COMS System Definition

This System Definition section contains information used to determine the most suitable way of implementing and integrating a COMS system which meets the requirements set out in the Mission Requirements.


COMPONENTS DEFINITION

The binary signal from the main board is encoded into AX.25 protocol before being passed to the COMS board. Converting to AX.25 protocol involves the data being ‘packetized’ and ‘keys’ the transmitter (adds satellite ‘call sign’ to the data). The packeted signal is modulated and superimposed onto a carrier wave by the Datatech PIC modem and transceiver. The modulated signal is sent to the antenna (transmitted). A radio operating at the UHF band at ground station receives the data signal and encodes the stream to the form required for data analysis. This can now be interpreted as such by a computer/laptop.
A beacon may used to identify the satellite to amateur radio operators and the CubeSat team. (Finding Our Cubesat by John Heath )


FREQUENCY BAND AND DATA RATE

International regulations for communications are required to control interference between different systems and to ensure compatibility between various national systems that may be connected end to end. It is only possible, therefore, to use certain frequency bands. Our satellite is intended to be monitored by amateur radio operators and we ourselves will be amateurs for radio communication.

It is necessary to consider Doppler shifting as too large a shift can result in a greater error in tracking the satellite. In the amateur band a suitable uplink we can use is 144-146 MHz and for downlink, 435 MHz. (A later decision was made to use ~435MHz for both the up & down link)

It is estimated that on a given pass one will have around 5-10 minutes of tracking time across the ground station. This largely depends on the orbit. It necessary to know exactly what capacity the data is likely to take up from the payload. Having a camera, the image properties need to be known as this largely will affect the consideration of the data rate we can use for the transceiver. The larger the data capacity, the larger the data rate will need to be to transmit this to the ground station. We estimate that our data rate should be around 360kb for a 10 minute pass, which would correspond to 5 photos of ~70kB each and 830 particle events at ~ 12B each.


TX

The selected transmitter/receiver hardware for the COMS subsystem is the LRT470-1 transceiver by RF DataTech, which will be operated in half duplex mode at an amateur band frequency of around 435 MHZ (exact frequency TBC). The transceiver will be operated at a nominal power of 350 mW, with the capacity of autonomously increasing this up to a maximum of 750 mW if needed.

3.1.1 Transceiver specifications

Frequency range: 406 – 475 MHz
Programmable bandwidth: 10 MHz slot
Power requirements: 1.9 W at (350 mW RF output) – see Fig 2.
Number of channels: Any number within programmable bandwidth
Minimum programmable channel step: 6.25 or 10 KHz
Channel spacing: 12.5 KHz (optional 20/25/30 KHz)
Operating temperature: -30 to +60ºC
Size: 78 mm W x 52 mm L x 20 mm H
Weight: 120 g
Connectors: Interface 15 Way 2.54 mm Pitch pins

Trasmitter:
RF output power: 10mW – 750 mW
Maximum deviation: ± 7.5 kHz
Adj. channel power: Better than 60dB
Spurious emissions: To ETS300-220 specification
Modulation input: DC – 2.4 kHz for a 12.5 kHz channel
Rise time: <9 ms

Receiver:
Receiver current: 22 mA at 5 V DC
Sensitivity: -120 dBm for 12 dB SINAD* de-emphasised response †
-118 dBm for 12 dB SINAD* flat response ‡
Spurious response: >75 dB
Blocking: >95 dB relative to 1 μV
Intermodulation: >65 dB
Adjacent channel: >65 dB at 12.5 kHz
IF frequencies: 45 MHz and 455 MHz
Spurious emissions: To ETS300-220 specification
Signal output: 250 mV, DC – 2.4 kHz for 12.5 kHz channel
RSSI output: -120 dBm to -40 dBm
Mute response time: <3 ms

  • SINAD: SIgnal-to-Noise-And-Distortion is the ratio of the signal power to the noise and distortion power components.

†This is the minimum signal that can be successfully received with a 12 dB SINAD when pre-emphasis (at transmitter) and de-emphasis (at receiver) techniques are used to artificially boost the SINAD of the high frequency components of the signal, which are more susceptible to noise.
‡This is the minimum signal that can be successfully received with a 12 dB SINAD when emphasis techniques are not employed (we will not be employing emphasis techniques therefore the minimum signal power we can receive is -118dB).


ANTENNA

*(More recent decisions have changed this section a lot see The Antenna page)*

TYPES
There are many types of antenna one could use. For instance:

The antenna however is presumed to be part of the micro-transceiver package that will be supplied as a COTS.

XI-IV Antennae

*(More recent decisions have changed this section a lot see The Antenna page)*


DEPLOYMENT METHODS

A tried and tested concept by University of Hawaii at Manoa

The whole team there decided to stick with the method that almost every other college used of using fishing line and nichrome wire. After it was decided where to mount the antenna on the CubeSat, the team discussed how to make the fishing line hold the antenna. It was decided that holes would be drilled at the end of the measuring tape antennas and then fishing line would be attached using that hold and then go into the CubeSat through a hold drilled in two of the faces of the CubeSat. The fishing line would then go through the nichrome wire, out another hold and be tied to the opposite antenna branch. Fig. 1 shows a detailed drawing of this setup.

Fig. 1: Picture of CubeSat antenna mounting with deployment mechanism. There are two setups, A and B, because the nichrome wire deployment has to be mounted on different parts of the CubeSat.

The nichrome wire was tested in several different ways that will be discussed in part 3.0. The results of that test were that the smaller the diameter of the coil the less voltage and time it takes to break the fishing line. So to get the nichrome wire coil as small as possible we wrapped the nichrome wire around a needle, resulting in a diameter of about 1mm. While wrapping the coil around the needle, it was found that it was extremely difficult and that the wire would probably not coil any smaller than that. After finding out how small the nichrome wire could be, it was found places on the CubeSat that the setup would be able to fit as shown in Fig. 2 and Fig. 3.

Fig. 2: Setup A of nichrome wire placement. The nichrome wire is mounted on the inside of the CubeSat.

Fig. 3: Setup B of the nichrome wire placement. The nichrome wire is placed on the TTC board.


REFERENCES:

• Dr. Nigel Bannister, University of Leicester. PA3613 - Space Mission Design - Lecture 7, Communications Subsystem
• ‘Spacecraft Systems Engineering’ Third Edition - Fortescue, P. & Stark, J.
• Alinco website – www.alinco.com/usa.html
• ‘EE 396 Report’, University of Hawaii at Manoa http://cubesat.eng.hawaii.edu/~ttc/x96/Antenna/EE%20396%20Report.doc


This section can be downloaded here

Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-Share Alike 2.5 License.