First, the rover had to make a "phone call" to Earth to correct for drift - changes in time - in the spacecraft clock.

When Spirit phones home using a direct-to-Earth, X-band communications link, the rover's transmitter has to be running, which requires a fair amount of energy.

During more typical, direct-from-Earth communications, only the rover's receiver has to be on. To set the spacecraft clock, Spirit transmits a data product called a time packet.

The time packet is used to synchronize the rover's clock back to Earth time (also known as Universal Time). A previous attempt to relay the time packet was unsuccessful, causing Spirit's clock to be off by as much as a minute and a half.

In addition to resetting the clock, Spirit completed a light schedule of activities on sols 1493-1494 (March 15-16, 2008). By sol 1496 (March 18, 2008) , Spirit had generated enough solar power to proceed with measurements of argon gas in the Martian atmosphere and studies of the Freeman rock target.

Interspersed with those activities, Spirit continued to work on the "Bonestell panorama" and take panoramic-camera images of a target dubbed "C.S. Lewis." The rover spent sols 1492, 1494, and 1497 (March 14, 16, and 19, 2008) recharging the battery, conducting only minimal science activities, and storing data for later transmission to Earth.

Spirit continued to have difficulty receiving spacecraft commands via the rover's high-gain, X-band, dish antenna as a result of the mast that holds the panoramic and navigation cameras getting in the way and partially obscuring the signal.

To help address this challenge, rover planners had Spirit complete a self-assessment to see if the rover could independently recognize an occlusion of the high-gain signal and respond by swiveling the high-gain antenna to a different position. The self-assessment, on sol 1493 (March 15, 2008), was successful.

Spirit used the technique prior to an actual uplink session on sol 1496 (March 18, 2008), when the rover's handlers were expecting a particularly severe occlusion. The activity was successful and the uplink did not appear to be impeded in any way. Currently, this activity involves having the rover use a temporary parameter that then goes away when the rover shuts down for a nap. Rover planners are considering making the temporary parameter permanent.

Looking forward, Spirit will go increasingly into "hibernate" mode as the Sun continues to dim. Rover planners predict Spirit will be able to conduct science activities until about late April. Spirit is healthy and all subsystems are performing as expected. The latest available power readings from sol 1496 (March 18, 2008) showed power at 249 watt-hours (100 watt-hours is the amount of energy needed to light a 100-watt bulb for one hour).

Spirit has no plans to move before the next Martian spring and is hard at work accomplishing as much as possible before power levels drop to a point that temporarily precludes use of the scientific instruments on the rover's arm.

Sol-by-sol summary:

To conserve energy, mission planners have restricted the number of sols on which Spirit receives direct-from-Earth instructions via the rover's high-gain antenna and transmits data to Earth via the Odyssey orbiter.

Spirit continues, on a daily basis, to monitor atmospheric dust levels with the panoramic camera, check for drift (changes with time) in the miniature thermal emission spectrometer, and survey the sky and ground with the instrument. In addition, during the past week, Spirit completed the following activities:

Sol 1491 (March 13, 2008): After communicating with Odyssey, Spirit studied the elemental composition of "Wendell Pruitt" with the alpha-particle X-ray spectrometer.

Sol 1492: In addition to monitoring atmospheric dust and conducting surveys with the miniature thermal emission spectrometer, Spirit recharged the battery.

Sol 1493: Spirit initiated a direct-to-Earth communications link using the X-band antenna and transmitted a data packet to correct the spacecraft clock.

Sol 1494: In addition to monitoring atmospheric dust and conducting surveys with the miniature thermal emission spectrometer, Spirit recharged the battery.

Sol 1495: In the morning, Spirit acquired column 3, part 1 of the full-color Bonestell panorama, using all 13 filters of the panoramic camera. Spirit positioned the alpha-particle X-ray spectrometer to measure argon gas in the Martian atmosphere. The rover took a single-frame image with the navigation camera. After communicating with Odyssey, Spirit measured argon with the alpha-particle X-ray spectrometer.

Sol 1496: Spirit monitored dust accumulation on the rover mast and acquired column 3, part 3 of the full-color Bonestell panorama. The rover acquired full-color images, using all 13 filters of the panoramic camera, of the Freeman rock target.

Sol 1497 (March 19, 2008): Spirit looked for changes in the "El Dorado" dune field with the panoramic camera and acquired column 4, part 1 of the Bonestell panorama. The rover recharged the battery. The following morning, Spirit was to acquire movie frames in search of clouds with the navigation camera, acquire super-resolution, panoramic-camera images of a target dubbed "C.S. Lewis half," and survey the horizon with the panoramic camera.

Odometry: As of sol 1496 (March 18, 2008), Spirit's total odometry was 7,528 meters (almost 4.7 miles).

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