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.

CubeSat Communications Transceiver for Increased Data Throughput

This paper describes the characteristics of a new CubeSat transceiver for increased data throughput. Here, the authors utilize a commercial transceiver chip from 900 – 928 MHz ISM band. This new transceiver can adjust its data rate, its bandwidth and RF frequency throughout a given LEO (Low Earth Orbit) pass by measuring its receiving signal power. An algorithm controls and optimizes all these features.

We know that all of the components of the CubeSat should consume very low power and be small in size, so we have these limitations when it comes to the design area. The antennas used on most CubeSats are microstrip patch antennas because of their small size and flexibility. In this paper they used  this kind of antenna, which provides a worst case loss of 10dB over an isotropic (spherical) coverage.

Specifications of the single chip transceiver:

• Texas Instruments cc1101
• It transmit up to 500kbps
• Provides 10mW RF output power while using 100mW of DC power
• Adjustable channel filter bandwidth between 58-812KHz (receiver)
• Consumes 50mW DC power (receiver)
• Designed to be interfaced with a PIC (programmable intelligent computer) for control and data flow purposes
• Can measure received signal strength and frequency offset

With just the features of the transceiver chip a person may know the capabilities of the communications system of the CubeSat. Another important element is the algorithm that controls this chip. It has to describe the operating mode in any condition that the CubeSat may experience. To confirm this paper, the authors ran a simulation of a 45-degree elevation pass and it increased data throughput performance by as much as 300%. They also recommend using a higher speed and more powerful PIC to optimize this system.

References: CubeSat-Communication Transceiver Paper

Exploiting Link Dynamics in LEO-to-Ground Communications

Adaptive Radio Technologies (ART) offers a new platform that could significantly improve the CubeSat’s downlink bandwidth, but is not without its limitations.

In this paper, the traditional methods of downlink are compared to the new method proposed by ART.

The Low Earth Orbit to ground radio link is highly dynamic. The traditional approach by which radios are designed assume the worst-case scenarios, which gives reliability but is inefficient because is not utilizing the full capacity of a given channel. Los Alamos National Laboratory has developed advanced technologies that can fully exploit the dynamic channels that are used, thus improving the efficiency of small satellite radios. By adaptively changing the data transmission rate during a LEO satellite pass the adaptive radio is capable of bit rates as low as 117 kbps and as high as 18.6 Mbps.

This is achieved by measuring the signal strength and then using a conventional bit rate uplink to command the satellite to change the downlink’s bit rate. It is not practical to adapt this bit rate continuously; instead it can be adjusted at discrete steps to optimize the results. Given identical power, weight, and volume using the adaptive radio method provides 10 times more improvement than using conventional methods. To obtain these results a 2.4 GHz downlink frequency was used.

While this new concept provided by Adaptive Radio Technologies gives the best results in the downlink bandwidth, it is by using the 2.4 GHz frequencies. This creates a limitation because past CubeSats have used the amateur frequencies, which are also used by the ground station and are the frequencies that we should focus on as stated on the previous post.

Reference: www.adaptiveradiotech.com/presentations/jmp_smallsat_2009.pdf

A Survey of CubeSat Communication Systems:

This paper, written in 2008, provides a summary of the communication subsystems utilized in various CubeSat missions, and compares their performance.

Some CubeSats utilize one radio for the data downlink and another one, with much lower power consumption, for the beacon. A different radio is utilized in almost every one of the missions discussed in the paper. Most of the downlink frequencies are around 437MHz, which is in the amateur radio band range, and most of them utilized a 1200 baud rate.

Out of the 23 CubeSats analyzed, two of them stand out for the amount of data downlinked. These are the QuakeSat-1 and CanX-2, both being 3U CubeSats.

QuakeSat-1 utilized two ground stations that were linked via the internet, so it could download more data. The transceiver on this satellite downlinked at 436.675 MHz with a 9600 baud rate.

CanX-2 utilized a custom S-Band radio, operating at 2.2Ghz up to 256 kbps. It took 4 years for the team to obtain a frequency in the licensed Space Research spectrum.

Some subsystem recommendations are included in the paper:

• Utilize a long beacon.
• Include a way to easily reset the satellite if it becomes non-responsive.
• Make sure the ground station is operating properly before launching the satellite.

From this paper we can gather that utilizing the amateur radio frequencies could be the best option for us, since obtaining a restricted frequency can take up to several years.

Reference: http://www.klofas.com/papers/CommSurvey-Bryan_Klofas.pdf

Hardware Specifications of the Ground Station

The objective of the ground station is to make possible the communication by receiving data of the satellite and sending commands to it. The minimum necessary hardware in a ground station is:

• Antenna – for receiving and transmitting electromagnetic waves.
• Radio – to receive and transmit data.
• Computer – for data handling and for decision making.

Hardware Specifications

Antenna:

• Able to operate within the chosen frequency band of the radios.
• Able to withstand the weather conditions at the location of the ground station.
• Able to operate with signals in the radio amateur band (433-438).
• An estimate 15dB antenna gain.

Radio:

• The operating frequency band have to be around 437 MHz
• A minimum 50 W transmitting capability.
• Must be controllable from the ground station main computer.
• Frequency scanning, automatic frequency control and adjustable squelch level.

These specifications can be used as a guide for our ground station model for the CubeSat.

Reference: http://www.cubesat.aau.dk/documents/gshw.pdf

CubeSat Research at the University of Puerto Rico in Mayaguez (UPRM)

Here at University of Puerto Rico in Mayaguez (UPRM), under the guidance of Dr. Lizdabel Morales-Tirado, we are part of the CubeSat Transmission Module Research. Our group is composed of enthusiastic Electrical and Computer Engineering students. A CubeSat is a pico-satellite, which can be built less expensively and faster than a traditional satellite. It provides a low budget solution for researchers to study more complex problems.