Ground Station

Communication and Ground Station 12 October 2008

AgendaComm. Work and RequirementsBeacon TelemetryLink BudgetGeneral DecisionGround Station

Communication Work Beacon: Low bit rate device on the satellite which is functional for the entire lifetime of the satellite. Telemetry Data: High bit rate (1.2kbps) Monopole for downlink of data. Ground Station: Which can receive signals We dont have UPLINK

Communication RequirementsBoth the antennas onboard must be transmitting linearly polarized radio signals.2 independent crossed yagi antennas at ground stations to receive data and measure their polarization. Low cost ground station for other universities to measure polarization.

BeaconIt is device which transmits low power signal by an omni-directional antenna which help us to locate our satellite Beacon circuit is not connected to OBC OOK transmitter and receiver at 437MHz and transmission power 0.4 Watt. Transmitter

OscillatorPower AmplifierAntenna

Beacon Contd.Receiver

We are using a Monopole Antenna to transmit and a yagi antenna to receive.

AntennaLNACircuitComp.

TelemetryWe are using Monopole 2 (another antenna) used for data transmission.Contact time: 13 15 minData rate: 1.2 kbpsOver India; we will transmit Position Data and transmit stored data of last 24 hrs over IITB GS.Modulation: FSK

Link BudgetAt what power we should transmit our signals so that we will definitely receive it?Through Link Budget AnalysisEstimate various types of losses for example : Ionosphere losses, atmospheric losses, polarization losses, pointing losses etc.

Decision making

Communication parametersModulationCarrier Wave FrequencyOn Board AntennaeTransmitter CircuitProtocol of Transmission

Modulation Technique.The three key parameters of a carrier wave are its amplitude, its phase and its frequency all of which can be modified in accordance with a low frequency information signal to obtain the modulated signal. Modulation Techniques can be broadly divided into 3 categories Analog Modulation and Digital Modulation and Pulse modulation.

Modulation Parameters.The selection of a particular modulation technique depends upon the following parameters:

a) Data Requirements b) Availability of Modules c) Minimum SNR d) Minimum Bandwidth required e) Intersymbol Interference

Frequency Shift Keying We decided to go ahead with frequency shift keying as it has the following characteristics :Spectrally Efficient Moderate value of Eb/NoModules Readily AvailableLow intersymbol interference as compared to FM.

Carrier Wave FrequencyThe carrier wave frequency was to be selected keeping in mind the following parametersAvailability of frequency bands Payload RequirementsAvailability of modulesInterband Interference and noise

We selected the Amateur band of 433-437 and the 400-405 band as the former is an amateur band and hence it is easy to get modules and sanctions for that band and the latter is a band reserved for Ionospheric studies which is what our payload deals with.

Onboard AntennaAn antenna is a transducer designed to transmit or receive electromagnetic waves.As we had done away with uplink the selction of the transmitting antennae, both the telemetry and the beacon, was a very critical decision for the team.

Antenna parameters Moderate to High Gain Moderate BeamwidthLinear Polarization For payload purposesPolarization PurityEase of FabricationCharacteristic length to be of the order of satellite dimensions.

Monopole Antennae We chose the monopole over the other antennae because:Being Omnidirectional it reduces the constraints on attitude controlIt produces a linearly polarized beam.High polarization purity can be obtained by careful design of the antenna.The length of the antenna is comparable to the satellite dimensionsIt is easy to fabricate .The loss in gain can be compensated by using a directional antenna on the ground station

Transmitter Circuit Parameters.Industrial grade .Low power requirements in accordance with the power budget.Wide bandwidth covering our operational range.Minimum and Maximum Bitrate should cover our requirementsCompatible with AX.25 protocol.

CC1020 We selected this circuit mainly because it had been used earlier in a student satellite and hence had some space heritage. The other parameters of CC1020 areIt is industrial grade.It is operational over a frequency band of 405MHz to 470 MHz and is tunable.Its power requirements are low.It supports packet data transmission.Maximum bitrate is 156.2 kBps which is much higher than our requirements.

Protocol It is a standard practice in communication systems to use a protocol for the transmission of data. The protocol tells the user regarding the start and end of the data stream and enables the end user to correct the data in case it has become corrupt due to noise or interference.

AX.25 The AX.25 protocol is a standard protocol used by the amateur radio enthusiasts and is robust enough for our purpose.

Ground station Requirements The ground station equipment can be divided into the following :Pre-pass ComponentsReal Time ComponentsPost-pass Components

Pre pass ComponentsNova Tracking Software We have selected this software as it is compatible with the rotor interface.

Real Time ComponentsCrossed Yagis G5500 RotorLNA Power DividersReceiver for both the beacon and the telemetry data

Post Data RequirementsSignal strength Analysers for payload purposesData storage and analysis equipmentComputers

Ground Station

Ground Station Contd.Communication horizon: -30 to 30 degLocation: GG Building terraceTracking accuracy: 1 degree Yagi 2 independent crossed yagis at 405MHz and 437MHz FSK CircuitOOK circuit

For Beacon and TEC Beacon Signals Block Diagram

For Beacon and TEC Contd. FSK Signals Block Diagram

Cost Analysis

Components Quantity Cost ($) LNA 4 320 Crossed Yagi 2 100 OOK circuit1 100 Power Divider1100 Extra 80 Total700

Ground Station for Beacon

Cost Analysis

Components Quantity Cost ($) LNA 180Yagi 2 50OOK circuit1 100 Extra 70Total300

Thank You

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Ground Station