On the use of a mobile surface radar to study the atmosphere and ionosphere of mars

Ady. Space Res. Vol. 10, No. 3—4, pp. (3)35—(3)38, 1990 Printed in Great Britain. All rights reserved.

0273-1177/90 $0.00 + .50 Copyright © 1989 COSPAR

ON THE USE OF A MOBILE SURFACE RADAR TO STUDY THE ATMOSPHERE AND IONOSPHERE OF MARS S. A. S. L.

I. Klimov,* V. V. Kopeikin,** V. V. Krasnoset~skikh,** M. Natanzon,** A. E. Reznikov,** M. P. Gough,*** P. Kingsley,t T. A. Lachlan-Cope,t H. 0. Mullert and J. C. Woolliscroftt

*ZKI..Space Research Institute, Academy of Sciences, Profsoyuznaya, Moscow, U.S.S.R. * *1zMIRAN Academy of Sciences, Troitsk, Moscow Region, U.S.S.R. * * ~University of Sussex, U. K. tUniversity of Sheffield, Sheffield SlO 2TN, U.K. INTRODUCTION It has been suggested in a previous paper, /1/, that a simple pulsed radar be included on a future Mars Lander mission to look at the subsurface of Mars. It is apparent that the transmitter and aerials used will radiate power upwards as well as downwards and this will present an opportunity to investigate the upper atmosphere of Mars by radar techniques. As the available power is limited it is suggested that the technique of meteor wind radar be used at higher frequencies, see /10/, and spaced antenna drift using total reflection at the ionosphere, at lower frequencies. The frequencies available from the radar will also be suitable for trans-ionosphere sounding (see /2/). This paper starts with a brief review of the atmosphere and ionosphere of Mars. The scientific objectives that ought to be addressed by a mission to Mars with an atmospheric radar are discussed. A possible outline design for such an atmospheric radar system is then presented. THE ATMOSPHERE OF MARS Although Mars is smaller than the Earth it has an almost identical rotation rate. Its surface pressure at around 7 mb is, however, very much less than that of the Earth. The surface pressure shows a seasonal variation as carbon dioxide at the poles freezes or sublimes. Carbon dioxide is the largest constituent of the atmosphere. The surface temperature is around 200 - 240K. A temperature profile of the Martian atmosphere does not show the temperature maximum in the middle atmosphere that is found on Earth (the stratopause). Near the surface of Mars, in the troposphere, the temperature drops with height as it does on Earth and at a high altitude it increases with height, in the thermosphere. Between these two layers the temperature is fairly constant. The absence of a stable stratosphere on Mars should have very interesting effects on the propagation of gravity and planetary waves from near the surface to the upper atmosphere. Also large diurnal and semidiurnal oscillations are predicted (see /3/). THE MARTIAN IONOSPHERE AND METEORS 3 at around 120 km during the day. The total The Martian ionosphere has a peak electron density of around io~cm ionospheric content in a vertical column is around 5 x io’~electrons cm2. Measurements of the Martian ionosphere have been very sparse so these figures must be looked upon as approximate, see /4/. These day-time values give a critical frequency of around 2.7 MHz. At night the maximum density is much lower at 5 x io~electrons cm3 giving a critical frequency of around 0.6 MHz see /5/. The flux of meteors on Mars will be very similar to that seen on Earth. Because of the different composition and surface pressure on Mars the meteors will tend to cause the maximum ionization at a different height than on Earth. Apshtein et al /6/ suggest that this maximum ionization should occur between 147 km and 390 km. This appears to be rather high and a better estimation might be that of Bauer /7/, around 90 km, very close to that on Earth. SCIENTIFIC OBJECTIVES In some ways the atmosphere of Mars bears a greater resemblance to the Earth’s atmosphere than that on any other planet in the solar system. The atmospheric pressure on Mars although 100 times less than on Earth, is still appreciable. Carbon dioxide in the Martian atmosphere seems to play a similar role to water vapour on Earth, producing clouds and acting as an energy sink. There has been some evidence for frontal type weather systems (see /8/). Despite these similarities the atmospheres of Earth and Mars do differ in some fundamental ways. One of the main differences is the absence of a stable martian stratosphere to act as a lid to the troposphere. It should be much easier for surface disturbances to propagate into the upper-atmosphere of Mars than it is on Earth.

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S. I. Klimov et al.

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A study of the meteorology of the atmosphere near the surface on Mars would be interesting, and should provide many insights into the Earth’s atmospheric system. Also because the Martian troposphere is probably strongly coupled to the upper atmosphere it is necessary to study the upper atmosphere as well if proper understanding of the atmosphere is to be obtained. The radars suggested in this paper should easily be able to detect long period waves such as Rossby or baroclinic waves (see /8/ and /9/) which should have periods of the order of two or three sols. Also tidal oscillations with periods of either one sol, half a sol or one third sol should easily be detectable by both radars. As well as these rather large scale waves it is likely that atmospheric gravity waves will be able to propagate up into the upper atmosphere. Gravity waves cannot occur with frequencies greater than the Brunt-Vaisaila frequency. Near the surface of Mars this is equivalent to a period of around two minutes. The gravity wave spectrum therefore covers a range of periods from two minutes up to several hours. Because of its low power the meteor wind radar will measure only limited numbers of meteors and this will make it difficult for it see the shorter period gravity waves. However the spaced antenna drift method should be able to detect short period gravity waves provided that the horizontal wavelength of the gravity wave is not much less than the wavelength of the transmitter signal. It should also be possible to use the meteor radar to do some astronomical observations of meteors and meteor streams. Although we have assumed for this study that the meteor flux on Mars is very much the same as on Earth any differences that are found will be informative about the structure of the solar system.

I Carrier Fsequency Mean Power Pap Geological Radar Atmospheric Radar Pulse length Geological Radar Atmospheric Radar Peak Power Nominal Resolution Geological Radar

0.3 MHz — 10 W

3 MHz — 10 W

30 MHz — 10 W

1.5 kHz 300 Hz

15 kHz _________

150 kHz 300 Hz

33.3 ssec 3.33 ~ssec psec 100 ~ssec _________ 100 p5cc 200 W 200 W 200 W 5000 in (1 500 m in rock) 15 km

Atmospheric Radar Unambiguous Range Geological Radar

100 km (25 km in rock) 500 km

Atmospheric Radar Integration Time Geological Radar only 10 sec External Noise dB W/Hz -143

500 in (150 in in rock) _________

10 km (2 50Gm in rock)

50 in (15 in in rock) 15 Km

_________

1 km (250 in in rock) 500 km

10 sec -164

10 sec -185

TABLE 1

THE MARS RADAR A Mars mission may already have on board a geological sounding radar. An atmospheric radar should use as many parts of this radar as possible. The general outline of a possible radar to do the geological sounding has been presented in a previous paper /1/, see Table 1. That paper suggested the use of three frequencies, 0.3 MHz, 3.0 MHz and 30.0 MHz and a bandwidth of 10% of the transmission frequency. We will now consider how the parameters of the existing radar can be adapted for use in atmospheric sounding. It is intended that the radar will see two types of atmospheric target; one will be the ionosphere of Mars and the other will be meteor trains. Although it is possible to obtain echoes from dust and turbulent cells in the lower atmosphere, the power available may not be sufficient for meaningful results. Both the upper atmospheric targets will be around 100 km away and so at the high P.R.F. that the geological sounder may be using, 1.5 kHz at 0.3 MHz, 15 kHz at 3.0 MHz and 150 kHz at 30.0 MHz, there will be a considerable problem with range ambiguity. Also, because of the large bandwidth proposed for the radar, the galactic noise seen by the receivers will relatively large. First we will consider the case of meteor trains seen using 30.0 MHz transmissions. This frequency will probably be the best for detecting meteors as it well above the critical frequency for the martian ionosphere and should suffer from little absorption. Also because of the short wavelength it should be possible to build aerial systems that can be used on Mars more easily. The power returned from an overdense meteor train, using the formula found in /10/, is

P.

=

11PTG2 (f)3q~

1.6 x 10

where q=electron line density (say 2x10’4 electrons m’), .\=wavelength (10 m), Ro=range (say 100km), G=aerial gain (say -4) and Pr=transmitted power (200W peak). This gives

F,

=

1.34 x l0~l W (-138.7 dBW)

Study of Mars using Mobile Surface Radar

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At 30 MHz the external noise at the input to the receiver will be around -185dBW/Hz which, with a receiver bandwidth of 3MHz, will produce a noise level of the order of 9 x iO—’~W. The meteor echo will not be detectable above the receiver noise. A meteor echo is a transient event normally lasting less than a second. It is not therefore possible to use integrating techniques that are proposed for the geological radar. The best solution would seem to be to use different receivers for the meteor radar, with a reduced bandwidth. It is likely that the receivers for the atmospheric radar will be at a different location to those for the geological radar. If the bandwidth of the receivers is reduced we can no longer use very short pulses with duration 0.3 microseconds. A solution would be to increase the length of every five hundredth pulse to 10 microseconds. This would only need a receiver bandwidth of 100 kHz and decrease the external noise to around 10—14 W and make the detection of meteors possible. The lengthening of one pulse in every five hundred will have the effect of marking it and so also removing the effect of range ambiguity. In fact the reduced bandwidth of the new receivers will reduce their sensitivity to the shorter pulses. Now we consider the other frequencies which may be available. The 3.0 MHz transmission will be above the critical frequency for the ionosphere of Mars for much of the time. However the 0.3 MHz frequency will mean that echoes cast be obtained from the ionosphere at most times of day. The 0.3 MHz frequency can be used to obtain ionospheric drift measurements of atmospheric wind. This method has been used by many workers over the years (see /11/ and /12/). It consists of having one transmitter and at least three spaced receivers. The receivers see a diffraction pattern caused by irregularities in the ionosphere. As this drifts past the receiving aerials the pattern is seen by first one and then another. By measuring the time delay in the pattern between each receiver it is possible to calculate the neutral atmosphere wind. As the transmitter can be considered a point source the diffraction pattern will appear to move twice as fast as the irregularities (see /13/). To calculate the power returned from the ionosphere we need to calculate the radar cross section of the ionosphere. We will estimate this by considering that only the first Fresnel zone contributes to the returned signal. If the wavelength is small when compared to the radar range we can approximate the radius of the Fresnel zone by the equation

2 The radius of the Fresnel zone is then approximately 7 km. This means the radar cross section will be around 157 km if the layer can be considered to be a perfectly conducting isotropic scatterer. In fact this will not be the case, the reflection coefficient being closer to 0.1 than 1.0 and this will give a radar cross section of 16 km2. Using the radar equation (see /14/) — —

PTG,GT)~2t~ (4ir)3R4

where GT = Transmitter aerial gain (say 1.0), G, = Receiver aerial gain (say 1.0), u = Radar cross section ( 16 10112), this gives a returned power of 1.6 x 10~ watts. Given that the external noise at 0.3 MHz is -143 dBW/Hz, a bandwidth of 30 kHz gives a receiver noise level of 1.5 x 10_50 W. We see that in this case the returned signal will be well above the noise. However, the pulse repetition frequency of the 0.3 MHz transmitter is 1.5 kHz and this will give problems with range ambiguity. The best way of dealing with this is probably to lengthen some of the pulses again. If every fifth pulse is lengthened to 100 microseconds we will be able to remove the problem. The receivers will again need a suitably restricted bandwidth of 10 kHz and this will lower the receiver noise even more. The 3.0 MHz frequency will appear to be close to the critical frequency during the day although it will be well above it during the night. It will not be useful for the spaced antenna drift method especially as the absorption will appear to be quite high. For use as a meteor radar the longer wavelength will be an advantage. Looking at equation (1) we see that the power in the receiver is proportional to A~and so the returned power at 3.0 MHz will be much greater than at 30.0 MHz. However, the longer wavelength of 100 m will make the construction of any kind of directional aerial very difficult. Also, absorption will probably be a problem. As the data processing available on the lander will be limited it is concluded that the 3.0 MHz frequency should not be used. AERIALS The aerial will be one of the most difficult problems in constructing a radar for use on the surface of Mars. For the lower frequencies the aerial size will have to be small when compared to the wavelength and this will make the matching of the receiver to the aerial difficult. It is thought that the best form of aerial will be a loop for the lower frequencies. At the higher frequency of 30.0 MHz the wavelength is only 10 m and this will allow the use of a folded dipole. It is possible to consider an aerial that behaves as a loop aerial at the lower frequencies and as a folded dipole at the higher. For use with meteor radar it is useful to have some form of directional aerial system to give an idea of the direction in which the meteor train lies. This could be done by two methods. One is to use several dipoles spaced out and used as an interferometer. There will be a slight problem in getting a reference signal to each receiver and this method will require more signal processing. A simpler method is to use some form of Yagi aerial array. Of course this limits the area of the sky being observed but does give more gain in the system. The aerials for the spaced antenna drift will have to be loop aerials spaced out on the surface of Mars. They should be spaced, if possible, in the form of a rough equilateral triangle with the length of the sides determined by the size

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S. I. Klimov et aL

of the irregularities in the ionosphere. In terrestrial spaced antenna radars the size of the disturbance seen in the D-region is around 50-lOOm and best results are found using spacing of 100-200m. The terrestrial radars tend to use slightly higher frequencies and this spacing is normally around one wavelength. The Martian radar is forced to use a much lower frequency and this will mean the amplitude of the diffraction pattern will be lower and the spacing of the aerials should be larger. Using a spacing of around 200-500 m it should be possible to see the disturbance. It is suggested that the receivers can be put oii the seismic packages that will be placed on the surface near the lander. However, the exact spacing of these is not known and they are probably restricted in the power available and the size of equipment that they can carry. Also there will be the additional complication of the telemetry required to send the data from the seismic packages to the processing unit in the main part of the lander. DATA PROCESSING The data processing requirements for the atmospheric radar will be much greater than for the geological radar as it requires the analysis of data in real time. With meteor echoes the main problem is one of identifying an echo from the background noise. Some help comes from making the hardware discriminate against very short pulses. The problem becomes more complicated when one wishes to discriminate between overdense and underdense meteors. Meteor radars on Earth can be run with real time data analysis programs written in compiled FORTRAN that occupy around 32 Kbytes of memory. It should be possible with careful programming in machine code to reduce this considerably. The spaced antenna drift method requires the output from three antennas to be cross-correlated with each other. There are various methods for doing this in tile literature of varying degrees of complexity (see /15/). The simplest method involves taking a fast Fourier transform of the output of each receiver and then comparing the phases of each component found. This method does not give reliable results especially when the outputs are not really correlated. As it will not be possible to check the original record on Earth this could be a great problem. A better method will probably be full correlation analysis (see /12/) although this will require more computing time and more memory. Assuming a realistic limit of space qualified ROM of around 32 Kbyte it should be possible to include the programs for the atmospheric radar, however it may not be possible to run them all at once. It is suggested that a microprocessor can control the radar using a scheduling table that defines the order in which the individual programs run. It should be possible to alter this table from Earth. This will be particularly useful as it is likely that the radar will need fine tuning after the first results have been seen. COMPATIBILITY WITH GEOLOGICAL RADAR The design of this radar has been suggested so that it fits with the geological radar as well as possible. The main impact it will have on the system is the lengthening of some transmitter pulses to reduce the bandwidth. This will increase the power the radar requires but not so much that it becomes a problem. It is likely that the main conflict between the two radars will be in the data processing. It is planned that both radars share the same microprocessor for control of the radars and analysis of the data and it will probably be fully used. CONCLUSIONS The calculations above show that both meteor radar and the total reflection radar should be possible with the limited power available. It is thought however that such a radar is unlikely to be flown on a Mars lander in the next five to ten years. The geological radar is more likely to be included. It is suggested that it might be easy to arrange for the control and analysis software of the geological radar to be modified so that meteor and ionospheric echoes could be resolved even if only limited measurements could be made. This would make the design of a full atmospheric radar. for a subsequent Mars lander, much easier. REFERENCES 1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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