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TOPEX/Poseidon: Perspectives on an Ocean Planet

- Zenith Omni Antenna

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The zenith omni antenna is a short, fixed, low-gain antenna mounted on the zenith side of the Instrument Module. It served as part of the primary communication link from the spacecraft to the TDRSS until the high gain antenna was deployed 22 hours after launch. Currently it is used during emergencies and for some command, telemetry, and tracking support.

Because the zenith omni antenna is mounted on top of TOPEX/Poseidon, it can only be used to communicate with satellites maintained in higher orbits, in this case the TDRSS satellites, and not with ground stations. To communicate with ground stations directly, TOPEX/Poseidon must use the nadir omni antenna.