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Radio Telemetry Design: Frequency Choices and Constraints

Explore the design choices and constraints of radio telemetry, including frequency selection, modulation and coding techniques. Learn about the propagation of radio waves, regulations, bandwidth limitations, and noise considerations.

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Radio Telemetry Design: Frequency Choices and Constraints

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  1. Radio Links II

  2. Sounding rocket telemetry Poker Flat telemetry dish

  3. Other telemetry design choices • Frequency – where (in “frequency space”) is information transmitted • Technological constraints: what can be built? • Natural constraints: how do different frequencies behave in the environment? • Bandwidth – how much information is transmitted?

  4. Frequency choices

  5. Propagation of radio waves

  6. Line of sight propagation • About 400 miles at 100,000 feet

  7. Atmospheric transmission • Transmission “window” in GHz range

  8. Regulations

  9. Bandwidth • Need more than one frequency to carry information – need a “band” of frequencies

  10. Bandwidth limitations • Available frequencies are limited – may be forced by regulations to stay in a narrow band • A higher bandwidth signal requires more power to transmit with the same signal-to-noise ratio

  11. Noise • Extrinsic – natural sources, interfering transmitters • Intrinsic – thermal noise caused by random motion of electrons • Noise power P = kTDf, Df = bandwidth • For best SNR, want to make Df as small as possible Whistler White Noise

  12. Modulation • Continuous radio wave “carrier” has zero bandwidth but carries no information • Want to change (modulate) the wave over time to convey a message • Will increase bandwidth: More information -> More bandwidth

  13. Modulation and coding • Low level: How can the carrier wave be modified to carry information? (modulation) • Higher level: How should the modulating information be formatted for best communication? (coding)

  14. Spark gap transmitter • This is a Marconi 1 1/2 kw quenched spark gap transmitter. This piece of equipment was installed on the yacht Elettra and is featured in photographs of Marconi in the radio room on board the ship. It is similar to the transmitter that was installed in the radio room of the liner Olympic, the sister ship to the Titanic. This transmitter was capable of sending messages over a distance of 4,500 miles. • Type Q.G. No. 356546, with eight-plate quenched spark gap, four flat copper-strip inductance coils with moveable leads, three-position adjuster switch, nickel-plated guard rail and cast nameplate Marconi's Wireless Telegraph Co. Ltd., on mahogany baseboard - 76cm (30in.) high. • Built c. 1920

  15. On-off keying (OOK) • Simplest/oldest form of modulation • Morse code (1837) – developed for telegraphy Modulation Coding

  16. Amplitude Modulation • AM radio, broadcast TV • Make amplitude of carrier wave proportional to the signal of interest (modulating signal) • Vulnerable to distortion from atmospheric attenuation Signal  Carrier 

  17. Signal  Carrier  Frequency Modulation • FM radio • Make frequency of carrier wave proportional to signal • More resistant to atmospheric effects

  18. ESS 205 Telemetry: Audio and Video • Live video, using standard television signal (AM)

  19. Television “raster scanning” • Electron beam illuminates one spot on the TV screen at a time, covering entire screen 30 times per second • Broadcast as an AM signal with modulation proportional to brightness – but where?

  20. Standard NTSC television signal Brightness profile (1 line) • In addition to brightness information, contains signals which allow the TV to locate the start of each line (horizontal sync) and the beginning of the first line (vertical sync)

  21. Audio FM balloon telemetry • Use FM radios designed for voice transmission • Radio link can transmit frequencies 300 Hz – 3 kHz • All information to be delivered from the payload must be coded into frequencies in this range. • Voltage to frequency conversion (CricketSat, 2003-2004 flights) • Speech synthesis/DTMF (2005-2006 flights)

  22. CricketSat signal generation: 555 timer chip • Simple silicon microchip for construction of timers & oscillators • Generates a square wave audio signal at a frequency determined by two resistors and capacitor “Ground”

  23. Signal generation with the 555 • Use components which are sensitive to their environment • Thermistors – resistance changes with temperature • Photoresistors – resistance changes with light • Capacitive humidity sensors – capacitance changes with humidity • Use in a 555 circuit to generate audio frequencies

  24. Voltage to frequency converter • Generic device for turning a voltage into a frequency • 1 V  100 Hz • 2 V  200 Hz • Etc., etc. • Example: Analog Devices AD537 1”

  25. Multiplexing • How to measure several sensors over one radio link? • Share the link by switching, or “multiplexing” between them • Simple technique: Each sensor takes turns modulating the transmitter • Radiosondes use this technique • Used in ESS205 (2004)

  26. Speech Synthesis • RC System’s V-stamp text-to-speech synthesizer “reads” English text • Pro: Simple to use, no special receiver required • Con: Not machine readable 1”

  27. DTMF (Dual Tone Multi Frequency) • “Touch Tones” encode digits 0-9, A-D, *, # as sounds containing two different audio tones • Low frequency indicates row, high indicates column • Machine readable - devices for encoding and decoding (tones back into numbers) are readily available DSchmidt Technologies’ DTMF Decoder II

  28. ESS205 audio telemetry (2005-6) • Interleave speech output (for human reception requiring no special equipment) with DTMF (for machine readability) . . . . . . . . . .

  29. Digital telemetry for scientific ballooning: The Sprite project (2002-2006) • Capable of high bit rate (3 MBps) • Inexpensive • Legal to use in USA & Brazil • Versatile • Transparent • Moderate design complexity

  30. Analog vs. Digital Modulation • Analog: Modulation is interpreted as a continuously varying parameter • Digital: Electrical signal is interpreted to be one of “N” (usually, two) possible values Received + Interpreted Modulating Signal Modulating Signal 1 Received + Interpreted = 0 Threshold

  31. Noise behavior of digital systems • Digital systems are immune to small quantities of noise • Larger amounts of noise cause complete system failure 1 = 0 Threshold 1 = 0

  32. Types of digital radio telemetry • Modulate carrier discretely to form 1’s and 0’s • Examples: OOK, FSK (frequency shift keying) • FSK: switches between two frequencies (“0” and “1”) at a certain bit rate (baud = bits per second) • Bell 103 (original 300 baud modem protocol): • Others: ASK, PSK, QPSK, OQPSK, MSK, QAM… • “Answer” • 0 = 2025 Hz, • 1 = 2225 Hz • “Originate” • 0 = 1070 Hz, • 1 = 1270 Hz

  33. Digital data transmission: Modulation is not enough 0101010011101010010101010101010111111101010101011111100000000000010111010010101001010100111101010100101010010100101001111101010101000101001010101010111111110001100101001010010101010010010101011111111111110100111100010101010001000100100010001001001010101000111111111111010010101000010100101001000100101000100111111111110101011010011110100101010100000000000001010101010010101010010010101111010101001001 … now what?

  34. Binary numbers • Decimal digits have values 0-9, binary digits (“bits”) only 0-1 • Combine multiple digits to form larger values • 8 bits (one byte) = 0…255 • 16 bits (one word) = 0…65536 • Example: 79 decimal = 01001111 binary Decimal Binary

  35. Synchronization • Add structure to transmitted data to allow interpretation Sync byte 0101010011101010010101010101010111111101010101011111100000000000010111010010101001010100111101010100101010010100101001111101010101000101001010101010111111110001100101001010010101010010010101011111111111110100111100010101010001000100100010001001001010101000111111111111010010101000010100101001000100101000100111111111110101011010011110100101010100000000000001010101010010101010010010101111010101001001 One 8 bit “sync byte” 01001111 inserted every 120 bits (15 bytes) creates a repeating “frame” pattern

  36. Telemetry Frames Temperature Pressure Etc. Sync byte 0101010011101010010101010101010111111101010101011111100000000000010111010010101001010100111101010100101010010100101001111101010101000101001010101010111111110001100101001010010101010010010101011111111111110100111100010101010001000100100010001001001010101000111111111111010010101000010100101001000100101000100111111111110101011010011110100101010100000000000001010101010010101010010010101111010101001001 • Define frame contents according to telemetry requirements

  37. Error Correction • Add redundant content to frames to allow detection & correction of bit errors 0101010011101010010101010101010111111101010101011111100000000000010111010010101001010100111101010100101010010100101001111101010101000101001010101010111111110001100101001010010101010010010101011111111111110100111100010101010001000100100010001001001010101000111111111111010010101000010100101001000100101000100111111111110101011010011110100101010100000000000001010101010010101010010010101111010101001001 Error correction byte • Some methods: Hamming, Reed-Solomon, Golay, Turbo

  38. Sprite telemetry design approach • Use newest technology – higher integration, higher performance, more features • Use consumer & amateur radio technology wherever possible • Frequency: 902-928 MHz • Amateur radio band in US & Brazil • Amplifiers/antennas readily available • Also used for non-licensed devices: cordless phones, wireless networks, etc.

  39. Sprite transmitter • Cell phone transmitter evaluation board generates & modulates low-level radio signal • Modified for 902-928 MHz operation • Amateur radio power amp boosts power to 3-5 W • Commercial dipole antenna

  40. Sprite receiver • Antenna modified from TV satellite dish • Commercial pre-amp • Commercial wide-band receiver • Custom intermediate-frequency (IF) amplifier • Digital demodulator – commercial evaluations boards • Custom digital interface • Ethernet single board computer • Laptop

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