Archive for the ‘GEO Subsystems’ Category

TCR Command

Due to the sensitive nature of this topic, I will only address this subject from a top level general process.

TCR commanding is accomplished by transmitting the modulated commands on a RF carrier that is tuned to the command frequency. The command carrier is received at the antenna and is applied to the input of the command receiver. By design the command receiver is a dual function unit, it will lock onto a receive carrier and demodulate valid command signals to produce a digital output. Once the command receiver is locked onto the command carrier, it verifies and demodulates commands and outputs them to a Command and Data Handling subsystem or Flight Computer for execution.

As additional units and antennas are added to this subsystem to provide redundancy and flexibility hybrid devices are installed to split the received signals. The use of switches in this receive path could result in a potential single point failure and are therefore avoided. The use of hybrids allows the signal to be divided and applied to each receiver and since hybrids are passive devices the chances of failures are minimized.

TCR Ranging

During the launch and early operations phase a path through the TCR subsystem is provided to collect phase angle range data for orbit determination. This path can also be used during the mission life if desired.

TCR ranging is accomplished by transmitting the ranging signal on a RF carrier that is tuned to the command frequency and measuring the phase angle difference of the receive signal on a RF carrier tuned to the telemetry frequency. For this reason it is normally recommended that no commands are transmitted while ranging is being preformed. By design the command receiver is a dual function unit, it will lock onto a receive carrier and demodulate the signal to produce a digital or analog output signal. Once the command receiver is locked onto the ranging carrier, it demodulated the range signal and outputs it to the input of the telemetry transmitter. The telemetry transmitter (another dual function unit) then modulates the ranging signal onto a transmit carrier and amplifies it to the transmit level and is then sent to the transmit antenna. Ranging carriers are received on the ground at the assigned telemetry frequency.

To accurately process the range data all variables must be accounted for. During the integration and test phase of the satellites assembly each range path through the TCR subsystem is calibrated. To calibrate each path the subsystem is configured for ranging, the path configuration is noted and ranging is preformed. The resulting range measurement is the delay through that path and is referred to as the satellite delay.

When processing range measurements, to improve accuracy you must account for delays through the satellite and ground station. Once these delays have been accounted for then the range measurement is the distance to and from the satellite. By dividing the measurement by 2 you get the range from the ground antenna to the satellite.

Ranging can also be preformed by transmitting and receiving the range signals through a given payload channel.

Flight Software systems

Just like any other computer the satellite has it’s own version of software. Each satellite has at least one computer on board used for its control along with RAM (random access memory) and PROM (programed read only memory) . The flight software is resident in PROM and is copied to RAM to run at system start-up.  Some satellites also have the ability to modify the software while on orbit.  This is accomplished by use of EPROM (erasable programed read only memory) or EEPROM. When EPROM’s are used there is still a PROM for start-up then it copies the software from EPROM to RAM to run.  Once the flight software (FSW) process have been started it will run in RAM until it is restarted. The flight software collects data or issue commands to and from each unit on a scheduled basis. Once collected the data is either processes it for transmission as telemetry or passes it to the C&DH system to complete that process.  Commands from the ground or commands passed from the C&DH system are processed and executed in FSW. The ACS software also run as part of FSW to collect sensor data, produce error control signals and apply the signals to the control actuators to maintain the Attitude and pointing of the satellite. The FSW in complex satellite architectures will also interface with independent processors for each subsystem to collect telemetry and process commands. Flight recorders have been employed to store data on the satellite for retrieval at a later time, or store Attitude parameters used at start-up to minimize transients. Without the use of EPROM ground commands may be issued to change the software running in RAM on a contingency basis to correct problems, if this is done then the same commands will be required after every re-start.

If the architecture includes a C&DH system the actual Flight Computer hardware is included in that system.

Satellite Payload systems

Satellites provide a platform for a wide variation of applications.  The equipment placed on the satellite to accomplish the intended mission is what pays for the satellite and is designated as the Payload. Although communications is one of the most common uses, payloads can include imaging of the Earth or objects in space, global positioning systems (GPS), ranging and altimeters used in mapping, spectral analysis devices to determine the composition of the atmosphere.  In the past the payloads have been selective in three primary categories, Communications, Imaging and Scientific. I would consider GPS satellites as a forth category unto itself. Some satellites are now being built with shared payloads to provide a more cost effective means to collect scientific data for long term studies.

Communications satellite payloads range in complexity from a few channels in one frequency band to hybrid systems that have multiple channels using a number of frequency bands including frequency conversion and switching systems to translate and rough the traffic to selective areas. These satellites can receive signals from the ground and retransmit them to the ground or another satellite. Applications are also in place to communicate with ships, trains, trucks and airplanes. The basic types of communications are voice, video and data.

Imaging satellite payloads are governed by the resolution of the ground image they can provide. The images can be produced in the visible light spectrum, normal color or black and white (for higher resolution), inferred spectrum for tracking temperature differentials or other ranges based on the design of the camera used. Space imaging also uses Ultraviolet and Gama-ray spectrum among other types. These satellite may also include mapping instruments that produce topographical mapping information utilizing for example, radar or laser mapping techniques.

Scientific payloads are just that, and include a diverse range of equipment. These satellites are designed to collect very specific information. Some of the applications have been to collect data and measure the irradiation from the Sun, Sun spot activity, solar flair and wind measurements, measure the ozone layer, map carbon dioxide concentrations, search the stars and the list goes on. Scientists are developing new missions every day to gain more knowledge.

Propulsion systems

On a satellite the propulsion system is designed to provide a means to control the satellite and maintain it’s orbit.  The two most common applications are mono-propellant and bi-propellant. The mono-prop system utilizes one type of fuel, where the bi-prop system uses a fuel and an oxidizer combination to produce thrust. In it’s simplest form this system is comprised of thrusters,  a fuel tank, tubing for fuel lines, heaters, valves and filters. More complex designs also include a dedicated processor, oxidizer tank, pressurization tanks and lines, additional power supplies, and safety relays and fuses. The propulsion system can be pressurized prior to launch and operate in a blow down mode. When pressurization tanks are added to the system it can operate in a combination of pressure regulated and blow down modes. Electric Ion thrusters have been used, they have a low specific impulse and require high current and long maneuver durations so are therefore normally reserved for use in planetary missions. The mission life is determined primarily by the efficient use of the fuel loaded into this system.

In some applications due to short mission life, on the order of 6 months to a year the propulsion system is or can be eliminated. These are normally proof of concept, flight qualification and special scientific missions that do not require long durations.

Satellites in LEO orbits use thrusters to adjust the satellites velocity, orbiting altitude and inclination.

On GEO satellites the designs normally include the use of thrusters for Attitude control as a back-up system. Under 2-Axis control the fuel tanks can be oriented to utilize the centrifugal effect of the spinning satellite to force the fuel out to the thrusters.  While in 3-Axis control the thrusters can be used to reduce the momentum stored in the reaction wheels. During the launch phase the use of solid fuel motors to achieve a circularized orbit at GEO has been replaced in some cases with bi-propellant thrusters to conserve weight and potentially extend mission life.

Command and Data Handling subsystems

Command and Data Handling subsystems are the portion of the satellite that acts on commands from the ground or internal commands generated by the Flight Computer (FC) and collects telemetry from the other subsystems and prepares it for transmission to the ground. This subsystem can be as simple as one Flight Computer processor that controls the entire satellite. In distributed architecture applications there can be a Flight Computer, a processor for each subsystem, a data bus controller that provides data and command transfer between subsystems and a C&DH processor to interface with the TCR subsystem. In the more complex systems there are backup systems to minimize impact due to unit failures. Flight recorders that can also be utilized to store data and memory areas to store command tables for scheduled execution.

This subsystem contains the processor or processors, interface bus, the means to receive, decode, process and execute commands and the ability to collect, process, store and transmit telemetry from the satellite through the TCR system.

Thermal Control Systems

The Thermal Control System (TCS) utilizes a number of approaches to maintain a stable operational temperature throughout the satellite. Temperature extremes can be dealt with by active or passive means.  Active temperature control is accomplished by use of thermostatically controlled heaters or monitoring thermocouples and actively cycling hearers on and off. In addition units can be turned off and on to affect the temperature in a specific area. Passive thermal control is accomplished by use of reflective surfaces to reject absorption of heat or to radiate excess heat away, or use of non-reflective surfaces to absorb heat. In addition thermally conductive materials can be employed to evenly distribute the heat over a specific area and thermal isolating materials are used to prevent conduction of heat into an area. Some of the reflective surfaces are Quarts Mirrors applied to a surface and milar blankets secured over areas. Non-reflective surfaces can be painted with material to increase the heat absorption, an example is the use of black paint. Thermally conductive surfaces include metal to metal contact and can be the use of a thermally conductive material between the unit and mounting surface to increase conductivity. Thermal isolation is accomplished by use of a thermally non conductive material between panels or the unit and the panel.

Temperatures in different areas of the satellite can drop to less than -180 degrees C at the end of the solar panels when the satellite is in an eclipse and other areas can exceed 80 degrees C when you reach solstice seasons so thermal control does play a major roll.

Telemetry Command and Ranging subsystem of a satellite

The TCR subsystem is comprised of all the equipment needed to transmit data from the satellite to the ground in the form of telemetry.  It is the interface to receive commands from the ground and provides paths to rough ranging signals received from the ground back down to the ground for range measurements used in orbit determination.  Ranging signals received have to be converted from the receive frequency to the transmit frequency and then transmit back to the ground.  Some of the units employed by this subsystem are transmit and receive antennas, command receivers, telemetry transmitters and range signal translators.

Electrical Power Systems on satellites

The Electrical Power subsystem (EPS) on the satellite is used to power the satellite form launch through end of the mission life cycle. This subsystem is made up of the units on-board the satellite used to generate, store, regulate and distribute the power needed to support the satellite. This system consists of power sources including solar arrays and batteries, charging systems, voltage and current regulation, power bus and fuse assemblies and EPS control systems.

On LEO satellites the EPS system is highly or fully automated. Since the satellite can be transitioning from solar array power to battery power every orbit it would have to replace the energy used from the batteries each time. Some LEO satellites are placed in a Sun synchronous orbit so they have power from solar arrays continuously and battery systems are used for emergency conditions.

On GEO satellites that are constantly in view,  some systems are controlled from the ground while others are automated. At GEO the satellites solar arrays will have to track the Sun or be clocked to keep them pointed at the Sun for maximum efficiency. These satellites also experience daily eclipses during approximately a 45 day period centered around the equinoxes (twice a year). The duration can be from a few minutes to about 70 minutes each. During these periods the batteries must have the capacity to support the power required and have a suitable margin in the event of anomalies.  After an eclipse the batteries must be recharged, based on the satellite design, this process can be done manually or in some cases is automated. Ground control software systems have also been used to automate this process.

3-Axis GEO Satellites

In this type of satellite all 3 axis of the satellite are controlled.  This is a general description of the concept of the 3-Axis stabilized GEO Satellite.  In this application the body movement of the satellite is controlled in Roll, Pitch and Yaw.  For example, Momentum wheels have been used to provide gyroscopic stiffness in Roll and Yaw and the speed is adjusted by positive and negative torques to control Pitch.   In another application a series of Reaction Wheels are mounted and aligned to control all 3 axis. In this application there are 4 Reaction wheels mounted off axis such that any 3 are capable of control of the 3 axises and providing a forth wheel for use in the event of unit failure or need for additional control.  The ACS system will collect the sensor information and calculate the the amount error in each axis. The error signals will be generated and applied to each axis to maintain the axis within it’s pointing requirements.  Earth sensors are used to provide Pitch and Roll errors throughout the Orbit and Sun sensors are used to provide Yaw errors when the Sun is in its field of view. With Sun and Earth presents in the sensors all 3 axises are actively controlled and calibrated.  While the Sun is not present in the sensor Pitch and Roll are actively controlled.  These satellites require 3 types of maneuvers to maintain their orbit. Inclination maneuvers are scheduled to maintain the alignment of the orbit plain with the equator minimizing North/South drift. Delta Velocity (Delta-V)  Maneuvers keep the orbit circular to minimize East/West drift.  Finely Momentum Adjustment Maneuvers to reduce the momentum stored in either the Momentum or Reaction wheels. By firing the proper thrusters this reduces the momentum in that axis and maintains the wheel speeds within it’s optimum operational ranges.  The frequency of each type of maneuver is determined by the orbital slot and the pointing requirements the satellite is being operated by.  Typically the frequency ranges from 1 to 3 weeks.

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INFORMATION

Shining light on satellites and how they operate. Drawing from over 30 years of knowledge and experience in all phases of the life of a satellite from concept, to operations, and through end of life. You will find short topics intended to give you an understanding of how they work, the general concepts, and principals used along with information on ground systems. There is also a section dedicated to topics that can be used as basic concept training along with links to animations and 3D models I have created.