) supply their own energy; pushbroom arrays out-collect whiskbroom mirrors.
PAN: one wide band, finest pixels; multispectral: few broad bands; hyperspectral: hundreds of narrow contiguous bands; thermal: emitted 8–14 µm; microwave: all-weather.
Radar: slant-range resolution cτ/2, ground range cτ/(2 sin θ), real-aperture azimuth λR/L, SAR azimuth about L/2; foreshortening, layover, shadow and speckle.
Visual interpretation: tone, size, shape, texture, pattern, shadow, site and association; in a standard FCC (NIR-red-green) vegetation is red.

Practice questions (14)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

The main reason land-imaging satellites are placed in sun-synchronous orbits is that
1. they pass over each latitude at the same local solar time, giving comparable illumination
2. they stay fixed over one point on the equator
3. they avoid the atmosphere altogether
4. they image every point of the Earth every hour
Show answer
Answer: A — they pass over each latitude at the same local solar time, giving comparable illumination
The orbit plane precesses once a year, keeping a constant angle to the Sun, so every pass over a given latitude happens at the same local time and images from different dates share similar sun angles. Staying fixed over the equator describes a geostationary orbit.
Compared with a whiskbroom scanner of the same pixel size, a pushbroom scanner
1. has a longer dwell time per pixel and therefore a better signal-to-noise ratio
2. needs a rotating mirror to sweep across the track
3. cannot suffer from striping
4. images only one pixel at a time
Show answer
Answer: A — has a longer dwell time per pixel and therefore a better signal-to-noise ratio
A pushbroom's linear array has a detector for every ground cell of the line, so each stares at its cell for the whole line time and gathers more signal. It has no scan mirror, but it can stripe because thousands of detectors must be calibrated to match.
An imaging spectrometer recording 224 narrow, contiguous bands between 0.4 and 2.5 µm produces
1. hyperspectral data
2. panchromatic data
3. thermal data
4. synthetic aperture radar data
Show answer
Answer: A — hyperspectral data
Hundreds of narrow bands that are contiguous — no gaps — define hyperspectral data, which gives a near-continuous reflectance spectrum for each pixel. Multispectral data has a few broad separated bands; panchromatic has one.
On a night-time thermal image of a lake and its surrounding dry land, the lake usually appears
1. warmer than the land, because water has high thermal inertia
2. cooler than the land, because water reflects moonlight
3. identical to the land, because both are at air temperature
4. invisible, because thermal sensors do not work at night
Show answer
Answer: A — warmer than the land, because water has high thermal inertia
Water heats and cools slowly (high heat capacity and conductivity, and mixing), so it is cooler than land by day and warmer by night. Thermal sensors record emitted radiation and work perfectly well at night; moonlight reflection is irrelevant at 8–14 µm.
A radar transmits pulses of length 0.1 µs. Taking c = 3 × 10⁸ m/s, its slant-range resolution, in metres, is ____.
Numerical answer — type the value.
Show answer
Answer: 15
Slant-range resolution = cτ/2 = 3 × 10⁸ × 0.1 × 10⁻⁶/2 = 30/2 = 15 m. The factor 2 is the two-way travel of the pulse; omitting it gives 30 m.
For the radar with 0.1 µs pulses (c = 3 × 10⁸ m/s), the ground-range resolution at a local incidence angle of 30°, in metres, is ____.
Numerical answer — type the value.
Show answer
Answer: 30
Ground range = slant range/sin θ = (cτ/2)/sin 30° = 15/0.5 = 30 m. Using cos 30° gives 17.3 m; the sine is correct because a small incidence angle (near range) stretches one slant cell over more ground.
A real-aperture radar with a 10 m antenna operates at a wavelength of 5.6 cm. At a range of 850 km its azimuth resolution, in metres, is ____.
Numerical answer — type the value.
Show answer
Answer: 4760
Real-aperture azimuth resolution = λR/L = 0.056 × 850 000/10 = 4760 m. That is why spaceborne imaging radars synthesise an aperture: a SAR with the same antenna resolves about L/2 = 5 m, independent of range.
On a radar image, the top of a steep mountain facing the sensor is displayed closer to the sensor than its base. This distortion is
1. layover
2. radar shadow
3. speckle
4. panoramic distortion
Show answer
Answer: A — layover
Radar places targets by their slant range. When the slope is steeper than the incidence angle allows, the echo from the summit arrives before that from the base, so the summit is laid over the foot. Shadow occurs on the back slope; speckle is a noise pattern, not a geometric distortion.
In a standard false colour composite, healthy dense vegetation appears
1. red
2. green
3. blue
4. black
Show answer
Answer: A — red
The standard FCC displays NIR in red, red in green and green in blue. Vegetation reflects NIR strongly and absorbs red, so the red gun dominates and it looks red. Green would be the true-colour appearance.
An interpreter identifies a building as a hospital because of an adjoining helipad and ambulance bays. The element of interpretation used is
1. association
2. tone
3. texture
4. shadow
Show answer
Answer: A — association
Association (site and association) identifies an object from the features found with it. Tone is brightness or colour, texture the frequency of tonal change, and shadow the outline cast by height — none of them identifies function the way context does.
A satellite imager has a PAN band of 2.5 m and multispectral bands of 10 m. How many PAN pixels fall within the ground area of one multispectral pixel?
Numerical answer — type the value.
Show answer
Answer: 16
The linear ratio is 10/2.5 = 4, and area goes as its square: 4² = 16 PAN pixels per multispectral pixel. Answering 4 compares lengths, not areas.
Which of the following are advantages of imaging radar over optical sensing?
1. It can image through cloud cover
2. It can image at night
3. Its backscatter is sensitive to surface roughness and moisture
4. Its images are free of speckle
Show answer
Answer: A — It can image through cloud cover; B — It can image at night; C — Its backscatter is sensitive to surface roughness and moisture
Centimetre wavelengths pass through cloud, the sensor supplies its own illumination so it works at night, and backscatter depends on roughness and the dielectric constant, which rises with moisture. Coherent illumination is exactly what causes speckle, so radar images are never free of it.
Which of the following statements about platforms and sensors are correct?
1. A geostationary satellite has an orbital period of one sidereal day
2. LiDAR is an active sensor
3. A thermal infrared sensor is passive
4. A sun-synchronous orbit is an equatorial orbit
Show answer
Answer: A — A geostationary satellite has an orbital period of one sidereal day; B — LiDAR is an active sensor; C — A thermal infrared sensor is passive
To stay over one longitude a geostationary satellite must turn with the Earth, once per sidereal day; LiDAR fires its own laser pulses, so it is active; a thermal sensor records emitted heat without illuminating anything, so it is passive. Sun-synchronous orbits are near-polar (about 98°), not equatorial.
Which of the following are correctly matched in visual image interpretation?
1. Texture — the frequency of tonal change, such as the smooth look of a paddy field against a rough forest
2. Pattern — the spatial arrangement of objects, such as the regular rows of an orchard
3. Shadow — helps estimate the height of a tower
4. Tone — the size of an object relative to known objects
Show answer
Answer: A — Texture — the frequency of tonal change, such as the smooth look of a paddy field against a rough forest; B — Pattern — the spatial arrangement of objects, such as the regular rows of an orchard; C — Shadow — helps estimate the height of a tower
Texture is how fast tone varies, pattern is the arrangement of repeated objects, and a shadow's length with the sun elevation gives height. Tone is the brightness or colour of an object; relative dimension is the element called size.