Objetivo específico 2

M agellan undertook four m apping cycles b efo re en terin g the v enusian atm osphere on the 12th O ctober 1994. Each cycle lasted for 243 days, the time Venus took to com plete one axial ro tatio n w ith respect to the orbital plane o f the spacecraft. Since M agellan d ata are classified by m apping cycles, it is h elp fu l to p ro v id e a b rie f sum m ary o f each cycle:

• C ycle 1 ran from 15th S eptem ber 1990 to 15th M ay 1991. T his cycle constituted the prim ary m ission. In to tal 83.7% o f the p lan et was im aged betw een 90^ N and 70® S. A ll radar im ages are left-lo o k in g ; rad ar illu m in a tio n o f the su rface is from the left. In cid en ce angles varied from approxim ately 45^ at periapsis (9.5^ N) to 160 at the highest latitudes. D ata gaps (black strips in SAR im ages) occurred owing to superior conjunction (w hen the Sun lies b etw een the E arth and V enus, hence no data can be tran sm itted b a c k to E arth ), g e o m e tric al lim ita tio n s and m e c h a n ic a l q u irk s

{Saunders an d Pettengill, 1991; Ford and Plaut, 1993).

• C ycle 2 (the b eginning o f the ex ten d ed m issio n ) ran from 15th M ay 1991 to 15th January 1992, fillin g gaps from C ycle 1 and obtaining coverage o f the south p o lar area. C ycle 2 im agery d iffers from Cycle 1 in that all C ycle 2 d ata are rig h t looking. F u rth erm o re, the in cid en t angle w as fix ed at 25^. Im ages w ere o b ta in e d b e tw e e n 75® N and 60® S. 54.5% o f the p lan e t w as m apped during cycle 2 w hich resu lted in the cu m u lativ e surface coverage reaching 96%.

• C ycle 3 lasted from 15th January 1992 to 15th M ay 1992 and e m p h asised the a cq u isitio n o f true stereo im ag e co v erag e. A p p ro x im ately 21.3% o f the surface w as covered by le ft looking im ag es at in cid en ce angles sm aller than th o se o f C ycle 1. An ad d itio n al 2% o f V enus w as im aged, resu ltin g in a total o f 98% co v erag e {Ford and Plant, 1993). R adar m apping w as term in ated after Cycle 3.

• C ycle 4 was d edicated to g rav ity o b serv atio n s o b tain ed by d irecting M ag ellan ’s HG A tow ards E arth and recording changes in the radio sig n al’s D oppler shift. G ravity data resolution varies as a fu n ctio n of altitude. Thus, in order to obtain m ore co n stan t high re so lu tio n data, the sp a c e c ra ft’s o rb it w as c irc u la rised by u sin g the v enusian atm osphere to aerobreak the probe.

1 . 4 The Magellan Data Set

T he M agellan m ission returned m ore data than all prev io u s p la n e ta ry m issio n s co m b in ed , p ro d u c in g the firs t g lo b al h ig h reso lu tio n data set o f V enus o f 4225 usable SAR im aging orbits

{Ford an d Plant, 1993). In total, approxim ately 98% o f the p lanet w as im ag ed betw een 120-300 m eter reso lu tio n ; som e areas w ere im a g ed on m ore th an one o ccasio n w ith d iffe re n t g e o m e trie s an d /o r directions o f illu m in atio n . A m ore d etailed d escrip tio n of data collection and reduction m ethods is found in A ppendix I.

1.4.1 SAR Data

In order to m axim ise the scientific value o f M ag ellan data, in d iv id u a l im ages w ere assem b led to p ro d u ce M o saiced Im age D ata R ecords (M IDRs). Raw SAR data received from M agellan w ere p ro cessed into full resolution basic im age data records (F-B ID R s). F -B ID R s rep re se n t one o rb ital strip and are ex tre m e ly n arro w sw ath s o f im age data, ty p ic ally 300 p ix els (22 km ) F -W , by 200,000 pixels (16,000 km ) N-S. Each p ix el is a m easure o f the re fle c ta n c e o f the rep re se n te d su rface elem en t, o f d iam ete r 75 m etres, ex p ressed in decibels (dB ), relativ e to the m odel o f the av erag e v en u sian re fle c ta n c e {Saunders et al., 1992; Wall et al.,

1995). T he surface area rep resen ted by each 75 m eter p ix el has been re sc a le d from the true reso lu tio n o f the rad a r in stru m en t, w hich is latitude dependent and never b etter than 120 m eters.

Im age m osaics consist o f 7168 pix els v e rtica lly (lin es) by 8192 p ix e ls h o riz o n ta lly (sam p les) and p ro v id e the b ase fo r stan d ard M agellan photographic im age products {F ord , 1993; W a l l et al., 1995). Each M ID R has a photom osaic id en tifie r given in general form by X-M IDR. yyHzzz; C, where:

• X is the resolution (F = full reso lu tio n data w here 1 pixel corresponds to 75 m eters; C l, C2 and C3 data, com pressed once, tw ice and thrice data, with 1 pixel corresponding to 225, 675 and 2025 m eters resp ectiv ely ).

• yy and zzz are the approxim ate latitude and longitude at the centre o f the mosaic in degrees (e.g. 15 N, 335 B).

• H represents the hem isphere (N for north and S for south)

• C denotes the Cycle in which the data were required e.g. 1 = Cycle 1 etc.

In Cycle 2 and 3 data, three digit num bers are used; the first d ig it indicates the Cycle and the last is the version num ber { F o r d ,

1 9 9 3 ). F ig u re 1.6 and T ab le 1.7 su m m arise s th e M a g e lla n p r o d u c ts .

T he M ag ellan data set includes p h o to p ro d u cts and d ig ita l C D -R O M im ag es. P h o to p ro d u cts are a v a ila b le fo r the M ID R p ro d u cts, although ow ing to financial restric tio n s only a lim ited area o f the p lan et is represented by p ro cessed F -M ID R im ages. H o w e v e r, fu ll re s o lu tio n le f t- lo o k in g F M A P s (75 m e tre s , re s a m p le d p ix e ls ) are a v a ila b le as p h o to p ro d u c ts ty p ic a lly m e a su rin g 1 by 1 m eter in size. R ig h t-lo o k in g FM A P s are cu rren tly being produced for the whole o f the planet.

F-M IDRs and C-MIDRs are also stored on CD-ROM . Each have M ID R s segm ented into 56 in d iv id u al fram elets m easu rin g 1024 lin es by 1024 sam ples, the com plete im age m easuring 7168 lines b y 8 1 9 2 sa m p le s. A sa m p le d l / 8 t h s c a le b ro w s e im a g e accom panies each M ID R m easuring 896 lines by 1024 sam ples.

P-M IDR