Geostationary orbit
A circular orbit over the equator that moves with Earth’s rotation and keeps a satellite above the same ground location.
- Geostationary Earth Orbit Satellite (GEO)Definition paragraph
Glossary
A shared vocabulary
The orbit, light, spacecraft, instrument, and evidence terms used in the explainer, with the public sources that ground them.
Personal educational project based on cited public sources. Not an official publication of my employer or of the agencies or companies discussed. Models are schematic; estimates and assumptions are identified.
Orbits and geometry · Light and thermal radiation · Instruments and architecture · Spacecraft services · Payload electronics and interfaces · Electronics and mechanism assemblies · Reading this project
Orbits and geometry
Geosynchronous orbit
An orbit with a period equal to Earth’s rotation relative to the stars. The circular, equatorial, prograde case is geostationary.
- Catalog of Earth Satellite OrbitsHigh Earth Orbit
HEO
Highly elliptical orbit: an elongated path with substantially different closest and farthest distances from Earth. This site’s HEO example is a generic teaching ellipse.
- Catalog of Earth Satellite OrbitsHighly Elliptical Orbits
LEO and MEO
Low Earth orbit and medium Earth orbit are orbit families. Their selected altitudes here are teaching inputs, not the parameters of a military program.
- Catalog of Earth Satellite OrbitsLow Earth Orbit; Medium Earth Orbit
Semi-major axis
Half the longest diameter of an ellipse. In the two-body model it sets the orbital period; it is measured from the ellipse’s center, not from Earth’s surface.
- How Orbital Motion is CalculatedOpening orbital-elements discussion
Eccentricity
A measure of an orbit’s shape. A circular orbit has zero eccentricity; increasing elliptic eccentricity makes the ellipse more elongated.
- How Orbital Motion is CalculatedOpening orbital-elements discussion
Apogee and perigee
The farthest and closest points, respectively, in an orbit around Earth. The HEO calculation displayed here uses apogee.
- How Orbital Motion is CalculatedPolar equation and motion near perigee and apogee
Inclination
The angle between an orbital plane and its reference plane. Earth’s equatorial plane is the usual reference for Earth satellites.
- How Orbital Motion is CalculatedThe Orbit in Space
Nadir
The direction toward the surface point directly below the observer. The model uses that point to define its reference distance.
Sidereal day
Earth’s rotation period measured relative to the distant celestial reference frame. It supplies the general geosynchronous reference period.
- Astrodynamic ParametersParameter table: mean sidereal day
Light and thermal radiation
Infrared
Electromagnetic radiation at wavelengths longer than visible light. Infrared instruments can measure thermal emission from warm objects.
- Electromagnetic MathElectromagnetic spectrum and The Energy of Light activities
Photon
A quantum of electromagnetic radiation. A photon’s energy depends on its frequency, and therefore on its wavelength.
- Electromagnetic MathActivity 47, PDF p. 112
Wavelength
The spatial repeat distance of a wave. In vacuum, wavelength multiplied by frequency equals the speed of light.
- Electromagnetic MathActivity 47, PDF p. 112, problem 4
Spectral band
A selected interval of wavelengths. An instrument channel has a particular spectral response; the model’s teaching intervals are ideal rectangles, not instrument responses.
- Thermal Infrared Sensor (TIRS)Spectral Bands
SWIR, MWIR and LWIR
Short-wave, mid-wave and long-wave infrared. The model uses these broad labels to organize illustrative wavelengths, without adopting universal boundaries or a real sensor bandpass.
- Infrared Detectors Overview in the Short Wave Infrared to Far Infrared for CLARREO MissionSections A.1–A.3 and Table 1
Molecular emission band
A region of the spectrum associated with radiation from a molecule. The cited civil combustion study identifies bands of carbon dioxide and water vapor.
- Determination of primary-zone smoke concentrations from spectral radiance measurements in gas turbine combustorsPrinted p. 5 / PDF p. 7, gaseous combustion products paragraph
Atmospheric absorption and windows
Atmospheric gases absorb some wavelengths more strongly than others. A window is a region with greater transmission; this site does not calculate a transmission curve.
- Remote SensingAbsorption Bands and Atmospheric Windows
Blackbody
An ideal thermal radiator whose spectrum depends on its temperature. The model’s uniform blackbody is a teaching source, not a measured Earth scene or plume.
- Planck FunctionFirst formula image
Spectral radiance
Radiant power per projected area, per solid angle, and per wavelength interval. Stating the wavelength unit matters: a per-meter density differs from a per-micrometer density.
- Planck FunctionWavelength calculator and first formula image
Band radiance
Spectral radiance integrated across a stated wavelength interval. The result retains area and solid-angle units; it is not power collected by a detector.
- Planck FunctionWavelength-form Planck law
Photon radiance
Radiance expressed as a photon rate per projected area and solid angle. Each wavelength’s energy radiance is divided by the energy of one photon before integration.
- Electromagnetic MathActivity 47, PDF p. 112
Diffraction
The spreading and interference associated with a wave passing through an aperture. The ideal circular-aperture example gives an angular scale, not the measured resolution of a real instrument.
- Electromagnetic MathActivity 46, PDF p. 110
Vacuum light time
The time light takes to cross a stated distance in vacuum. Processing and communications routing add other delays, so this is not a warning-system latency.
- Fundamental Physical Constants: complete listing, 2022 CODATA adjustmentSpeed of light in vacuum
Instruments and architecture
OPIR
Overhead persistent infrared: the term used in the cited public architecture documents for space-based infrared observation supporting warning and related missions.
SBIRS
Space Based Infrared System. The cited public GAO report describes spacecraft in geosynchronous and highly elliptical orbits and an associated ground segment.
Bus and payload
The bus is the spacecraft platform supporting the mission equipment. An infrared payload, a mission processor, and communications equipment have distinct roles in the public architecture described by GAO.
- Missile Warning Satellites: Space Development Agency Should Be More Realistic and Transparent About Risks to Capability DeliveryPWSA-Enabling Technologies and Processes, opening description
Ground segment
The receiving, spacecraft-operations, and mission-data-processing parts of a space system on the ground. A drawn ground marker represents those roles, not an actual facility layout.
- Missile Warning Satellites: Comprehensive Cost and Schedule Information Would Enhance Congressional OversightPrinted p. 1 / PDF p. 5 and printed p. 8 / PDF p. 12
FORGE
Future Operationally Resilient Ground Evolution. GAO’s historical program description assigns the planned system spacecraft-operations and mission-data-processing roles.
- Missile Warning Satellites: Comprehensive Cost and Schedule Information Would Enhance Congressional OversightPrinted p. 1 / PDF p. 5, opening paragraph
ABI
Advanced Baseline Imager, the civil imaging instrument of the GOES-R satellite series. Its published channel and observing-cadence figures describe that instrument alone.
- Advanced Baseline Imager (ABI)Instrument description
TIRS-2
Thermal Infrared Sensor 2, the Landsat 9 civil thermal instrument. Its published detector, optics, and cooling specifications remain attached to that named instrument.
- TIRS-2 Testing: A Landsat 9 Instrument Takes ShapeInstrument design and cryocooler paragraphs
Focal plane
The region where an optical system forms its image and detector assemblies receive the light. The civil examples show that instrument-specific optical and detector designs vary.
- GOES-R Series Data Book, Revision APrinted pp. 3-8 and 3-11 / PDF pp. 36 and 39
QWIP
Quantum well infrared photodetector, a detector technology used in the named TIRS civil instruments. Selecting this material family does not assign an operating temperature.
- Thermal Infrared Sensor (TIRS)Design section
HgCdTe and InSb
Mercury cadmium telluride and indium antimonide: infrared detector materials discussed in the opened NASA detector review. A material name alone is not a temperature or performance specification.
- Infrared Detectors Overview in the Short Wave Infrared to Far Infrared for CLARREO MissionPDF pp. 3–7, sections A.1–A.3
Cryocooler
A refrigerator that removes heat from a cold region. The cold-stage temperature, heat lift, and required input power depend on the particular device and operating conditions.
- Review of Refrigeration Methods (submitted chapter)Opening refrigeration discussion, PDF pp. 2–3
Carnot coefficient of performance
The ideal refrigeration limit relating heat removed at the cold temperature to required work. A real cooler’s efficiency and heat load are separate quantities.
- Review of Refrigeration Methods (submitted chapter)PDF pp. 2–3, equation (1) and definitions
Spacecraft services
Solar cell, string, and array
A photovoltaic cell produces electrical current from absorbed light. Cells connected in series form a string; strings can be connected in parallel as part of an array circuit. Mechanical panel boundaries and electrical circuit boundaries are different descriptions.
- State-of-the-Art of Small Spacecraft Technology: 3.0 PowerSection 3.2.1 Solar Cells, opening paragraphs; incidence and multi-junction cells
- GOES-R Series Data Book, Revision ASolar Array, printed p. 11-3 / PDF p. 135, paragraph above Figure 11-3
Slip ring
An electrical connection across a rotating interface. GOES-R uses a slip-ring assembly to transfer power from its rotating solar wing to the spacecraft; the motor and resolver belong to the associated drive assembly.
- GOES-R Series Data Book, Revision ASolar Array Drive/Slip Ring Assembly, printed p. 15-4 / PDF p. 164
Power conditioning and distribution
Conditioning regulates or converts the available electrical supply. Distribution delivers it to equipment through feeds and switches, with sensing and protection. GOES-R assigns these functions to different modules in its Power Regulation Unit.
- GOES-R Series Data Book, Revision APrinted p. 11-2 / PDF p. 134; PRU module list, printed p. 11-5 / PDF p. 137
- GOES-R Series Data Book, Revision APower Regulation Unit and Fuse Board Assemblies, printed pp. 11-5–11-6 / PDF pp. 137–138
Buck and boost converters
DC power converters that step voltage down or up. GOES-R uses buck/boost converters for battery charge and discharge, and buck converters to derive its lower-voltage bus from the higher-voltage supply.
- GOES-R Series Data Book, Revision APrinted p. 11-2 / PDF p. 134; PRU module list, printed p. 11-5 / PDF p. 137
Eclipse power
Stored electrical energy supports spacecraft loads while Earth blocks sunlight from the array. In GOES-R, batteries also supply demand beyond the available array output and recharge when excess array power is available.
- GOES-R Series Data Book, Revision APrinted pp. 11-2–11-4 / PDF pp. 134–136, Batteries and EPS operation
Attitude versus orbit
Attitude is the spacecraft’s orientation; its orbit describes its path around Earth. GOES-R’s guidance, navigation, and control subsystem determines both, using attitude-reference sensors for orientation and separate navigation information for orbital position.
- GOES-R Series Data Book, Revision AGuidance Navigation & Control, printed p. 12-1 / PDF p. 141, attitude-determination paragraph
Star tracker and inertial reference
A star tracker uses celestial observations as an attitude reference. An inertial unit measures motion with onboard sensors. GOES-R combines star trackers and an inertial measurement unit to determine attitude.
- GOES-R Series Data Book, Revision AGuidance Navigation & Control, printed p. 12-1 / PDF p. 141, attitude-determination paragraph
- Basics of Space Flight: Chapter 11, Onboard Systems, page twoCelestial Reference and Inertial Reference
Reaction wheel
A motor-driven wheel exchanges angular momentum with the spacecraft to control its attitude. Propulsion can help manage accumulated wheel momentum. A wheel is an actuator; a star tracker is a sensor.
- Basics of Space Flight: Chapter 11, Onboard Systems, page twoAttitude and Articulation Control, 3-Axis paragraphs; momentum desaturation
- GOES-R Series Data Book, Revision APrinted p. 12-1 / PDF p. 141; Propulsion Subsystem, printed p. 13-1 / PDF p. 151
Multilayer insulation (MLI)
Thermal blankets used to reduce unwanted heat transfer. GOES-R uses MLI alongside coatings and low-conductivity supports; radiators provide a deliberately different path for rejecting heat.
- GOES-R Series Data Book, Revision APassive thermal control, printed p. 14-2 / PDF p. 156
Tracking, telemetry, and command (TT&C)
Spacecraft communication functions for tracking, reporting engineering status, and receiving commands. GOES-R documents these responsibilities separately from instrument-data transmission and relay services.
- GOES-R Series Data Book, Revision ACommunications Subsystem, printed pp. 9-1–9-3 / PDF pp. 117–119; distinct service functions
- GOES-R Series Data Book, Revision ACommand and Telemetry Processor, printed p. 10-2 / PDF p. 126
Payload electronics and interfaces
FPA and FPM
In ABI, a focal-plane array combines the detector array and its associated readout integrated circuit for one spectral channel. A focal-plane module groups the filtered channels for a spectral region. These are different levels of assembly.
- GOES-R Series Data Book, Revision APrinted p. 3-9 / PDF p. 37, Focal Plane Modules
Readout integrated circuit (ROIC)
The integrated circuit associated with a detector array that reads its electrical response. In ABI this belongs to the focal-plane array; the video-processing electronics are a separate stage.
- GOES-R Series Data Book, Revision APrinted p. 3-9 / PDF p. 37
Video Processor
ABI electronics that generate array timing and bias, collect detector samples, and format them for transmission to the Data Processor in the Electronics Unit.
- GOES-R Series Data Book, Revision APrinted p. 3-16 / PDF p. 44, Sensor Unit Electronics
Detector bias and timing
Electrical operating voltages and clock signals supplied to read out the detector arrays. ABI assigns these functions to its Video Processors.
- GOES-R Series Data Book, Revision APrinted p. 3-16 / PDF p. 44, Video Processors paragraph
Sensor Unit Electronics (SUE)
In ABI, electronics in the Sensor Unit that digitize focal-plane data and control mechanisms. Video Processors and Peripheral and Thermal Control electronics form the SUE.
- GOES-R Series Data Book, Revision APrinted pp. 3-15–3-16 / PDF pp. 43–44
Electronics Unit (EU)
ABI’s main electrical interface to the spacecraft. Its chassis and circuit cards provide power supplies, command and control, data processing, telemetry, and scan control.
- GOES-R Series Data Book, Revision APrinted p. 3-16 / PDF p. 44, Electronics Unit
Packetization
Organizing data into packets for further handling. ABI’s Data Processor formats and packetizes detector data received from the Video Processor; this is distinct from the earlier digitization of the detector signals.
- GOES-R Series Data Book, Revision APrinted p. 3-15 / PDF p. 43 and Table 3-6, printed p. 3-17 / PDF p. 45
Cryocooler Control Electronics (CCE)
ABI electronics that operate each cryocooler. A platinum resistance thermometer measures the cold-head temperature; the controller adjusts power-amplifier duty cycle to maintain its set point.
- GOES-R Series Data Book, Revision APrinted p. 3-17 / PDF p. 45, Cryocooler Control Electronics
Thermal interface
A physical connection whose thermal role must be accounted for. GOES-R isolates ABI from its platform with titanium mounting feet while providing a separate loop-heat-pipe connection to its dedicated radiator.
- GOES-R Series Data Book, Revision APrinted p. 14-5 / PDF p. 159, Earth Pointing Platform
Electronics and mechanism assemblies
Scanner Interface & Motor Driver (SIMD)
The ABI Electronics Unit card that drives its scan-mirror motors. Optical encoders measure position through separate encoder-processor cards.
- GOES-R Series Data Book, Revision ATable 3-6, printed p. 3-17 / PDF p. 45; printed p. 3-7 / PDF p. 35
- GOES-R Series Data Book, Revision ATable 3-6, printed p. 3-17 / PDF p. 45, Encoder Processors row
Peripheral and Thermal Control (P&TC)
ABI Sensor Unit electronics for thermal hardware, calibration targets, covers and focus control. Scan-mirror motion has its own drive electronics in the Electronics Unit.
- GOES-R Series Data Book, Revision APrinted p. 3-16 / PDF p. 44, P&TC paragraph
- GOES-R Series Data Book, Revision ATable 3-6, printed p. 3-17 / PDF p. 45; printed p. 3-7 / PDF p. 35
High Speed I/O (HSIO)
ABI’s SpaceWire interface to the spacecraft. This Electronics Unit card carries data downstream of the Data Processor’s formatting and packetization.
- GOES-R Series Data Book, Revision ATable 3-6, printed p. 3-17 / PDF p. 45, High Speed I/O
Telemetry and Timing (TNT)
The ABI Electronics Unit card that generates system clocks and handles instrument telemetry.
- GOES-R Series Data Book, Revision ATable 3-6, printed p. 3-17 / PDF p. 45, Telemetry and Timing
Thermal Dynamic Unit (TDU)
The thermomechanical portion of an ABI cryocooler: an integral cooler, a remote cold head and a transfer line. Its control electronics are a separate assembly.
- GOES-R Series Data Book, Revision APrinted p. 3-14 / PDF p. 42, Cryocooler
- GOES-R Series Data Book, Revision APrinted p. 3-17 / PDF p. 45, CCE
Solar Array Drive Assembly (SADA)
GOES-R’s motor-driven mechanism for rotating its solar wing, with resolver circuits to measure its position. A separate slip-ring assembly transfers electrical power across the rotating joint.
- GOES-R Series Data Book, Revision APrinted p. 15-4 / PDF p. 164, SADA/SRA
Power Regulation Unit (PRU)
The GOES-R assembly that regulates power from the solar array and batteries to spacecraft loads. It contains modules for regulation, conversion, distribution and monitoring.
- GOES-R Series Data Book, Revision APrinted p. 11-5 / PDF p. 137, Power Regulation Unit
Command and Telemetry Processor (CTP)
GOES-R’s gateway for validating received spacecraft commands and formatting health telemetry. It passes accepted commands onward to the flight-computer architecture.
- GOES-R Series Data Book, Revision APrinted p. 10-2 / PDF p. 126, CTP
Remote Interface Unit (RIU)
A GOES-R interface between the flight computer’s data bus and distributed equipment. It routes commands and collects engineering measurements and status; the Sun Pointing Platform has a related interface unit.
- GOES-R Series Data Book, Revision APrinted p. 10-4 / PDF p. 128, RIU/SIU
Current Sensor Unit (CSU)
GOES-R hardware that measures electrical feed current and produces an analog telemetry voltage. An RIU digitizes that voltage; this is a spacecraft-health measurement, separate from detector readout.
- GOES-R Series Data Book, Revision APrinted pp. 10-6–10-7 / PDF pp. 130–131, Current Sensor Unit
Transient Suppression Unit (TSU)
A passive GOES-R harness interface that supplies a path for electrostatic discharge to protect susceptible circuitry. It has no command or telemetry functions of its own.
- GOES-R Series Data Book, Revision APrinted p. 10-7 / PDF p. 131, TSU
Focal Plane Electronics (FPE)
In TIRS-2, the electronics between the detector assemblies and their connection to the main electronics. The published design distinguishes the FPE from the focal-plane interface board.
- Landsat 9 TIRS-2 Architecture and DesignPDF pp. 14–15, new FIB and FPE cross-strapping; PDF p. 19, FPA/FPE/FIB/MEB paths
Focal-plane Interface Board (FIB)
A TIRS-2 board in the connection between the focal-plane electronics and main electronics. The published design uses selected cross-connections between redundant electronics.
- Landsat 9 TIRS-2 Architecture and DesignPDF pp. 14–15, new FIB and FPE cross-strapping; PDF p. 19, FPA/FPE/FIB/MEB paths
- Landsat 9 TIRS-2 Architecture and DesignPDF pp. 14–15, MEB/CCE/FPE/encoder cross-strapping and FIB/RSE; PDF p. 19, redundant sides
Main Electronics Box (MEB)
The TIRS-2 assembly containing separate boards for command and data handling, power, thermal and mechanism control, and high-speed interfaces.
- Landsat 9 TIRS-2 Architecture and DesignPDF p. 6, GSFC electrical responsibility; PDF p. 19, MEB-A/B board labels and interface legend; PDF p. 20, Electrical row
Redundancy Switch Electronics (RSE)
TIRS-2 electronics that connect the cooler-control electronics to its thermomechanical unit through the published redundant switching arrangement.
- Landsat 9 TIRS-2 Architecture and DesignPDF p. 6, Ball cryocooler responsibility; PDF pp. 14–15, RSE linking CCE to TMU; PDF p. 19, cryocooler paths
- Landsat 9 TIRS-2 Architecture and DesignPDF pp. 14–15, MEB/CCE/FPE/encoder cross-strapping and FIB/RSE; PDF p. 19, redundant sides
Reading this project
Civil twin
This project’s name for a specifically identified civil instrument used to explain published engineering. Its specifications are never substituted for an unknown military payload.
As drawn
A representative illustration. Its dimensions, spacecraft placement, colors, and exploded spacing are visual choices, not hardware specifications.
Level
A place in the visual journey, from Earth and orbit families to an instrument or detector element.
Layer
One of the explorer’s Light, Data, and Heat views, each highlighting different component roles in the same schematic geometry.
Calc.
A value calculated by this site. Its formula, chosen inputs, and source references are available separately from published specifications.
Assumed
A documented teaching or drawing choice. The label identifies a chosen input, not a verified property of a real system.
Unchecked
A source that has not been successfully verified for use here. A failed fetch, an access challenge, or an unresolved source pointer cannot support a claim.