- Brad Norman
The MEROPE communications subsystem consists almost entirely of commercial off the shelf components. Simple and robust, it is centered around a Paccomm Picopacket terminal node controller (TNC) operating continuously in "transparent" mode, whereby all serial data from the processor are immediately packetized and transmitted through a Yaesu VX-1R dual-band handheld transceiver (HT). The entire subsystem weighs less than 140 grams and occupies a total volume (including antennas and interconnects) of 180cm3--less than 1/5 of the total spacecraft weight and volume budgets—-with a hardware cost of less than $400. Downlink is at a frequency of 145.980 MHz with 20 kHz of available bandwidth. Uplink is at 437.445 MHz with a 30 kHz bandwidth. Communications flow is controlled by the Motorola HC12 flight processor, which is linked through a 9600 baud RS232 serial connection to the TNC. The entire communications link (ground-MEROPE-ground) is seamless, initialized by a single encrypted uplink command. Upon contact with MEROPE, the ground station instructs the processor to dump the contents of its memory into the TNC, which packetizes the binary data and keys the transmitter. Return to the top of the page
End-to-end link analysis was not complete at press time, due to some ambiguity in ground station design. However, a preliminary downlink analysis was performed. In the analysis, system temperature, line losses, flight antenna efficiencies, space loss, atmospheric attenuation were taken into account to estimate he MEROPE-ground station bit energy signal to noise ratio (SNR). The downlink bit energy SNR (in dB) can be estimated using Equation (1),
SNR = P + Ll +Gt + Ls + La + Gr + 228.6 - 10logT - 10logR (1)
where P = transmitter power ~1 dBW,
Ll = line losses ~ -1.1 dB,
Gt = antenna gain factor ~1.5 dB,
Ls = space loss ~ -150 dB,
La = atmospheric losses ~ -3 dB,
Gr = receiver gain (unknown),
T = receiver system temperature (unknown),
R = data rate = 1200 bps.
MEROPE’s transmitter power output peaks at 1W RF. Line losses from transceiver to antenna are minimal, given the compactness of the subsystem. For a 650km-high circular polar orbit, the MEROPE-ground station distance, S, is within the bounds of 650km < S < 2950km, yielding a space loss Ls between 141.53 dB < Ls < 154.67 dB. Given these figures, the MEROPE-ground station link SNR is on the order of 40 dB worst-case, which is well within the capabilities of most COTS ground station equipment. Also, note that the bit energy SNR for the uplink will be at least 20 dB above the downlink SNR, since the receiver noise temperature will be lower (on average) and since the downlink transmitter power will be at least 10 dB above the MEROPE transmitter power. Return to the top of the page
The TNC consists of a single shielded printed circuit board (PCB) measuring 8.45 cm long by 6.17 cm wide, weighing 57 grams. It is powered at 7-14Vdc and draws between 50mA and 70mA during continuous operation. Here is an image of the Pico-packet.
The transceiver consists of the "guts" of a Yaesu VX-1R, arguably one of the smallest and lightest handhelds on the market. The HT is one double-sided PCB measuring 8.32cm long and 4.20cm wide, weighing only 47 grams. The shielded RF module stands 1.20cm high and occupies one half of the PCB. Power is supplied from a 3.7V bus for 0.5W of RF output. Current consumption for the receiver and transmitter is 150mA and 400mA, respectively. Transmission duration for the expected 100 Kbytes/pass of telemetry and payload data at 1200 baud is about 11 minutes, comparable to the above-the-horizon window. Here is an image of the Yaesu radio.
The MEROPE antenna is a center-fed dipole tuned to the 2m uplink, which is nearly harmonic with the 70cm downlink. The antenna consists of two 48cm-long nickel-titanium (“Nitinol”) tape measure to avoid binding to the attitude control magnets. Each element is uncoiled on orbit from opposing sides of the spacecraft. The choice of a single dipole is fourfold: (1) it allows for enhanced directional gain over an omni antenna since a dipole does not require a ground plane and is not shadowed by the spacecraft; (2) the use of one antenna for both up-/downlink frequencies avoids the addition of duplexer hardware; (3) it is relatively simple to tune to both uplink and downlink frequencies; (4) it reduces the number of potential points of failure, and will radiate if only one element deploys. The MEROPE project ground station will consist of a COTS amateur base station radio and antenna assembly. Here is an image of the entire communications subsytem assembly, showing the two (coiled) dipole antennas in the (gray) antenna deployers. The green block on the top (left) side of the card is the TNC. The radio is mounted on the reverse.
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[1] Hunyadi, G., D. M. Klumpar, S. Jepsen, , B. Larsen, M. Obland. “A Commercial Off the Shelf (COTS) Packet Communications Subsystem for the Montana EaRth-Orbiting Pico-Explorer (MEROPE) Cubesat” 2002 IEEE Aerospace Conference Proceedings, March 9-16, 2002.
[2] J.R. Wertz, W. Larson (eds.), Space Mission Analysis and Design, 3rd Ed., Microcosm, Inc. and W.J. Larson, 1999.
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