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Radar Cross-Section Observations of the Echo I Communications Satellite

UNCLASSIFIED

AD 274,065

Reproduced by the Armed Services Technical Information Agency

Arlington Hall Station

Arlington, Virginia

UNCLASSIFIED

NOTICE: When government or other drawings, specifications or other data are used for any purpose other than in connection with a definitely related government procurement operation, the U. S. Government thereby incurs no responsibility, nor any obligation whatsoever; and the fact that the Government may have formulated, furnished, or in any way supplied the said drawings, specifications, or other data is not to be regarded by implication or otherwise as in any manner licensing the holder or any other person or corporation, or conveying any rights or permission to manufacture, use or sell any patented invention that may in any way be related thereto.

Massachusetts Institute of Technology

Lincoln Laboratory

AFESJ - TDR - 62 72

While performing searches for various engineering topics, it is often difficult to locate the desired material. That is because many documents are either buried deep within the results, or they are never indexed by the search engines in the first place. Government websites are good sources of information, but it typically takes doing multiple hit-and-miss searches within the website's database to find information. As time permits, I have been re-publishing documents that I find so that the search engines will index them.

Unless otherwise marked, U.S. government documents may be freely copied so long as the content is not altered from the original. Warning: Some of these documents have been processed with optical character recognition (OCR) software and might contain errors.

Building 821: Radar Test Building

Development of a Radar/SAR Assimilation System for
    Internal Wave Prediction
Early U.S. Navy Experimental Radars

Improved Doppler Radar/Satellite Data Assimilation

• Learning Morse Code Characters: A Replication of
   the Keller Method

Meteorological Studies with the Phased Array Weather
    Radar and Data Assimilation Using the Ensemble
    Kalman Filter

Pearl Harbor Revisited: U.S. Navy Communications
    Intelligence, 1924-1941

Radar Cross-Section Observations of the Echo I
    Communications Satellite

Radio and Radar World War I & World War II,
    U.S. Army Signal Corps

What Is Radar?

Abstract

This report contains radar cross-section measurements conducted on Echo I (60 Iota 1), a passive communications satellite, from the first revolution on 12 August 1960 to revolution 5159 on 6 October 1961. Tracks on the early revolutions, up to about 92, generally displayed a fairly constant amplitude, indicating little deformation of the spherical shape of the balloon. The smoothest observations noted had approximately ±0.5 db (peak-to-peak) fading variations over the duration of track. Later recordings show amplitude patterns with various mounts and rates of fading. Fast scintillations, with a period of the order of 0.5 - 1 second, can be seen during certain runs as well as slow long-term fading during others.

Beginning with about revolution 1944 (18 January 1961), a gradual deterioration in the shape of the balloon wan indicated by frequent deep fades and a generally rough amplitude pattern. Fades of ±4 db can be seen throughout most of these rum, with the most severe fades (>20 db in depth) occurring during revolution 5159, the last covered in this report.

The average cross sections measured were comparable to the theoretical value of about T50 square meters, with the average becoming somewhat less during the later revolutions when fluctuations were more pronounced.

Amplitude records for fourteen of the observed passes are included. A summary is also included for a voice communications experiment conducted between Millstone and the Prince Albert Radar Laboratory during revolution 12.

Radar Cross-Section Observations
Of The Echo I Commutations Satellite
at 440 Mc/s
D. P. Hynek
Lincoln Laboratory, Massachusetts Institute of Technology

 

Introduction

Since the launch of the Echo I communications satellite much interest ban been expressed In the degree to which the 100-foot diameter balloon maintained its spherical. shape. Several questions have been raised as to the possible distortion of its shape In a space environment. Of chief concern is the puncturing of the thin skin by micrometeorites and the effect of thermal shock upon entering the earth's shadow. Some insight into the amount of distortion can be obtained by observing the fading characteristics of reflected radar signals. This report presents amplitude data recorded on a number of revolutions during Its first fourteen months in orbit.

Radar Parameters

Table I

Pertinent parameters of the Millstone Kill Radar during these observations were as shown in Table I.

Frequency: 440 Mcs
Antenna: 84-ft paraboloid., 35-db tracking gain
Peak transmitted power: 1.25 to 2.5 megawatts
Pulse repetition frequency: 28 or 30 pps
Pulse length: 2 milliseconds
Transmitted polarization: right circular
Received polarization: left circular
Conical scan rate: 7.5 rev/sec, (approx.)
Receiver noise figure: 2 db (approx.)
System losses: 2 db (approx.)

Data Recording

For recoding purposes the IF signal was range-gated and passed through a logarithmic amplifier. In the majority of cases, multiple data were recoded on 35-mm strip film by photographing the "x"-axis of an oscilloscope to which the detected signal had been applied. The film travel speed was l14.8 inches per minute. This speed allows sufficient space on the film for sequential pulses to be distinguished. The horizontal lines appearing an the film are range marks but can also be used as grid lines in estimating relative signal strength from. the calibration.

Several runs also appear on a pen-recorded strip chart. In these cases the digital output of an amplitude sampler was reestimated as an analog signal for recording.

Averaged target and doppler data print-outs were obtained from a real-time digital sampler which is part of the Millstone Radar system

[missing]

In all, nearly 100 revolutions of Echo I were tracked by the Millstone Radar from revolution 01 (12 August 1960) through revolution 5199 (6 October 1961). In many cases the satellite was tracked from horizon t horizon. Amplitude data were recorded on about 35 of those.

 The passes for which the amplitude are available are listed in Table II, along with the average cross sections and fading characteristics. The average cross section given were calculated by averaging several representative regions throughout the run. The values listed in Table II should be accurate to within ±2 db. Amplitude recordings of a selected number of runs appear in Figs. 1 through 14. Cross-section samples at various points during each run are shown directly in the recordings.

 

RF Cafe - Table IIa - Radar Cross-Section Observations of the Echo I Communications Satellite

RF Cafe - Table IIb - Radar Cross-Section Observations of the Echo I Communications Satellite

* The theoretical radar cross section is approximately 70 square metes.

Of more interest perhaps than the average cross-section measurements are the fading characteristics which are indicative of the balloon deformation. Early revolutions recorded, up to number 92, displayed smooth amplitude pattern with signal variations of the order of ±1 db. Right-circular polarization was transmitted and left-circular recorded. A spot check from time to time, during these early passes, of the relative signal strength of the two orthogonal received polarizations indicated a ratio of about 20 db. The small magnitude of the depolarized component indicated that the balloon had fully inflated and maintained its sphericity to a high degree during this time (and, incidentally, also served as a check on the circularity of the transmitted signal). Figures 1 through 4 (revolutions 01, 09, 11 and 45) are representative amplitude recordings of this early group of passes.

During revolution 01 rapid scintillations, at a rate of 3 or 4 per second, can be seen at 06h 5Ta l0s and may arise from interference with the final stage of the launching rocket which probably was still within the radar beam at this time. This effect can be seen to a certain extent at other times during the run. Occasionally a regular variation is seen every four pulses. This variation is due to the conical-scan modulation of the radar and is most apparent at time of acquisition.

Revolution 09 displayed the most fading (±1.5 db) of the early passes and also appears to contain reflected ray interference, with signal enhancement and cancellation, new the horizon. This noticeable near the beginning and end of track. It in also evident to a degree during other runs.

Revolution 11 contains the smoothest amplitude record made. It was tracked for about 19 minutes with fading of only ±0.5 db.

Following revolution 92, awe fading was observed (±1.5 - ±3 db), with the ratio of the two orthogonal received signals at times approaching 6 db.

Fading was generally slow, occurring at irregular intervals, until revolution 156 when a periodicity of 0.5 - 1 second became apparent. It is interesting to note that revolution 156 occurred soon after the satellite first entered the earth's shadow. This rapid scintillation was last observed during revolution 167 (Fig. 5) when sharp nulls (4-5 db) were recorded. The short-term fading was not seen during nine passes tracked between revolutions 179 (Fig. 6) a 636 (Fig. 7). Instead, long term, irregular variations of ±1.5 - ±3 db were observed.

During the two revolutions 941 and 942, a faster fading rate, of the order of one second, was again observed and was mot pronounced during the first half of the observation of revolution 912 (Fig. 8). This short-term fading was not observed during the last half of this track but instead gave way to a slow rolling amplitude pattern.

This change in fading characteristic, over the duration of a single track, suggests a relatively soother surface for a portion of the balloon. A very pronounced ground effect can be seen again at the and of this track.

Amplitude data were not available between revolutions 942 and 1448.

Four tracks conducted between revolutions 1448 and 1862 indicate a smoother target with a longer fading interval and somewhat shallower depth (±1.5 db). No short-term scintillations were observed during this time. Figure 9 (revolution 1862) is typical of these runs.

On ten tracks taken between revolutions 1944 and 5159, a gradual deterioration of the balloon was indicated by occasional very deep fades and a generally rough amplitude pattern. (Revolution 1944, 3147, 3941, 4268 and 5159 appear in Figs. 10 through 14) Fades of the order of 10 db can be seen throughout most of the runs, with the most severe fading recorded during revolution 5159, the last covered in this report. During this revolution two deep fades (>20 db) were observed having similar characteristics and occurring 6 min 40 sec apart. The track was not long enough to determine whether the effect was repeated. The cross section at the depth of the fades was of the order of 10 square meters.

These observations are summarized in the approximate groupings shown in Table III.

In the Appendix is a summary of a UHF communications experiment conducted during revolution 12 between Millstone and the Prince Albert Radar Laboratory.

Computer print-outs of position and doppler observations are not included in this report.

 

Conclusion

The series of radar observations conducted on Echo I suggest that the balloon was full inflated soon after launch and maintained its spherical shape to a reasonable degree until approximately the time it first entered the earth's shadow. The rapid and regular fading observed soon afterward w have resulted from the interference between reflections from the individual gores making up the structure. This effect might have been accentuated by shrink of the balloon following los in internal pressure, giving it a pumpkin-like appearance.

The irregular and more pronounced fading after this time suggests a further deterioration of the general shape of the balloon with, however, either a modest recovery to sphericity or a relatively smooth surface for only a portion of the balloon, in observations following this. The change in fading pattern, from rapid short-term fluctuations to smoother long-term fading, during the track of revolution 942 would suggest a slow rotation to a smoother portion of the balloon. 

RF Cafe - Table III - Radar Cross-Section Observations of the Echo I Communications Satellite

Sine the geometry explored, during the course of an any is limited and, in the face of an unknown rotation rate and orientation, it is difficult to say whether the changes noted in the fading pattern from run to run are characteristic of the condition of the balloon or of the geometry.

Beginning with the observation of revolution 1944, a steady degradation of the balloon is evident with, perhaps, multiple punctures taking their toll. The last revolution included in this report (number 5159, 6 October 1961) points to a grossly distorted structure.

ECHO I Amplitude
Revolution 01
12 August 1960

[All recordings redacted]

ECHO I Amplitude
Revolution 09
12 August 1960

[All recordings redacted]

ECHO I Amplitude
Revolution 11
13 August 1960

[All recordings redacted]

ECHO I Amplitude
Revolution 45
15 August 1960

[All recordings redacted]

ECHO I Amplitude
Revolution 167
25 August 1960

[All recordings redacted]

ECHO I Amplitude
Revolution 179
16 August 1960

[All recordings redacted]

ECHO I Amplitude
Revolution 636
3 October 1960

[All recordings redacted]

ECHO I Amplitude
Revolution 942
28 October 1960

[All recordings redacted]

ECHO I Amplitude
Revolution 1862
11 January 1961

RF Cafe - Chart 1a - Radar Cross-Section Observations of the Echo I Communications Satellite

ECHO I Amplitude
Revolution 1944
18 January 1961

RF Cafe - Chart 2a - Radar Cross-Section Observations of the Echo I Communications Satellite

RF Cafe - Chart 2b - Radar Cross-Section Observations of the Echo I Communications Satellite

ECHO I Amplitude
Revolution 3147
25 April 1961

RF Cafe - Chart 3a - Radar Cross-Section Observations of the Echo I Communications Satellite

RF Cafe - Chart 3b - Radar Cross-Section Observations of the Echo I Communications Satellite

ECHO I Amplitude
Revolution 3941
29 June 1961

RF Cafe - Chart 4a - Radar Cross-Section Observations of the Echo I Communications Satellite

RF Cafe - Chart 4b - Radar Cross-Section Observations of the Echo I Communications Satellite

ECHO I Amplitude
Revolution 4268
25 July 1961

RF Cafe - Chart 5a - Radar Cross-Section Observations of the Echo I Communications Satellite

ECHO I Amplitude
Revolution 5159
6 October 1961

RF Cafe - Chart 6a - Radar Cross-Section Observations of the Echo I Communications Satellite

RF Cafe - Chart 6b - Radar Cross-Section Observations of the Echo I Communications Satellite

APPENDIX

Millstone-PARL UHF Communications Experiment*

During revolution 12 (13 August 1960) of the Echo I satellite, a communications experiment was carried out at 440 Mcs between the Millstone Hill Radar and the Prince Albert Radar Laboratory (PARL), Saskatchewan, Canada.

A pre-taped voice message was transmitted from Millstone via the satellite to PARL for a duration of some ten minutes using narrow-band FM. During the first portion of the pass, the Millstone dish was manually trained using predicted position data from Space Track as corrected by Millstone tracking data gathered on revolution 11. The last portion of the track was effected with an optical antenna director controlling the dish. PARL first acquired the target with its radar at 448 Mcs, and then automatically tracked on the Millstone communication signal.

During revolution 11, and earlier tracks, it was ascertained that the balloon had maintained its spherical shape to a very high degree and would be a good passive reflector for the experiment.

The quality of the received signal at PARL was good and possessed the expected signal-to-noise ratio (approximately 16 db under ideal conditions). The chronological sequence of events and signal strengths logged for this experiment are as shown on the following page.

*Data provided by D. R. Hansen, PARL.

RF Cafe -  - Radar Cross-Section Observations of the Echo I Communications Satellite

RF Cafe -  - Radar Cross-Section Observations of the Echo I Communications Satellite

 

RF Cafe -  - Radar Cross-Section Observations of the Echo I Communications Satellite

*Millstone-type antenna

 

See the original Radar Cross-Section Observations of the Echo I Communications Satellite document here.

 
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