ASIGNACIÓN DE FUNCIONES

Magellan carried one prime instrument, a radar sensor, designed to collect data by a

process known as 'burst mode operation'. The radar system operated sequentially in three

modes: synthetic aperture radar (SAR), altimetry and radiometry, and collected a total o f

4225 usable imaging orbits (Ford and Pettengill, 1993). The SAR operated at a wavelength

o f 12.6 cm (S-band, 2.385 GHz) with horizontal parallel transmit/receive polarisation (HH)

to enable penetration o f the thick venusian cloud cover and allow discrimination o f small-

scale surface roughness (Ford, 1993) (Table 1.6).

Following processing, the primary SAR image products (full-resolution basic image

data records, or BIDRs, covering an area o f 22 km by 16000 km) were assembled into

m osaicked image data records (MIDRs) for scientific analysis. F-MIDRs use the full

resolution (75 m /pixel) o f the input data and cover an area o f approximately 5° x 5°. C-

MIDRS are obtained by compressing the resolution o f the original data and cover larger

areas. C l-M ID R s cover 15° x 15° at 225 m resolution, C2-MIDRs cover 45° x 45° at 675

m resolution and C3 MIDRs cover 120° x 80° at 2025 m resolution (Figure 1.6). Image

products are identified by their resolution and the centre point latitude and longitude, e.g.

C l-M ID R 15N215 (compressed once data, 15° north, 215° east). Full resolution F-MAP

images have also been produced which cover an area o f 12° x 12°.

The Magellan data set includes both hard copy photoproducts and digital CD-ROM

images. The global altimetry, emissivity, reflectivity and rms slope data sets are provided

in 32 Mercator and 8 polar stereographic framelets contained on CD-ROM. A full

description o f Magellan data collection and reduction methods can be found in Ford et al.

INTRODUCTION 34

Param eter

Radar Imaging Radar frequency

Polarisation

Slant range resolution'

Azimuth resolution'

Radar image pixel size

Antenna look angle'

Swath width'

Image strip length

Number o f looks per pixel'

Value 12.6 cm /2.385GHz HH 120 — 360 m 1 2 0 - 150 m 75 m (Full resolution)

13 to 45 degrees from nadir

20 - 25 km

17,000 km

5 to 17

Altimetry

Footprint variation'

Spacing o f footprints along orbit

Error o f altimetric resolution

10x12 km to 20x29 km'

8 km, greater at pole

> 5 m

P assive R adiom etry Footprint variation'

Sample interval

16x24 km^ to 83x87 km^

5 km X swath width

D ependant on spacecraft altitude w hich changes with latitude

Table 1.6. Parameters o f the Magellan SAR instrument.

L 8 ,l SAR Data

To interpret SAR data correctly, a knowledge o f how radar interacts with natural

surfaces is needed. Radars depend on the measurement o f range to create an image and are

therefore forced to look to the side to allow the sensor to differentiate between objects to

IN T R O D U C T I O N 35

(i.e. r adar illuminat ion c o m e s from the left side o f the image) , but right looking data, wh er e available, is useful for a c o m p a r i s o n and an aid to g e o lo gi ca l interpretation. R a d a r return, or backscatt er, is d e p e n d e n t on a n u m b e r o f factors: sur f a ce r o ug h ne s s, i nci dence angle and the di el ectric properties o f the surface (Elachi, 1983; Drur y, 1993). S mo o t h surfa ces cause the m a j o r i t y o f r a d ar to be re fl ect ed a w ay f r om the a n t e n n a , and so a p p e a r ' radar dark', w h e r e a s r ough s urfa ces ca us e the r a d a r to be s cattered c a u s i n g m o r e signal to be returned, and i m a g e s a p p e a r ' radar bright' ( Fi gur e 1.7). V a r i a t i o n s in i nc i d e n ce angl e al so vary the a m o u n t o f signal r e tu r n e d to the s pa c ec r af t and h a v e a s t r o ng i n f l u en c e on b a c k s c at t er cross-section. P-MIDR C3-MIDR (120° X 80°) 80° 75° 60° 45° C2-M1DR (45° X 45°) 30° C l-M ID R (1 5 °x 1 5 °) 15° F-MIDR (5° X 5°) 60° _60° 45° 45' 30° _{15° 15°} 30° 30° F-BIDR (20 to 25 km) 45° 60° 75°

F-BIDR Full-Resolutlon Basic Image Data Record F-MIDR Full-Resolutlon Mosaicked Image Data Record C l-M ID R Com pressed-Once Mosaicked Image Data Record C2-MIDR Com pressed-Twlce Mosaicked Image Data Record C3-MIDR Com pressed-Thrlce Mosaicked Image Data Record P-MIDR Polar Mosaicked Image Data Record

INTRODUCTION 36

« X

Smooth: no return Sightly rough: slightly diffuse

Moderately rough: moderately diffuse

» X

Very rough: very diffuse

Figure 1.7. The effect o f surface roughness on radar backscatter. Surfaces which are less rough than the radar wavelength scatter the beam in the specular direction. Rough surfaces scatter energy in all directions, and back to the sensor. They therefore appear bright in radar images. After Farr (1993).

Several abnormalities are common in radar images that can cause problems during

interpretation (Figure 1.8). Low incidence angle o f the radar beam can cause some areas to

be in shadow because they are masked by features in the foreground. Radar foreshortening

compresses the image causing upstanding features to appear to have a steep or shortened

foreslope and a shallow backslope. This effect is often exacerbated by very steep terrain,

creating an effect called layover, where the top o f the feature is imaged before the bottom.

Image data in the laid-over area are subsequently lost. For more information about radar

I N T R O D U C T I O N 37 (a) SAR L o o k A n g le R a d a r B e a m R a d a r-Im a g e P la n e R a d a r-Im a g e N e a r R a n g e F a r R a n g e (b) SAR L o o k A n g le R a d a r B e a m R a d a r-Im a g e P la n e R a d a r-Im a g e F o rm a t F a r R a n g e N e a r R a n g e C A (C) S A R L o o k A n g le — R a d a r B e a m R a d a r-Im a g e P la n e R a d a r S h a d o w F a r R a n g e N e a r R a n g e C D A

F igur e 1.8. G e o me t r i c distortions in r a dar images; a) for eshor t eni ng, w h e r e b y the slope AB is c o m p r e s s e d in the i ma g e plane; b) l ayover, w h e r e b y the top o f the t o po gr ap hi c obst acl e is i m a g e d be f or e the base; c) s h a d o w , w h e r e the ba c k s l o pe o f the o b st ac l e BC and surface C D are not illuminated by the ra dar return a nd data are not acquir ed. Af t er Farr (1993).

INTRODUCTION 38