number of stations may use the reflector simultaneously—and the reliability of a passive reflector satellite is inherently good because it has no electronics to fail.

The experimental flight programs described have demonstrated the technical feasibility of employing satellites for intercontinental communications. However, this is not the only objective of our communications satellite program. Not only do the currently available systems have many technical shortcomings, but there are other potential uses for communications satellites which have not begun to be explored. Examples of such areas include use of higher frequencies to extend the frontiers of the useful frequency spectrum and to minimize interference problems; investigation of highpowered satellites which would permit use of very small ground facilities which might be carried on ships, emergency mobile land units, and aircraft (this capability could result in an application of communications satellites to air and sea traffic control procedures); incorporation into stationary communications satellites of additional equipment to provide for other observations in the national interest such as scientific experiments or meteorological sensors; and investigation of high-powered broadcasting satellites which could broadcast radio or television programs directly to all the home receivers in a very large area on the Earth.

Toward these ends, NASA conducts an extensive program of supporting research and advanced technical development, and here is where the resources, imagination, and talents of private industry and universities can be applied to good advantage.

The general objective of this supporting research and development program is to provide the foundation for new systems having substantially greater capabilities than those now attainable.

We will aim for intermediate-altitude active repeater satellites with increased communications capabilities, multiple access, a passive or semipassive control system for orienting the satellite toward the Earth, and other improve

ments.

In the area of synchronous satellites, we hope to achieve greater orbital stability, antennas with greater directivity, multiple access, longer orbital lifetimes, improved systems for stationkeeping and attitude control, and systems providing more onboard power for the satellite electronics.

For passive reflector satellites, we plan research and development of shapes and structures having substantially greater radio reflectivity for a given size, and having less weight per unit area, than the current passive reflectors.

This supporting research and technology program will not necessarily result in flight programs of complete spacecraft. It will consist of studies and investigations of components and techniques-antennas, transmitters, receivers, power supplies, new modulation schemes, and new and better techniques for maintaining satellites in a predetermined position relative to the Earth. Last, but by no means least, will be efforts to improve the reliability and thus the useful lifetime of satellite systems. Individual components must have extremely long life; subsystems must be as simple and straightforward as possible, consistent with the function to be performed; and systems must be analyzed and designed to minimize the consequences of a failure when one does occur.

These programs are discussed in more detail as follows:

METEOROLOGICAL SATELLITES

A summary of our objectives is given as follows:

(1) Develop an operational meteorological satellite system

(2) Develop and apply space technology to satisfy meteorological observational requirements

(3) Develop meteorological satellite and sounding rocket systems

(4) Maintain a continuing program of modification and improvement.

We are currently engaged, together with the U.S. Weather Bureau, in the development of an operational meteorological satellite system. This system is to be used by the Weather Bureau in the fulfillment of its service responsibilities. Our objective is to develop various aspects of space technology and integrate this technology into appropriate systems which will provide the meteorological observations required by both the operational and the research meteorologists. In more specific terms, our objective is the development of meteorological satellite and sounding rocket systems. Finally, it is our objective to maintain a continuing program for modifying and improving the developed systems.

Table 9-I shows some of the characteristics of the six Tiros satellites that we have already launched successfully. Tiros V, now operating on one camera, has been providing usable data for more than 10 months. Tiros VI, also operating with one camera, has completed over 7 months of successful functioning. Tiros has severe limitations from the point of view of data coverage. Briefly, they are that, as shown on the left side of figure 9-12, since Tiros is space oriented it does not see the Earth continuously, and it is also limited in coverage by the angle of its orbital inclination. On the other hand, Nimbus will give more extensive coverage for, as shown on the right side of the figure, it is Earth oriented and in a near-polar orbit.

TABLE 9-I.-Tiros Satellites

The hardware elements of our flight program are ilustrated in figure 9-11. In the lower left corner is a representation of the small and large sounding rockets which are used; then Tiros, the spin-stabilized satellite which has demonstrated the capabilities of satellite technology in meteorological flight systems, and Nimbus, which incorporates as many as possible of the techniques now available to us to improve the quality of the satellite systems, are shown. In the upper right is a synchronous satellite orbiting at an altitude of 23,300 miles. Such a spacecraft, now under study, should make it possible to observe the weather conditions continuously over a selected segment of the Earth's surface. These programs are discussed in more detail as follows:

Table 9-II is a compilation of some of the operational support that Tiros has given. This type of support has, of course, proved extremely valuable to the Weather Bureau and the DOD

712

300

10

21

Typhoons observed and tracked__

The Tiros project is on schedule in all of its phases. We now have relative freedom to schedule the next few launches, not on the basis of spacecraft availability, but on the basis of the most favorable time with regard to meteorological coverage. The present NASA program

In

calls for five additional Tiros launches. order to insure the availability of operational data until the next family of satellites-Nimbus-is able to provide observations on a regular basis, the Weather Bureau has scheduled two additional launches in the Tiros series. These are referred to as the "operational" Tiros satellites. In view of the continuing excellent performance of Tiros V and VI, our original launch date for the next Tiros has been intentionally delayed twice. While Tiros V and VI are providing data in one part of the globe, the next satellite will provide data in another. This additional coverage will increase the frequency of observation over the South Atlantic during the hurricane season and thus assist in maintaining a continuous vigil over the area where these severe storms are born.

As was the case with all the previous R&D Tiros satellites, the planned additional five Tiros satellites will be able to provide data for operational use in current analysis and forecasting while at the same time executing their R&D missions. Some of these missions are listed as follows:

(1) 15-micron radiometer to assist in development of Nimbus horizon scanner (2) Automatic picture transmission for test purposes

FIGURE 9-13.-Automatic picture transmission (APT).

system is designed to transmit continuously pictures of local cloud conditions to weather stations near the track of the satellite. Weather satellite cloud pictures are usually transmitted to elaborate Command and Data Acquisition stations at a relatively rapid rate. This requires the use of wide radio bandwidths and large antennas. In the APT system we use slow transmission rates and thus permit the use of small bandwidths and a relatively small antenna. The picture is snapped in a fraction of a second. The rest of the 3-minute cycle time is used to slowly scan the picture electronically and transmit it to the ground station. At the ground station, the console is used to point the antenna towards the satellite; to receive the picture; and to display it on a facsimile recorder. Each picture will cover an area a bit larger than the Tiros pictures. The ground equipment costs less than $50,000 per set and so will ultimately permit use by many major U.S. and foreign weather stations.

The Automatic Picture Transmission System was designed for the Nimbus satellite, but a flight test on Tiros will permit checkout of the

system using a number of ground stations. When used later with Nimbus, APT will enable ground stations to obtain direct cloud cover pictures of their local area.

The Nimbus satellite is shown in figure 9-14. In Nimbus we are doing much more than merely improving the sensors. In essence, Nimbus represents a major step forward over Tiros in these five elements:

(1) Orientation.-Only during a very small portion of its orbit does Tiros look straight down. The oblique angle of the majority of pictures causes difficulty in rectifying the pictures and in interpreting the cloud patterns due to the changing scale. Nimbus will view the Earth vertically during its entire lifetime.

(2) Coverage.-Tiros provides somewhere between 10 and 25 percent of the global cloud cover per day. Meterological requirements are for total global observations. Nimbus is to be launched in a near polar orbit and is to view every portion of the Earth every day.

(3) Direct local readout.-The automatic picture transmission subsystem developed for Nimbus has already been discussed. Eventually, this system may represent one of the greatest contributions of the meteorological satellite program to local forecasting.

(4) Lifetime.-An operational system must have a reasonably long life. The Tiros lifetime was estimated to be about 3 months. In general, we have exceeded this. An operational system should have a lifetime of at least 6 months and eventually 1 year or more. In ad

dition to carefully engineered systems and reliable quality control in manufacturing, design redundance plays a major role in achieving useful long life. Due to weight limitations imposed on it, the first Nimbus will be launched without this redundancy. However, the future developed operational spacecraft will include redundant systems.

(5) Growth Potential.-One of the more important elements of Nimbus is its potential for growth. Cloud pictures and the infrared radiation measurements satisfy the operational meteorological requirements only in part. Other requirements for operational systems include the need for instrumentation to provide many more kinds of observations. Thus, it is important that a meteorological satellite have the flexibility to accept without too much difficulty new instruments when developed. Nimbus is designed to do just that.

Each one of these five elements represents a major advance over Tiros. Nimbus, including all these, represents an extremely large step forward beyond the Tiros capability.

The prototypes of the various subsystems have been tested successfully. It remains now to combine the developed Nimbus subsystems and to check them out together as operating prototype and flight system. When the spacecraft passes the testing procedures, we will be ready to launch Nimbus and we estimate this to be before the end of this year.

The data acquisition hardware development has also been moving forward. Figure 9-15 is a photograph of the Command and Data Acquisition Station in Fairbanks, Alaska. An

FIGURE 9-14.-Nimbus spacecraft.

FIGURE 9-15.-Command and Data Acquisition Station,

Fairbanks, Alaska.

agreement has been signed with the Canadian government for the establishment of a similar antenna installation in the northern part of Nova Scotia. The combination of the Alaskan and Nova Scotian stations will give us coverage of practically every orbit of Nimbus. One antenna also backs up the other antenna to a sufficient extent to maintain reasonable service, should one station be temporarily inoperative.

Sounding Rockets

We are also concerned with the development of sounding rockets for the exploration and measurement of the atmosphere in the region accessible to neither satellites nor balloon-borne instruments.

Experimental sounding rockets as applied to the region of 20 to 40 miles have revealed the potential value of systematic study of this atmospheric region by means of a network of sounding rockets. We are engaged in developing a small meteorological rocket sounding system for this purpose.

The development requirements for the system are rather stringent. The motor must be reliable. It must be launched at a specific time under a variety of adverse weather conditions and its flight path must be reliably predicted; that is, there must be a small impact area. The wind sensor should accurately respond to the vertical variations of the wind rather than have an integrated response over a large vertical interval. The sensors of temperature, density, and/or pressure should provide accurate and as direct as possible measurements of ambient atmospheric conditions with minimum reaction to outside influences.

To permit use in a network, the data acquisition component should be a self-sufficient unit independent of the present range support required. The data reduction components of the system should permit rapid conversion of the telemetry records into the necessary meteorological units to allow quick dissemination of the data for analysis and utilization.

Our work with large sounding rocket systems involves techniques which extend our knowledge of the atmosphere to altitudes up to about 100 miles. The experiments are distributed throughout the seasons of the year, and at several locations to sample the geographical and

The first item in the figure indicates our continuing effort toward a system capability for continuous monitoring of atmospheric events particularly for the tracking of short-lived storms. During the current fiscal year, contract and in-house activities are being conducted to uncover and evaluate the problems associated with the subsystem developments required to provide this kind of a capability. The results of these studies will be applied in the initiation of the development of preprototype hardware requiring long leadtimes.

A second item is the dielectric (or electrostatic) tape system development. The dielectric tape camera is a technique whereby the image which is normally formed in the TV camera of our meteorological satellites is formed not on a fixed screen within a vacuum tube, but on a piece of plastic tape which has been appropriately coated to be photoresponsive. This is in contrast to the TV camera unit where we would