Ionospheric total electron content (TEC) data derived from dual‐frequency Global Positioning System (GPS) signals from 30 globally distributed network sites are fit to a simple ionospheric shell model, yielding a map of the ionosphere in the northern hemisphere every 12 hours during the January 1–15, 1993 period, as well as values for the satellite and receiver instrumental biases. Root‐mean‐square (RMS) residuals of 2–3 TEC units are observed over the 20°–80° latitude band. Various systematic errors affecting the TEC estimates are discussed. The capability of using these global maps to produce ionospheric calibrations for sites at which no GPS data are available is also investigated.

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Abstract.Measurements of Mars' rotational variations can be conducted via Earth-based radio tracking observations of the Mars Pathfinder lander during an extended mission. Two-way range measurements between an Earth tracking station and the lander will enable precise monitoring of the planet's orientation, allowing details of Mars' internal structure and global surface/atmosphere interactions to be determined. An analysis has been performed to investigate the accuracy with which key physical parameters of Mars can be determined using the Earth-based radio tracking measurements. Acquisition of such measurements over one Martian year should enable determination of Mars' polar moment of inertia to 1% or better, providing a strong constraint on radial density profiles (and hence on the iron content of the core and mantle) and on long-term variations of the obliquity, which influences the climate. Variations in Mars length of day and polar motion should also be detectable, and will yield information on the seasonal cycling of carbon dioxide between the atmosphere and the surface.

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To support the coming decade of Mars exploration, NASA is establishing a telecommunications relay and navigation infrastructure in Mars orbit, supporting increased science data return, providing energy-efficient relays for small scout-class mission concepts, gathering engineering telemetry during critical mission events, and providing in situ radiobased navigation. A key element of this vision is the Electra Proximity Payload, a telecommunications and navigation payload that will fly on each Mars orbiter, beginning with the 2005 Mars Reconnaissance Orbiter (MRO) and with subsequent flight on the 2009 Mars Telecommunications Orbiter (MTO). We present here the functional requirements, design characteristics, and implementation status of the Electra Payload and its role as a telecommunications node in an evolving Mars orbital infrastructure.

An orbiting spacecraft and ground observatories have been used to obtain interferometric observations of cosmic radio sources. The Tracking and Data Relay Satellite System (TDRSS) was used as the orbiting observatory in conjunction with two 64- meter radio telescopes at ground observatories, one in Australia and one in Japan. The quasars 1730-130 (NRAO 530), 1510-089, and 1741-038 were observed at a frequency of 2.3 gigahertz, and a maximum projected baseline of 1.4 earth diameters was achieved. All quasar observations for which valid data were acquired resulted in detected fringes. Many of the techniques proposed for a dedicated very long baseline interferometry observatory in space were used successfully in this experiment.

A series of three crosslink occultation experiments have been acquired between the Mars Odyssey and Mars Reconnaissance Orbiter spacecraft to probe the Martian atmosphere in 2007. While crosslink occultations between Earth‐orbiting satellites have been used to profile the Earth's atmosphere and ionosphere since 1995, this represents the first demonstration of crosslink occultation measurements at another planet. These measurements leverage the proximity link telecommunication payloads on each orbiter, which are nominally used to provide relay communication and navigation services to Mars landers and rovers. Analysis of the observed Doppler shift on each crosslink measurement reveals a clear signature of the Martian atmosphere, primarily the ionosphere. Inversion of the observed Doppler data yields vertical profiles of the Martian refractivity and electron density. The electron density profiles show the presence of two layers with peak densities and peak heights that are consistent with empirical model results. Our study demonstrates the feasibility and future potential of the crosslink radio occultation technique in the exploration of planetary atmospheres.