Archive for January, 2011


Frequencies and Antennas

Common CubeSat Transmission Frequencies

The most common CubeSat transmission frequency is 437 MHz, which is in the Ultra High Frequency (UHF) band. It is often utilized because it is in the amateur radio frequency band, thus not requiring a special license to operate. Another reason for this is that the antenna can be relatively small.

Two other common transmission frequencies are 2.2 GHz, which is in the S-band, and 5.8 GHz, which is in the ISM band. A license is required to operate in these frequencies, which can take months or years to get. Transmitting in these frequencies may require more power, but they have some advantages. Sky background noise reaches its lowest level between 1 and 10 GHz, there is lower attenuation of the signal due to atmospheric phenomena in higher frequencies.

Common Antenna Types Utilized

One of the most common antennas used is the dipole antenna. A single dipole antenna can be utilized for both uplink and downlink. It has to be deployed after the satellite is in space. If only one element deploys, the antenna will still radiate.

Another widely utilized antenna is the patch antenna.  The satellite must be facing the Earth during transmission.  Two or more antennas may be required, which can take up a significant amount of space in the satellite’s surface, which might be needed for solar cells. They also have limited bandwidth, compared to other types of antennas. They don’t require deployment, as they are embedded in the satellite’s surface.

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Packet Telemetry

In order to receive data from the satellite it is helpful to design a packet scheme that can guarantee reliable and redundant communication. The Consultative Committee for Space Data Systems (CCSDS) provides its recommendation for space data system standard, which we have chosen to follow and apply to our project.

Packet Telemetry

Is a concept which facilitates the transmission of satellite-acquired data from the source to a user in a standardized highly automated manner. This is done so the ground system can recover the individual data units with high reliability by using Source Packets and Transfer Frames, which are data structures.

Source Packet

The Source Packet contains a Primary Header that is used to route the packet to its destination, and it contains information about the length, sequence, and other characteristics of the packet. There can be a secondary header which carries a standardized time-tagging, the satellite position, attitude and data that can support the primary functions of the packet.

Source Packet Illustration

 

Transfer Frames

This data structure provides an envelope for transmitting data packets over a noisy satellite-to-ground channel. It also contains a header that provides information that helps in the data routing and classification. This frame has a fixed length and it is used to encode various variable length Source Packets together for a reliable transmission to the ground. By using this method there is no need to have a predetermined size of each packet and guarantees that all the information is transmitted and if not, the users would be able to recognize what data was missing.

Transfer Frame Illustration

Idle Packets

What happens when a Source Packet is too small to fit in a Transfer Frame? Idle Packets are used, which are data that carries no information and have no specified bit pattern. They are used to meet synchronization requirements and error protection techniques for the transmission of data.

 

Source: public.ccsds.org/publications/archive/102x0b5s.pdf

CubeSat Abstract

CubeSat Transmission Module

A CubeSat is a miniaturized satellite usually measuring 10cm3. It has the ability to carry a variety of payloads in a very small package. For the Puerto Rico CubeSat project we have the following requirements for the Telemetry, Tracking, and Communication (TT&C) Subsystems: data rate should be approximately 1Mbps, maximum weight of the TT&C subsystems should be 0.218 kg, maximum power consumed by the transceiver and receiver and the data command and handling of the satellite should be less than 2.2 watts and frequency of the satellite should be in the S band.

We propose the Adaptive Radio Technologies Firehose Communications System transceiver as an alternative because it complies the required features. This system has a 1 Mbps average downlink rate for a 2.5m dish, weighs less than 160g, has a ½ watts radio frequency output power. A patch antenna should be utilized with the transceiver. Quadrature Phase Shift Keying (QPSK) appears to be the best modulation method for the satellite data transfer. The satellite should have a GPS module to enable accurate tracking of the satellite’s orbit, and allow acquired data to be correlated to a location.

Our project will emphasize in the CubeSat transmission module, which is responsible of all the satellite’s communications.

Note: This abstract was created following the guidance provided by Dr. Lizdabel Morales-Tirado.