Communication System
Objective
The objective of this system is to establish communication link with ground station.The telemetry and payload data is transmitted to the ground station .A customized protocol is used for representing the data. The communication system should be developed within the constraints imposed with respect to power, size and mass.
Specification
Beacon specification
Telemetry format
Modulation:Morse code will be used to modulate the signal. Table 1 in the appendix. Shows the morse code representation of various alphabets, numbers and punctuation marks by a combination of dots and dashes
It should be noted that:
* A DAH (-) is 3 times longer than a DIT (.).
* The space between a DIT and a DAH is one DIT.
* The space between two alphabets is a DAH.
* The space between two words is seven DITs.
The beacon message is divided into 4 parts. Each part is transmitted at an interval of 30sec. hence, it takes 2 minutes to transmit the complete telemetry. The contents of the telemetry are described below. The numbers transmitted follow the octal representation.
Part 0:
Part 0 contains the satellite ID. The format of part 0 is as shown below:
Header-Space-Satellite ID-Space
The Header contains the Part number. In this case Header contains '0'.
The header is followed by a space then the satellite ID. The satellite ID contains 6 characters in alpha-numeric form and the corresponding morse code is transmitted.
Part 1 contains some of the status parameters of the satellite and battery 1 voltage. The format of Part 1 is as shown below:
Header-Space-Status Parameters-Space-Battery 1 Voltage
Part 1 starts with a header which contains the part number, In this case it is '1'.
The header is followed by a space then the Status parameters of the satellite. This field provides information about the status(ON/OFF) various subsystems of the satellite. The different status parameters are as follows:
* EPS ON/OFF
* ADCS ON/OFF
* Communication ON/OFF
* Antenna Deploy. ON/OFF
* Image ready to Tx. Yes/No
There are six parameters which can be represented by one bit each. Say 0 representing OFF or NO and 1 representing ON or Yes. Consider an example where the "Status Parameter" field contains "111010", this in octal form is 72 and it means the following:
- Payload : ON- EPS: ON
- ADCS: ON
- Communication :OFF
- Antenna Deploy.: ON
- Image ready to Tx.: NO.
The status parameter field is followed by a space, followed by Battery1 Voltage.
The battery voltage is represented using an ADC value represented by 10 bit number. To decode the actual value of the battery voltage, the following formula is used:
V = (-0.00939 * ADC) + 9.791 V.
Part 2
Part 2 contains the coil currents and battery 2 voltage. The format of part 2 is as follows:
Header-Space-Coil Current-Space-Battery 2 voltage
Part 2 starts with a header which contains the part number, In this case it is '2'.
The header is followed by a space, followed by the coil currents of the 3 coils placed on 3 different axis. The coil current varies from -20mA to +20mA with a step size of 1mA. This can be represented using 6 bits as follows.
-20mA is represented as '0' (in binary 000000) and +20mA is represented as '40'(in binary 100000).
Thus a total of 18bits to represent the current flowing in 3 coils.
The battery voltage is represented using an ADC value represented by 10 bit number. To decode the actual value of the battery voltage, the following formula is used:
V = (-0.00939 * ADC) + 9.791 V.
Part 3:
Part 3 contains panel current and RTC value. The format of Part 2 is as shown below:Header-Space-Panel current-space-RTC
The different panels from which the current is measured and transmitted are X1, X2, Y1, Y2 and Z1.
Each parameter has an ADC value represented using 10bits, associated with each of the panel currents. It is decoded using the following formula.
I = ((-0.486*ADC) + 502.524) mA.
Antenna system
* Two Perpendicular monopoles configuration for data Transmit/Receive
* Two perpendicular monopoles configuration for beacon Transmit
This kind of configuration maintains stability of spacecraft after deployment and provides a good near omnidirectional pattern for successful communication link.