Payload (HSCOM/PLD)

Subsystem Top Level Requirements
-- The system shall communicate on the S-band frequency range
-- The system shall follow all FCC and AMSAT communication regulations for wireless communications
-- The system shall be capable of working with the Boulder Colorado Space Grant Consortium S-Band ground station

Subsystem Functional Requirements
--
The system shall not exceed a mass of 150 grams and a volume of 200 cm^3
-- The system shall provide a downlink data rate of at least 50kbps
-- The system shall maintain a link margin of at least 3dB at elevations greater than 45 degrees
-- The system bit error rate shall be less than 1*10^-5 at elevations greater than 45 degrees
-- The system shall not exceed 8 Watts of peak power during transmit
-- The system shall not exceed 0.25 Watts of standby power
-- The system shall provide a control method for the Command and Data Handling (CDH) subsystem to control the state of the system with functionalities including push-to-talk, data rate control, flow control, and shutdown control
 

The Science subsystem, internally known as High Speed Communications (HSCOM), is responsible for proving the effectiveness and reliability of an S-band communications link for CubeSat applications.

MicroHard MHX2400 Modem
The heart of the HSCOMM system is the MicroHard MHX2400 modem, a 2.4GHz frequency-hopping spread-spectrum transceiver. The modem operates from 2.4000GHz to 2.4835GHz in the Industrial, Scientific, and Medical (ISM) band which spans from 2.4000GHz to 2.5000GHz centered on 2.4500GHz. By default, the modem also operates in the amateur radio band available from 2.4000GHz to 2.4500GHz.

The modem essentially acts as a very long serial cable, making it easy to implement with the satellite. CDH sends data across a UART connection operating at 115 kbaud and the modem does all of its own frequency hopping, packet formation, and error correction. An identical modem at the ground station receives the signal, decodes it, and sends a serial stream of data to a ground station computer.

Antenna Design
The antenna for the HSCOM system is a 3.09cm quarter-wave monopole mounted in the center of one face of the structure. The antenna is constructed of spring steel to aid in its deployment. The rather strange antenna gain pattern can be seen below. Since the aluminum structure is 10cm on a side and the wavelength of the S-band signal is approximately 12 cm, the structure causes significant distortion to the antenna gain. Many antennas were investigated, but once the structural effect is accounted for, the monopole appears to give the most omni-directional gain pattern. The antenna is aligned so that the large pink lobe on the gain pattern below is most frequently facing the ground station during ground passes.

 

Link Margin and Analysis
The link budget for the HSCOM system was completed using a Matlab script in conjunction with ground pass data from Satellite Tool Kit (STK). The final analysis of the HSCOM link budget depends upon the outcome of the ADCS dynamics model which is still under construction. This model will provide the expected spin rate and oscillation rate about the magnetic field vector. Both of these motions will affect the pointing of the HSCOM monopole antenna and thus affect the link margin.

The antenna will be aligned with the magnets, and thus with the magnetic field vector. Since the satellite will primarily spin around this vector, this will provide the most consistent gain pattern. If the antenna was not aligned with this vector, it would rotate with the satellite, creating a sweeping cone of gain pattern that would be non-ideal for maintaining a communication link.

   

Links & Downloads

MicroHard MHX2400 Manual Rev 1.56

4NEC2X Antenna Modeling Software

HSCOM Monopole NEC Model w/ Isogrid CubeSat Structure