state of art summary

8/3/2019 State of Art Summary

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Detectors for radiance-balancing sensors are limited to thermopiles and metal bolometers.

Thermistor bolometers exhibit wide variations in resistance and responsivity and in the

effects of ambient temperature on these parameters. The precise temperature control re-

quired to prevent mismatch in two balancing detectors makes the use of thermistorsimpractical (ref. 15).

Another factor that has to be considered when thermistor bolometers are used is thermal

runaway. A thermistor bolometer changes its resistance with temperature. The thermistor

material has a negative temperature coefficient. At a temperature called the detector critical

temperature, the bolometer impedance has been reduced to the point that the high bias cur-

rent causes detector self-heating and thermal runaway.

Thermistor bolometer behavior (ref. 18) for a detector biased at 60 percent of peak bias

voltage and base temperature maintained at room ambient is as follows:

Detector signal/noise ratio Behavior

Output is proportional to input.

Output is approximately proportional to input.

Output is not proportional to input.

Detector heating produces a large resistance and voltage

imbalance in bolometer bridge.

Detector heating causes degradation of cements used in

construction, and the detector burns out.

io1 to 105

105 to i o 6

i o 6 to 107

108

109

2.5 State of the Art Summary

Table I summarizes a number of sensor characteristics. Care must be exercised in interpreting

this table because the best valuesgiven are not necessarily applicable to any one known sensor.

3. CRITERIA

Horizon sensors should be designed to indicate the angular position of the optical dis-

continuity, near where the Earth profile meets the space background, for determination of

the spacecraft local vertical under all specified operational conditions.

The design should achieve required accuracy, lifetime, and reliability within imposed size,

weight, and power constraints. The design shall minimize adverse effects of the natural

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TABLE I.-State o f the Art Summary

Characteristic

Sensor type

Spectral band

Optical material

Aperture

Field of view

Accuracy

Sun rejection

Detector type

Reliability

Weight

Power

Possible choices or range of values

Conical scan

Edge trackerRadiation balancing

Hor izon -crossing indicator

CO;! ab sorption band 14-16 pm

H 2 0 absorption band 20-35 pm

Germ anium or silicon

1 t o 3 i n .

0.75' to 3 rectangular, square,

or circular

0.05' to 2'

Use of separate detectorUse of main de tector by n oting

signal level, pulsewidth , etc.

Thermistor, metal bolometer, or

thermopile

M T B F a = 1 5 0 0 0 h r t o 1 2 0 y r

0.5 to 25 Ib

0.5 to 20 W

Principal factor s involved

Accuracy required, operational altitude; length

of mission; ype o f satellite, i.e., spinn ing or3-axis stabilized; and degree of mechan ical

complexity perm itted

Adequate signal/noise ratio for desired scan

type, good cloud rejection, and horizon

stability

Desired spectral band

Large enough t o give adeq uate signal/noise

ratio fo r selected spectral band and scantype

Accuracy required, must give adequate signal/

noise ratio for selected spectral band

Altitude of operation

Spectral band selected

Length of mission

Degree of mechanical and electrical com plex-

ity permitted

Degree of com plexity permittedAbility to anticipate Sun's presence before

entering field o f view

Selection depends mainly on type of sensor,

whether scanning or nonscanning

Accuracy required, weight and power limits

Accuracy required, type of scan desired

Accuracy required, type of scan desired

aMean time betwe en failures.

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environment, including cold clouds, atmospheric scattering, Sun interference, Moon inter-

ference, and Earth radiance nonuniformities. Susceptibility to spacecraft adverse environ-

mental effects including mechanical obscuration, electromagnetic interference, thermal

radiation, and thermal conduction must be minimized. The output signals should be com-

patible in level, dynamic range, data rate, and format with the control system of which theyare functional elements.

The simplest, most reliable design capable of performing the specified function should be

provided. Designs developed with flight-proven hardware and techniques should be used

whenever practicable.

3.1 Input Phenomena

3.1.1 Spectral Region

The spectral region of operation should be selected to be consistent with performance re-

quirements, considering effects caused by clouds, Earth spatial and temporal temperature

differences, and radiance profile shape variations.

3.1.2 Sun Interference

Detector damage by solar heating must be prevented. Output errors caused by the Sun

being in or near the detector field of view shall be minimized. An alarm signal should be

considered to indicate actual or impending Sun errors. This alarm signal should be com-

patible with switching logic circuits so that appropriate action may be taken consistent with

the horizon-sensor design and the requirements of the spacecraft attitude-control system.

For radiance-balancing sensors, output errors and detector-plane thermal gradients must be

considered when the Sun is in or near the total field of view encompassed by all detectors.

3.1.3. Internal Reflections

The optical design should be such that multiple reflections from external sources, such

as the Sun, Moon, Earth, or spacecraft, and from sources within the sensor itself do not

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generate spurious detector signals sufficient to cause output errors greater than a pre-

determined tolerable magnitude.

3.1.4 Moon Interference

Effects of Moon interference shall be analyzed and considered. Moon interference effects

on the horizon-sensor output and the influence of the erroneous output on the spacecraft

mission should be considered.

3.2 Design Interfaces

The following factors, derived from vehicle system and operational interfaces, shall be

considered in the design and by the user.

3.2.1 Optica I Interface

The optical interface with the spacecraft must be designed so that no part of the spacecraft

or its appendages can appear in the field of view of the sensor or be close enough to the field

of view to cause output errors. Alternatively, the sensor must contain sufficient logic to

reject spurious signals if the field of view of the sensor must include part of the spacecraft,

expended reaction jet fuel, or jettisoned parts.

3.2.2 Electr ical Interface

Control shall be imposed on the electrical interface with the spacecraft to prevent spurious

signal and noise inputs to the detector preamplifier. The prevention of electromagnetic

interference is vitally important because of the low signal levels usually encountered in

horizon sensors.

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3.2.3 Thermal Interface

The thermal design must include special considerations of the radiant thermal interface

defined by radiative inputs from Sun, Earth, space, and spacecraft in addition to the normal

thermal-conduction interface with the spacecraft, because of the exceptional thermal sensi-

tivity of all types of sensors.

3.3 Other Design Considerations

3.3.1 Sca n Mechanization Protection

Sensor designs that use gears and/or bearings that require lubrication must be provided with

reliable seals. The optical elements must be protected from contamination by evaporated

lubricant, whether from seal failure or other causes. Sensors with torsional-suspension

scanning elements, e.g., flex-pivot or taut-wire suspension, must be adequately protected

from the launch environment. Fatigue in flex pivots must be considered.

3.3.2 Thermal Design

For sensors using thermistor bolometer detectors, safeguards must be included to prevent

thermal runaway of the detector. For radiance-balancing sensors, the thermal design must

consider the effects of output errors caused by temperature gradients across individual de-

tectors, thermal gradients between detectors, and the temperature range over which the

detectors shall operate.

3.3.3 Detector Life

The thermal environment of detectors, especially immersed thermistor bolometers, should

be consistent with required operating time and life versus temperature characteristics of the

detectors.

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3.3.4 Alinem ent Provisions

Means for alining the sensor to the required spacecraft references and for verifying this

alinement after installation on the spacecraft shall be provided in the design.

3.3.5 Contamination an d Degrad ation of Optical Elements

Contamination and degradation of internal and external optical elements must be prevented.

If dust covers are used, positive dust cover removal procedure shall be provided. In all

phases of design, including the specification of assembly, test, and handling procedures, atten-

tion must be given to the prevention of degradation of any optical element or surface. This

includes prevention of contamination during fabrication and testing as well as during mission

orbit.

’

3.3.6 Corona Suppression

The design shall prevent the occurrence of corona discharge at all specified ranges of tem-

perature and pressure. Any electrical potentials above approximately 100 V should be

suspect. The low pressure of orbit shall not be depended upon t o prevent discharge becauseof the contingency of accidental turn-on before outgassing is complete.

3.4 Performance

3.4.1 Ac qu is i tion

The sensor design must provide the required probability of initial Earth acquisition. Loss

of track resulting from variations in apparent Earth diameter over the expected mission

altitude range and over the required range of vehicle attitudes must be prevented. Alarms

to indicate loss of track and/or automatic reacquisition sequence if track is lost should

be considered.

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3.4 .2 Performance Tests

In addition to tests conducted to verify sensor performance, the test plan should specify

that each sensor be subjected to at least one field of view test over the dynamic range of ex-pected radiant inputs and to a cold-wall test with the sensor operated over its expected

environment.

3.4.3 Launch-Site Checkout

Required launch-site performance testing should be defined early in the design phase.

RECOM MENDED PRACTICES

The selection or design of an Earth horizon sensor for a particular application involves a

complex tradeoff between altitude range of operation, accuracy, reliability, size, weight,

power, and cost. The user’s requirements for operational altitude, linearity, and cross

coupling will dictate the scan pattern. The specifications on accuracy and response to

spurious signals from cold clouds and the Sun will govern the spectral band, aperture, and

field of view. The size, weight, and environment will influence the lenses, mirrors, and ma-

terial selection. The environment and life requirements will affect the choice of rotationalor dither rate, construction, and alinement.

Within the broad infrared spectrum of 2 to 30 pm, the Earth presents a variety of horizon

scenes, depending on the specific portion of the spectrum used. Where accuracy is limited

by horizon noise, the preferred spectral region is the 15-pm C 0 2 absorption band embracing

the spectral region from about 14 to 16 pm-although the mechanization employed may

include radiation of longer wavelengths.

If lifetime and reliability are primary requirements, a static (i.e., no moving parts) sensor is

indicated. However, accuracy may dictate the need for dynamic scanning or tracking. Design

approaches for dynamic scanning devices range from sealed units with gears and bearings tounits with no metal-to-metal friction.

Early plans for appropriate spacecraft performance tests and launch-site checks should be

made. Detailed plans should provide for avoiding Sun and Moon interference, thermal

runaway, optical ghosts, and EMI.

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4.1 Input Phenomena

4.1.1 Spectral Region

The selection of the spectral region of operation is most important step in the design process

and is based on a tradeoff that is influenced by many factors, the most important of which are

the properties of the horizon radiation, the Sun’s radiation, and the available components.

The correct spectral region for a particular sensor depends greatly on orbit parameters and

scan pattern, and represents the best achievable compromise between accuracy and volume.

Operation at wavelengths shorter than about 12.5 pm is to be avoided if possible. The

optimum spectral region for high accuracy is the 15-pmCOz band from about 14 to 16 pm.

For cases where lower accuracy can be tolerated, the 12.5-to-40-pm region may be more

advantageous.

4.1.1.1 Earth Spectral Distribution

The Earth-emitted spectral distribution is given in figure 10 (from ref. 1). These approxi-

mate curves, which should be considered as illustrative of the Earth’s spectrum rather than

as hard design data, show tha t the Earth radiates significantly only at wavelengths greater

than about 6 pm.

The spectral distribution of reflected solar radiation is contrasted with blackbody distribu-

tions comparable to the Earth’s spectral radiance in figure 11 (from ref. 19). Below 5 pm

the reflected solar radiation is much greater than self-emitted Earth radiation. Thus a sensor

field of view scanning across a partially illuminated Earth would see three large gradients,

one at each limb and one at the terminator, if its spectral response extends below 5 pm.

The gradient at the terminator could be misinterpreted as the horizon unless the sensor signal

processing (or ground data reduction, where applicable) compensates for this error.

Sensors with a response below 6 pm have to be designed to eliminate erroneous signals caused

by reflected solar radiation.

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I I I I I I I I I I I I I I I6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38

Wavelength, pm

OPPelark _ _ _ _ _ - _

Figure 10.-Comparison of estimated and measured upwelling radiation, temperate atmosphere,

and vertical path.

1.5

IN o = Total blackbody radiance

Reflected solar radiat ion

6000" K Albedo=l.O

No ( 1 . 8 , ~ ~ m + ~ ) = 3 . 0 W/cm2-sr

0 5 10 15 20

Wavelength, pm

Figure 11 -Reflected solar and simu lated Ear th blackb ody/

spectral energy/distribution curves.

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4.1.1.2 Horizon Stability

In those applications where high accuracy is required, a spectral region should be se-

lected to minimize the angular instability of the horizon. The 15-pm COz band satis-

fies these objectives; the specific spectral width with best stability encompassing this

band was shown in reference 20 to extend from 14.0 to 16.3 pm, with potential detected

horizon stability better than 1 km. The radiance profile in that spectral band exhibits

deterministic variations with time and latitude as well as random variations. The deter-

ministic changes have been demonstrated analytically and verified experimentally as shown

in references 21 through 24. For missions in which extreme accuracy is required, pro-

gramed compensation for the deterministic effects can improve the local vertical accuracy

as indicated in reference 25. For other spectral regions, horizon profiles can be generated

from data in reference 26 to assess the effects of atmospheric changes.

In addition, amplitude effects can be compensated for by normalizing the time-varying

detector output signal to the measured peak radiance or peak detector signal. This

has the effect of producing what have been called normalized radiance profiles (refs.

1, 27, and 28). The radiance amplitude effects can be eliminated as shown in figure

12 (from ref. 9). However, care must be taken that the spectral region selected does

not allow the radiance compensation technique to be confused by atmospheric

anomalies.

4.1.1.3 Horizon Detection

As a detector field of view sweeps across the discontinuity between Earth and space,

it generates an output signal that is a function of the magnitude and shape of the limb

radiance profile, as distorted by the transfer function of the sensor. The relationship

between the defined infrared horizon and the hard Earth horizon depends on the detec-

tion technique used.

A fixed threshold on the detector signal is shown in reference 28 to be one of the more

stable techniques. Other features of the detector waveform can be used for horizon

identification. Techniques such as sensing a particular value of slope or the waveform

inflection point, or extrapolation of the linear portion of the waveform to zero signal

on the time or scan angle axis may be employed. However, these techniques require

more complex electronics than simple threshold sensing and might offer advantages only

in certain special instances. References 1, 27, and 28 discuss many of these signal-processing

techniques and their related accuracies. References 29 to 36 provide additional data regard-

ing Earth horizon characteristics tha t affect the horizon detection problem.

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20

14 -

Q) 12 - Arct ic w in ter0c

8 -

6 -

4 -

2 - Original curves

0 0.2 0,4 0.6 0.8 1.0 1.2 1.

Degrees above horizon at 320 n. mi.

I ' " ~ ' " U ,

/Arctic summer

5 eser t

I

0 0. 2 0.4 0.6 0.8 1.0 1.2 1.4

Degrees above horizon at 320 n. mi.(b)

Figure 12.4a) C 0 2 band horizon profiles. (b) C02 band horizon profiles,

normalized.

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4.1.2 Sun Ef fec ts

The two primary Sun effects to be considered are detector damage in sensors using

thermistor bolometers that are not adequately shielded, and output errors, which exist evenwhen adequate protection is provided.

4.1.2.1 Detector Damage

Thermistor bolometers can reach a critical temperature where the bolometer impedance has

been reduced to the point that the resulting bias current causes detector self-heating and

thermal runaway.

Care must be taken that the sum of the temperature rises caused by the radiation of

the Sun and the operating temperature of the sensor does not exceed the detector critical

temperature.

Thus, in any particular application using thermistor bolometers, detector heating by the Sun

must be carefully analyzed to determine potential detector damage. The margin of safety

can be increased by narrowing the spectral region, by decreasing the temperature of the

detector heat sink, and by operating the detector at less than the recommended 60 percent

of peak bias. If these steps do not provide sufficient safety margin, a Sun shutter may be

used to block the optical path until the Sun is no longerin

the field of view, or a vehicle

maneuver can be executed to change the sensor/Sun line of sight while still maintaining

adequate sensor/Earth geometry to retain sensor function.

Thermopile detectors do not exhibit the thermal self-heating from bias currents that affect

thermistor bolometers. They would not be damaged until incident radiation caused the

detector to reach the sintering temperature of the detector materials or the softening tem-

perature of the backing materials.

4.1.2.2 Output Errors

When the Sun is in or near, in some instances, the detector field of view, detector output will

confuse the sensor signal processing unless some form of discrimination exists. Discrimina-

tion is based on amplitude sensing, pulsewidth logic, separate Sun sensing detectors, or a

combination of these.

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When the Sun is in such a position relative to the horizon and to the detector field of view

that amplitude discrimination and pulsewidth discrimination are not effective, one of the

following steps can be taken:

(1) The resulting pointing error may be accepted.

(2 ) The sensor field of view can be caused to change its relative Earth/Sun orientation

by a vehicle maneuver or by reorienting the scan planes within the sensor.

(3) A redundant sensor head or detector field of view can be placed into operation,

as in the OGO sensor (ref. 13).

(4) The sensor output can be inhibited until the Sun problem is eliminated by the

changing orbit position of the vehicle.

If a separate Sun detector is used to sense actual or impending solar interference, the output

of the Sun detector must be sufficiently related to the Earth sensor field of view so that the

critical Earth-Sun field-of-view orientation can be determined. When the Sun is on the

horizon, atmospheric attenuation may prohibit sensing of solar interference by a separate

Sun detector. In that case, if resulting horizon-sensor output errors must be avoided, the

situation must be predicted and steps (2 ) or (3 ) above used.

The solar image presents a considerable thermal gradient problem to static or radiance-

balancing sensors. This problem can be minimized by operating in as longwave a spectral

region as possible to reduce the effective solar signal. Placement of the spectral filter

outside the collecting aperture of the optics minimizes the amount of solar flux entering

the sensor aperture and the asymmetric solar heating of the collecting optics and lens barrel

(ref. 16).

Aside from these general statements, specific practices are too intimately related to sensor

design details to be generalized. Exceptional optical and thermal design practices must be

observed and the internal solar heating problem must be considered carefully.

4.1.2.3 Sun Rejection

The ratio of Earth spectral radiance to Sun spectral radiance using the data of figure 10 for

the Earth and a 6000" K blackbody for the Sun, is illustrated in figure 13. Note that theSun radiates about 2000 times as much as the Earth at the shortest wavelength at which

there is significant Earth emittance, (Le., 8 pm). Thus, a small field of view detector (less

than 0.5" X 0.5" or 7.61 X 10-6 sr) with a spectral response centered at about 8 pm would

receive a signal 2000 times greater than the Earth signal for which it was designed, when

scanning across the Sun. The curve in figure 13 shows that operating at the longest possible

wavelength minimizes the problem, and that the Sun radiates considerably more than the

Earth, regardless of the infrared spectral region.

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0

m:L, 10 20

Wavelength, pm

Figure 13.-Ratio of Earth spectral radiance to

Sun spectral radiance.

By eliminating requirements for spectral filtering other than that provided by the inherent

spectral transmission of the sensor optical materials, overall optical efficiency can be im-proved. Increased efficiency leads to a greater signal-to-noise ratio or a smaller aperture

diameter, and consequently smaller overall size. Thus, if sensor operation and required

accuracy permit, spectral response down to about 2 pm is permissible as in the Tiros horizon-

crossing indicator (HCI) of reference 9.

The large Sun-to-Earth radiance ratio causes more problems when changing from the Tiros

HCI-type sensor, to wide-angle mechanical sensors, or to dithering edge trackers This ratio

has the largest effect on nondithering, nonmechanical-scanning sensors (e.g., electronic-scan,

wide-angle scanners or radiance-balancing sensors, or any sensor in which the detector field

of view is relatively stationary in object space). As much sunlight as possible must be filtered

out to prevent detector damage when the solar image is directly on the detector and to mini-mize thermal gradients across the detector planes, which cause output errors (refs. 11 and

17). The required magnitude of solar rejection depends on mission requirements and sensor

type. The only guidelines on spectral region is that where strjngent solar rejection require-

ments exist, filtering must be incorporated t o limit the sensor spectral response to the longer

wavelengths.

29



4.1.3 In ter na l Reflections

Improper control of internal reflections can cause ghost images of the Sun, and such solar

ghost images can produce spurious detector signals. The unwanted internal reflections occur

whenever refracting optical elements, such as lenses, are used, Unwanted reflections are sup-

pressed through the use of thin film antireflection coatings on the offending refractive sur-

faces. Where possible, reflecting optics can be used in place of the refracting element. The

number of refracting surfaces should be minimized and proper employment o? light baffles

and optical stops is important. Both geometrical optics and physical optics must be con-

sidered in the design. The optical design must be given an operational test in a cold-wall test

chamber at the proper signal levels before the design can be considered adequate.

4.1.4 Moon Interference

If the field of view of the sensor scans across the Moon, the detector output signal will be a

function of the lunar illumination phase-sometimes exceeding Earth threshold for a detec-

tor with a small field of view. These lunar signals can be discriminated against by measuring

signal pulsewidth when the Moon is far enough from the horizon to produce a complete

pulse. There will be times, however, when the Moon is so close to the horizon and the sensor

field of view that it cannot be discriminated and a pointing error will result. Edgstracking

sensors are particularly susceptible to large errors during moonrise. The courses of action

are the same as those listed in section 4.2 for Sun near the horizon.

Lunar interference can be eliminated by increasing the detector field of view. As the field

of view increases beyond the angular subtense of the Moon, the detector signal from Earth

increases but the lunar signal remains constant. However, use of a large field of view must

be carefully evaluated with respect t o other considerations, primarily accuracy.

The best-known method for determining when the Moon is in a location to cause interference

is prediction based on the spacecraft orbital parameters.

4.2 Design Interfaces

4.2.1 Optical Interface

The horizon sensor responds to all the reflected and emitted radiation in its environment.

Optical interface is defined to be the total optical environment of the sensor as it is mounted

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