QUESTION:
What are the X-band antenna gains (low and high) for the lander? What type
of design is each antenna? What was the transmit power on the omni during
the landing, and what is the transmit power used during the high gain
downlink sessions? What modulation is used?

I would like to do a set of MPF X-band link calculations. I have found
everything else I need (DSN antenna gains and system temps, RS/Viterbi
decoder characteristics, etc) but I have not been able to find any
information on the lander antennas.

My best estimate for the high gain antenna gain is about 17dB, based on all
the numbers above plus an item from your FAQ that gave the expected signal
power on July 4 as -146 dBm on the 34m DSN antenna. How close did I get?

ANSWER from Gordon Wood on July 15, 1997:
The low gain antenna is a choked circular waveguide which has about 8 dBic
of gain (relative to an isotropic radiator) at the peak of the beam which
points straight up (toward zenith) from the lander.

The high gain antenna is a flat plate array and has a peak transmit gain of
about 24dBic.

Both the low gain and high gain antennas are right-hand circularly
polarized.

The transmit power is similar for both the low gain and high gain with the
radiated power being only slightly different because of different losses in
transmission lines and switches coupling the transmitter to the antennas. 
The radiated X-band power is approximately 10 watts.

The TOTAL power at the Goldstone 34 meter DSN HEF (DSS-15) on July 4th was
about -145.4 dBm which is very close to your number.  However, since the
antenna gain that you assumed is significantly lower than the HGA true gain
the discrepancy is a bit larger.

Gordon Wood, Mars Pathfinder Chief Engineer for Mission Communications


ANSWER from Guy Beutelschies on July 19, 1997:
Mars Pathfinder uses a 13 Watt X-Band transmitter.

Guy Beutelschies        Mars Pathfinder Flight Engineer

ANSWER from Gordon E. Wood on July 23, 1997:
The uplink frequency is 7175 MHz and the downlink is at 8429 MHz. These
frequencies are in a microwave region generally termed as "X-band" and
occupy a deep space communications frequency allocation that is reserved for
weak signals to avoid interference.  The performance of X-band
communications links takes advantage of the ability to focus centimeter
wavelengths with the large tracking antennas we use on earth.

On Mars the lander has a flat plate array for X-band communications. It is
physically slightly larger in diameter than a soccer ball.  It posseses
+24.9 dB of gain which means it focuses the downlink power such that the
received signal is about 300 times stronger at earth than if it was radiated
equally in all directions.

On the earth we are currently using the 70 meter diameter parabolic
antennas of the Deep Space Network.  These are located in Madrid,
Spain; Canberra, Australia; and Goldstone, California, USA.

The lander radiates about 10 watts of X-band signal power.  On earth we use
18 kilowatts for uplink transmissions to the lander.  Uplink transmissions
are sent from smaller antennas (34 meters versus the 70 meters used for
receiving).

PCM-BPSK-PM; i.e. pulse-code-modulation, binary-phase-shift-keyed, phase
modulation.

What all that means is that we send only digital binary data (PCM), it is
bi-phase modulated (BPSK) on a subcarrier (22.5 kHz or 360 kHz, depending on
the data rate), and phase modulated (PM) onto the X-band carrier.

Gordon E. Wood
Chief Engineer for Mission Communications
Mars Pathfinder Project