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

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

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Part I

• DX from Mars

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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.

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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 relays signal. The path loss and antenna
  gains must be computed to see if the signal
  reaching Earth is above the background noise.

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The Inner Solar System
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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
• apath is the path loss
• Grecvant is the receiving antenna gain
• afeedline is the feed line loss.
• All powers, gains and losses must be expressed in
  dB.  

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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.

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Path Losses

• Next it is necessary to compute the path loss
• where a 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)
• ? 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. 

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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.

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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

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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

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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

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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

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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

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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 2546-6-26846-6-144 dBm
• SNR 10 dB which is sufficient for good copy on
  PSK-31

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Part II

• DX on Mars

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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.

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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
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Critical Frequency

• Because Mars atmosphere is composed almost
  entirely of carbon dioxide, there is only one
  layer in its ionosphere
• The peak electron density, 1.210 5 cm 3 , is
  only 5 of the peak electron density or Earths
  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).

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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 MUFs reach 10 MHz.
  Daytime terrestrial MUFs can reach 40 MHz.
• At night, Martian MUFs drop to 2 MHz, compared
  to 5 to 10 MHz on Earth.

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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

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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.

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Cyclical Propagation Variations

• In addition to diurnal variations, propagation on
  Mars also has a seasonal variation.
• Mars orbit is more elliptical than Earths 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, MUFs would
  be relatively constant throughout the year.
• Mars ionosphere, like Earths, is also affected
  by the solar cycle, but because Mars has no
  magnetic field, the solar wind could wreak havoc
  with HF communications there.

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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.