Mission period: 28 november 1964 - 07 december UT 1964

Canopus search, acquisition and reacquisition

Attitude control

    The spacecraft, fully stabilized in attitude, used the Sun and the star Canopus as references. Cold-gas jets pointed the spacecraft in all three axes (pitch, yaw, and roll), and external torques were counteracted in two of the axes by changing the positions of movable solar pressure vanes. Gyroscopes were available for initial acquisition and for inertial control during the midcourse maneuver.

    The roll, or Z axis is a reference line paralell to the low-gain antenna mast, through the center of the spacecraft. "Roll motion" referes to a spin about this axis. Spinning motion can be used in terms of rotation about the Z axis, but description of "rocking" motion necessitates the use of two more coordinate axes. Spin about the Y axis is termed "yaw" motion, about the X axis "pitch" motion.


Canopus sensor

The Canopus sensor produced, when a star was in its field of view (11° in cone and 4° in clock), error signals proportional to the angular position displacement of the star from the spacecraft roll axis. An electronic logic was set to respond to any object more than one-eighth as bright as Canopus. Including Canopus, there were seven such objects visible to the Canopus sensor as the spacecraft swung around in its search mode. A Canopus gate was used to switch the roll-axis control system to the acquisition mode.


It was anticipated that star identification would be a major problem since the only information from the Canopus sensor, other than the error signal, was a brightness measurement and the cone angle of an object within ± 5°. Therefore, a map-matching technique was developed to identify objects seen during the roll search mode. Fundamental to the plan was an a priori telemetry-type map of the expected sensor brightness output as a function of clock angle. A mathematical model of the Canopus sensor and the sky, including the Milky Way, was developed so that, with available trajectory information, an IBM 7094 computer could be programmed to print, before launch, a map of the expected telemetry output of the brightness channel during roll search. This then could be matched with an actual telemetry map to identify observed objects. Initially the actual telemetry map produced by a complex computer program after receipt of the telemetry was statistically correlated with the a priori map and other data available from the spacecraft. Whenever an object was acquired after a roll search, the probability that each acquirable object had been acquired was calculated. If the object were not Canopus, another roll search was instituted and another computer run made until Canopus could be identified as the object acquired. This procedure was necessary because of the lack of knowledge concerning the absolute calibration of the Canopus sensor for all stars and the integrated background of stars in the field seen by the sensor.

Mars - Mariner 1964 Final Project Report

Canopus search

The CC&S command to initiate acquisition of the star Canopus (L-3 event) occurred on November 29, with the acquisition sequence beginning at a clock angle of 60°. At a clock angle of 119° (in the vicinity of the star Markab), an acquirable object entered the Canopus sensor field of view and was acquired. Telemetry indicated immediately that this object was not the star Canopus. (Subsequent analysis revealed that it was probably a cluster of stars whose brightness was augmented by earth light reflected into the sensor optics.) Later on November 29, the spacecraft was in a roll search in its automatic reacquisition mode. Another acquirable object entered the sensor field of view, but the brightness of this object was shown by telemetry to be approximately one-quarter of the expected Canopus brightness. It was decided to proceed with Canopus acquisition by means of ground commands the following day.


Canopus acquisition

The first such roll override command (DC-21) was transmitted on November 30, and the spacecraft went into a roll search. After nearly 60° of roll, another star was acquired. Comparison of the data received with the values expected resulted in the first positive indication of the spacecraft roll orientation between the star Regulus and the star Naos. A second command (DC-21) moved the roll reference to the star Gamma Velorum. The final roll override command (DC-21) resulted in Canopus acquisition.


Command: DC-21
Function : Roll override, negative increment
Effect : Simulates Canopus acquisition logic violation. Turns on gyros. Applies negative roll search signal to roll gas jet electronics. Causes spacecraft to begin counterclockwise roll search to acquire a new target.

Engineering data Channels

Channel Measurement Units DN = 0 DN = 60 DN = 126
108 Canopus sensor output Volts 0.0 1.41 2.95
107 gyros on: roll gyro rate
gyros off: Earth sensor output		 mr/sec
millifootcandle		 +16.8
-27.8		 +1.05
-2.63		 -16.3
+41.4
114 gyros on: roll search
gyros off: roll position		 mr/sec
mr		 -
+26.2		 -
+1.65		 -
-25.4
105 gyros on: pitch gyro roll rate
gyros off: pitch Sun sensor		 mr/sec
mr*		 +19.5
+16.1		 +1.21
+0.66		 -18.5
-14.8
112 pitch position mr* 30 DN = +50 60 DN = +10 100 DN = -175
106 gyros on: yaw gyro roll rate
gyros off: yaw Sun sensor		 mr/sec
mr*		 +20.1
+15.5		 +1.25
+0.42		 -19.5
-14.0
113 yaw position mr* 30 DN = +60 60 DN = +10 100 DN = -175
An asterisk(*) in the unit column indicates a grossly nonlinear conversion from Data Number to Engineering Unit. Approximate values for 3 DN are given where applicable.

Engineering Document No. 338 by William L. Momsen

Critical event and times of the Mariner IV mission


Table from "Mars-Mariner 1964 Final Project Report"

Last Updated: 2002.07.01