The Ultimate DX Experience: Mars! By Marc. C. Tarplee, Ph.D. N4UFP

Introduction In 2001, NASA launched the Mars Odyssey orbital probe. This orbiter has a space-to-ground link operates at 437.1 MHz. NASA is able to use this frequency, because the 70 cm band is not an amateur allocation on Mars (the FCC has no jurisdiction on Mars ). The use of terrestrial amateur frequencies of Mars raises two interesting questions: would be possible to hear the Mars Odyssey orbiter on Earth, as it communicated with probes on the surface of Mars? (DX from Mars) What types of surface-to-surface communications are possible on Mars? (DX on Mars)

Part I DX from Mars

Hearing the Mars Odyssey Relay from Earth Communications from Mars to Earth are line of sight, through 38 million to 250 million miles of interplanetary space. Earth and Mars rotate around the Sun, the Earth in 365 days, and Mars in 687 days. Earth revolves in 24 hours, and Mars in 24 hrs 38 min. For at least part of each day, it is possible to have a line of sight between the relay and Earth.

Hearing the Mars Odyssey UHF downlink from Earth It will be assumed that reception will be attempted only after dark, so that question of solar noise does not have to be considered. Mars appears in the evening sky for weeks at a time, so there would certainly be many opportunities to listen for the probe Although there is a line of sight between Earth and Mars, that does not guarantee reception of the relay’s signal. The path loss and antenna gains must be computed to see if the signal reaching Earth is above the background noise.

The Inner Solar System

Received Signal Strength The received signal strength (in dBm) can be computed from the following equation: Precv if the received power Pxmit is the transmitted power Gxmitant is the transmitting antenna gain αpath is the path loss Grecvant is the receiving antenna gain αfeedline is the feed line loss. All powers, gains and losses must be expressed in dB.  

Transmitter Specifications The transmitted power will be low, since the probes depend on solar cells for electric power. Typical output powers are approx. 20 watts (43 dBm). To save weight, lander antennas tend to be very simple. The estimated transmit antenna gain is 5 dBi.

Path Losses Next it is necessary to compute the path loss: where α = the path loss in dB D = the distance traveled by the RF D varies from 38 to 250 million miles (61 billion to 402 billion meters) λ = the wavelength of the RF (0.69 meters) The corresponding path loss ranges from 251 to 267 dB. An average path loss of 259 dB will be used. 

UHF Receiving Antenna Specifications The gain of the receiving antenna should be a large as possible. It will be assumed that the receiving antenna is a commercially available 70 cm yagi with approximately 25 elements. The gain will be approximately 18 dBi. Feed line losses can be a problem at 437 MHz. Good coax, such as Belden 9913 will have a loss of about 2.7 dB per 100 ft. A 3 dB feed line loss will be assumed.

Expected Received Signal Strength for the UHF uplink signal Now the received power can be calculated: This is extremely weak! The thermal noise floor of a receiver with a 100 Hz BW is -154 dB; thus the signal would be lost in the noise. The Mars Odyssey signal could be received if: The receive antenna were made larger (>300 ft dia dish) More power were used at the transmitter end (> 100 W (50 dBm)) Cool the receiver front end to reduce thermal noise

Hearing the Mars Odyssey X-band link Mars Odyssey transmits data and telemetry back to Earth on the X-band. (~10GHz) Based on data from the JPL website: output power ~ 25W = 44 dBm antenna gain ~ 40 dB bandwidth ~ 10 kHz (based on a data rate of 21.3 kb/s and a coding efficiency of > 2 bit/Hz

X-band Receiving Antenna Specifications Yagis are not practical at 10 GHz. The best approach is a paraboloidal reflector antenna It will be assumed that the receiving dish has a diameter of 8 feet and an illumination efficiency of 50% at 10 GHz. The estimated gain of the dish is 43 dBd Feedline losses will be assumed to be 6 dB

Expected Received Signal Strength for the X-band downlink signal Received Power: The thermal noise floor of a receiver with a 10 kHz BW is -134 dB The signal is still below the noise floor. If the receive antenna were made much larger (~ 400 ft dia dish) the RSL would now be~ -130 dBm. Receive dishes used by NASA are considerably smaller, on the order of 100 ft in diameter. Smaller dishes are probably made possible by a narrower bandwidth than was estimated and cryogenically cooled receiver front ends that have low noise floors

ME (Mars-Earth) Operation Path losses on the Mars-Earth link are similar to those encountered in EME (moonbounce) operation. When amateur operation does commence on Mars, ME operation should be possible. Station requirements resemble those for EME: High gain antennas at both ends of link (> 25 dBi = 16x14 el Yagis or a dish) Ability to rotate the antenna in azimuth and elevation High transmitter power (> 25 W = 44 dBm) Received signal levels would be in the –140 dBm range, at least 10 dB above the noise floor. Antenna arrays of this type are already in use for EME

1296 MHz ME Link Analysis Path losses at 1296 MHz = ~ 268 dB. Antennas at both ends are 10 ft (3.3m) diameter dishes with 50% illumination efficiency (G ~ 46 dB) Feedline losses at each end are 6 dB Output Power of the transmitter is 44 dBm (25W) Bandwidth is 100 Hz ( suitable for PSK-31 ) Noise floor = -154 dBm RSL = 25+46-6-268+46-6=-144 dBm SNR ~ 10 dB which is sufficient for good copy on PSK-31

Part II DX on Mars

Possible Propagation Modes Line of sight communications are possible on Mars. However, for a given height, Mars’ smaller diameter gives a shorter range. Over-the-horizon VHF/UHF modes such as tropospheric scatter are dependent on the presence of water vapor, which is not part of the Martian atmosphere. Propagation modes such as Trans-Equatorial F and Field-Aligned Irregularities are dependent on a planetary magnetic field, which Mars does not have. Initially, the dominant mode of RF propagation may be HF sky wave.

The Martian Ionosphere The upper atmosphere of Mars, like Earth is bombarded by high energy radiation from the sun. Although the average intensity of this radiation is about 44% of what Earth receives, there still should be enough energy to create an ionosphere on Mars. Martian Ionospheric Electron Density vs Altitude

Critical Frequency Because Mars’ atmosphere is composed almost entirely of carbon dioxide, there is only one layer in its ionosphere The peak electron density, 1.2*10 5 cm –3 , is only 5% of the peak electron density or Earth’s F-layer. The critical frequency of the ionosphere and the electron density are related as follows: where Ne is the electron density fcr is the critical frequency. On Earth, the critical frequency of the F2 layer varies from 5 MHz (night) to 14 MHz (day). On Mars, it is varies from 0.6 (night) to 3 MHz (day).

Maximum Usable Frequency For DX paths, in which the radiation angle is near zero, the maximum frequency for sky wave propagation (MUF) is given by:   R is the radius of the planet h is the effective height of the ionosphere. During daytime, Martian MUF’s reach 10 MHz. Daytime terrestrial MUF’s can reach 40 MHz. At night, Martian MUF’s drop to 2 MHz, compared to 5 to 10 MHz on Earth.

Sky Wave Path Comparison To cover a given distance, more hops are needed on Mars Minimum number of hops needed to reach all points on the surface: -5 on Earth -6 on Mars

HF Communications on Mars If amateur frequencies were allocated on Mars as they are on Earth, only the 160, 80, 40 and 30 meter bands could be used for long-haul communications. Since there is no D-layer on Mars that absorbs lower frequencies, all bands could be used during daylight hours. After dark 160m would be the only possible band for DX. The 20m band on Mars would act much like 6m on Earth; during periods of intense solar activity there could be DX openings on 20 during the day.

Cyclical Propagation Variations In addition to diurnal variations, propagation on Mars also has a seasonal variation. Mars’ orbit is more elliptical than Earth’s and during the northern hemisphere winter, solar irradiation is 40% higher than it is during the northern hemisphere summer. In the northern hemisphere, there would be tremendous seasonal change in MUF. However, in the southern hemisphere, MUF’s would be relatively constant throughout the year. Mars’ ionosphere, like Earth’s, is also affected by the solar cycle, but because Mars has no magnetic field, the solar wind could wreak havoc with HF communications there.

Closing Comments It is unknown whether sporadic phenomena similar to sporadic-E occur in the Martian atmosphere The effects of global sandstorms that sometimes engulf the planet on propagation are not known. When amateurs finally get the chance to operate on Mars, there will probably be new propagation modes discovered that are unknown here on Earth. Operating on Mars could be the most exciting amateur activity of the 21st century, should we decide to go.