Radio Links II Sounding rocket telemetry Poker Flat

Radio Links II

Sounding rocket telemetry Poker Flat telemetry dish

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?

Frequency choices

Propagation of radio waves

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

Atmospheric transmission • Transmission “window” in GHz range

Regulations

Bandwidth • Need more than one frequency to carry information – need a “band” of frequencies Morse code 500 Hz Telephone 3, 000 Hz CD audio 22, 000 Hz Television 5, 500, 000 Hz Ethernet (10 Mb) 10, 000 Hz DBS Television 33, 000 Hz

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-tonoise ratio

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

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

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)

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, nickelplated guard rail and cast nameplate Marconi's Wireless Telegraph Co. Ltd. , on mahogany baseboard 76 cm (30 in. ) high. • Built c. 1920

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

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

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

ESS 205 Balloon Telemetry • Two types: – Live video, using standard television signal – Readout of sensor values, using audio FM telemetry

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?

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)

Audio FM balloon telemetry • Use FM radios designed for voice transmission • Radio link can transmit frequencies 300 Hz – 3 k. Hz • All information to be delivered from the payload must be coded into frequencies in this range. – Voltage to frequency conversion (Cricket. Sat) – Speech synthesis/DTMF (Moses Lake launch)

Cricket. Sat 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”

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

Voltage to frequency converter • Generic device for turning a voltage into a frequency Ø 1 V 100 Hz Ø 2 V 200 Hz Ø Etc. , etc. • Example: Analog Devices AD 537 1”

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 ESS 205 (2004)

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”

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

ESS 205 audio telemetry (2005) • Interleave speech output (for casual reception requiring no special equipment) with DTMF (for machine readability) Speech GPS . . . DTMF GPS Speech Expt 0 -3 DTMF Expt 0 -15 Speech Expt 4 -7 DTMF Expt 0 -15 Speech Expt 8 -11 DTMF Expt 0 -15 Speech Expt 12 -15 Speech GPS . . .

Digital telemetry for scientific ballooning: The Sprite project (2002 -2005+) • Capable of high bit rate (3 MBps) • Inexpensive • Legal to use in USA & Brazil • Versatile • Transparent • Moderate design complexity

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

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

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): “Originate” 0 = 1070 Hz, 1 = 1270 Hz “Answer” 0 = 2025 Hz, 1 = 2225 Hz • Others: ASK, PSK, QPSK, OQPSK, MSK, QAM…

Digital data transmission: Modulation is not enough 0101010011101010101010111111101 010111111000000101110100101010011110100101010010100111 110101000101010111111110001 10010100101001001011111010011110001010001001000 100100101000111111010010101000 010100100010011111101 01011010011110100101000000010101001001010111101001001 … now what?

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 107 106 105 104 103 102 101 100 0 0 0 7 9 27 26 25 24 23 22 21 20 (128) (64) (32) (16) (=8) (=4) (=2) (=1) 0 1 0 0 1 1

Synchronization • Add structure to transmitted data to allow interpretation Sync byte 0101010011101010101010111111101 010111111000000101110100101010011110100101010010100111 110101000101010111111110001 10010100101001001011111010011110001010001001000 100100101000111111010010101000 010100100010011111101 01011010011110100101000000010101001001010111101001001 One 8 bit “sync byte” 01001111 inserted every 120 bits (15 bytes) creates a repeating “frame” pattern

Telemetry Frames Temperature Pressure Etc. Sync byte 0101010011101010101010111111101 010111111000000101110100101010011110100101010010100111 110101000101010111111110001 10010100101001001011111010011110001010001001000 100100101000111111010010101000 010100100010011111101 01011010011110100101000000010101001001010111101001001 • Define frame contents according to telemetry requirements

Error Correction • Add redundant content to frames to allow detection & correction of bit errors 0101010011101010101010111111101 010111111000000101110100101010011110100101010010100111 110101000101010111111110001 10010100101001001011111010011110001010001001000 100100101000111111010010101000 010100100010011111101 01011010011110100101000000010101001001010111101001001 Error correction byte • Some methods: Hamming, Reed-Solomon, Golay, Turbo

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.

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

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