pro 

The Use of Active Microwave Sensors 

in Search and Rescuc Operations 

July 1986 ~~ ~ 

Prepared for: 

NASA Herrdy:ra:rters 

Office of Space Science ApylicaLions 

Cede EC, Jd:n Kieblelr 

Washington, D.C. 2,0546 

Contrsct No. NASW - 3973 

PrEpareO by Tom Tillotson 

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,' {bASA-CE-180346) !XBE USE CF ACTIVE 

BICBOYAVE SEHSOPS IN SEARCH A&D BESCUE 

C€ERAIIC%S (Syrtcnatics General Corp.) 40 

F Av6i1: NTIS HC A03/8P A01 CSCL Y4B Unclas 

G3/32 0063936 

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SYSTEMATICS GENERAL CORPORATION 

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COMMUNICATW,AND FLECTPC$!!CS DNk;!a 1636 Ofd Cx Road 

7:;. Bim 28 

Sterling, Virginia ?%170 

(703) +7 1 -23 30 

187-27832 
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The Use of Active Microwave Sensors 

in Search and Rescue Operations 

July 1986 

Prepared for: 

NASA Headquarters 

Office of Space Science Applications 

Code EC, John Kiebler 

Washington, D.C. 20546 

Contract No. NASW - 3973 

Prepared by Tom Tillotson 

SYSTEMATICS GENERAL CORPORATION 

~IcATK)NsANDaEcTRoMcsDMsloN 1606 Old Ox Road 

P.O. Box 28 

Sterling, Virginia 221 70 

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ABSTRACT 

This report studies the possibility of utiliking low-orbit active microwave sensors in Search And 

Rescue (SAR) operations to detect and positionally locate scenes of distress. Particularly this 

report addresses the use of a scanning type system, i.e. rotating a highly directional antenna as 

opposed to a more complex synthetic aperture type system. Targets for this system will be passive 

reflectors, not active tranmitters like EpIRBs; reflectors which could be easily stowed, require little 

to no maintenance, could be quickly erected and can be more affordable than battery powered 

alternatives. Presented are parametric analyses to determine the required antenna gains and reflector 

areas for various frequencies and orbit altitudes. Also, major issues are raised as well as indicating 

areas requiring further study. 
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The Use of Active Sensors in Search and Rescue Operations 

A system has been proposed whereby active microwave sensors (radar) in low orbit may be used in 

Search and Rescue (SAR) operations to detect distress signals. Distress signals will be in the form 

of abnormally high reflectivity due to the presence of a passive reflector. If a sufficient resolution 

can be obtained distress signals can be indicated by specific reflector arrangements. 

Calculating the Total Power Received at Low Orbit 

from Earth Scattering and Passive Reflecting 

Figure 1 shows the geometry for the proposed low orbiting active sensor system. The total power 

received (less background radiation) can be calculated from the following equation: 

where: 

Pr = the power received at the output of the receiving antenna. 

P, = the transmit power at the input of the transmit antenna. 

Gt = the gain of the transmit antenna over an isotropic radiator. 

Aeff = the effective area of the receiving antenna. 

R= the slant range distance to the sensor resolution cell. 

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,Aclva snu 

Eulh 

Flgurr 1. Sensor Geometry 

4-A Q @ 

Flguro 2. Trlhedralr 

flour. 3. Mainborn Footprint Art. Cieomtry 

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Afootprint = the area of the antenna's -3 dB contour on the surface of the earth (resolution 

cell). 

o= the "scattering coefficient" which represents the equivalent scattering cross 

section per unit area of land. 

dtarget = the "scattering cross section" of a passive relfector which represents the 

equivalent cross section of a conducting sphere which would return an echo 

equal in strength from the target in question. 

The relationship between 0 and d is: 

(2) d = (o).(area of land in view). 

A target (passive reflector) can be defined in terms of its effective area. The effective area is the area 

of an equivalent flat conducting plate oriented so as to be perpendicular to the direction of the 

incident radiation which and returns an echo equal in strength as from the target in question. The 

scattering cross section, cr', is related to the equivalent flat plate area (Afp = ab where a and b are the 

edge dimensions) by: 

(3) 

4 R Afp2 = 

k2 

where his the wavelength at which measurements are made. 

A reflector does not have to be a conducting sphere or flat plate. An efficient, broadband, 

broadbeam reflector is the trihedral or comer reflector (figure 2). The planes forming the trihedral 

are perpendicular to one another, hence the name comer reflector. 

For a triangular trihedral reflector the equivalent flat plate area is: 

(4) AfP = a2/ 

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where (a) is the length of the edge from the vertex to opening. For a square trihedral reflector: 

Afp= ca2 

where (a) is the length of the edge from the vertex to opening. 

Equation (1) is the sum of the contributions of normal backscatter and the additional reflected 

energy from a passive reflector. In the equation (oAfoopfit) represents reflection and scattering of 

incident radiation due to normal backscatter and (GImget) represents the additional reflected energy 

from a passive reflector. 

Consider first the contribution to the received power due to normal backscatter: 

The effective area can be written as: 

(7) Aeff= G - 

47r 

where it is assumed the same antenna is used for both receive and transmit and has gain (G). 

Utilizing the geometry of figure 3 the area of the sensor's -3 dB footprint can be approximated by 

the area of an ellipse formed by the intersection of a cylinder and a plane where the minor and major 

axes are: 

e resolution 

L L 

4 
where: 

(1 1) e= the -3dB beamwidth of the transmitlreceive antenna = 

30,000 ' 

degrees. 

/G 

and e the depression angle measured from the orbit tangent to the antenna boresight. 

The area of the footprint is then given by: 

(12) 

2 xa 

cos ($ 

Afootprint = nab - 

(13) 

(14) 

x 30,000 x 

x3 R2 0.2315 

- 

Afootprint - G cos4 

Substituting equations (7) and (14) into equation (6) yields: 

(15) pr' [z2 1 [r G2] 3 PtGt Gh2 1 x3 R2 0.2315 [ G cos+ 

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if 0 = ./27,oOo//G’ then: 

An alternate method of calculation using a different form of the equation for antenna effective area 

and mainbeam beamwidth yields a slightly different result. Assuming the effective area to be: 

where (h) is the antenna efficiency and (D) is the physical antenna diameter, and the beamwidth to 

be: 

71 h 

e= - 

D 

it then follows by similar calculations that: 

n3 R2 h20.0389 

and: 

If q = 65% then: 

(22) 

or if q = 55% then: 

- Afootprint - D2 cos 4 

PtGt IF h2 

pr = [z21 [ 64cos4 1 

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Equation (23) differs from equation (16) in that it allows the antenna efficiency to be used more 

directly in the calculations. A ten percent increase in the antenna efficiency from 55% to 65% 

results in an 18% increase in the received power, or about 0.7 dB. 

Since [ Q Afootprint + Q' ] represents the portion of the incident energy reflected back towards the 

source we can say that to receive an amount of power 'p' percent above the normal backscatter 

would require that: 

or 

Utilizing equation (20) d becomes: 

3 22 x R h 0.0389 

d = PO[ 

D2 cos @ 

The relationship between gain, resolution, and antenna diameter given by equations (7), (8), (1 1) 

and (18) were used to produce graphs 1 and 2. Graph 1 shows the required gain to achieve the 

desired resolution from various orbit altitudes. This reflects the required antenna size for a 

physically scanning system, i.e. not a synthetic aperture, and as can be seen from the graph, 

without the significantly increased processing gain of synthetic aperture techniques relatively high 

gain antennas will be required to perform SAR. Graph 2 shows the required antenna diameter vs 

resolution for some of the presently allocated active sensing bands. Table 1 below indicates the 

bands allocated below 100 GHz. 

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D Table 1. Allocations for Active Microwave Sensors 

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Frequency Band (GHz) Wavelength (cm) 

1.215 - 1.3 24.69 - 23.08 

3.1 - 3.3 

5.25 - 5.35 

8.55 - 8.65 

9.5 - 9.8 

13.4 - 14.0 

17.2 - 17.3 

24.05 - 24.25 

33.5 - 35.6 

78. - 79. 

9.68 - 9.09 

5.71 - 5.61 

3.51 - 3.47 

3.16 - 3.06 

2.24 - 2.14 

1.74 - 1.73 

1.25 - 1.24 

0.90 - 0.84 

0.38 

Utilizing equations (3), (4), (16) and (26) graphs 3,4,5 and 6 were produced. Graph 3 shows the 

required passive reflector equivalent flat plate area to achieve an echo 15% above normal 

backscatter. This is plotted against the land scattering coefficient. Typical scattering coefficients 

will be between -30 and -5 dF3 for most land areas when sensing from 10 to 40 GHz. This was 

determined from data available in the "Manual of Remote Sensing", Vol l., American Society of 

Photogrammetry, and Figure 3 of CCIR Report 850. As can be seen by graph 3 a relatively small 

(4 mete?) effective area is required to detect a 15% greater backscatter over most land areas when 

utilizing a 1 kilometer resolution. Graphs 4 and 5 show the edge dimension of a triangular and 

square trihedral reflector required to produce an echo at15% above normal backscatter. Graph 6 

shows the required reflector effective area vs resolution. Appendix A contains similar graphs 

showing parametric relationships at single frequencies. 

Discussion of Analysis 

Certain general observations can be made from the calculations and graphs produced in this report. 

They are: 

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1. To be effective for search and rescue resolution must be kept to within a few kilometers. If 

the resolution was worse than a few kilometers it will be defeating its usefulness for locating 

distress sites. In addition a larger resolution will require a larger target in order to be 

detected; this is shown by graph 6. 

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2. Relatively high antenna gains are required to produce the desired resolutions when a 

scanning system, i.e. not synthetic aperture, is used to detect distress signals. This is 

apparent from graph 1 where it is shown that at least 54 dBi gain is required to produce a 2 

km resolution from an orbit altitude of 300 km. 

3. The antenna diameters necessary to produce the required gain, as shown in gaph 2, may 

prohibit the use of a scanning system in the micowave region for SAR. The nature of the 

service and the high resolution that is required will necessitate the use of a low orbit sensor 

able to scan with a swath width many times greater than its resolution. The ability to rotate 

or scan with such a large antenna (greater than 3 meters in diameter) on a spacecraft is not 

presently feasible. There may, however be a way to utilize a modified parabola or circular 

reflector and a rotating antenna feed in a manner similar to the TDRS single access antennas 

to provide a scanning beam. Another alternative may be to use a zero momentum spacecraft 

where the large counter-rotating body of the spacecraft can be used to offset the momentum 

of the rotating antenna. The intent of this report is to address the possibility of using a 

simple scanning system, however another alternative is the use of synthetic aperture radar. 

4. Irrespective of the sensor altitude, if a resolution less than 3 kilometers can be obtained the 

size of the reflector can be kept to a reasonable size, i.e. an effective area of less than 2 

square meters, or trihedral reflectors with edge dimensions less than 2 meters. It has been 

shown in an experiment by NASA outlined in the publication "A Global Search and Rescue 

Concept Using Synthetic aperture Radar and Passive User Targets", NASA TN D-8172, that 

aluminized mylar can be supported by a light framework to produce a viable reflector target. 

Such a target can be be stowed in a compact form and set up in times of distress. This 

experiment was conducted using an aircraft flown at a 1.9 kilometer altitude utilizing 

synthetic aperture radar, and plans for a space shuttle test were being formulated. 

5. Without the use of synthetic aperture radar resolution will be measured in kilometers. This 

will not allow the grouping of distress targets in known geometric patterns for better 

detection and identification of actual distress. This is a key issue in the synthetic aperture 

radar experiment. False indications of distress from a system that has a resolution on the 

order of kilometers may come from almost any metallic object such as ships, drilling 

platforms, airplanes at sea, or cars, trucks, buildings, or towers on land. Such a 

preponderance of false indications would render the system worthless. 

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6. A low orbiting system providing resolution in the kilometer range may prove useful for 

narrowing the search area for terrestrial SAR craft. Instead of searching wide areas of Ocean 

or land, searches can be confined to within an area the size of the sensor resolution. Radar 

on terrestrial SAR craft can then be used within these areas to pin-point distress sites. 

Conclusions 

At least three points can be identified which will require significant efforts to produce a viable 

search and rescue system. One, sufficient antenna gain will be required to achieve adequate 

resolution on the order of one to three kilometers. Two, a means of scanning will have to be 

devised, whether by rotating the entire antenna or only the antenna feed. And three, a means of 

positively identifying distress signals from false echos will have to be devised. Distress signals 

will most likely come from remote areas on land or at sea. Objects producing high reflectivity will 

probably be manmade and will for the most part be located near populated areas, areas where a 

global SAR system will not be required. Therefore, prior knowledge of an area being sensed, and 

an indication of potential distress, Le. overdue communications, distress calls, etc., will aid in 

reducing false distress signals and speed location detection. Aid in locating a known distress 

situation may be sufficient to warrant development of such a system, however, some new 

procedure or technique will still have to be developed in order to produce a system that can provide 

the initial detection of distress. 

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Appendix A 

Parametric Relationships at Specific Sensing Frequencies 

Between Antenna Diameter and Resolution 

and Between Passive Reflectors and Scattering Coefficients 
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