JUNE 20, 1965

       For 204 days the Mariner IV spacecraft has been charting the vast expanse between the planets Earth and Mars on the longest deep space mission in history.

       Since Mariner IV was launched last November 28, four American astronauts have orbited the Earth, two Ranger spacecraft have photographed the moon and dozens of other missions have begun and ended.

       Mariner must fly yet another four weeks before it passes within 6000 miles of Mars for close-up photography of the planet's surface and as many weeks beyond Mars for transmission of the pictures and other scientific data back to Earth.

       Although the spacecraft is now more than 113 million miles away, it remains in constant touch with its home base, the Jet Propulsion Laboratory in Pasadena, California, through a remarkable space communications link.

       In a sense, Mariner IV is at the end of a long, taut, invisible kite string, reeling itself out hundreds of thousands of miles each day. It's along this straight line that Mariner's radio signal travels to Earth.

       For about nine hours each day, engineers and technicians at the Goldstone Space Communication Station in Southern California have a firm grip on the imaginary length of string. One of Goldstone's 85-foot dish antennas is trained on Mariner, gathering in its faint broadcast and stands ready to transmit commands to the spacecraft should they be required.

       As the Earth turns under Mariner, the antenna moves slowly from east to west, finally losing radio contact on the horizon. Before the spacecraft gets out of earshot, another station near Canberra, Australia, picks up the string for the next nine-hour stint. From Canberra, control is handed over to Johannesburg, South Africa, and again back to Goldstone.

       Goldstone, Canberra and Johannesburg are stations of NASA's Deep Space Network, which handles communications for America's unmanned exploration of the moon, planets and interplanetary space. The stations are located approximately 120 degrees apart around the glode so that at least one antenna is in Mariner's line of sight at all times.

       Control center for the Mariner mission is headquartered in the Space Flight Operations Facility at JPL in Pasadena. It is linked to the DSN stations by a ground communications system.

       The control center has been operating on a 24-hour basis since launch day, the Friday following Thanksgiving Day, 1964. It is here that engineers and scientists have almost immediate access to Mariner's constant flow of messages--about 100,000 each day--that include measurements made by the scientific instruments and those indicating the current condition and performance of the spacecraft. It is here also that the data is analyzed and decisions are made to send commands to Mariner.

       Latest engineering event monitored in near real-time was a command issued by the Mariner's on-board central computer and sequencer to the Canopus tracker last Monday morning to compensate for the changing relationship between the spacecraft, the sun and the star Canopus. The command electronically changed the "look angle" of the tracker so that the star will stay in view through the encounter sequence next month and beyond.

       The Johannesburg station was tracking Mariner when the "update" occurred right on time at 11:40 a.m. EDT. It was about 10 minutes later that Mariner's keepers in Pasadena learned that the command was issued and acted upon properly. Because of the communications distance on that day, Mariner's radio signal needed 10 minutes to reach the big antenna in Johannesburg.

       As Mariner IV gets closer to Mars and to the critical encounter sequence, activity will heighten in the Space Flight Operations Facility and at the tracking stations. During the week prior to Mariner's July 14 planet fly-by, the three stations now tracking Mariner will follow it from horizon to horizon, increasing the viewing time of each to about 12 hours a day and hence the overlapping coverage. Three additional stations of the DSN will be "on line"--at Woomera, Australia, Madrid, Spain, and a second station at Goldstone.

       Power level of Mariner's radio signal, if measured at the spacecraft antenna, is about 10 watts. On July 14, at the point of Mariner's closest approach to Mars where the distance to Earth will be more than 134 million miles, power of the signal received at Goldstone will be, in engineering language, 10 ?-19 watt, or .0000000000000000001 of one watt. This means that the signal strength will dwindle from 10 watts at Mars to one-billionth-of-one-billionth-of-one-watt at Earth.

       Super-sensitive receivers, coupled with the big 85-foot antennas at the DSN stations are able to home in on this signal, as faint as it is, and amplify it for telemetry processing, recording and relay via the ground communications system to the control center in Pasadena.

       Before the technical personnel at any one of the tracking stations can get hold of Mariner's long kite string by "locking up" the signal with the station receiver, they must know where to point the antenna when their turn comes around. This information is supplied by the radio signal itself. As the tracking data is received at one of the stations and is relayed to the Space Flight Operations Facility, it reveals to trajectory experts its exact location in space and its velocities relative to the sun, Earth and Mars. With the aid of computers of the data processing center in the SFOF, predictions are made and transmitted to the station waiting to lock up the signal. These predictions tell the station personnel where to expect Mariner to appear in the sky as it clears the horizon and at what radio frequency to tune the receiver.

       Accurate tracking of Mariner IV is based on the Doppler shift of its radio signal, or the apparent change in frequency of the signal as the spacecraft moves farther away from Earth. Twoway Doppler, used by the DSN for tracking lunar and planetary spacecraft, utilizes a signal transmitted from the station to the spacecraft receiver-transmitter where it is converted to a new frequency in an exact ratio with the ground frequency and then retransmitted to Earth. Since the frequency of the signal sent from the ground can be determined with great precision, the resulting Doppler information and velocity calculations are very accurate.

       Transmitter power at each of the stations is 10,000 watts. The transmitter is used both for two-way Doppler tracking and for sending commands to Mariner IV. The spacecraft has received and acted upon 42 commands from Earth since launch. Others may be transmitted during the encounter phase of the mission.

       The two-way Doppler technique is the key to one of Mariner IV's planetary investigations--the occultation experiment. About an hour after the point of closest approach, the spacecraft will pass behind Mars as viewed from Earth. The Doppler effect upon the radio signal as it penetrates the Martian atmosphere will permit scientists to determine the density and scale height of the atmosphere.

       If Mariner IV is still operating at Mars, its television camera system will take and record as many as 21 black-and-white still pictures of the planet's surface for later playback to Earth beginning about 10 hours after the fly-by.

       Because of the data rate possible at the Earth-Mars distance--8 1/3 bits per second--it will take more than eight hours to transmit one picture. Each picture contains about 250,000 bits of information. It is planned to play back each picture twice, requiring nearly three weeks for return of all picture data.

       Since the picture data will be received in binary form-ones and zeros which form values representing light intensity from white to black--it is possible to receive part of a picture at Johannesburg or Madrid and another part at Goldstone, losing nothing in the transfer. The time-coded digital information can be matched at JPL where it will be converted into a photograph of the surface of Mars. The conversion process, involving computer programs and specialized equipment, may take several days.

       On April 29, 1965, Mariner IV established a new space communications distance record of 66 million miles. The mark will more than double at encounter.

       Mariner project officials do not anticipate a break in the long communications thread connecting Earth and the spacecraft for several months. It is probable that Mariner IV will continue broadcasting for a long time as it orbits the sun, but out of range of the Earth.