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c. Remote instrumentation and physics.

i. Active vs passive sensors

Passive Sensors:

- Detects external natural radiation (reflected sunlight or emitted thermal radiation)

- Definition: A passive microwave instrument designed to receive and to measure natural emission/radiation (microwave emission) coming from constituents of the Earth’s surface and its atmosphere.

- The power (or natural emission) measured by passive sensors indicates surface composition, physical temperature, surface roughness, and other physical characteristics of the Earth.

- The frequency bands for passive sensor measurements are determined by fixed physical properties (molecular resonance) of the substance being measured. These frequencies do not change and information cannot be duplicated in other frequency bands.

- They are patterned after radio astronomy instruments, which detect emissions with very low power (or weak natural signals). 

- They are particularly sensitive to accumulated radiation from a multitude of emitters on the ground, both from within the frequency band in which measurements are being made, and from out-of-band.

- Spaceborne passive sensors provide the ability to obtain all-weather, day and night, global observations of the Earth and its atmosphere. These spaceborne passive sensors operate in frequency bands allocated to the Earth Exploration-Satellite Service or to the Space Research Service. They are also made to measure naturally emitted Earth energy and detect natural thermal infrared radiation emitted from Earth’s surface by using moonlight and artificial lights as an illumination source.

- Passive sensors often work better in the daytime, though an exception are thermal infrared sensors. This is because the ambient temperature is lower at night and thus there is a higher contrast between objects and their surroundings.

- Downward emission by clouds is extraneous and should not be considered in passive sensing analysis. As opposed to multiscatter in foliage canopies, upward emission from vegetation, and double-bounce scattering by tall, human-made objects, downward emission by clouds is downwards, and passive sensors do not capture it, and therefore it is extraneous. 

Active Sensors: 

- Produces its own radiation/energy.

- A radar instrument used for measuring signals transmitted by the sensor that were reflected, refracted or scattered by the Earth’s surface or its atmosphere.

- Radar is the most common type of active sensor. It operates in the microwave and radio wavelength regions of the electromagnetic spectrum. The wavelengths range from 1 mm to 20 m.

- Spaceborne active sensors operate in the Earth Exploration-Satellite Service or in the Space Research Service. Active sensor frequency allocations are often shared with other radar systems, as such systems are normally compatible with the operation of the sensors.

- Spaceborne active sensors applications: meteorology and observation of the Earth’s surface and atmosphere. Ex: precipitation radars measure the radar echo from rainfall to determine the rainfall rate over the Earth’s surface; and cloud profile radars measure the radar echo return from clouds to provide a three dimensional profile of cloud reflectivity over the Earth’s surface.

Passive Microwave Sensing

Operating principle - Measures natural microwave emissions produced by the Earth’s surface and atmosphere. Atmospheric gases, liquid water, and surface features emit weak microwave energy based on molecular resonance and physical temperature

 Types of Passive Microwave Sensors: 

• Imaging sensors - Many environmental data products are produced using multivariable algorithms to retrieve a set of geophysical parameters simultaneously from calibrated multi-channel microwave radiometric imagery

• Atmospheric sounding sensors - Atmospheric sounding is a measurement of vertical distribution of physical properties of a column of the atmosphere such as pressure, temperature, wind speed, wind direction, liquid water content, ozone concentration, pollution, and other properties

• Microwave limb sounding sensors - Limb sounders observe the atmosphere in directions tangential to the atmospheric layers and are used to study low to upper atmosphere regions where the intense photochemistry activities may have a heavy impact on the Earth’s climate. 

Passive Sensor Data Products

• Measured radiation: occurs naturally; Very low power levels; Contain essential information on the physical processes

• Radiation peaks indicate presence of specific chemicals

• Absence of radiation from certain frequencies indicates absorption by atmospheric gases

• Strength or absence of signals at particular frequencies is used to determine whether specific gases are present and, if so, in what quantity and at what location

- Environmental information is obtained through passive sensor measurements: 

• Use spectroscopy to obtain information from frequencies.

• Signal strength at a given frequency may depend on several variables

• Use of several frequencies necessary to match the multiple unknowns

Use of multiple frequencies is primary technique used to measure various characteristics of the atmosphere and surface of the Earth

Key frequency allocations & Environmental Applications:

Overview

1.4 Ghz (L-band): Optimal sensitivity for measuring surface soil moisture and surface salinity

5.0 - 10 Ghz (C-band / X-band) - Estimates vegetation canopy biomass once soil moisture contribution is isolated

17.0 - 19.0 Ghz - Minimum signature for water vapor and ocean temperature; optimal for determining ocean surface emissivity

23.0 - 24.9 GHz - Centered on the water vapor absorption line: determines total atmospheric water vapor column

36.0 - 37.0 GHz - Highly sensitive to liquid cloud water content and snow cover extent

Used over Oceans

1.4 Ghz - Optimal sensitivity for measuring surface soil moisture and ocean surface salinity

6 Ghz - Optimal sensitivity to sea surface temperature

17 - 19 Ghz - Signature of sea surface temperature and atmospheric water vapour is the smallest; optimal for ocean surface emissivity

24 Ghz - Optimal for measuring total content of water vapor

36 Ghz - Liquid clouds are obtained via measurements around this frequency

Five frequencies (around 6 GHz, 10 GHz, 18 GHz, 24 GHz and 36 GHz) are necessary for determining the dominant parameters.

Used over Land

1.4 Ghz - Used to measure soil moisture

5 - 10 Ghz - Needed to estimate vegetation biomass once the soil moisture contribution is known

18-19 Ghz & 23-24 Ghz - Needed around the water vapour absorption peak to assess the atmospheric contribution

37 Ghz - Needed in combination with these frequencies

Downward cloud emissions - Downward atmospheric microwave emissions reflect off the ground away from spaceborne sensors or are reabsorbed, making them extraneous background noise in upward-looking passive satellite retrieval algorithms 

Whisk Broom vs Push Broom Scanners:

- Whiskbroom move backwards while scanning back and forth while pushbroom scans everything linearly and moves forward 

• Whisk Broom is used for passive remote sensing from space. In a whisk broom sensor, a mirror scans across the satellite’s path (ground track), reflecting light into a single detector (photosite) which collects data one pixel at a time. The moving parts make this type of sensor expensive and more prone to wearing out, such as in the Landsat 7. Whisk broom scanners have the effect of stopping the scan, and focusing the detector on one part of the swath width. Because the detector is only focused on a subsection of the full swath at any time, it typically has a higher resolution than a push broom design for the same size of scan swath.

• Push broom scanner, also known as an along-track scanner, is a device for obtaining images with spectroscopic sensors. The scanners are regularly used for passive remote sensing from space, and in spectral analysis on production lines. The moving scanner line in a traditional photocopier is an example of a push broom scanner. In orbital push broom sensors, a line of sensors arranged perpendicular to the flight direction of the spacecraft is used. Different areas of the surface are imaged as the spacecraft flies forward. A push broom scanner can gather more light than a whisk broom scanner because it looks at a particular area for a longer time, like a long exposure on a camera. One drawback of push broom sensors is the varying sensitivity of the individual detectors. Another drawback is that the resolution is lower than a whisk broom scanner because the entire image is captured at once.

Basic Vocabulary of Imaging “Systems”

- Along track direction: direction of flight

- Cross track direction: perpendicular to flight direction

- Swath width: total width sensor can image (width of IFOV for framing camera, larger for scanner)

- Dwell time: amount of time camera shutter open for each image

3 Basic Types of Imaging Systems

1. Framing Camera: Single field of view, does not scan. Size of detector chip proportional to total field of view.

2. Cross Track (whisk broom) scanner: Records entire IFOV on a “point detector” (a single photosite!). Rotating mirror moves IFOV across track. 

• Mechanism: Uses a rotating or oscillating mirror to sweep a singular detector (or small array) perpendicular to the satellite’s flight direction (cross track)

• Advantages: Only calibrate one detector. Easy to record many Xs by splitting a single beam of light. 

• Disadvantage: Rotating mirror: breaks, requires power, slow (takes time to scan).

• On a cross-track scanner, objects seemingly fall towards the nadir line. This is due to a phenomenon called "bow-tie distortion," a purely geometric effect of the scanner's design. A cross-track scanner uses a rotating mirror to sweep across the Earth perpendicular to the satellite's direction of motion. The scanner captures data in a series of equal angular increments. This means the Instantaneous Field of View (IFOV) is constant. However, the physical area on the ground represented by each IFOV is not constant. Pixels at the edges of the scan are wider and longer than those in the center, near the nadir line. As a result, objects appear to be stretched and displaced towards the center of the image, creating the illusion that they are "falling" toward the nadir line.

• Bow-Tie Distortion: Occurs because the Instantaneous Field of View (IFOV) projects a constant angular swath. As the scanning mirror swings off-nadir toward the edges, the distance to the ground increases, causing edge pixels to expand in size compared to nadir pixels.

3. Along track (push broom scanner): Linear array of CCD photosites (pixels); Image collected in strips. 

• Mechanism: Employs a fixed linear array of Charge-Coupled Device (CCD) or CMOS detectors oriented perpendicular to the flight path, capturing an entire ground line simultaneously as the satellite moves forward

• Advantages: Longer dwell time (stronger signal = better SNR); Geometric integrity (not vulnerable to scanning mirror); Lower power demands and longer lifetime (no moving parts). 

• Disadvantages: many photosites must be x-calibrated. Ex: ASTER sensor: 15 - 30 m resolution; Similar spectral band to LandSat; Terrestrial based application.

ii. Optical and infrared imagers, radiometers, LIDAR, space-based gravimetry

Optical imagers -  senses visible light and create photos based on it

• Wavelengths from about 380 - 750 nanometers

• Measure reflected solar light across discrete wavelength bands (Blue ~ 450 nm, Green ~ 550 nm, Red ~ 650 nm) to derive true color imagery, cloud albedo, and surface cover composition

Infrared imagers - sense infrared light

• Uses are helping astronomers learn about the temperatures of celestial bodies and places on Earth, and for tracking storms.

• Infrared waves have longer wavelengths than visible light and can pass through dense regions of gas and dust in space with less scattering and absorption. Thus, infrared can also reveal objects in the universe that cannot be seen using optical telescopes.

• Near-Infrared NIR (0.75 - 1.4 m): Differentiates live vegetation structural health from water and soil

• Shortwave Infrared (SWIR, 1.4 - 3.0 m): Differentiates soil moisture,mineral compositions, and snow from cloud cover

• Thermal Infrared (TIR, 8.0 - 14.0 m): Detects kinetic temperature emissions from surfaces and cloud top day or night

Radiometers - Instrument used for measuring radiometric quantities such as radiant flux (power), irradiance, or radiance, usually optical radiation

- A microwave radiometer (MWR) is a radiometer that measures energy emitted by microwaves. 

- Over a large wavelength range = broadband; over narrow range = spectroradiometers

• When made to match the human spectral photopic efficiency function, the V-lambda curve, it is called a photometer

- Microwave radiometers are very sensitive receivers designed to measure thermally-emitted electromagnetic radiation. 

• They are usually equipped with multiple receiving channels to derive the characteristic emission spectrum of planetary atmospheres, surfaces or extraterrestrial objects.

• Microwave spectral range between 1 and 300 GHz provides complementary information to the visible and infrared spectral range. The atmosphere and also vegetation is semi-transparent in the microwave spectral range. This means components like dry gases, water vapor, or hydrometeors interact with microwave radiation but overall even the cloudy atmosphere is not completely opaque in this frequency range.

- For weather and climate monitoring, microwave radiometers are operated from space as well as from the ground. They are designed to operate continuously and autonomously often in combination with other atmospheric remote sensors like, for example, cloud radars and lidars. They allow the derivation of important meteorological quantities such as vertical temperature and humidity profiles, columnar water vapor quantity, and columnar liquid water path with a high temporal resolution on the order of minutes to seconds under nearly all weather conditions.

- Microwave radiometers are also used for remote sensing of Earth's ocean and land surfaces, to derive ocean temperature and wind speed, ice characteristics, and soil and vegetation properties.

Broadband Radiometers: Integrate total incoming radiant energy over wide spectral regions (e.g., CERES measuring total shortwave 0.3 - 5 m and a lonewave 5 - 100 m flux)

Spectroradiometers: Measure radiant energy in numerous discrete, narrow spectral bands (e.g., MODIS with 36 channels)

Microwave Radiometers (MWR): Sensitive heterodyne receivers designed to measure thermal microwave emissions (1 - 300 GHz). They operate in limb-sounding modes (looking tangentially through atmospheric layers) or nadir-looking nodes to retrieve the vertical profiles of temperature, humidity, liquid water path, and ocean surface wind speeds.

LIDAR:

- Light detection and Ranging. (active)

- Operating principle: Transmits rapid, high-energy laser pulses (1064 nm NIR or 532 nm green) and measures the precise elapsed flight time of returning backscattered light

- Combined with other data recorded by the airborne system — generate precise, 3D maps about Earth’s surface (including covered by vegetation) + data to measure height of ground surface and height of above objects (ex. trees, buildings) 

- A lidar instrument principally consists of a laser, a scanner, and a specialized GPS receiver. 

- Airplanes and helicopters are the most commonly used platforms for acquiring lidar data over broad areas. 

- Two types of lidar are topographic and bathymetric. 

• Topographic lidar typically employs Near-Infrared (1064 nm) lasers to map terrestrial topography and canopy structural profiles

• Bathymetric lidar uses water-penetrating green light (532 nm) to map and measure shallow seafloor and riverbed elevations.

- Can be used for measuring vegetation height and topography mapping.

- Compared to radar: Radar is able to “see” through clouds while LiDAR is not. This is because LIDAR provides extremely fine vertical and spatial sampling for thin cloud and aerosol profiling (e.g., CALIPSO) but cannot penetrate thick clouds or heavy precipitation, which microwave RADAR can bypass

Space-Based Gravimetry

• The study of Earth’s gravity field using satellites. It focus on how changes in the distribution of snow, ice, and the Earth’s crust affect Earth’s gravitational field

• Physical Basis: Employs Newton’s Law of Universal Gravitation (F=Gm1m1r2) to detect local Earth mass variations (ice sheet thickness changes, groundwater storage fluctuations, deep ocean mass shifts) by observing orbital perturbations of LEO satellites. The gravitational field between two objects has a linear relationship with the product of their masses and an inverse relationship with the square of the distance between them. 

• If a satellite passes above a mass inhomogeneity or anomaly, its orbit will be perturbated, such as increasing or decreasing its distance from the Earth. The closer a satellite is to the Earth, the more sensitive it is to the gravity of the object below it.

- Researchers typically use data from satellites that:

• Orbit as low as possible (usually 200 – 500 km from the ground)

• Uninterrupted tracking over large orbital arcs in three spatial dimensions.

• Have separation between gravitational and non-gravitational forces (e.g. atmospheric drag, solar radiation pressure, albedo, thrust, etc.) acting on the satellite.

- The satellite motion and orbit statistics are observed to generate insights about the forces acting on the satellite.

- The main observation principles are:

• Satellite-to-satellite tracking in the high-low mode (SST-HL): A low Earth orbiting (LEO) satellite is tracked by high orbiting GNSS satellites, relative to a network of ground stations. The satellite is a probe within the Earth’s gravity field, which can be precisely tracked without interruption. The observed 3-D accelerations correspond to the gravity accelerations. The non-gravitational forces acting on the satellite are measured by accelerometers. An example of this measuring principle is given by the mission CHAMP (CHAllenging Minisatellite Payload)

• Satellite-to-satellite tracking in the low-low mode (SST-LL): Two LEO satellites are placed in the same orbital plane, separated by several hundred kilometers (~ 220 km), and the range (distance) between them is measured with the highest possible accuracy. Basically, the acceleration difference between the two satellites is measured. The two satellites can be considered as one instrument in which:

• (1) Variations in the gravity field cause variations in the range between the two satellites: areas of stronger gravity affect the lead satellite first and accelerate it away from the second satellite,

• (2) Range variations are measured by a high-accuracy microwave link or laser-ranging interferometry,

• (3) The observed range variations are corrected for non-gravitational effects by precise accelerometers.

SST-LL provides a very much higher sensitivity than SSL-HL. Examples of this measuring principle are the GRACE (Gravity Recovery and Climate Experiment) and GRACE Follow-On (GRACE-FO) missions.

• Satellite gravity gradiometry (SGG): Measures 3D gravitational acceleration gradients directly inside a single LEO spacecraft using paired accelerometer sets. Acceleration differences in all three dimensions are measured directly within a LEO satellite. One important advantage, compared to SST-HL and SST-LL, is that non-gravitational accelerations are the same for all measurements inside the satellite and hence vanish by differentiation. An example of this measuring principle is the mission GOCE (Gravity field and steady-state Ocean Circulation Explorer).

iii. Radar Altimetry, precipitation radar, Doppler radar (ground, airborne, and satellite-based)

 General Radar:
- Radar is a system that uses radio waves to determine the distance (ranging), direction (azimuth and elevation angles), and radial velocity of objects relative to the site

- It is a radiodetermination method used to detect and track aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations and terrain.

- Can be used for tracking and predicting severe storms, and determining the location and velocity of an object.

Radar Altimetry

- Principle: Measures the precise round-trip travel time of a nadir-directed microwave pulse (typically Ku-band ~ 13.6 GHz, C-band ~ 5.3 GHz, or Ka-band ~ 35.5 GHz) from a satellite to the ocean or ice surface

- Measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it takes a beam of radio waves to travel to ground, reflect, and return to the craft. 

- This type of altimeter provides the distance between the antenna and the ground directly below it, in contrast to a barometric altimeter which provides the distance above a defined vertical datum, usually mean sea level.

- The altitude above the ground is calculated from the radio waves' travel time and the speed of light. Radar altimeters required a simple system for measuring the time-of-flight that could be displayed using conventional instruments, as opposed to a cathode ray tube normally used on early radar systems.

- To do this, the transmitter sends a frequency modulated signal that changes in frequency over time, ramping up and down between two frequency limits, Fmin and Fmax over a given time, T. In the first units, this was accomplished using an LC tank with a tuning capacitor driven by a small electric motor. The output is then mixed with the radio frequency carrier signal and sent out the transmission antenna.

- Since the signal takes some time to reach the ground and return, the frequency of the received signal is slightly delayed relative to the signal being sent out at that instant. The difference in these two frequencies can be extracted in a frequency mixer, and because the difference in the two signals is due to the delay reaching the ground and back, the resulting output frequency encodes the altitude. The output is typically on the order of hundreds of cycles per second, not megacycles, and can easily be displayed on analog instruments. This technique is known as Frequency Modulated Continuous-wave radar.

- Radar altimeters normally work in the E band, Ka band, or, for more advanced sea-level measurement, S band. 

- Radar altimeters also provide a reliable and accurate method of measuring height above water, when flying long sea-tracks.

- The altitude specified by the device is not the indicated altitude of the standard barometric altimeter. A radar altimeter measures absolute altitude: the height "Above Ground Level" (AGL).

- The Honeywell KRA-405B is a common type of radar altimeter. It is an airborne instrument that provides accurate altitude information above the ground, especially during the approach and landing phases of flight. 

- Jason altimetry data show long-wavelength ocean height anomalies that gradually increase over decades. It is most commonly used to measure long-term global sea-level rise.

- A limitation is that it can only give data on the area below the sensor.

Precipitation Radar

-  Type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail, etc.)

- Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the intensity of the precipitation. Both types of data can be analyzed to determine the structure of storms and their potential to cause severe weather.

- Wavelength Bands:

- S-band (10 cm / 2 - 4 GHz): Minimal attenuation by heavy rainfall; used in ground stations (e.g., NEXRAD)

- C-band (5 cm / 4 - 8 GHz): Trade-off between antenna size and atmospheric attenuation

- X-band (3 cm / 8 - 12 GHz): High attenuation; restricted to short-range units

- Ku/Ka-Bands (13.6 GHz / 35.5 GHz): Used on spaceborne platforms (e.g., DPR on GPM) for fine vertical profiling of rain rates and ice structures

Sending Radar Pulses:

• Weather radars send directional pulses of microwave radiation, on the order of one microsecond long, using a cavity magnetron or klystron tube connected by a waveguide to a parabolic antenna. The wavelengths of 1 – 10 cm are approximately ten times the diameter of the droplets or ice particles of interest, because Rayleigh scattering occurs at these frequencies. This means that part of the energy of each pulse will bounce off these small particles, back towards the radar station.

• Shorter wavelengths are useful for smaller particles, but the signal is more quickly attenuated. Thus 10 cm (S-band) radar is preferred but is more expensive than a 5 cm C-band system. 3 cm X-band radar is used only for short-range units, and 1 cm Ka-band weather radar is used only for research on small-particle phenomena such as drizzle and fog. W band (3 mm) weather radar systems have seen limited university use, but due to quicker attenuation, most data are not operational.

• Radar pulses diverge as they move away from the radar station. Thus the volume of air that a radar pulse is traversing is larger for areas farther away from the station, and smaller for nearby areas, decreasing resolution at farther distances. At the end of a 150 – 200 km sounding range, the volume of air scanned by a single pulse might be on the order of a cubic kilometer. This is called the pulse volume.

• The volume of air that a given pulse takes up at any point in time may be approximated by the formula v=hr^2θ^2 where v is the volume enclosed by the pulse, h is pulse width (in e.g. meters, calculated from the duration in seconds of the pulse times the speed of light), r is the distance from the radar that the pulse has already traveled (in e.g. meters), and θ is the beam width (in radians). This formula assumes the beam is symmetrically circular, "r" is much greater than "h" so "r" taken at the beginning or at the end of the pulse is almost the same, and the shape of the volume is a cone frustum of depth "h".

- Listening for Return Signals:

• Between each pulse, the radar station serves as a receiver as it listens for return signals from particles in the air. The duration of the "listen" cycle is on the order of a millisecond, which is a thousand times longer than the pulse duration. The length of this phase is determined by the need for the microwave radiation (which travels at the speed of light) to propagate from the detector to the weather target and back again, a distance which could be several hundred kilometers. The horizontal distance from station to target is calculated simply from the amount of time that elapses from the initiation of the pulse to the detection of the return signal. The time is converted into distance by multiplying by the speed of light in air:

where c = 299,792.458 km/s is the speed of light, and n ≈ 1.0003 is the refractive index of air.

• If pulses are emitted too frequently, the returns from one pulse will be confused with the returns from previous pulses, resulting in incorrect distance calculations.

- Determining Height

• Since the Earth is round, the radar beam in vacuum would rise according to the reverse curvature of the Earth. However, the atmosphere has a refractive index that diminishes with height, due to its diminishing density. This bends the radar beam slightly toward the ground and with a standard atmosphere this is equivalent to considering that the curvature of the beam is 4/3 the actual curvature of the Earth. Depending on the elevation angle of the antenna and other considerations, the following formula may be used to calculate the target's height above ground:

Where:

r = distance radar–target,

ke = 4/3,

ae = Earth radius,

θe = elevation angle above the radar horizon,

ha = height of the feedhorn above ground.

Pulse Volume Calculation: The spherical volume scanned by a single radar pulse is modeled as:

V=h*r2*2

Where h is the pulse length in meters (h = c𝜏), r is the target range in meters, and is antenna beamwidth in radians

Beam Height Correction: Accounting for Earth curvature (a_e) and atmospheric refraction and atmospheric refraction (k_e = 4/3), target height H above the ground at range r and elevation angle e is

H = r2+(keae)2+2rk_eaesine-keae+ha

Doppler radar (ground, airborne, and satellite-based):

- Doppler Shift Principle: Detects the radial movement velocity v_r  of targets (rain drops, winds) toward or away from the radar by observing the phase/frequency shift of reflected pulses: