-- Sainath Aher

Department of Geography,

Sangamner College, Sangamner

Email: saigeo23@gmail.com

Introduction to

Remote Sensing

The several historic images of Earth taken, especially NASA’s “Earth Rise” which was taken by Apollo 8 astronaut Bill Anders in 1968 but a new image taken by a Russian weather satellite may have set a new record for detail.

Russian weather satellite set a new record…

Remote Sensing Overview

Basics…

RS ..?

Sensor.. ?

Energy .. ?

Scanning.. ?

Satellite .. ?

Sat. Image ..?

Basics…

RS ..?

Sensor.. ?

Energy .. ?

Scanning.. ?

Satellite .. ?

Sat. Image ..?

Ground Base RS Airbase RS Space base RS(Normal Photo) (Aircraft) (Satellite)

Active RS Passive RS

Microwave RS Thermal RS

Geosynchronous Sun synchronous(Geostationary) (Polar Orbit)

Along Track Across Track

Orbit

Scanning

Introduction

The term RS was first used in the early 1960s.

Remote sensing is the science and technology

for acquiring information about object

without physical contact it.

Acquire and measure the information of

object by device (sensor) which is not in

physical contact with the objects.

Our eays also sense the object informationwithout contact with it, hence it is true sensor

Definition: Remote sensing is the science and art

of acquiring information about the Earth's

surface without physically contact with it.

Varioues Satellite, Aircraft and

Balloons working for the RS.

Science

Art

Technology

P+

E+

T+

G

Remote Sensing by Laser-Induced PlasmasThe ChemCam Instrument Suite on the Mars

2. Interaction of energy with

the earth’s surface

3. Sensing of reflected energy by the sensor

4. Conversion of energy

received into photographic/digital data

5. Conversion of information into image

Stages in RS (Satellite Based RS Process)

Essential component of RS-3

The Signal (from an object or phenomena)

The Sensor (from a platform)

The Sensing (Acquiring knowledge about object)

The interaction of the signal with the object by which we obtain information about it.

Remote sensing is the branch of Physics, which is extensive application in almost every field of human activity

Sensor Height and Spatial coverage

Satellite

Aircraft

Balloon

Remote sensing platforms

Ground-based Airplane-based Satellite-based

Remote Sensing : The science and art of acquiring information about objects from a

distance using various sensors mounted on platforms which are at a considerable heightfrom the earth surface.

Platforms

Ground borne Air borne Space borne

Spectral radiometer, Pilot-Balloons

Aircraft Balloon

Satellite

Sensors -Camera

-Radar

Historical Remote Sensing Balloons Remote Sensing

Ground Borne Remote Sensing

(Aerial Photography)

Varioues Satellite, Aircraft and Balloons

working for the RS.

Satellite RS.

RS by Aircraft (Aerial Photography).

RS by Balloons.

Aerial photography is the taking of photographs of the ground from an elevated position.

Platforms for aerial photography include fixed-wing aircraft, helicopters, multirotor

Unmanned Aircraft Systems (UAS), balloons, blimps and dirigibles, rockets, kites, stand-alone

telescoping and vehicle mounted poles.

Mounted cameras may be triggered remotely or automatically; hand-held photographs may

be taken by a photographer.

Air Borne Remote Sensing

AircraftCamera system

Aerial Photographs

How AP taken ?

Sun-synchronous polar orbits

Most earth imaging satellites is polar-orbiting, meaning that they circle the planet in a roughly north-south ellipse while the earth revolves beneath them.

Therefore, unless the satellite has some sort of "pointing" capability, there are only certain times when a particular place on the ground will be imaged global coverage, fixed crossing, repeat sampling

typical altitude 500-1,500 km

example: Terra/Aqua, Landsat

Non-Sun-synchronous orbits

Tropics, mid-latitudes, or high latitude coverage.

typical altitude 200-2,000 km

example: TRMM, ICESat

Geostationary orbits

regional coverage, continuous sampling

over low-middle latitudes, altitude 35,000 km

example: GOES.

Space Borne Remote Sensing

Types of remote sensing

Passive: source of energy is either the Sun or Earth/atmosphere Sun

- wavelengths: 0.4-5 µm

Earth or its atmosphere

- wavelengths: 3 µm -30 cm

Active: source of energy is part of the remote sensor system Radar

- wavelengths: mm-m

Lidar

- wavelengths: UV, Visible, and near infrared

Passive RS Active RS

Camera takes photo as example no flash and flash

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Direction of Satellite Motion Active remote sensing instruments send out a signal of radiation at a particular wavelength.

Active Remote Sensing

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Passive Remote Sensing

Passive remote sensing instruments either use the Sun as the source of radiation…… or use radiation emitted by the Earth’s surface or atmosphere.

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By stitching together a continuous series of push broom images, a contiguous swath or ribbon of data encircling the Earth can be achieved.

Push-Broom Sensors(Along Track)

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In “Cross-Track Scanning,” a scan mirror swings back and forth along

the sub-orbital track, allowing the sensor to sequentially observed pixels and trace out a small swath or ribbon

of the Earth’s surface along the direction of the satellite’s motion.

Cross-Track Scanning Sensors

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Nimbus-7 TOMS Orbital Altitude: 955 km

EarthProbe TOMS (original) Orbital Altitude: 500 km

If the orbit is too low and/or the FOV is too small, complete global coverage cannot be obtained with only 16 orbits in a single day.

Spatial Coverage

Nadir

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Horizon

Solar Zenith Angle

Zenith

Elevation Angle

Orbital Geometry

IKONOS SATELLITE

IKONOS is a commercial earth

observation satellite, and was the first to

collect publicly available high-resolution

imagery at 1- and 4-meter resolution. It

offers multispectral (MS) and

panchromatic (PAN) imagery. The

IKONOS launch was called by John E.

Pike “one of the most significant

developments in the history of the space

age”. IKONOS imagery began being sold

on January 1, 2000.

Orbit height: 681 kmSpeed on orbit: 7.5 km/sLaunch date: September 24, 1999Launch site: Vandenberg Air Force Base (US)

The IKONOS-2 satellite is a fine resolution craft operated by GeoEye,(formerly Space Imaging). IKONOS was launched on September 24,1999, and provides imagery beginning January 1, 2000. IKONOS is usedto obtain both urban and rural mapping of natural resources and ofnatural disasters, tax mapping, agriculture and forestry analysis,mining, engineering, construction, and change detection. IKONOSprovides relevant data for nearly all aspects of environmental study.

SensorSatellite Sensor Bands Spectral Range Scene Size Pixel Res

IKONOS-2Multi-spectral

1=Blue 455 - 520 µm

11 X 11 km4 meter

2=Green 510 - 600 µm3=Red 630 - 700 µm4=NIR 760 - 850 µm

Panchromatic Pan 760 - 850 µm 1 meter

IKONOS SATELLITE

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QUICKBIRD

QuickBird imagery is a fineresolution remotely sensedproduct available to the publicthrough Digital Globe.QuickBird's ultra fine resolutionmakes this valuable imagery forvalidation and land coverassessment.

Sensor

Satellite Sensor Bands Spectral RangeScene Size Pixel Res

QuickBird-2Multi-spectral

1=Blue 450 - 520 µm

16.5 km X 16.5 km

2.44 - 2.88 meter

2=Green 520 - 600 µm3=Red 630 - 690 µm4=NIR 760 - 900 µm

Panchromatic Pan 760 - 850 µm 61 - 72 cm

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Weather Satellite -Russia

Sputnik first artificial satellite of INDIA U.S. military WGSS communications satellite

GPS Satellite NASA Milstar Satellite _ US

IRS-P4 Ocean-Colour Sensor (OCM)

IRS Satellite Series:

The initial versions are composed of the 1 (A,B,C,D). The later versions arenamed based on their area of application including OceanSat, CartoSat,ResourceSat. Some of the satellites have alternate designations based on thelaunch number and vehicle (P series for PSLV).

True Color (Sensor) Image FCC (Sensor) Image

Bird foot delta -Mississippi River Three Gorges Dam - China

Satellite Remote Sensing product

Satellite Based Remote Sensing CALIPSO Satellite

Scanner, Linear Array Detectors

IRS 1D PAN+LISS III

Pune City

How to Interpret a Satellite Image: 5 Tips and Strategies

Look for a scale Look for patterns, shapes, and textures

Define the colors (including shadows)

Find north

Consider your knowledge

Satellite images are likemaps: they are full ofuseful and interestinginformation, provided youhave a key.They can show us howmuch a city has changed,how well our crops aregrowing, where a fire isburning, or when a stormis coming.To unlock the richinformation in a satelliteimage, you need to fallowabove said 5 tips:

Photographic interpretation is “the act ofexamining photographic images for the purpose ofidentifying objects and judging their significance”(Colwell, 1997).

This mainly refers to its usage in military aerialreconnaissance using photographs taken fromreconnaissance aircraft.

Principles of image interpretation have beendeveloped empirically for more than 150 years.

The most basic of these principles are the elementsof image interpretation. They are: location, size,shape, shadow, tone/color, texture, pattern,height/depth and site/situation/association.

These are routinely used when interpreting anaerial photo or analyzing a photo-like image.

A well-trained image interpreter uses many of theseelements during his or her analysis without reallythinking about them.

However, a beginner may not only have to forcehimself or herself to consciously evaluate anunknown object with respect to these elements, butalso analyze its significance in relation to the otherobjects or phenomena in the photo or image.

How to Interpret a Satellite Image: 5 Tips and Strategies

Location: There are two primary methods to obtain precise location in the form of coordinates. 1) survey in thefield using traditional surveying techniques or global positioning system instruments, or 2) collect remotelysensed data of the object, rectify the image and then extract the desired coordinate information. Most scientistswho choose option 1 now use relatively inexpensive GPS instruments in the field to obtain the desired locationof an object. If option 2 is chosen, most aircraft used to collect the remotely sensed data have a GPS receiver.This allows the aircraft to obtain exact latitude/longitude coordinates each time a photo is taken.

Size: Most commonly, length, width and perimeter are measured. To be able to do this successfully, it isnecessary to know the scale of the photo. Measuring the size of an unknown object allows the interpreter to ruleout possible alternatives. It has proved to be helpful to measure the size of a few well-known objects to give acomparison to the unknown-object. For example, field dimensions of major sports like soccer, football, andbaseball are standard throughout the world. If objects like this are visible in the image, it is possible todetermine the size of the unknown object by simply comparing the two.

Shape: There is an infinite number of uniquely shaped natural and man-made objects in the world. A fewexamples of shape are the triangular shape of modern jet aircraft and the shape of a common single-familydwelling. Humans have modified the landscape in very interesting ways that has given shape to many objects,but nature also shapes the landscape in its own ways. In general, straight, recti-linear features in theenvironment are of human origin. Nature produces more subtle shapes.

Shadow: Virtually all remotely sensed data is collected within 2 hours of solar noon to avoid extended shadowsin the image or photo. This is because shadows can obscure other objects that could otherwise be identified. Onthe other hand, the shadow cast by an object may be key to the identity of another object. Take for example theWashington Monument in Washington D.C. While viewing this from above it can be difficult to discern theshape of the monument, but with a shadow cast, this process becomes much easier. It is good practice to orientthe photos so that the shadows are falling towards the interpreter. A pseudoscopic illusion can be produced ifthe shadow is oriented away from the observer. This happens when low points appear high and high pointsappear low.

Elements of Interpretation

Tone and color: Real-world materials like vegetation, water and bare soil reflect different proportions ofenergy in the blue, green, red, and infrared portions of the electro-magnetic spectrum. An interpreter candocument the amount of energy reflected from each at specific wavelengths to create a spectral signature.These signatures can help to understand why certain objects appear as they do on black and white or colorimagery. These shades of gray are referred to as tone. The darker an object appears, the less light it reflects.

Texture: This is defined as the “characteristic placement and arrangement of repetitions of tone or color in animage.” Adjectives often used to describe texture are smooth (uniform, homogeneous), intermediate, and rough(coarse, heterogeneous). It is important to remember that texture is a product of scale. On a large scaledepiction, objects could appear to have an intermediate texture. But, as the scale becomes smaller, the texturecould appear to be more uniform, or smooth. A few examples of texture could be the “smoothness” of a pavedroad, or the “coarseness” a pine forest.

Pattern: Pattern is the spatial arrangement of objects in the landscape. The objects may be arranged randomlyor systematically. They can be natural, as with a drainage pattern of a river, or man-made, as with the squaresformed from the United States Public Land Survey System. Typical adjectives used in describing pattern are:random, systematic, circular, oval, linear, rectangular, and curvilinear to name a few.

Height and Depth: Height and depth, also known as “elevation” and “bathymetry”, is one of the mostdiagnostic elements of image interpretation. This is because any object, such as a building or electric pole thatrises above the local landscape will exhibit some sort of radial relief. Also, objects that exhibit this relief will casta shadow that can also provide information as to its height or elevation. A good example of this would bebuildings of any major city.

Site/Situation/Association: Site has unique physical characteristics which might include elevation, slope, andtype of surface cover (e.g., grass, forest, water, bare soil). Site can also have socioeconomic characteristics suchas the value of land or the closeness to water. Situation refers to how the objects in the photo or image areorganized and “situated” in respect to each other. Most power plants have materials and building associated in afairly predictable manner. Association refers to the fact that when you find a certain activity within a photo orimage, you usually encounter related or “associated” features or activities. Usually there are multiple largebuildings, massive parking lots, and it is usually located near a major road or intersection.

Elements of Interpretation

Remote Sensing of our Campus

Kollar Area -Google Image

Date : 11/5/2013

Agency: CNES/Astrium

Flying Height – 1701 ft

Eye Altitude: 8372 ft

Latitude & Longitude

Interpretation:White patches in rectangular and square shape is settlements, water is in Black blush, agriculture in green, Soil in red, roads in black(True colure)

In earlier Civilizations-Navigators, Pictoral Form

Collection of Data Geographers, MAP CARTOGRAPHYSurveyors,

Use of Maps - To Describe far off/Distant Places- As an Aid of Navigation- Military Purposes

Govt. Institutes Major Preparation of Topographical MapsMilitary Depts. Role on the Basis of Spatial Data

collected from actual Field Survey

Ex. India- S.O.I., England –O.S., U.S.A.-USGS

Evolution of Remote Sensing Technique

18th Century – Need of Systematic Mapping of Land/Earth Surface

Purpose of Mapping

Natural Resource Evaluation,Management & Development Tourism Development Infrastructure Development Rural Development Transport & Communication Environmental Management etc.

In order to make an effective study of these thrust and emerging fields, since last two decades –

REMOTE SENSING : as a new innovative technology

Roots of Remote Sensing Conventional Photography

Ist Photograph - Daguerre & Niepce (France-1839) : permanent impression of image created by lightIst Aerial Photograph – G. F.Tournachon (1858) : PARIS city

(From Balloon)

Ist recorded A/Photo – W. Wright (1909) : CENTOCELLI city, Italy(From aero plane)

During II World War -Extensive Use of A/Photographs for Topographic Mapping,

Military Intelligence, Surveillance of enemy areas

After II World War- Transfer of Technology to non military application Geological Studies

Forest SurveySoil SurveyRoute Alignment

Important Events in the History of Remote Sensing

1957 – Launch First Artificial satellite : SPUTNIK

1960 - The word “Remote Sensing” was used first time

Launch of TIROS 1: weather satellite

1972 – LANDSAT 1 (ERTS-Earth Resource Technology Satellite)

1973 - Launch of SKYLAB, the first American Space station

1970-80- Advancement in Image Processing due to availability of

microprocessor based computers

1978 – SEASAT 1

1980 – METOSAT by USSR

1986 – SPOT( System Pourl’ Observation dela Terre) French Satellite

France, Sweden and Belgium jointly

Landmarks of Indian Space Programme

1975 - ARYABHATTA (Launched from Soviet Union)

1976 - SITE (Satellite Instructional Television Experiment) Telecast Services

1979 - BHASKARA (Launched from Soviet Union)

1980 - ROHINI (Ist satellite launch vehicle : SLV-3, developed by India)

1981- APPLE (Ariane Passenger Payload Expt.)Launched by Europe’s Ariane

rocket

1983 - ROHINI : sensor camera for transmitting pictures

1988 - IRS-1A : Launched from Baiknur, Khazakistan (72.5m-LISS I)

1991 - IRS-1B

1993 - IRS- P2 : PSLV-D2, Launched from Sriharikota, India (36.25m-LISS II)

1995 - IRS-1C : (188m -WiFS)

1996 - IRS- P3: PSLV- D3, Launched from Sriharikota, India

1997 - IRS-1D

1999 - IRS-P4: OCEANSAT, (350m- Ocean Color Monitor)

2004 - IRS-P6 : RESOURCESAT (23.5m-LISS III, 5.8m-LISS IV)

2005 - IRS-P5: CARTOSAT 1 (PAN, 2.5m )

Future IRS MissionsCARTOSAT 2 (1m resolution)RISAT 1: Radar Imaging Satellite

History of Remote Sensing

INTERNATIONAL:

During the American civil war, the photograph taken from a Balloon were extensivelyused for Military purpose

Aerial Photography took place during the two world war, mainly for Military purpose

The use of Aerial photographs for peaceful purpose, e.g. in forestry, geology,agriculture, land use planning, disaster management started soon after 1st world war.

The collection of data from space is started in 1960 with orbiting US metrologicalsatellite.

The photograph taken by the astronauts of Gemini and Apollo satellites which isuseful for Earth-Resource Survey.

NATIONAL:

RS practiced in INDIA as early as 1920 when B&W aerial photographs were used were used for

survey & Geological Exploration.

1926-The aerial photographs used for Flood Assessment of Indus river-1st RS Application.

1970-RS use the sensor was first at tempered in Agricultural by the ISRO.

False color infrared (IR) image of coconut plantation in Kerala state ware abtained from

Helicopter.

Basics of EMR/Atmospheric Affects Foundations

of Remote Sensing

EMR is a form of energy that reveals its presence by the observable effects it produces when it strikes the matter. EMR is considered to span the spectrum of wavelengths from 10-10 mm to cosmic rays up to 1010 mm, the broadcast wavelengths, which extend from 0.30-15 mm.

Electromagnetic Waves

Electromagnetic waves may be classified by frequency or wavelength. - the velocity of ALL electromagnetic waves is equal to the speed of light "c" c = 299,792,458 meters per second*3.28 ft/m*1/5280 mi/ft = 186,234 miles/sec *60 sec/min*60 min/hr = 670,444,951 miles per hour

Wave Phenomena Concepts

Electromagnetic waves are generated by hydrogen fusion taking

place in the sun.

When the electromagnetic energy encounters anything, even a

very tiny object like a molecule of air or water, one of three

reactions will occur.

The radiation will either be reflected off the object, be absorbed

by it or it could be transmitted through because the energy

manifested by electromagnetic waves can behave both like a

particle (the proton) and like a wave (the oscillating

electric/magnetic fields).

In remote sensing, information transfer is

accomplished by use of electromagnetic

radiation (EMR).

The Electromagnetic Spectrum:

The ranges from the shorter wavelengths (including gamma and

electromagnetic spectrum x-rays) to the longer wavelengths

(including microwaves and broadcast radio waves).

There are several regions of the electromagnetic spectrum

which are useful for remote sensing.

Electromagnetic radiation is energy propagated through space

between electric and magnetic fields.

The electromagnetic spectrum is the extent of that energy

ranging from cosmic rays, gamma rays, X-rays to ultraviolet,

visible and infrared radiation including microwave energy.

weather and climate forecasts.

CALIPSO Satellite:

The Cloud-Aerosol Lidar and Infrared Pathfinder SatelliteObservation (CALIPSO) satellite provides new insight into therole that clouds and atmospheric aerosols (airborne particles)play in regulating Earth's weather, climate, and air quality.CALIPSO combines an active lidar instrument with passiveinfrared and visible imagers to probe the vertical structure andproperties of thin clouds and aerosols over the globe. CALIPSOwas launched on April 28, 2006 with the cloud profiling radarsystem on the CloudSat satellite.

CALIPSO and CloudSat are highly complementary and togetherprovide new, never-before-seen 3-D perspectives of how cloudsand aerosols form, evolve, and affect weather and climate.CALIPSO and CloudSat fly in formation with three othersatellites in the A-train constellation to enable an even greaterunderstanding of our climate system from the broad array ofsensors on these other spacecraft.CALIPSO is a joint U.S. (NASA) and French (Centre Nationald'Etudes Spatiales/CNES) satellite mission that has been inoperation for four years.

Astrium was an aerospace subsidiary of the European Aeronautic Defence and Space Company (EADS) that provided civil and military space systems and services from 2006 to 2013. In 2012, Astrium had a turnover of €5.8 billion and 18,000 employees in France, Germany, the United Kingdom, Spain and the Netherlands. Astrium was a member of Institute of Space, its Applications and Technologies.

In late 2013 Astrium was merged with Cassidian, the defence division of EADS and Airbus Military to form Airbus Defence & Space.[1] EADS itself reorganized as the Airbus Group, with three divisions that include Airbus, Airbus Defence & Space, and Airbus Helicopters.

GPS IS Global Positing System

• The GPS- 3 Components:

• 1. The Control Segment

• 2. The Space Segments

• 3. The User Segment

Indian RS Indian Remote Sensing satellites (IRS) are a series of Earth

Observation satellites, built, launched and maintainedby Indian Space Research Organisation. The IRS seriesprovides many remote sensing services to India.

Applications of remote sensing dataConventional radar is mostly associated with aerial traffic control, early warning, and certain large scale meteorological data. Doppler radar is used by local law enforcements’ monitoring of speed limits and in enhanced meteorological collection such as wind speed and direction within weather systems in addition to precipitation location and intensity. Other types of active collection includes plasmas in the ionosphere. Interferometricsynthetic aperture radar is used to produce precise digital elevation models of large scale terrain (See RADARSAT, TerraSAR-X, Magellan).Laser and radar altimeters on satellites have provided a wide range of data. By measuring the bulges of water caused by gravity, they map features on the seafloor to a resolution of a mile or so. By measuring the height and wavelength of ocean waves, the altimeters measure wind speeds and direction, and surface ocean currents and directions.Light detection and ranging (LIDAR) is well known in examples of weapon ranging, laser illuminated homing of projectiles. LIDAR is used to detect and measure the concentration of various chemicals in the atmosphere, while airborne LIDAR can be used to measure heights of objects and features on the ground more accurately than with radar technology. Vegetation remote sensing is a principal application of LIDAR.Radiometers and photometers are the most common instrument in use, collecting reflected and emitted radiation in a wide range of frequencies. The most common are visible and infrared sensors, followed by microwave, gamma ray and rarely, ultraviolet. They may also be used to detect the emission spectra of various chemicals, providing data on chemical concentrations in the atmosphere.Stereographic pairs of aerial photographs have often been used to make topographic maps by imagery and terrain analysts in trafficability and highway departments for potential routes.Simultaneous multi-spectral platforms such as Landsat have been in use since the 70’s. These thematic mappers take images in multiple wavelengths of electro-magnetic radiation (multi-spectral) and are usually found on Earth observation satellites, including (for example) the Landsat program or the IKONOS satellite. Maps of land cover and land use from thematic mapping can be used to prospect for minerals, detect or monitor land usage, deforestation, and examine the health of indigenous plants and crops, including entire farming regions or forests.[3] Landsat images are used by regulatory agencies such as KYDOW to indicate water quality parameters including Secchi depth, chlorophyll a density and total phosphorus content. Weather satellites are used in meteorology and climatology.Hyperspectral imaging produces an image where each pixel has full spectral information with imaging narrow spectral bands over a contiguous spectral range. Hyperspectral imagers are used in various applications including mineralogy, biology, defence, and environmental measurements.Within the scope of the combat against desertification, remote sensing allows to follow-up and monitor risk areas in the long term, to determine desertification factors, to support decision-makers in defining relevant measures of environmental management, and to assess their impacts.[6]Geodetic[edit]Overhead geodetic collection was first used in aerial submarine detection and gravitational data used in military maps. This data revealed minute perturbations in the Earth’sgravitational field (geodesy) that may be used to determine changes in the mass distribution of the Earth, which in turn may be used for geological studies.Acoustic and near-acoustic[edit]Sonar: passive sonar, listening for the sound made by another object (a vessel, a whale etc.); active sonar, emitting pulses of sounds and listening for echoes, used for detecting, ranging and measurements of underwater objects and terrain.Seismograms taken at different locations can locate and measure earthquakes (after they occur) by comparing the relative intensity and precise timings.To coordinate a series of large-scale observations, most sensing systems depend on the following: platform location, what time it is, and the rotation and orientation of the sensor. High-end instruments now often use positional information from satellite navigation systems. The rotation and orientation is often provided within a degree or two with electronic compasses. Compasses can measure not just azimuth (i. e. degrees to magnetic north), but also altitude (degrees above the horizon), since the magnetic field curves into the Earth at different angles at different latitudes. More exact orientations require gyroscopic-aided orientation, periodically realigned by different methods including navigation from stars or known benchmarks.