Ultimate IR Horizon Sensor Correspondence Absact The accuracy of presently available IR horizon sensors is not suf- ficient to meet the stringent attitude sensing and control require- ments for future remote sensing and meteorological satellites. The different sources of error in a horizon sensor are analyzed. The accuracy of the sensor is presently limited by the detector noise. Use of HgCdTe in place of an immersed bolometer detector, which is used in conventional horizon sensors eliminates many of the errors. Hence, it is possible to design an ultimate IR horizon sensor whose accuracy is limited only by the uncertainty of the Earth horizon. Comparison of performances of the two types of detectors for horizon sensing is given and possible configu- rations of sensor using this detector are discussed. I. Introduction Highly precise attitude control and attitude determina- tion is required for the current generation of application satellites carrying very high resolution radiometer (VHRR) or multispectral scanner (MSS) type payloads. At present high accuracy attitude sensing is carried out by means of interferometer [1] or star sensors in conjunction with landmark data 12, 3]. The first approach requires two or three ground stations along with a heavy on board system. The second approach necessitates complex ground data analysis and a high accuracy star sensor on board the satellite. This paper describes the concept of a horizon sensor having a high accuracy to meet stringent requirements. The presently available horizon sensors have an accuracy limited by the noise and slow response of the immersed bolometers which are generally used as the detector. This can be improved by means of the HgCdTe detector which has a much better D* and a very low time con- stant. In fact the accuracy achievable is limited only by the uncertainity of the horizon. The cooling of the detector to 1000 K, which is necessary to achieve its high D* performance, can be done by sharing the passive coolers used for the thermal imaging systems of the VHRR or MSS. Alternatively, an independent cooler can be provided for the horizon sensor alone. Horizon sensor configurations using this detector may be realized for spinning satellites as well as for three-axis stabilized sat- ellites. II. Principle of Horizon Sensing The horizon sensor consists of an infrared telescope having germanIium collecting optics to focus the Earth Manuscript received September 1, 1978; revised April 16, 1979. 0018-9251/80/0300-0233 $00.75 i) 1980 IEEE IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. AES-16, NO. 2 MARCH 1980 233
Transcript
Ultimate IR Horizon Sensor
Correspondence Absact
The accuracy of presently available IR horizon sensors is not suf-ficient to meet the stringent attitude sensing and control require-ments for future remote sensing and meteorological satellites.The different sources of error in a horizon sensor are analyzed.The accuracy of the sensor is presently limited by the detectornoise.
Use of HgCdTe in place of an immersed bolometer detector,which is used in conventional horizon sensors eliminates manyof the errors. Hence, it is possible to design an ultimate IR horizonsensor whose accuracy is limited only by the uncertainty of theEarth horizon. Comparison of performances of the two typesof detectors for horizon sensing is given and possible configu-rations of sensor using this detector are discussed.
I. Introduction
Highly precise attitude control and attitude determina-tion is required for the current generation of applicationsatellites carrying very high resolution radiometer (VHRR)or multispectral scanner (MSS) type payloads. At presenthigh accuracy attitude sensing is carried out by means ofinterferometer [1] or star sensors in conjunction withlandmark data 12, 3]. The first approach requires twoor three ground stations along with a heavy on boardsystem. The second approach necessitates complex grounddata analysis and a high accuracy star sensor on board thesatellite.
This paper describes the concept of a horizon sensorhaving a high accuracy to meet stringent requirements.The presently available horizon sensors have an accuracylimited by the noise and slow response of the immersedbolometers which are generally used as the detector.This can be improved by means of the HgCdTe detectorwhich has a much better D* and a very low time con-stant. In fact the accuracy achievable is limited only bythe uncertainity of the horizon. The cooling of thedetector to 1000 K, which is necessary to achieve itshigh D* performance, can be done by sharing the passivecoolers used for the thermal imaging systems of the VHRRor MSS. Alternatively, an independent cooler can beprovided for the horizon sensor alone. Horizon sensorconfigurations using this detector may be realized forspinning satellites as well as for three-axis stabilized sat-ellites.
II. Principle of Horizon Sensing
The horizon sensor consists of an infrared telescopehaving germanIium collecting optics to focus the Earth
Manuscript received September 1, 1978; revised April 16, 1979.
0018-9251/80/0300-0233 $00.75 i) 1980 IEEE
IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. AES-16, NO. 2 MARCH 1980 233
detector time constant, the dwell time of the instantaneousfield of view (IFOV), aberrations of the optical system,and the slope of the CO2 band at the horizon. The hori-zon detection error due to noise (ed) is given by the fol-lowing relation to a first order of approximation [5].
ed = trlIn (1)
radiation inputto detector
-U-
p ,/\ +Vm+V,'KV7 J 1 amplifier output
comporator ouputearth pulse
Fig. 1. Principle of horizon sensing.
radiation onto an infrared detector. An IR filter in the14-16 ,u band is used for limiting the band of IR energycollected since this CO2-absorption band is most suitablefor horizon sensing applications [4].
As the satellite spins, the sensor field of view scansacross the Earth disc in the 14-16 ,u band and gives out-puts corresponding to the "Earth to space" and "space toEarth" discontinuities (Fig. 1). An automatic thresholddetection circuit, wherein the threshold of edge detectionis kept at an optimum percentage of the peak signal, isused to minimize the errors in the measurement of thescanned Earth chord width.
III. Errors in Horizon Sensing
Measurement of the scanned Earth chord width by thehorizon sensor involves various types of errors, whichcan be classified generally as random and systematicerrors. The systematic errors can be accounted for inground computations, but it is desirable to reduce theseerrors, especially when the sensor output is used for onboard attitude control purposes. The major sources oferror of both types and the manner in which they canbe reduced using the new detector are described below.
A. Random Errors
These are contributed by the noise of the detector,amplifier and processing electronics, change in the align-ment of the sensor axis, and drifts in the amplifiers dueto aging or environmental changes.
Noise. The output of the amplifier as the sensor scansacross the Earth has a finite rise time depending upon the
where tr is the rise time of the amplified signal, and on isthe signal-to-noise ratio at the amplifier output (S/N ratio).
The detection error can therefore be reduced by havingas small a rise time as possible at the amplifier output.This could be achieved by using a detector of low timeconstant and by reducing the field of view of the sensor.But for the same aperture size, a smaller field of view meansa lower power received by the detector and hence a poorersignal-to-noise ratio. Therefore a compromise has to beachieved to attain minimum detection error by selectingproper detector size. The use of a HgCdTe detector makesit possible to have a very low field of view (0.10 X 0.10)as compared to (10 X 1 0) typically used with immersedbolometers and yet achieve the same order of S/N ratiodue to its higher D*. Also, this detector has a time con-stant which is very much less compared to a bolometer.The reduced time constant results in lower noise errors.
Misalignment of Optical Axis. This error results fromchanges in dimensions of the detector and the telescopehousings due to temperature changes, vibrations, etc.The present state of the art has reduced these errors tovery low values by the use of proper materials and goodmechanical designs. Therefore this error can be neglected.
Electronics Drifts, Gain Changes, Etc. The variationof threshold, bandwidth, gain, etc. due to temperaturechanges and aging of components contributes to therandom errors but again, these are small enough to beneglected.
B. Systematic Errors
The systematic errors can generally be eliminated inground computation, but for the on board attitude con-trol system it is necessary to minimize these errors.
Error Due to Change in Spin/Scan Rates. The electri-cal signal developed by the detector lags with respect tothe optical input due to the nonzero response time of thedetector. This rise time of the output can be given as
(2)tr = (K1 2 + t2 + t2)/2
where rd is the detector time constant, t1 is the dwelltime of the sensor field of view which equals ' 5/Swhere 0 is the sensor field of view in degrees and S isthe spin rate, th is the rise time due to the slope of theCO2 band which equals 1 Gh /S where Oh is the angularwidth of the sloping portion of the CO2 band as seen
IEEE TRANSACTION ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. AES-16, NO. 2 MARCH 1980234
from the satellite and K1 is a constant. The rise timetr can be expressed in terms of the angle as seen fromthe satellite as Or = 6S X tr. Using (2)
or = (36 K1 s2 2 + 02 + 02 )1/2. (3)
When the edge detection is done using a thresholdwhich is a fraction of the peak signal, a delay is intro-duced between the true horizon crossing and the detectedhorizon crossing pulse as shown in Fig. 2. This delayis dependant on the spin rate. If rd can be made verysmall so that the term 36 K1 S2 2d is negligible, thisdelay becomes independant of spin rate. For a ther-mistor bolometer, rd is around 2.5 ms and hence theterm has a value comparable to the other two terms in(3). Therefore the output of the sensor has an error whichvaries with spin rate. This error is a systematic error andcan be eliminated by using a calibration procedure, butfor on board control it would be preferable if this vari-ation with spin rate is zero. For a HgCdTe detector, 'rdis about 200 ns which makes the spin-rate dependentterm negligible in satellites whose spin rates are below1000 r/min. (Most of the present day satellites fallin this group.) The constant angular delay can also bemade zero by selecting the proper percentage of threshold.
Error Due to Lack of Uniformity ofEarth Radiation.The seasonal variation in radiance from the Earth causesa gradient of signal between the two horizon edges [5 ] .This slope is maximum for a north-to-south scan direction(or vice versa). Due to this gradient, the differentiatedamplifier output does not go down to zero but remainsa finite value. This residual value can be given as
where V1 is the amplitude of the Earth signal at the lead-ing edge, V2 is the amplitude of the Earth signal at thetrailing edge, T is the width of the horizon-crossing pulse,T1 is the frequency cut-off time constant of the amplifier.
Due to the residual value present at the trailing edge,the trailing edge output does not reach its actual peakvoltage and has a smaller or larger value as shown in Fig. 3.This causes an error in the edge detection carried out by theautomatic threshold circuitry. This error could be ex-pressed as
er=(dV/2V2K)Xt (5)
where tr is the rise time of the amplified signal, and K isthe attenuation of the amplifier. Substituting from (4)
er [(2 - VV1)/K V2] T1 trIT. (6)
In the worst case, the radiance ratio for a north-to-southscan could be 1:2. Then
er = I4T t,/K T. (7)max
It can again be noticed that the error will be minimizedby having as small a rise time as possible.
ErrorDue to Change in Chord Length. For a satelliteat a particular altitude, the length of the chord subtendedby the Earth and measured by the sensor is a function ofits attitude. The dwell time of the field of view crossingthe horizon varies with attitude depending upon whetherthe crossing is equatorial or at a higher latitude, as shownin Fig. 4. This causes a variation in the rise time of theamplifier output and hence an error in the detected edge.Obviously a very small field of view would reduce thiserror to a negligible value.
Variation in Height ofCO2 Band. The mean altitudeof the CO2 band above the Earth is about 50 km [7],which undergoes seasonal variations. This altitude vari-ation is of the order of ± 5 km and causes a change in thepulsewidth of the horizon-crossing pulse and therebycauses an error in attitude determination.0018-9251/80/0300-0235 $00.75 01980 IEEE
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It
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field of fiew
i
7/45ihordI
,Idiawneter
-
radkgce input(45chord)
radiance irput(diameter)
mately 0.0070. This error could be eliminated in groundcomputations by taking the known models of the CO2band and its seasonal variation into account.
IV. Design of Sensor Using HgCdTe
The immersed bolometer used in the conventionalhorizon sensor can be replaced by a cooled HgCdTe detec-tor having the following parameters (typical values):
Size =0.1 mmX 0.1 mm
D*= 1 X 1010 cm Hzl/2/W,
_ 1~~~~I.err-DrIr Time constant = 200 ns.
Fig. 4. Error due to change in chord length.
TABLE IComparision of Horizon Sensors Using Immersed Bolometerand HgCdTe
Type of Errors Error in Degrees (1 a)
Using Immersed UsingBolometer HgCdTe
Random errors
Noise 0.01-0.03 0.001
Misalignmentsdrifts, etc. -
Systematic errors
Change in spinrate 0.03-0.05 0.0015(typically ± 10%o)
Change in chordlength (typically 0.02-0.03 0.001± 10o)
CO2 -band error 0.007 0.007
Overall
Random errors (1 a) 0.02-0.04 0.0015Systematic errors (1 a) 0.05-0.08 0.0073
The maximum value of this error can be given as
eh SI(R+h)2 -R2]/2
where eh is in radians, R is the radius of the Earth in
kilometers, and h is the height of the satellite in kilo-meters.
For a geosynchronous satellite, this error is approxi-
0018-9251/80/0300-0236 $00.75 1980 IEEE
The optics is comprised of a single-element germaniumlens having 50 mm diameter and an f/no of 1.25 Byusing an aspheric lens a blur circle of less than 0.1 mmdiameter can be achieved. The power collected by thedetector when observing the Earth can be given by
P =NAQ77
where n is the overall transmission efficiency, A is theaperture area, Q2 is the field of view in steradian, N is theradiance of the Earth in the 14-16 , band which equals0.5 mw/cm2/steradian (average).
The upper and lower cut-off frequencies of the ampli-fier are chosen for minimizing the detection and radianceerrors and to have a minimum of "1/f noise" of theHgCdTe detector. The corresponding errors of the hori-zon sensor for use in a geostationary satellite with a spinrate of 60 r/min are compared with the results that are
achieved with an immersed bolometer detector (Table I).This calculation neglects the amplifier noise since it ismuch less than the detector noise.
Table I shows that a reduction in errors by a factorof 10 is achieved by use of the HgCdTe detector andthe dominating source of error is due to the variationin height of the CO2 band. Thus the horizon sensor per-
formance is limited by only the source geometry ratherthan detector noise or other sensor parameters.
V. Mechanical Construction Details
One of the designs for the mechanical housing of thesensor is shown in Fig. 5. This design has an independentcooler which is pointed north/south for a geosynchronoussatellite. The optical axis is folded out from the detectoraxis using a plane mirror. A two-stage passive cooler isused which brings down the detector temperature to
1000 K. In another possible configuration the coolerused for the VHRR or the thermal sensor of the MSScan be shared by the horizon sensor detector (Fig. 6).
An additional sensor using a thermistor bolometerwill be required for the initial phase of operation andas a back up since the passive cooler needs a few daysto achieve the desired temperature.
IEEE TRANSACTION ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. AES-16, NO. 2 MARCH 1980
(9)
VP
-L
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Is CW
Fig. 5. Horizon crossing sensor using HgCdTe detector.
VI. Conclusion
Use of HgCdTe in place of an immersed bolometerdetector improves the performance of a horizon sensorconsiderably. Even if an independent passive cooler isused for the sensor, the sensor would weigh about 4 kg,and if it is incorporated in the existing passive cooler ofthe thermal imager payload, the extra weight require-ment would be about 1.5 kg. The accuracy of the hori-zon sensor is about ten times that of a conventional sen-sor and is comparable to the interferometer used inAmerican Technology Satellite F which weights 8 kg andconsumes approximately 16 W. This order of accuracycould very well be the ultimate that could be achievedby a horizon sensor based on C02-band detection.
Acknowledgment
The authors are thankful to Director, ISRO SatelliteCentre, Bangalore, India, for all the encouragementgiven for this work.
Y.K. JAIN
T.K. ALEX
BABU KALAKRISHNANIndian Space Research OrganizationBangladore, India
[2J R.L. White, N.B. Adams, E.G. Geisler, and F.D. Grant,"Attitude and orbit estimation using stars and landmarks,"IEEE Trans. Aerosp. Electron. Syst., vol. AES-1 1, p. 195,Mar. 1975.
[3] A.F. Fuchs, C.E. Velez, and C.C. Goad, "Orbit and attitudestate recoveries from landmark data," J. Astronaut. Set.,vol. 4, pp. 369-381, Oct.-Dec. 1975.
[41 J.R. Thomas et al., "The analysis of 15 micron infraredhorizon radiance profile variation over a range of meteoro-logical and seasonal conditions," Nat. Atmospheric SpaceAdministration, Tech. Rep. NASA CR-725, 1967.
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detctor, i I. IntroductionI"-
Fig. 6. Radiometer incorporation horizon sensor for spinningsatellite.
[51 P.V. Dijk, "Infrared detection for satellite attitude sensing:The ANS horizon sensor," Proc. First European Electro-optics Markets and Technology Conf., vol. 64, 1972.
[61 D. Sciacovelli, "Measurement errors of pencil beam infraredsensors in transfer orbit," European Space Research andTechnology Organization, Noorewijk, The Nederlands,ESTEC Internal working paper 1066, Mar. 1977.
[71 M.J. Brewer and T.H.P. Jones, "Modelling of Earth radiancefrom ESRO IV-HCI data," European Space Research Organi-zation, Noorewijk, The Nederlands, Tech Rep.ESS/SS722, Sept. 1966.
Improved Range Resolution Filters for Rectangular-Pulse Radar Systems
Abstract
The performance of several new clutter-reduction filters suit-able for rectangular-pulse radar systems is investigated. Thenew filters consist of various approximations and modificationsof two filters known to be optimal for certain criteria: the well-known Urkowitz filter which optiizes the clutter improvementratio, and the newer sidelobe reduction filter which minimizesoutput noise power subject to peak sidelobe constaints. Thenew filters are compared usig five basic criteria: clutter im-provement ratio, signal-to-noise ratio, sidelobe peak ratio, pulsecompression ratio, and filter complexity. The results are sum-
marized in tabular and graphical form.
Manuscript received February 21, 1978; revised August 18,1978 and July 20, 1979.
Despite the considerable attractions of pulse-com-pression techniques, the additional cost and complexityare often not warranted and a great many radar systemscontinue to use a finite-length rectangular pulse of con-stant frequency [ 1 1, 12]. Unfortunately, however, thechoice of pulse length T is usually determined by manycompromises. On the one hand, T must be large in orderto achieve the desired long-range detection and movingtarget indicator (MTI) performance; but on the otherhand, long pulses mean poor range resolution and poordetection in weather clutter. It is possible to resolve par-tially the dilemma by using long Dulses so that long-rangedetection in noise is not compromised, and using specialreceiver filtering techniques to improve the detectabilityof short-range targets in clutter.
Two basic approaches have been used to improve de-tection in weather clutter in radar systems. The first isto maximize the ratio of peak signal-to-clutter power ina linear portion of the receiver. This was the approachtaken by Urkowitz [1] and the optimum filter, whenthe thermal noise is neglected and no Doppler knowl-edge is assumed, is the well-known inverse filter [1, 6, 13].This work has been extended to include thermal noise,Doppler information, etc., and is summarized in [19].The second approach does not actually improve the basicsignal-to-clutter ratio, but instead attempts to normalizethe clutter power and provide a clutter-free video pre-sentation; this produces the so-called constant false-alarm rate (CFAR) receivers [6, 7].
The basic idea in the latter case is that the clutterbackground is "averaged" and this average clutter levelis used to derive a varying threshold level for target de-tection [14-18]. These cell-averaging CFAR receivershave been shown to provide effective CFAR action fora variety of clutter distributions including the well-knownWiebull, log-normal, and Rice distributions. When con-sidering weather clutter, however, and assuming thatthere are many scatterers per resolution cell, it is knownthat the Rayleigh model is satisfactory and log-CFARsystems are often used in preference to cell averagingsystems. They rely upon the fact that statistically-dis-tributed clutter can be modeled as Rayleigh noise witha changing variance. A log-amplifier converts the clutterto constant variance noise with a varying mean, and high-pass filtering is then used to remove the varying mean.The best-known log-CFAR receivers are log- fast timeconstant (FTC) [2], log pulse length discrimination (PLD)[3], and cell-averaging log-CFAR [4, 7].
It is important to note that the cell-averaging CFARreceivers can be thought of as high-pass fiters. The back-ground averaging is essentially a low-pass action and theoverall effect of subtracting this averaged signal fromthe original signal is to create a high-pass function.
There are several difficulties with the currently avail-able clutter-reduction techniques. First, the output ofthe Urkowitz filter consists of a single spike, which does
IEEE TRANSACTION ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. AES-16, NO. 2 MARCH 1980238
