Tuesday, 27 May 1997
Shortly after 9:00 p.m. PDT last Saturday, operators staffing the Goldstone antenna complex in the Mojave desert announced that they had locked up on a signal transmitted from Surveyor at a data rate of 2,000 bits per second. This milestone marked the transition out of safe-mode and back to normal operating conditions. Since early in the month, Surveyor's safe-mode orientation had limited the maximum data transmission rate to 250 bits per second or less.
The spacecraft automatically entered safe-mode on the morning on Thursday, May 7th when the onboard computer encountered an infinite loop in flight software. Entry into safe mode placed Surveyor in a configuration that guaranteed adequate power, thermal, and communications margins. This mode is intended to be a benign operating state favorable for diagnostic and recovery activities if an unexpected event occurs in one or more of the spacecraft's systems.
Recovery operations involved a multi-step process that began on Friday. First, the flight team sent a series of instructions to Surveyor's backup flight computer. These instructions initialized the backup computer to begin using its normal flight software rather than the limited software set utilized in safe mode. Then, the flight team commanded the backup computer to control the spacecraft while performing the same software initialization procedure on the Surveyor's primary computer.
The next step required reestablishing the spacecraft's ability to point at targets in space. In safe mode, the flight computer assumes that its ability to find and point at targets other than the Sun has been compromised. Restoration of pointing capability involved commanding the spacecraft to rotate in a cone-shaped pattern around the Sun for several hours. This action allowed Surveyor's star scanner to lock-up on distant guide stars in space. The spacecraft determines its orientation in space by using these stars as reference points.
Pointing capability was restored early Saturday evening. At that time, the flight team commanded Surveyor to rotate from its safe-mode, Sun-pointed orientation to an Earth-pointed orientation. Aiming the spacecraft's antenna directly at the Earth enabled Surveyor to begin transmitting data using any one of its standard rates of 2,000 bits per second or faster. Early next week, the flight team will transmit modifications to Surveyor's flight software to prevent the infinite-loop condition from occurring again.
Surveyor would have been stable in safing for an indefinite period of time even if no corrective action had been taken. However, the flight team worked on restoring standard operations as quickly as possible because normal command sequences, such as those that control science calibration activities, are prohibited from executing in safe mode.
After a mission elapsed time of 201 days from launch, Surveyor is 124.64 million kilometers from the Earth, 27.15 million kilometers from Mars, and is moving in an orbit around the Sun with a velocity of 22.89 kilometers per second. This orbit will intercept Mars 107 days from now, slightly after 6:00 p.m. PDT on September 11th (01:00 UTC, September 12th). All systems continue to be in excellent condition.
Status report prepared by:
Office of the Flight Operations Manager
Mars Surveyor Operations Project
NASA Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109
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