Communications

Introduction

The main functions and responsibilities of the communications subsystem include:

Receiving commands from the ground station
Transmitting bus telemetry to the ground station
Transmitting payload telemetry to the ground station
Signal modulation and demodulation
Communicate and exchange data with Command and Data Handling subsystem
Deploy the antenna

The communications system is the main and only link between us and the satellite while it's in orbit. Retrieving the experimental payload results from space back to earth and maintaining the spacecraft’s health are both critical to mission success. Figure 1 shows the communication functional block diagram.

Figure 1: Communications Functional Block Diagram

Model Philosophy

Engineering Model

Engineering model of antenna and deployment mechanism to be tested in Phase C and D
Used for antenna characterization and deployment testing
Phase C verification: V-COM-0080
Phase D verification: V-COM-0160, V-COM-0180, V-COM-0480, V-COM-0530, V-COM-0560, V-COM-0600,

Flight Model

Flight model of antenna, deployment mechanism, and radio to be tested in Phase C and D
Used for transmission tests with radio and FlatSat tests of the system
Phase C verification: V-COM-0140, V-COM-0220, V-COM-0250
Phase D verification: V-COM-0320, V-COM-0030, V-COM-0230, V-COM-0260, V-COM-0280, V-COM-0080, V-COM-0150, V-COM-0520, V-COM-0521, V-COM-0580, V-COM-0590, V-COM-0130

Design Philosophy

The communications system is responsible for transporting valuable scientific data from the satellite to earth. To receive this data with minimal noise and with accuracy, we need to establish a robust communication link. In order to achieve this, we use link budget and link access analyses to determine the required performance for the radio power and antenna gain.

Key Design Requirements:

R-MIS-0060 - Spacecraft shall transmit payload telemetry to the licensed ground station.
R-MIS-0061 - Spacecraft shall transmit bus telemetry to a licensed ground station.
R-MIS-0062 - Spacecraft shall receive telecommands from a licensed ground station
R-COM-0010 - The frequency band selected for communication shall comply with Innovation, Science and Economic Development Canada (ISED) regulations.
R-COM-0020 – Transmit payload and bus telemetry using UHF
R-COM-0021 – Receive telecommands using UHF
R-COM-0080 – Antennas shall be capable of minimum gain of 1 dB
R-COM-0130 – Minimum uplink margin of 3 dB
R-COM-0131 – Minimum downlink margin of 3 dB

Communications Design

Architecture

The Iris mission architecture is data collection of the on-board scientific payload with the ground station at the University of Manitoba. In addition, we aim to collaborate with other participating CCP teams to create a shared network of all our ground station across Canada.

The radio architecture is a half duplex system, where UHF frequencies are used for uplink and downlink. The communication link parameters are listed below in Table 1.

Table 1: Communications Design Specs

Comms Design Parameters HF

System Block Diagram

The UHF transceiver is connected to a U-loop that facilitates testing of the radio and antennas with the spacecraft structure. The transceiver communicates with the command and data handling subsystem via CAN bus. The power supply for the system is the unregulated battery voltage, and therefore needs to be stepped down to the nominal transceiver voltage.

A block diagram of the system is illustrated in Figure 2 below.

Figure 2: Communications Architecture Block Diagram

Figure 3: Concept of Operations

Concept of Operations

The concept of operations for the communication system designates UHF as the primary uplink and downlink channel. The satellite will be actively listening in receive mode by default. When transmit mode is initiated, data will be downlinked via the UHF channel while periodically checking for additional commands. This is illustrated in figure 3.

The GOMSpace AX100 transceiver has a minimum delay in the RF relay of approximately 15 milliseconds when switching between receive and transmit modes.

Link Budget

The uplink and downlink budget is evaluated below in tables 2 and 3.

Table 2: Downlink Budget

LinkBudgetHiddenFrequency

Table 3: Uplink Budget

LinkBudgetHiddenFrequency

Link Access

Using orbital simulations from AGI's Satellite Tool Kit (STK), the communications link times between the satellite and the University of Manitoba can be summarized. These simulations were taken over a 1-year mission lifetime.

The link access timetable can be found here

Average contact duration - 234 seconds

Average number of contacts per day - 4

Average contact time per day - 949 seconds

Transfer capability per day (@9600bps) - 1139 kB

Minimum number of contacts per day - 2

Minimum contact time per day - 762 seconds

Worst case data transfer capability per day - 915 kB

Data and Telemetry

The data (image and telemetry) is compressed on CDH before transmitting to the ground station. Compression is performed before being stored on CDH flash memory. The bit stream is sent as CAN messages to the transceiver. Huffman algorithm will be used to compress images by 37% and differential-finite-context-method predictor (DFCM) will be used to perform 60% compression on the telemetry data. The table below shows the expected daily data transfer between the satellite and the ground station.

Table 4: Data transfer

CDH information Request

Hardware

The spacecraft antenna consists of a deployable quarter wave monopole in the 70cm band constructed from a spring steel alloy. The antenna is stowed using a mechanism that utilizes ultra high weight molecular polyethylene (UHWMPE) and deployed using burn resistor.

The communications subsystem connects the command and data handling (CDH) subsystem via CAN-Bus using the CubeSat Space Protocol (CSP) in order to exchange commands and telemetry. When information is requested by the ground station, the command is relayed to CDH which will access the on board memory and encode the data to prepare for transmission.

The radio selected is the GOMSpace AX100-U transceiver for UHF communications. This COTS component was chosen for its compliance with our system requirements, as shown in table 4 below. The radio includes a transceiver, front end (amplifiers, filters), and a micro controller as shown in Figure 4.

Source: AX100 Datasheet

Table 4: COTS radio requirements compliance

AX100 Properties

Figure 4: Internal block diagram of AX100 transceiver

UMS-0076 Electrical schematics (FM)

Key requirements for the schematic design are identified with compliance notes in the table 5 below.

Table 5: UMS-0076 Compliance Notes

comms FM .xlsx

Comms Control Board

The schematic in Figure 5 illustrates the communications control board which includes the twisted wire pair for CAN Bus and power, the interface connector for the radio, and a voltage step down regulator.

UMS-0076_Schematic_v2.pdf

Figure 5: Comms control board schematic

Part List

The parts that will be used in comms are listed below. Parts marked as N/A are manufactured in house:

COMMS parts list

Bill of Materials (BOM)

BOM is listed below. Parts marked as N/A are manufactured in house.

BOM

Mass Budget

The mass budget for comms subsystem is listed below

Communications Subsystem Mass Budget

Licensing

The required licenses for this mission and their statuses are listed below

1. ISED CPC-2-6-01 (Earth Station) - Approved

2. ISED CPC-2-6-02 (Space Station) - Approved

Key Risks

Some key risks to the subsystem are highlighted in Table 6.

Table 6: Key Risks

COMMS Key Risks

Assembly Plan

The communications board is assembled by attaching the transceivers to the dock PCB using fasteners. The command and data handling on-board computer must be installed first at the bottom of the COMMS/CDH module before any communications hardware. After successful installation of CDH, the transceiver assembly/dock is mounted on top. Necessary harnessing for power and data is connected to the interface connectors, and then coaxial cables are attached to their respective transceivers.

Afterwards, the antenna deployment housing is installed in the upper module, and the power harness is connected. The deployment mechanism is set to hold the antenna down and the UHMWPE line is secured through the resistor. Once the deployment mechanism is loaded, the other ends of the coaxial cables (connected to transceivers) and the antenna interface (coming from deployment housing) are connected to the U-loop interface.

Pre-Close-Up Verification Activities

Before the module is closed, the following verification activities must be performed:

Test the successful deployment of the antenna from the satellite structure. V-COM-0080.

Test Plan

Test plans can be found here

Page updated

Google Sites

Report abuse