Communications
Note: An extensive analysis of the communication systems, including plots, detailed simulations, and references, can be found in pdf form. A summary appears below:
Overview
The distance between Earth and Mars varies widely depending on the orbital position of each planet. Our goal is to maintain a data rate of at least 1 Mbps at the greatest Earth-Mars distance, which is approximately 401 million kilometers. Within the overall fleet of 18 active satellites, each constellation of 6 will contain one "master" satellite with a large Ka-band dish to communicate with Earth.
Deep Space Network
There are three Deep Space Network complexes around the globe, each equipped with a 70m dish antenna and several smaller 34m and 26m antennas. They are located at:
- Goldstone, CA
- Madrid, Spain
- Canberra, Australia
To support NASA's Mars Reconnaissance Orbiter, launched in 2005, several changes were made to the Deep Space Network. It now supports operation in the Ka-band, transmitting around 34.2 GHz and receiving at 32 GHz. Transmit power can reach 400 kilowatts. Using the Ka-band (as opposed to X-band, for example) allows for higher gain and bandwidth, with the trade-off being more disruption due to Earth's weather.
Turbo-coding
Beginning in 2003 the Consultative Committee on Space Data Systems (CCSCS) implemented Turbo Codes compatible with the Deep Space Network. Such codes can dramatically improve error correction methods used in the past, and are relatively easy to implement in hardware or software onboard our satellites. Using a (255;223) Reed-Solomon code with (rate-0.4998, length-8920) Turbo Codes will provide a final bit error rate of 10^-5 given a 1dB SNR.
Uplink Budget
Selected values are included here. For full details, view the pdf report.
| Specification | Value |
Notes |
| Frequency | 34.45 GHz |
Ka-band, compatible with DSN |
| Actual bit rate | 1 Mbps |
|
| Physical bit rate | 2.287 Mbps |
|
| Modulation | BPSK |
|
| Bandwidth | 3.43 MHz |
|
| Physical Noise Temp | 40 K |
|
| Satellite Antenna Gain | 59.7 dB |
|
| Satellite Antenna Diameter | 3m |
|
| Transmit Power | 50 dBW |
|
| Maximum Distance | 401 billion km |
|
| Received Power | -106.5 dBW |
|
| CNR | 40.7 dB |
Downlink Budget
Selected values are included here. For full details, view the pdf report.
| Specification | Value |
Notes |
| Frequency | 32.05 GHz |
Ka-band, compatible with DSN |
| Actual bit rate | 1 Mbps |
|
| Physical bit rate | 2.287 Mbps |
|
| Modulation | BPSK |
|
| Bandwidth | 3.43 MHz |
|
| Physical Noise Temp | 28 K |
|
| Satellite Antenna Gain | 59.7 dB |
|
| Satellite Antenna Diameter | 3m |
|
| Transmit Power | 18 dBW |
(63 watts) |
| Maximum Distance | 401 billion km |
|
| Received Power | -138.5 dBW |
|
| CNR | 3.64 dB |
Mars-Orbit to Mars-Surface Link
The positioning signal transmitted from satellites in Mars orbit to positioning devices on the planet's surface is centered at 2.23 GHz and is constructed with Gold codes of length 2^13 - 1 = 8191. The autocorrelation of this code is nearly perfect. Processing gain is 39.1 dB. A series of simulations was performed in SIMULINK/MATLAB to evaluate the performance of the positioning system, and full details are provided in the pdf report. As an example, the simulation setup and results for the autocorrelation are shown below:
Using a system of 18 active satellites divided into 3 constellations allows for a maximum of 8 visible satellites at any one time. With an altitude of 17,708 km and transmit power of 4.92 dBW (3.1 W), the average received power on the ground is -172.22 dBW per satellite.
Our signal contains a 50 bps data signal riding on top of the positioning signal, just like Earth-based GPS. The coarse acquisition signal has a chip rate of 8191 x 50 = 409,550 chips/sec.
Spread carrier-to-noise ratio is -26.4 dB, which increases to 12.77 dB after despreading. After removing 1 dB for link margin, our estimated BER is an excellent 2.07E-8. At 50 bit/sec this represents 1 error in 11 days.
A full Simulink simulation of the GPS system can be downloaded here.
Satellite to Satellite Link
The master satellite in each constellation can act as a relay for signals from Earth. It will receive the high data rate signal with its large Ka-band antenna and relay a down-converted X-band signal to the satellites nearby when necessary, or use that transmitter to send other specific instructions as directed by Earth. This system will transmit with a 60cm dish antenna that broadcasts 5 dBW of power at 8 GHz.