Communications

Downlink (UHF) antenna radiation pattern, shown with our 20rpm spin about the Z axis

Downlink (UHF) antenna radiation pattern, shown with our 20rpm spin about the Z axis

ELFIN is a 20 rpm spinner, with its spin axis roughly orbit normal (as dictated by science requirements). The antennas are positioned in between the spin axis and the spin plane to mitigate spin fading in all orientations.

We are requesting IARU coordination for an experimental license within the amateur satellite service’s 435-438 MHz band so that we may beacon globally. We will use an AstroDev radio to provide a 19,200 baud GFSK AX.25 UI downlink.

 

Link Budgets

Our uplink and downlink budgets, using W3GEY’s Link Model spreadsheet, is available here (001-LinkBudget-02-Q.xls). Our beacons will be receivable a typical OSCAR station, however slightly more capable stations will be utilized for routine downlinks so that we can conserve power aboard ELFIN.

We consider ourselves a medium-throughput mission, generating far too much science data for 1200 baud AFSK, but not warranting the high speed megabit-per-second systems currently being pioneered on the CubeSat scale by other missions.

 

Antenna radiation patterns

Simulations were done using 4NEC2, with separate models for the uplink and downlink. You can download them here (ELFIN_NEC.zip).

 

Downlink

Downlink radiation pattern highlighting dipole's null. Our magnetometer stacer boom, seen here along the Y axis, results in some atypical patterns.

Downlink radiation pattern highlighting dipole’s null. Our magnetometer stacer boom, seen here along the Y axis, results in some atypical patterns.

The downlink (UHF) antenna is a bent dipole, which for the purposes of these simulations is assumed to be tuned to 436.5 MHz. Results show a gain of 1.1dBi and a null of 8.1 dB.

We are currently developing a field test procedure, equipment, and software to verify these results. This setup uses a pair of economical RFM22 ISM radios, coupled with a 6DOF sensor (LSM303 magnetometer & accelerometer) to plot out antenna radiation patterns in real time using MATLAB. Once validated we will make these tools available to the community.

Thanks to AstroDev’s Fast Power Amplifier setting, we can perform dynamic link adjustments to match the performance of a particular Earth Station. This will allow power consumption to be reduced around the time of closest approach, and increase the power output near the horizon.

Downlink radiation pattern, viewed from the spin axis

Downlink radiation pattern, viewed from the spin axis

Slice with worst case UHF performance

Slice with worst case UHF performance

 

 

 

 

Uplink

tunacan

VHF and UHF antenna elements stowed in 3U+ bonus volume (“tuna can”)

The uplink (VHF) antenna is also a bent dipole, tuned to 144.4 MHz. Results show a gain of 3.05dBi, with a null of 6.6 dB.

Both the uplink and the downlink antennas consist of bistable fiberglass tape springs with embedded beryllium copper strips, made by the AFRL for their VPM Cubesat. This material is useful for a variety of CubeSat-scale deployables, however for ELFIN the most important feature is their magnetic cleanliness. This non-magnetic requirement is what prevents us from using the prevalent spring steel carpenter tape elements or NiTi (Nitinol) wire.

The tape springs are not held in their secondary stable state, allowing them to provide their own kick force, and are retained with Spectra fishing line, burned with a conventional 1/2 W axial resistor.

All antenna elements are stowed in the new bonus volume offered by the new 3U+ specification (often referred to as the tuna can).

 

Uplink (VHF) antenna radiation pattern, shown with our 20rpm

Uplink (VHF) antenna radiation pattern, shown with our 20rpm spin

Uplink radiation pattern highlighting dipole's deepest null.

Uplink radiation pattern highlighting dipole’s deepest null.

 

Slice with worst VHf performance

Slice with worst VHF performance

Uplink radiation pattern, viewed from the spin axis

Uplink radiation pattern, viewed from the spin axis