Radio Science Experiment
The Nozomi radio science experiment has
two primary objectives, to study the vertical structure of the ionosphere and atmosphere
of Mars, and to obtain estimates of the gravity field and constrain the internal structure
of the planet. The experiment will be performed using the normal on-board radio
communications equipment and an ultrastable oscillator.
Equipment
The normal spacecraft communications with Nozomi are via X-band (downlink) at 8410.93 MHz and S-band (downlink and uplink) at 2293.89 Hz. The quartz ultrastable oscillator has an output frequency of 19.11574 MHz. The operational temperature range is -20 to +40 degrees C and the frequency variation as a function of temperature is 7 x 10**-13 per degree C. The Allan variance at 1 second is 4.1 x 10**-13 s. The mass of the oscillator is 0.460 kg and it uses 0.5 W of power during normal operations. Maximum turn-on power is 0.78 W.
Atmosphere and Ionosphere Occultation
The atmosphere and ionosphere can be studied when an occultation occurs: the spacecraft moves behind Mars' atmosphere as viewed from Earth. As the spacecraft moves behind the limb, the signal penetrates deeper into the ionosphere and atmosphere. Refraction results in an increase in the Doppler shift and refractive defocusing, absorption, and scattering cause a decrease in the signal power. A limb tracking maneuver is also required of the spacecraft during this period to keep the antenna focused on the uplink signal. Finally, changes in polarization of the signal occur in the atmosphere. All these data can be related to the refracting and absorbing properties of the atmosphere as a function of raypath penetration depth and inverted to give the temperature and pressure structure and the density of absorbing species.
Gravimetry
Mars' gravity is studied by accurate measurements of the Doppler shift of the spacecraft transmissions. The Doppler shift indicates the velocity of the spacecraft relative to the Earth along a line-of-sight (LOS) between the spacecraft and Earth. Changes in this velocity give the LOS accelerations. After removal of normal orbital, dynamical, and atmospheric effects, the remaining accelerations are due to variations in the gravity field of the planet. The gravity field is due to the mass distribution within the planet, so constraints on the internal structure of the planet can be made using this derived gravity field.