L15_Telemetry_Comms. - Swedish Institute of Space Physics

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L15_Telemetry_Comms. - Swedish Institute of Space Physics

L15 Telemetry & Communications

Henrik Lindén, March 9, 2010

Swedish Institute of Space Physics, Uppsala, Sweden


Overview

Telemetry, Tracking & Command

(TT&C)

• Function

• Requirements and Design

Communications Architecture

• Link Design

• Modulation and Coding


TT&C subsystem functions

Telemetry

• Wireless transfer of measurement data.

Tracking

• Tracking the s/c position through Doppler shift,

and locking on to ground station carrier signal.

Commands

• Send commands to instruments and

components on the s/c.

• Critical commands can be stored, verified and

delayed.


Requirements & Design

Signal Type; audio, video, data

Capacity; bandwidth, channels

Coverage Area; local, national, mobile

Signal Strength


Requirements & Design

Connectivity; direct link, relay, crosslinks

Availability; times per day

Mission Lifetime


Requirements & Design

Data rates

• Commands

• Health & Status

• Mission

2-8 kbps

40 bps -10 kbps

> 1Gbps

Data Volume & Storage

• Compression, transmission duration

Frequency

• Assigned or existing


Requirements & Design

Bandwidth

• Data rate, modulation

Beamwidth

• Ground coverage, multibeam

Polarization

Mass & Power

• Larger antennas or more power consuming

amplifiers.


TT&C Block Diagram


8-12 GHz

2-4 GHz

12-18 GHz


Communications Architecture

The arrangement, or configuration, of

satellites and ground stations, and the

communication links between them.


• Uplink: Ground station to satellite

• Downlink: Satellite to ground station

• Crosslink/Intersatellite: satellite to satellite

• If the signal is relayed:

• Forward link: Ground station to satellite

• Return link: Satellite to ground station


Examples of Architectures


Link Design

Links are digital as standard.

• Less distortion and interference.

• Have low error rates, Bit Error Rate (BER) of

10^10 is possible.

• Easy to relay without loss.

TT&C must digitize data.


Link Bit rate examples


Link Design

Link-size equation

Mesurement of how strong the signal is.

Used to compute the signal to noise

ratio of the system.


5–10 is enough for a digital signal.


Link Design











E b

/N o

: ratio of received energy per bit to noise density

P: transmitter power

L l

: transmitter to antenna line loss

G t : transmit antenna gain

L s : space loss

L a : transmission path loss

G r : receive antenna gain

k: boltzmann´s constant

T s : system noise temperature

R: data rate


Modulation and Coding

Modulation varies the carrier wave.

Mostly frequency and phase modulation,

amplitude modulation is large and

expensive.

Modulation give different BER and E b /N o .

Only transmit changes to reduce data.


Common Modulation Types


Link Budget

Power and

antenna gain

values, key items

in a space

system design.

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