TT & C, RF and OBDH Flashcards
What does TT&C stand for in spacecraft operations?
Tracking, Telemetry, and Command.
What is the purpose of a TT&C system?
To maintain communication between the spacecraft and the ground station for control and data exchange.
What are the two main segments in a TT&C system?
The space segment and the ground segment.
What type of antennas are used in the ground segment of TT&C?
High-gain antennas like dish and Yagi antennas.
What type of antennas are used on spacecraft for TT&C?
Low-gain omni-directional antennas.
Why is forward error correction (FEC) important in TT&C systems?
It ensures link reliability by correcting errors in data transmission.
Name the frequency bands commonly used for TT&C.
VHF
UHF
L-Band (1-2 GHz)
S-Band (2-4 GHz)
X-Band (8-9 GHz)
Ka-band for deep space comms (20-30 GHz)
What is the role of packetized TT&C?
To provide robust and secure communication links, and to interface with on-board data handling systems.
What organization sets standards for packetized TT&C systems?
The Consultative Committee for Space Data Systems (CCSDS).
What are the basic principles of spacecraft tracking and ranging?
Using techniques like Doppler tracking, tone ranging, and GPS to determine the spacecraft’s position and movement.
What is G/T, and why is it important?
G/T is the gain-to-noise-temperature ratio, which measures the performance of a receiving system.
What is the simple link budget formula?
C/N=EIRP − [LD + La + Le] + (G/T) − K − 10log(B).
What is the “sinc(x)” function, and how is it related to antennas?
It represents the diffraction pattern of an antenna, showing main lobes and side lobes.
Define system noise temperature (Ts).
A measure of the noise generated by the receiver system and environment, expressed in Kelvin.
What is the role of a low-noise amplifier (LNA) in RF systems?
To amplify weak signals while minimizing additional noise.
What is
E<sub?b</sub> / N<sub?0</sub>
and why is it important?
It is the energy-per-bit-to-noise-density ratio, which measures the quality of a digital communication link.
How does ionospheric Faraday rotation affect RF links?
It rotates the polarization vector of signals, especially at VHF and UHF frequencies.
How can circular polarization mitigate Faraday rotation effects?
It minimizes polarization mismatch caused by ionospheric rotation.
Name two common OBDH network topologies.
Star topology and bus topology.
What are the advantages of a star topology in OBDH?
High reliability and centralized control.
What are the disadvantages of a bus topology in OBDH?
Vulnerability to single-point failures.
What is the role of CTUs and RTUs in the ESA OBDH bus?
CTUs (Central Terminal Units) manage data exchange, and RTUs (Remote Terminal Units) interface with subsystems.
What are Single Event Effects (SEEs), and why are they a concern in OBDH systems?
SEEs are disruptions caused by cosmic rays or radiation, which can lead to errors or damage in electronics.
What is EDAC coding, and how does it help mitigate SEEs?
Error Detection and Correction coding detects and corrects bit or byte errors in data storage and transmission.
What is memory washing in OBDH systems?
Periodically reading, correcting, and rewriting memory contents to prevent error accumulation.
How do Hamming codes and Reed-Solomon codes differ?
Hamming codes correct bit-level errors, while Reed-Solomon codes correct byte-level errors.
What is Triple Modular Redundancy (TMR), and how does it enhance reliability?
TMR uses three parallel systems and voting logic to detect and correct errors.
Why is software engineering critical for OBDH systems?
To ensure reliability, predictability, and real-time performance.
Name two programming languages commonly used for spacecraft software.
Ada and C/C++.
Why is Ada preferred for critical spacecraft applications?
It offers strong typing, modularity, and built-in exception handling for reliability.
What are the benefits of using FPGAs in OBDH systems?
High performance, reconfigurability, and integration of multiple functions.
How does radiation affect COTS components in OBDH?
Radiation can cause errors or damage, requiring mitigation techniques like shielding and error correction.
What is the difference between rad-hard and rad-tolerant components?
Rad-hard components are designed to withstand high radiation levels, while rad-tolerant components are less resistant but still functional in moderate radiation environments.
Why is systematic software development important for OBDH systems?
To meet mission schedules, ensure quality, and provide fault detection and recovery.
What is the role of intellectual property (IP) cores in FPGA-based OBDH systems?
IP cores provide reusable building blocks for implementing processors and other functionalities.
How does shielding mitigate radiation effects in OBDH?
It reduces the radiation dose absorbed by sensitive components.
What is the main trade-off of using Commercial Off-The-Shelf (COTS) components in space?
COTS components are cost-effective and high-performance but require additional radiation-hardening measures.
What are the three topology and three access protocols to consider for an OBDH network?
Topology:
- Single point failures
- Reconfigurability
- Complexity
Access protocols:
- Latency
- Flexibility
- Predicitability
What are two disadvantages of a centralized topology such as the mesh or star?
- Addition of new modules is not easy
- Wiring harness becomes large
what are the three common buses/networks mentioned?
- MIL_STD-1553B
- OBDH data bus (ESA standard)
- I2C data and control bus (inter-integrated circuit)
What are the three standards mentioned?
- IEEE1394/Firewire
- CAN (controlled area network)
- Spacewire
For the OBDH bus what are the 5 roles of the CTU and the three roles of the RTU?
CTU:
- Command handling
- Data gathering
- housekeeping
- clock reference generation
- Dedicated high speed interface
RTU:
- acquisition and multiplexing telemetry (A/D)
- remote distribution of commands
- event datation
What are the 5 primary radiation effects?
What are their effects?
TID - accumulated radiation damage leading to leakage currents and eventual circuit failure
SEU - Particle hit causes ‘bit flip’ soft error
SEL - Can cause power to ground short and permanent damage
SET - transients resulting in overstressed components
SEFI - Unexpected malfunction usually requiring a power cycle
Why is aluminium a good choice of material for radiation shields
Because it has a low Z (proton) number. It must be backed up with a high Z material such as Tantalum (Ta) or Tungsten (W)
semiconductor processes are modified to make components rad-hardened. What are the three main trade offs?
- Power
- Density
- Speed
Name one example of a SOS proccessing unit?
MIL STD 1750 processor architecture
What are three advantages of SOS
SEU immune
SEL immune
rad-tolerant
What are three disadvantages of SOS
- Specialist technology, small market and expensive
- Power hungry versus COTS
- Low level of integration versus COTS
What effects do SEU have on memory
corruption of stored data or variable
What two techniques are used to protect memory from SEUs?
Triple module redundancy (TMR)
Error detection and correction (EDAC)
Describe how TMR works to protect memory from SEUs?
- Sequential circuit elements are implemented three times in parallel
- They are compared by a ‘voting’ circuit
- If two outputs are the same, but one is different it will be outvoted
What are three disadvantages to TMR?
- Requires three times as much circuitry (mass, volume and cost)
- 200 % memory overhead
- If an SEU occurs to two of the parallel circuits then the voting circuit wont be able to deduce the correct output.
How is a typical triple voted controller usually implemented?
Using an FPGA
What does the FPGA do when an error is identified by the voting circuit?
It captures the address with the error and asserts an interrupt so that the correct data can then be rewritten to that memory address
What are advantages of TMR?
- Most comprehensive EDAC
- Uses simple logic operations to determine where the error is
- fast, low propagation delay
With EDAC codes, what is the main principle that allows it to make corrections?
Adds additional bits to data words
What are the 4 ECC schemes and which one decodes fastest and which one the slowest?
Hamming
Cyclic
Parity
Reed-Solomen
Hamming decode speed: 187.8 MBytes/s
RS decoding speed: 24.41 MBytes/s
Which ECC has the largest bit overhead and which the smallest?
- Hamming and Cyclic have a bit overhead of 12.5 %
- RS has a bit overhead of 3.125 %
What main benefit do the error correcting codes yield in terms of mission capabilities?
Spacecraft can travel further and withstand higher radiation environments.
What is meant by the numbers in ‘Reed-Solomen (255, 252)?
- 2nd number refers to the number of bytes being read/written
- 1st number is the sum of bytes being read and additional code bytes used for correction
What are the consequences of memory scrubbing?
Regular read, correct and rewrite of memory needs to happen to prevent accumulation of errors
- This results in adequate CPU overhead
- EDAC + scrubbing only protects memory external to the CPU, not data currently being processed at the CPU
- Disabling the cache can reduce SEU errors since CPU must fetch from the EDAC protected memory
For state of the art FPGAs, what is the maximum TID and how many logic gates do they possess?
<= 300 krad
<= 6 million
What are the effects of adding software to OBC?
- Increase in complexity and cost
- increases flexibility of the OBC
- both increases and decreases risk
What are the four software requirements?
- Phased predictable development to meet mission schedule
- verifiable high quality
- need fault detection isolation and recovery (FDIR)
- hard real-time performance (HRT)
What is the name of the MIL-STD programming language?
ADA
What two philosophies are practiced when developing software
- Top down development
- Object oriented development
