apollo lm guidance computer software for the final lunar descent.pdf

8/14/2019 Apollo LM Guidance Computer Software for the Final Lunar Descent.pdf

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AutomaticaVol. 9, pp. 243-250. Pergam on Press, 19 73. Printed in Great Britain.

Ap ollo L M G uidance Computer Software for

the Final Lu na r DescentP r o g r a m m e s D o r d in a t e u r P o u r le G u i d a g e d u M o d u l e L u n a i re A p o l l o

au Cours de L a lun i ssage F ina l

A p o l l o L M - F ii h r u n g s- R e c h n e r-P r o g r a m m e fiJ r d a s E n d e d e r M o n d l a n d u n g

Cpe)ICTBa nporpaM M uposaaH~CqeTHO BbIql4CJIbHO~MatUrtrIbi ~.q ~ naBe~eH rI~J I y I I a o r o o T c e r a a n o . r L a o l aO C .rIe RrIefi c Te i l e m 4 c n y c r a

D O N A L D E Y L E S t

A gu idance co mpu te r can bes t s e rve an As t ronau t du r ing f ina l descen t by ca l cu la t ingspacec ra f t pos i t i on l e s s f r equen t ly ca l cu la t ing ve loc ity more f r equen t ly and a t tach ingguida nce a lgor i thm s to the veloci ty ca lcula t ion as needed in rea l t ime.

Summary In all manned lunar landings to date, the lunarmodule Com man der has taken partial m anual control of thespacecraft during the final stage of the descent, below rou ghly500 ft altitude. Because of the irregularity of the lunarsurface and the inevitability o f som e error in the on-boa rdestimate of the spac ecraft s position with respect to the localterrain, fully automatic landings, with men aboard, cannotbe anticipated until landing fields are prepared on the moo n.

This report describes programs developed at the CharlesStark D raper L aboratory, M IT, for use in the LM s guidance

com puter during the final descent. At this time compu ta-t ional demands on the on- board comp uter are at a maximum,and particularly close interaction with the crew is necessary.The emphasis is on the design of the computer softwarerather than on justification of the particular guidance al-gorithm s employed. Afte r the compu ter and the missionhave been introduced, the current configuration of the finallanding programs and an advanced version developed ex-perimentally by the author are described.

I N T R O D U C T I O NT H E L u n a r M o d u l e G u i d a n c e C o m p u t e r ( L G C )c o n t a i n s 2 0 48 w o r d s o f e r a sa b l e m e m o r y , a n d3 6 ,8 6 4 w o r d s o f f ix ed m e m o r y f o r p r o g r a m s , o f 1 5u s a b l e b it s e a c h . I t i s p r o g r a m m e d i n t w o l a n -

g u a g e s , m a c h i n e l a n g u a g e , w i t h a n a d d t i m e o f 2 4/ ~ s e c , a n d a l i s t - p r o c e s s i n g i n t e r p r e t i v e l a n g u a g e , i nw h i c h , f o r e x a m p l e , a v e c t o r c r o s s - p r o d u c t , w i t he a c h c o m p o n e n t i n d o u b l e- p r e c is i o n , c o n s u m e s 5m s e c . T h e L G C s w o r k is d o n e i n c h u n k s ca l le dj o b s o f v a r y i n g l e n g t h , w h i c h a r e a s s i g n e d p r i o r i t i e ss o t h a t a h i g h e r p r i o r i t y j o b c a n b r e a k i n o n a l o w e r

* Received 12 July 19 71 ; revised 7 April 197 2. Theoriginal version of this paper was presented at the 4th IFA CSymposium on Automatic Control in Space which was heldin Dubrovnik, Yugoslavia during September 1971. It wasrecommended for publication in revised form by AssociateEditor H. A. Spang III .

t Principal Engineer, M.I.T. Charles Stark Drap erLab orato ry, Cambridge, Massachusetts, U.S.A .

p r i o r i t y j o b . J o b s a r e r e q u e s t e d o f a n d d i s p a t c h e db y a n E x e c u t i v e r o u t i n e . T h e L G C a l s o h a s i nt e r -r u p t s u s u a l l y c a l le dt a sks . T h e c h a r a c t e r i s t i c s o f t -w a r e e n t i t y f o r t h e p o w e r e d f l i g h t p h a s e s i s t h es o - c a l l e d S e r v i c e r j o b . S e r v i c e r is r e q u e s t e d e v e r y 2s e c b y th e t a s k t h a t r e a d s t h e L M a c c e l e r o m e t e r s .S e r v i ce r c o m p u t e s t h e v e h i cl e m a s s , i t s p o s i ti o n a n dv e l o c i t y v e c t o r s , a m a t r i x r e l a t in g t h e a t t i t u d e o f t h es p a c e c r a f t t o t h e o r i e n t a t i o n o f t h e in e r t ia l p l a t f o r m ,a n d a s s o r t e d o t h e r p a r a m e t e r s , s u c h a s a l t i t u d ed u r i n g l i n e a r d e s c e n t a n d a s c e n t. D u r i n g d e s c e n to n l y, S e r v i c e r u s e s d a t a f r o m t h e l a n d i n g r a d a r t oc o r r e c t t h e p o s i t i o n a n d v e l o c i t y e s t im a t e s . S e r v i c ert y p i c a l l y e n d s b y t r a n s f e r r i n g c o n t r o l , s t i l l i n t h es a m e j o b , t o t h e g u i d an c e p r o g r a m a p p r o p r i a t e t ot h e p h a s e in p r o g r e s s : P r o g r a m 6 3 , 6 4 o r 6 5.G u i d a n c e u s e s t h e i n f o r m a t i o n f r o m S e r v i c e r t oc a l c u l a t e a t h r o t t l e s e t t i n g , i f t h e t h r o t t l e a b l e d e s c e n te n g i n e i s b e i n g u s e d , a n d a t h r u s t a x i s d i r e c t i o n .A f t e r G u i d a n c e , s e le c te d p a r a m e t e r s a r e d i sp l a y e do n th e k e y b o a r d a n d d i s pl a y u n i t ( D S K Y ) . D u r i n gd e s c e n t a n d a s c e n t , a t a s k o c c u r s f o u r t i m e s as e c o n d t o m a i n t a i n d i s p l ay s o f a l t it u d e , a l t i t u d e -r a t e , f o r w a r d v e l o c i t y, i n d e s c e n t o n l y, a n d l a t e r a lv e l o c i t y o n s p e ci a l c o c k p i t in s t r u m e n t s . A l s o r u n -n i n g i n t h e b a c k g r o u n d i s a r o u t i n e w h i c h t r a n s m i t sL G C v a r i a b l e s to th e g r o u n d . Te n t i m e s a s e c o n d ,i n i n t e r r u p t , t h e D i g i t a l A u t o p i l o t ( D A P ) i s p r o -c e ss e d . T h e D A P f i re s a t t i t u d e c o n t r o l t h r u s t e r s a n dg i m b a l s t h e d e s c e n t e n g i n e to b r i n g t h e L M t oei thert h e a t t i t u d e c a l c u l a t e d b y g u i d a n c e , i f t h e a t t i t u d em o d e s w i t c h i s i n th e A u t o p o s i t i o n , o r t h e a t t i tu d e

c o m m a n d e d b y th e A s t r o n a u t t h r o u g h h is c o nt r o ls t i c k , i f t h e s w i t c h i s i n A t t i t u d e - h o l d . I n t h e l a t t e r

2 4 3


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244 DONALD EYLES

mode , p rope r ly ca l l ed r a t e - command a t t i t ude -ho ld ,a de f l ec t ion o f t he con t ro l s t i ck p roduces a ro t a -t i ona l r a t e p ropor t io na l t o t he deg ree o f de f l ec t ion ;when the s t ick i s not def lec ted , the vehic le a t t i tudeis he ld constant .

An A po l lo l una r l and ing beg ins w i th P rog ram 63,know n as t he b rak ing phase . Fu l ly au toma t i ca l ly,P rog ram 63 gu ides the LM f rom the pe ri aps i s o f a60 x 9 mi le l una r o rb i t t o a s e t o f t a rge t cond i t i onscal led h igh-gate , usual ly ta rgeted near 7000 f t a l t i -tude , 25 ,000 f t sho r t o f the landing s i te , wi th a for-ward ve loc i ty o f 400 f t /s ec and a do wn wa rd r a t e o f160 f t / sec . W hen these targets have been achieved,P rog ram 64, t he v i s ibi li t y phase , t akes o ve r andbeg ins w i th a maneuve r wh ich b r ings t he l and ings it e i n to t he v iew o f t he As t rona u t . Dur ing th isphase the As t ro nau t can r edes igna te t he l and ing s i te

to t he a r ea where he wishes t o land . P rog ram 64gu ides t he spac ec ra f t t o l ow-ga te , t yp i ca l ly ta rge t edat 1 00-200 f t d i rec t ly abo ve the s ite . At low-gate ,i f no t s e l ec t ed soone r by the A s t ronau t , t he f ina ll a n d i n g p r o g r a m s c o m m e n c e .

P r o g r a m s 6 3 a n d 6 4 h a v e b r o u g h t t h e L M t o th ene igh bou rhood o f t he l and ing si te . The t a sk o f thef ina l l and ing p rog ram, P rog ram 66 , i s t o g ive thehum an p i lo t a s muc h he lp a s poss ib l e in mane uve r-ing the LM to a pa r t i cu l a r, s a f e t ouchdown po in t .The spec i a l so f tware p rob lems conf ron ted in t h i sphase s t em f rom the f ac t t ha t compu ta t iona l de -m a n d s o n t h e g u i d a n c e c o m p u t e r a r e a t a m a x i -mum fo r an Apo l lo mi s s ion , and f rom the in t ima tein t e rac t ion wi th t he c r ew which i s r equ i r ed .

CURRENT CONFIGURATION

In the cu r r e n t con f igu ra t ion o f t he f ina l l and ingprogram, P rog ram 66 , t he As t ronau t has a cho iceo f fou r mod es o f con t ro l , d i f f e r ing in t he d iv i sionof du t ie s be tween As t rona u t and gu idance com-pu te r. W hen bo th the a t t i t ude and th ro t t l e mo deswi tches a r e i n Au to , the co mp u te r co n t ro l s t he LMat t i tude , t he t h rus t ax i s , t o p rodu ce ze ro ho r i zon ta lve loc i ty r e l a t ive t o the su r f ace , and man ipu -l a t e s t he LM th ro t t l e t o ma in t a in t he r a t e -o f -d e s c e n t c o m m a n d e d b y t h e A s t r o n a u t . W h e n t h ea t t i t ude m ode swi tch i s in A t t i t ude -ho ld , t he A s t ro -nau t i s r e spons ib le fo r con t ro l l i ng the LM a t t i tude ,when the th ro t t l e mode swi t ch i s i n Manua l , hemus t co n t ro l t he t h ro t t le . W hen ne i the r swi tch isin Au to , t he As t ronau t has t o con t ro l bo th a t t i t udeand throt t le , a job which i s d i ff icul t for one man,b u t c a n b e d i v i d e d b e t w e e n C o m m a n d e r a n d L MPi lo t, whe re the Co m ma nde r occup ie s t he l e ft -handpos i t i on in t he LM and the LM P i lo t i s on ther ight. I t i s con ven ient to d iscuss the hor izon ta l

and ver t ica l chann els of the f ina l landing guid-ance sepa ra t e ly.

In the ve r ti ca l channe l , t he com ma nde d r a t e -o f -descent i s se t to the ac tual ra te a t the in i t ia t ion of

P rogram 66 , and wheneve r t he t h ro t t l e i s r e tu rnedt o th e L G C a f t e r b e i n g o p e r a t e d m a n u a l ly. F r o mthen on i t i s subjec t to modif ica t ion , in s igned 1f t / s ec i nc remen t s , by the As t ronau t t h rough h i sr a t e -o f -descen t (ROD ) swi tch , a t h ree pos i t ion

togg le w i th a sp r ing r e tu rn to de t en t . The cu r r en tr a t e -o f -descen t a lgo r i t hm i s a s fo l lows

Arc = ( ( ( / : / o - / : / + L a g ( G - r cos ~)) / r ) - G) /cos

(1)

where Arc i s t he commanded th rus t acce l e ra t ionmagn i tude , / : / o i s t he des i r ed ve r t i ca l ve loc i ty, / : / i sthe present ver t ica l ve loci ty, G is the ver t ica lacce l e ra t ion due to g rav i ty, ~ i s t he ang le be tweenthe ve r t ica l and the LM th rus t ax i s , and Lag ex -p re s ses t he de l ay be tween va l id i ty o f da t a an d

veh ic le r e sponse in gu idance com ma nds . AF i sca l cu l a t ed f rom the measu red , ave rage acce l e ra t ionove r t he p receed ing sample in t e rva l and r ep re sen t sthe in s t an t aneous , p re sen t t h rus t acce l e ra t ion mag-n i tude . The th ro t t l e change com m and ed i s s imp lyAec-A r sca led by the spacec ra f t mass .

To speed i t s re sponse , t he r a t e -o f -descen t a l -go r i t hm is execu ted a t tw ice the Se rv i ce r f r equen cy :onc e ever)~ secon d. I t is processed onc e every 2 secin t he conven t iona l spo t a t t he en d o f Se rv ice r, andaga in in a spec ia l j ob , o f h ighe r p r io r i t y t han theSe rv i ce r j ob , r eques t ed to occu r 1 s ec l a t er. Thus ,in s t eady s t a t e , one execu t ion fo llows Se rv i ce r, theo the r i s imbedded in the beg inn ing o f the succeed ingServ ice r pas s . The r a t e -o f -desce n t a lgo r i t hm in -c ludes it s own acce l e ro me te r r ead ing and ve loc i tycom pu ta t ion . In t he cu r r en t con f igu ra t ion , t he ra t e -o f -descen t a lgo r i t hm is wr i tt en in t he L GC s s lower,i n t e rp re t ive l anguage , and t akes 0 .3 sec t o com-p le t e . The quan t i t y Lag in t he equa t ion above i smad e up o f th i s compu ta t ion pe r iod p lus t heth ro t t l e r e sponse time . Re fe rence [ I] may be con -su l ted fo r j u s t i fi ca t ions o f t he gu idance a lgo r i t hmsemployed in (he cu r r en t con f igu ra t ion .

3 5

3 0 0 0

~ _ 2 5 0 0k -

v

-7: :

2 0 0 0

1 5 -

I 0

0 5

0

- 0 5

I I I

+ , R O D L - R O D' r : ~ l r l ] ~ : I : E l : I ~ I ' I T I [ I I l I T I I

T i m e , s

FIG. 1. Current configuration ROD response.


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A p o l l o L M g u i d a n c e c o m p u t e r s o f t w a r e f o r t h e f in a l l u n a r d e s c e n t 2 4 5

F igu re 1 , t aken f rom a s imu la t ion o f t he f li gh tc o m p u t e r p r o g r a m f o r A p o l l o 1 4 , s h o w s t h ere sponse o f t he cu r r en t - con f igu ra t ion ve r t i ca lchanne l t o pos i t i ve and nega t ive ROD swi t ch in -pu t s . He re t he p lo t t ing f r equen cy i s 0 .1 s ec ; i .e .

t h e p l o t c o n ta i n s t e n p o i n t s f o r e a c h h a t c h m a r k o nthe absc is sa . Th e no i s ines s in t h rus t , and conse -quen t ly i n a l t i t ude - r a t e , be tween ROD inpu t s , i sdue to t he g ranu la r i t y o f acce l e rom e te r s (0 .01me ter /see) an d c locks (0 .01 see) , wh ich effec t thec o m p u t e d a l t i t u d e - r a t e / : / a n d t h e t h r u s t a c c e l e r a -t i on e s t ima te A t .

P r o g r a m 6 6 c a n b e e n t e r e d a t a n y t i m e i n t h edescen t by (1 ) pu t t i ng a t t i t ude mode in A t t i t ude -ho ld , and (2 ) de f l ec t ing the RO D swi t ch . A RO Dinpu t i s added to mode in A t t i t ude -ho ld a s a con -d i t i on fo r en t e r ing P rog ram 66 to a l l ow the As t ro -

nau t t o ge t t he f ee l o f t he LM 's ha nd l ing qua l i t i e sea r l i e r i n t he l and ing by maneuve r ing in A t t i t ude -ho ld , cued by a t t i t ude e r ro r s d i sp l ayed by thed ig i ta l au top i lo t , and , i n add i t i on , t o p reven t p rem a-tu re s e l ec t ion o f P rog ram 66 in t he even t o f a s ing le-p o i n t f a il u re . T h e t r a j e c to r y fl o w n b y P r o g r a m 6 4h a s a l w a y s b e e n t a rg e te d to h a v e a f l a r e - - s o t h a ta round 500 f t a l t i t ude the descen t - r a t e i s modera t ea n d s u i t a b l e f o r p a r t i a l m a n u a l t a k e o v e r u s i n gPro gra m 66 : a ro un d 15 f t /s ec . Ap o l lo 11 en t e redProgram 66 a t 410 f t a l t i t ude and ach ieved touch -dow n 140 sec l a t e r ; Apo l lo 12 used P rog r am 66 fo r110 sec s tar t ing a t 368 f t a l ti tude .

Du r ing Ap o l lo IT s f ina l l and ing , dus t causedmo re d i f f i cu lty t han expec ted and , a s a r e su l t , aho r i zon ta l channe l was added to t he f i na l l and ingg u i d a n c e. D u s t w a s fi rs t k i c k e d u p w h e n t h e L Mwas a t 300 f t a l t i tude , a nd la ter, be low 100 f t , i t wasd e n s e e n o u g h t o m a k e t h e v i e w f r o m t h e w i n d o wuse le s s a s an a t t i t ude r e f e rence . A t t i t ude f luc tua -t i ons wh ich r e su l t ed in t e r f e red wi th ma in t a in ingze ro ho r i zon ta l ve loc i ty, a l t houg h dus t d id no tin t e r f e re w i th t he A s t rona u t ' s ab i l i ty t o j udge h i sve loc i ty ac ros s t he su r f ace , s ince rocks were v i s ib let h r o u g h t h e d u s t.

W h a t w a s n e e d e d w a s a m o d e i n w h ic h t h e c o m -p u t e r w o u l d m a n e u v e r t h e s p a c e c r a f t t o m a i n t a i nze ro ho r i zon ta l ve loc i ty r e l a t i ve t o t he su r f ace .Accord ing ly, when the a t t i t ude mode swi t ch i s i nA u t o , t h e g u i d a n c e c o m p u t e r e m p l o y s t h e f o ll o w i n ga l g o r i t h m i n t h e h o r i z o n t a l c h a n n e l

A rc = ( (VD-- V ) /0 - 0 .4A FCold G (2)

w h e r e AFC i s t h e c o m m a n d e d t h r u s t a c c e l e r a t i o n ,Vo i s the ve loc i ty o f t he l una r su r f ace be low theLM V i s the p re sen t ve loc i ty, G i s t he acc e l e ra t iondue to g rav i ty, and ~ i s t he t ime co ns t an t . I t was

fou nd tha t 5 s ec was a su i tab l e va lue fo r 3 . Th ev e r t i c a l c o m p o n e n t o fArc i s se t to lunar gravi ty.To p reven t a t t i t ude excu r s ions i n p i t ch o r ro l lg rea t e r t ha n 20 away f rom the e r ec t , t h rus t ax i s

v e r ti c a l, p o s t u r e , t h e h o r i z o n t a l c o m p o n e n t o f A ~ cis l imi ted to ,4U~lm, an acc eler a t ion ch ose n so th a tAalim G is t he t an gen t o f 20 . No te t ha t a l l t hev e c t o r s m e n t i o n e d a r e i n t h e c o o r d i n a t e f r a m e o fthe ine r t i a l p l a t fo rm , w h ich i s a l i gned so tha t a t t he

t ime o f t he l and ing i ts x -ax i s i s ve r t i ca l , f ac i li t a t ingsepa ra t ion o f t he ve r ti ca l and ho r i zo n ta l channe l s .T h e h o r i z o n t a l c o n t r o l a l g o r i th m , w h i c h c o n s u m e sabo u t 0 .3 s ec is execu ted eve ry 2 s ec a t t he end o fServicer.

W hen the cu r r en t co n f igu ra t ion in i ts f i na l fo rmwas f i r s t t r ied , i t r an wi th a du tycyc le o f ove r 96p e r c e n t - - w o r s e t h a n t h e p r e v i o u s m a x i m u m -d u t y c y c l e c a s e, a b o u t 9 0 p e r c e n t i n P r o g r a m 6 4.As t ronau t i n i t i a t ed ac t iv i t i e s , such a s DSKYm o n i t o r i n g o f c o m p u t e r p a r a m e t e r s , c o u l d i nc r e a s eth i s t o ove r 100 pe r cen t , and the re was a fu r the r

i n d e t e r m i n a t e l o a d d u e t o in c r e m e n t s a n d d e c r e-m e n t s o f t h e L G C ' s i n t e r f a c e c o u n t e r s , e a c h o fw h i c h r e q u i r e d o n e c o m p u t e r m e m o r y c y c l e . I tshou ld be no ted he re t ha t exces s ive in t e r f acea c t iv i t y c a u s e d a s h o r t a g e o f c o m p u t a t i o n t i m edur ing the l and ing o f Ap o l lo 11. To m ee t th i sd i f fi cu lty, l og i c was p rov ide d w h ich om i t s one cyc l eo f P r o g r a m 6 6 g u i d an c e , t w o p r o c e ss i n gs o f th ev e r ti c a l a l g o r i t h m a n d o n e o f t h e h o r i z o n t a l , w h e nthe t ime avai lable i s insuff ic ient .

ADVANCED CONFIGURATION

To see i f the capab i l i t ie s o f t he gu idance co m-pu te r cou ld be u t i l ized m ore fu l ly in a s s i s t ing theA s t r o n a u t d u r i n g t h i s p h a s e , a n e x p e r i m e n t a l , a d -v a n c e d v e r s i o n o f t h e f i n al l a n d in g p r o g r a m w a sd e v e l o p e d . T h i s w a s a n a t t e m p t t o p r o f it f r o mA p o l l o e x p e r i en c e a n d w a s n o t i n t e n d e d t o f l y o nan ac tua l Apo l lo mi s s ion . Ho we ver it was Codedf o r t h e g u i d a n c e c o m p u t e r , a n d i t s d e v e l o p m e n tben i f i t ed f rom use o f a l l -d ig it a l a nd r ea l - t imeh y b r i d s im u l a t o r s a t M I T , a n d t h e L M M i s s i o nS i m u l a t o r a t C a p e K e n n e d y.

T h e p r o g r a m u s e d a s a t e s t b e d f o r t h is w o r k w a sa n o f f q in e v e r s io n o f t h e L M f li g ht p r o g r a m i nwhich the au tho r dev e loped a so lu t ion to d i f f icu l ti e sc a u s e d b ) a s h o r t a g e o f e x e c u t i o n t i m e w h i c h w a sm ore gene ra l t han the omis s ion s t r a t egy, me n t ioneda b o v e , a d o p t e d i n t h e c u r r e n t c o n f i g u r a t i o n a f t e rt h e h o r i z o n t a l a l g o r i th m w a s i n tr o d u c e d . S u c hd i ff i cu l ti e s a r i s e when m ore than 2 s ec o f ac t iv i tya re s chedu led ev e ry 2 s ec by the Se rv i ce r j ob . In t heo ff - l i ne p rog ram, t he Se rv i ce r cyc l e i s a l l owed tot ake longe r when i t i s ove r loade d . A l l com pu ta -t i ons wh ich depended on Se rv i ce r r ecu r r ing a texac t i n t e rva l s a r e mod i f i ed to u se a compu ted ,v a r i a b le g u i d an c e p e r i o d i ns t ea d . T h e c o m p u t a t i o n -t ime d i f f i cu l t i e s o f t he cu r r en t con f igu ra t ion sug -g e s te d t h a t f u r t h e r i m p r o v e m e n t s o f P r o g r a m 6 6wou ld be ea s i e r where exec u t ion - t ime was l es scri t ical .


4/8

246 DONALD EYLES

In the advanced conf igu ra t ion the gu idance a l -g o r i th m s f o r P r o g r a m 6 6 a r e al l d e t a c h e d f r o m t h eSe rv ice r j ob . Th ey a re a t t ached to t he ta sk p ro -ces sed fou r t imes a s econd to ma in t a in d i sp l ays o fa l t i t ude , a l t i t ude - r a t e , and fo rward and l a t e r a l

ve loc i ty on coc kp i t i n s t rumen t s . Bes ides t he d is -p l ayed va lues , t h is rou t ine ou tpu t s a ve loc i ty vec to rc o r r e c t e d f o r lu n a r r o t a t io n . N o n e o f th e g u i d a n c eequa t ions i n u se i n P rog ram 66 r equ i r e a pos i t i onv e c t o r a s in p u t . T h r o u g h o u t P r o g r a m 6 6 , a j o b i sr eques t ed eve ry sec by the d i sp l ay rou t ine . Inth is j ob , a gu idance a lgo r i t hm , o r a D SK Y d i sp lay,o r no th ing , i s p roces sed ac co rd ing to dec i s ion log icexec uted a t the t ime. This logic i s show n in Fig . 2 .

Every I14 ec

,:,,I

Y

1 1o n t r o l l e r d e f l e c t i o n ~

Y

N los t p~ ?

t

- y -

I H Z c o . = . Z . . . . . I

~

J

Flo. 2. Program 66 guidance decision logic.

e r t i c a l c h a n n e l

The f ir s t pa r t o f t he l og ic conce rns t he ve r t i ca lchanne l . Th i s i s t he cu r r en t con f igu ra t ion s r a t e -o f -descen t equa t ion , g iven in equa t ion (1 ) , r ecoded inmach ine l anguage so tha t i t s execu t ion t ime i sr e d u c e d b y a n o r d e r o f m a g n i tu d e , t o a b o u t 2 5msec . Because the quan t i t y Lag is co r r e sp ond ing lyreduced , t he v e loc i ty com pen sa t ion t e rm is e limi -

n a t e d . A s p r o g r a m m e d th i s e q u a t i o n b e c o m e ss imply

A FC = (((/:/o --/:/)/Z ) -- G)/COS. (3)

Fur the r, an a t t empt i s made to speed up there sponse to As t rona u t i npu t s , and to ach ieve g rea t e rsmoo thness i n t he t h ro t t l e p ro f i l e , and thus i na l t i tude - r a t e , i n t he ab sence o f As t ro nau t i npu t s.Th e r e sponse t ime o f the cu r r en t r a t e -o f -desce n t

logic is l imi ted (1) by the 1 sec in terval a t whichROD swi t ch inpu t s a r e coun ted , and (2 ) by thedes i r ab il i ty o f u s ing a t ime cons t an t l onge r t han thesample in t e rva l fo r r ea sons o f s t ab il i ty. Sm oo th -ness in the thro t t le prof i le is mi l i ta ted agains t by theg r a n u l a r it y o f L M a c c e l e r o m e t e r r e a d in g s a n d o fthe LGC c locks . Fo r sho r t e r s ample in t e rva ls ,p re fe rab le i n t e rms o f r e sponse t ime , t hese g ranu-lar i t ies are re la t ive ly exaggera ted in the throt t lec h a n ge c o m m a n d e d .

A scheme was ado p ted in wh ich , when theve loc i ty e r ro r is g rea t e r t ha n a ce r t a in l imi t , t he

ra t e -o f -descen t equa t ion i s execu ted immed ia t e ly,excep t t ha t execu t ions a r e no t a l l owed more f r e -quen t ly t ha n eve ry sec . I f t he ve loc i ty e r ro r i s le s sthan the l imi t , t he r a t e -o f -descen t equa t ion i sexecu ted T sec a f t e r t he p recee d ing pas s , whe re T isthe t ime cons t an t u sed the p receed ing pas s . Th i sa r r a n g e m e n t b o r r o w s t h e i de a o f a d e a d b a n d f r o ma u t o p i l o t p r a c t i c e - - o n l y t h e s y s t e m i s n o t d e a dwi th in t he dead band , on ly qu ie t . Add i t i ona l ly,l og ic was p rov ided so tha t t he t ime cons t an t u sedwhen ve loc i ty e r ro r i s ou t s ide o f the qu ie tband (TI )can be d i f f e r en t f rom th a t u sed in s ide the qu ie tban d(T2). No te t ha t i n t h i s a r r ang em en t r e spo nse toROD swi t ch inpu t s i s f a s t e r t han in t he p rev iousconf igu ra t ion bo th because (1 ) RO D inpu t s a r elooked fo r, and in gene ra l r e sponded to , fou r t imesas f r equen t ly a s be fo re , and (2 ) a sho r t e r t ime con-s t an t can be used because the s ample f r equency,whe n ve loc i ty e r ro r i s ou t s ide the qu ie tband , istw ice a s h igh . The no i se in com m and ed th ru s t isr educed by the use o f a s ample in t e rva l , whenwi th in the qu ie tband , l onge r t han the 1 s ec s amplein t e rva l o f t he cu r r en t con f igu ra t ion . No i se wasa l so r educed by the use o f a f i ne r-g ra in c lock fo rmea su r ing the s ample in t e rva l. Op t imu m cho icesfo r t he t ime cons t an t s ~ l and T2, and fo r t heve loc i ty e r ro r l imi t L have no t been sough t . G oo dpe r fo rmance has been ob ta ined in s imu la t ions w i thz l = 1 s ee , z 2 = 2 s ec a n d L = 0 . 2 f t/ s ec .

R o t e - o f - d e s c e n ta l g o r i t h

t f r y c. . . . . . . . . un t i l n e x tR O D i n p u tR O D s w i t c h i n p u t

5 4 -3 -2 - I 0 I 2 3 4 5 6 7

T i m e s

F I c 3 R O D algorithm timing.


5/8


F igu re 3 shows the pa t t e rn o f r a t e -o f -descen ta lgo r i t hm exe cu t ions fo l lowing a RO D swi t ch inpu ttha t occu r s a t t ime ze ro . I f t he p rev ious execu t ionw e r e a t ti m e - 0 . 2 5 t h e s a m e p a t t e r n w o u l d o c c u r0 .25 sec la ter.

T h e d y n a m i c s o f th e a d v a n c e d c o n f i g u r a ti o n o fthe r a t e -o f -descen t sys t em a re a s fo l lows : t he RO Di n p u t c h a n g e s c o m m a n d e d v e r ti c a l v e l o c it y b y 1f t /s ec . W hen nex t checked , o rd ina r i l y w i th in sec, ve loc i ty e r ro r exceeds 0 .2 f t / s ec and the RO Dalgor i thm i s execu ted im med ia t e ly, and aga in secla ter, wi th z se t to I sec . Qu ick m odif ic a t ion of thedescen t r a t e is t he a im now. I t does no t ma t t e r t ha tthe acce l e ra t ion e s t ima te AF, based on a s amplein t e rva l o f 0.5 s ec, is inaccu ra t e . W hen ve loc i tye r ro r becom es l e ss t han 0 -2 f t/ s ec , t he t r ans i t i on tothe qu ie t mode beg ins . De adbe a t r e sponse is

sough t by l e t t i ng the nex t s ample in t e rva l equa l t hecu r ren t t ime cons t an t , 1 s ec. The acce l e ra t iones t ima te fo r t h i s s ample in t e rva l i s improved be -cause the i n t e rva l i s l eng thened . In t he nex t execu -t ion o f t he r a t e -o f -descen t a lgo r i t hm, t he t h i rd s incethe ROD inpu t , z i s s e t t o 2 s ec , and the fou r thexecu t ion , wh ich thus occu r s 2 s ec l a t e r, bene f i t sf rom a s t il l l onge r s ample in t e rva l. Th e r a t e -o f -descen t a lgo r i t hm con t inue s t o be execu ted eve ry 2sec un t i l exc i t ed by ano the r R O D inpu t . Th i ssys t em t akes advan tag e o f i ts s ample da t a n a tu reb y m a n i p u l a t i n g t h e t i m e c o n s t a n t a n d t h e s a m p l e

in t e rva l t oge the r, t o ach ieve qu icke r r e sponse whenve loc i ty e r ro r is ou t s ide a qu ie tband , and s mo o the rcontro l when i t i s ins ide .

3 5 0 0

3 0 0 0

25I---

2 0 O O

15

I 0

0 5

, r

+ R O D - R O D

: : I : , r i p i [ ' , 1 [ ' , 1 i p I 11 [

T i m e

FK;. 4. Advan ced configuration ROD response.

Ta k e n f r o m a s i m u l a t io n o f t h e a d v a n c e d c o n -f igura t ion , F ig . 4 shows thrus t and ver t ica l ve loci tyreac t ing to pos i t i ve and nega t ive ROD swi t ch in -puts , and as in Fig . 1 , to which Fig . 3 should be

com pare d , t he p lo t t i ng f r equenc y is 0 .1 s ec . Thev a r y i n g i n t e r v a l b e t w e e n c o m m a n d e d t h r o t t l echanges i s c lear ly seen in the thrus t p lo t , as i s them o d u l a t i o n o f t he t h r o t t l e i n m o m e n t s o f e x c it a ti o n ,

and the r educ t ion o f no ise du r ing qu ie t pe r iods .By chance , t hese p lo ts show the r ea c t ion speed o fthe advanced conf igu ra t ion in t he bes t poss ib l el i gh t ; r e sponse to t he RO D inpu t s beg ins a lmo s t afu l l s econd so one r t han in the cu r r en t con f igu ra t ion .

F ina l ly, i t shou ld be po in t ed ou t t ha t a gu idanceor con t ro l sys t em such a s t h i s , p roces sed more o rl e s s f r equen t ly acco rd ing to need , o ff e r s po ten t i a lcompu ta t ion - t ime sav ings ove r a sys t em p roces seda t a c o n s t a n t f r e q u e n c y i n t e r m e d i a te b e t w e e n t h efa s t e s t and s lowes t f r equenc ie s o f t he va r i ab l esys t em. Whe the r t hese s av ings can be r ea l i zedd e p e n d s o n t h e a v e r a g e f r e q u e n c y o f e x c it a t io n , o nthe t ime-consumpt ion o f t he dec i s ion log ic wh ichm u s t b e p r o c e s s e d e v e r y p a ss , a n d o n w h e t h e r t h eou te r l oop , t he Se rv i ce r, c an to l e r a t e t empora rype r iods o f h igh ac t iv ity in t he i nne r l oop . As a

gene ra l s t a t emen t , i t i s compu ta t ion t ime wh ich i st r a d e d f o r t h e i m p r o v e d p e r f o r m a n c e o f t he a d -vanced con f igu ra t ion o f t he f ina l l and ing p ro g ram .How ever, a s a p rac t i ca l f ac t , th i s is t rue on ly whenthe i ,mer l oop , desc r ibed above and be low, i sexc i t ed by As t ronau t i npu t s .

orizontal channel

Th e r em a inde r o f the l og ic shown in F ig . 2 con -ce rns t he ope ra t ion o f the ho r i zo n ta l channe l o f t heP r o g r a m 6 6 c o n t r o l s y st e m . T h e h o r i z o n t a l c o n t r o lequa t ion i s bas i ca l ly unchanged f rom the cu r r en tconf igura t ion . In genera l i t i s exe cute d every 2 sec .A n i n d ex , c a l le d H Z C O U N T , i s i n c r e m e n t e d e v e ry sec, and the h o r i zon ta l con t ro l equa t ion i sexecu ted w hen the index equa l s ze ro m odu lo 8 .A d i sp l ay com pu ta t ion is execu ted eve ry second ,when the index equa l s 2 mo du lo 4 , and the en t i r epa t t e rn can be sh i f t ed a t any t ime by r e in i t i a l i z ingthe index r eg is t e r. Th i s mec han i sm i s neces sa ryto the s it e des igna t ion sys t em p rov ide d fo r P ro -g ram 66 in the adva nced conf igu ra t ion a s, w i ll beseen. Ref . [2] g ives a desc r ip t ion of the landin gp o i n t d e s i g n a t i o n ( L P D ) s y s t e m f o r P r o g r a m 6 4 ,to wh ich th i s sys t em fo r P ro g ram 66 can be com -pa red .

Th e cen t r a l i dea o f the P r og ra m 66 des igna t ionscheme o r ig ina t ed with an As t ron au t sugges t ion . I tw a s p r o p o s e d t h a t t h e g u id a n c e c o m p u t e r c o m p u t eand d i sp l ay on the DSKY the ang le , a long theLPD re t i c l e on the l e f t -hand window, a t wh ich theL M C o m m a n d e r c a n s e e t h e s p o t w h e r e h e w o u l dtouch d own i f a t t ha t i n s t an t he swi t ches f romAt t i t ude -ho ld to A u to , r e tu rn ing con t ro l o f t heveh ic l e 's a t t i t ude to t he ho r i zon ta l ve loc i ty pu l l inga lgo r i thm. In e ffec t t he As t rona u t wo u ld en t e r a

m ark , by swi t ch ing to Au to , w hen the s i te hedes i res passes a spot o n h is win dow speci f ied byt h e c o m p u t e r. F i g u r e 5 s h o w s t he a r r a n g e m e n t o fthe LM co ckp i t , i nc lud ing the LP D re t i c le on thel e f t -hand window.


6/8

2 4 8 D O N A L D E Y LE S

LM ontroller~

commander

omputer omputerdispl y keybo rd

FIG. 5 . LM cockpi t conf igura t ion.

. . S o z .0 ~ ; ~

H

j

spot ---:

F IG . 6. P rog ram 66 s it e de s igna t ion geome t ry.

See F ig . 6 . The ang le 0 wh ich is to be d i sp l ayedto the As t ronau t depends on the veh ic l e ' s a t t i t ude ,spec i f i ca l ly on the body z -ax i s , and on the twod i s t ances H and D . H i s t he LM a l t it ude . S ince the

LG C' s i dea o f t he spacec ra f t ' s a l t i t ude has beenc o n t i n u o u s l y c o r r e c t e d b y l a n d i n g r a d a r r e a d in g s ,th is a l t i t ude i s va l id fo r t he l and ing r ada r a n t enna ,w h i c h is m o u n t e d o n t h e b o t t o m o f th e L M d e s c e n ts t age , so t he quan t i t y H mus t be co r r e c t ed by add -ing the d i s t ance be tween the an t en na an d the As t ro -nau t ' s eye , abou t 4 m . D i s t he d i s t ance the space -c ra f t w i l l t r ave l wh i l e t he h o r i zon ta l con t ro la lgo r i t hm nu l ls i ts fo rw ard ve loc ity t o ze ro . I tdepend s on the ho r i zon ta l r a t e , wh ich is know n , andthe cha rac t e r i s t ic s o f t he ho r i z on ta l ve loc i ty nu l l inga lgo r i thm.

The quan t i t y D i s ca l cu l a t ed by mode l ing theacce l e ra t ion p ro f i l e f l own by the ho r i zon ta l a l -g o r i t h m w h e n m o d e i s r e tu r n e d t o A u t o , a s f o ll o w s

D = Dela y Vu + (V 2 + ,2u)/2Amim (4)

where Vn i s t he LM hor i zon ta l ve loc i ty. V 'n i su

or zAmlm, whichever i s smal ler, and Delay i s theave rage t ime i t t akes fo r t he a t t i t ude maneuve r t obeg in a f t e r mod e i s swi tched to A u to , p lus ha l f the

t ime i t t akes t o r each the b rak ing a t t i t ude once thema neu ve r has s t a r t ed . Ann m i s t he h o r i zon ta lacce l e ra t ion l imi t wh ich i s app l i ed to t he ou tpu t o fequa t ion (2 ). G iven D , t he l i ne -o f- s it e vec to r canbe eas ily com pu ted , and f rom tha t 0 can be ca l cu -l a t ed . The t ime qua n t i t y De lay i s ava i l ab l e fo rtun ing the ca l cu l a t ion fo r t he g rea t e s t accu racy inthe ve loc i ty r ange c on tem pla t ed fo r u se o f t hePro gra m 66 s it e des igna t ion capab i l i t y.

No te t ha t i f the ho r i zon ta l con t ro l a lgo r i t hm i sexecuted s t r ic t ly every 2 see , then there i s a 2-secva r i ab i l it y i n when , fo l lowing the m ark , t he

spacec ra f t r eac t s t o i t. Thus the ac tua l b rak ingd i s t ance cou ld va ry by 2 Vu . Fo r a ho r i zon ta lve loc i ty o f 50 f t / s ec - - a t yp i ca l one fo r t he u se o ft h i s c a p a b i l i t y - - D c o u l d v a r y t h r o u g h a r a n g e o f


7/8


100 f t. Because th is was cons ide red an unac cep -t ab l e l imi t a t i on on accu racy, t he i ndex ing mech-h a n i s m d e s c r i b e d a b o v e w a s i m p l e m e n t e d . I ta l l ows the pa t t e rn o f t he ho r i zo n ta l ca l cu l a t ions t obe r e in i t ia l i zed a t any t ime , so an execu t ion o f the

h o r i z o n t a l a l g o r i t h m c a n b e c o m m a n d e d i m -m e d i a t e l y w h e n t h e A s t r o n a u t i s s e e n t o h a v eswi t ched to Au to . Th i s r educes t he va r iab i l i ty int ime to 0 .25 sec - - ac tua l ly 0 .35 sec i nc lud ing theu n c e r t a i n t y i n h o w s o o n t h e D A P w i ll a c t o ng u i d a n c e c o m m a n d s - - a n d t h e v a r i a b i l i t y i n d i s -t ance i s r educed co r r e spo nd ing ly. F igu re 7 showsthe t iming o f ho r i zon ta l channe l execu t ions , i nc lud -i n g a s w i t c h f r o m A t t i t u d e - h o l d t o A u t o a t t i m ez e r o . Va l u e s o f H Z C O U N T a r e in p a r e n th e s e s .I t i s pos s ib l e fo r two execu t ions o f the ho r i zon ta la lgo r i t hm, wh ich consumes 0 .3 s ec , t o be r eques t ed

sec apa r t . Th e job p r io r i t y s t ruc tu re i s u sed toinsu re t ha t t he f i r s t f i n i shes be fo re t he s econds t a rt s . Se rv i ce r, r unn ing in t he backgrou nd , i sd e l a y e d b y t h i s m o m e n t a r y h i g h c o m p u t a t i o n a lload , bu t i n t he t e s t -bed p rog ram th i s has no i l le f f ec t . Du r ing th i s phase the ou te r, Se rv i ce r l oopcon ta ins no gu idanc e ; i ts func t ion i s occas iona l lyt o p r o v i d e n e w b a s e v a l u e s , i n c o r p o r a t i n g r a d a rda t a , fo r t he s imp le r nav iga t ion , u s ing on lya c c e l e r o m e t e r d a t a , p e r f o r m e d f o u r t i m e s a s e c o n din the gu idance loop .

1 H o r i z o n t a l v e l o c i t ynullina l g o r i t h m

J ~ e c o m p u t a t i o n a n dS K Y d i s p l a y

A l t i t u d e - h o l d t o a u t o1 1 1 /

- 5 - 4 -.7 -2 - I O I 2 3 4 5

T i m e s

FIG. 7. Horizontal algorithm timing.

Bes ides a new ra t e -o f -descen t s cheme and the s i t edes igna t ion sys t em jus t desc r ibed , ano th e r capa -b i l it y was added in t he advan ced conf igu ra t ion : t heA s t r o n a u t i s e n a b l e d t o m o d i f y t h e h o r i z o n t a lve loc i ty ma in t a ined by the ho r i zon ta l a lgo r i t hm. Inthe cu r r en t c on f igu ra t ion th i s ve loc i ty is a lwaysze ro . In t he adva nced con f igu ra t ion i t i s ze roedwhen P rog r am 66 is s el ec ted , and whe neve r a t t i t udem o d e i s s w it c h ed f r o m A t t i t u d e - h o l d t o A u t o , b u ti t c an be m od i f i ed by the As t rona u t , i nc rem en ta l ly,

by d i sc re t e de f l ec t ions o f t he a t t i t ude co n t ro l s t i ck .T h i s c a p a b i l it y o p e r a t e s o n l y w h e n m o d e i s A u t o ,so manua l maneuve r ing wi th t he s t i ck i s no ta ff ec t ed . I t shou ld be no ted tha t P rog ram 64 s i t e

r edes igna t ions a r e a l so e ff ec t ed by d i sc re t e c l i ck so f the a t t i t ude s t ick . Ve loc i ty i nc rem en t s a r e i npu tin a spacec ra f t - f i xed coo rd ina t e sys t em con s i s t en twi th t he fo rw ard a nd l a t e r a l ve loc i ty i nd ica to r s i nthe LM c ockp i t . A p i t ch de f l ec t ion o f t he s t i ck

mod i f i e s ve loc i ty i n t he fo rward o r r ea rwa rd d i r ec -t i on ; a ro l l de f lec t ion adds ve loc i ty t o t he l e f t o rr igh t . Tw o f t /s ec was found to be a su it ab l e magn i -t u d e fo r e a c h v e l o c it y i n c r e m e n t . C o m m a n d e dhor i zon ta l ve loc i ty can be r e se t t o ze ro e i t he r bypu t t i ng in oppos i t e s t ick de f l ec tions , o r by sw i t ch ingb r ie f ly t o A t t i tu d e - h o l d a n d b a c k t o A u t o . T h i sc a p a b i l i t y a d d s n o n e w e l e m e n t t o t h e s o f t w a r eo rgan iza t ion , excep t fo r an in t e r rup t t r igge red bythe s t ick def lec t ion to count i t .

sage of the ad vance d configurationHe re i s an exa mple o f the u se o f t he capab i l i t ie s

o f t he advance d conf igu ra t ion . A t 500 f t a l ti t ude ,wi th a d escen t r a t e o f 15 f t /s ec , fo rwa rd r a t e o f 80f t / s e c , t h e L M C o m m a n d e r s w i t c h e s m o d e t oAt t i t ude -ho ld and c l i cks t he ROD swi t ch , s e l ec t ingP r o g r a m 6 6 a n d r e d u c i n g t h e d e s ce n t r a t e. H el o o k s f o r w a r d a n d , i f n e c e ss a r y, m a n u a l l y m a n e u -ve r s t he spacec ra f t t o e s t ab l i sh a head ing towardt h e la n d i n g s p o t h e p ic k s o u t. W h e n t h e A s t r o n a u te n t e r e d A t t t i t u d e - h o l d t h e L M w a s p i t c h e d b a c ka b o u t 2 5 ; w h e n f o r w a r d v e l o c i ty h a s d r o p p e d t o40-50 f t /s ec , he p i tches t he veh ic l e fo rw ard so tha tthe t h rus t ax i s i s ve r t i ca l and the fo rward ve loc i ty

b e c o m e s c o n s ta n t . T h e L M P i l o t r e a d s a lo u d t h ea n g l e 0 di s p la y e d o n t h e D S K Y, w h i c h i s c o n s t a n ti f a l t i tude i s cons t an t . Ma k ing su re t ha t the p l aneo f t he L PD re t ic l e and h i s eye inc ludes t he ve loc i tyvec to r, and , i f neces sa ry, yawing the LM a ro und i tsth rus t ax i s so t ha t i t does , t he Commander wa tchesthe su r f ace a t t he po in t on the r e t i c l e i nd i ca t ed byt h e c o m p u t e r. H e p u t s h is h a n d o n t h e a t ti t u d em o d e s w i tc h , a n d w h e n t h e s p o t w h e r e h e w a n t s t otouch dow n c ros ses the i nd ica t ed po in t , he swi tchest o A u t o . T h e c o m p u t e r r e s p o n d s b y p i tc h i n g t h espacec ra f t back 20 As the ho r i zon ta l ve loc i ty i s

n u l le d t o z e r o th e c o m p u t e r r e t u r n s t h e L M t o t h ee rec t a t t i tude . I f t he L M i s no t no w p rec i se ly ove rthe des i r ed spo t , the C om ma nde r de f l ec t s h i s s ti ckto e s t ab li sh a sma l l r a t e i n it s d i r ec t ion . W hen thedes t ina t ion i s r eached , he f li p s mo de b r i e f ly toA t t i t u d e - h o ld a n d b a c k to A u t o a n d h o r i z o n t a lve loc i ty i s aga in nu l l ed to ze ro . The n , u s ing theR O D s w i t c h t o c o m m a n d a n a p p r o p r i a t e d e s c e n tra te , he lands .

CONCLUSION

Tw o s y s te m s h a v e b e e n d e s c r ib e d f o r u s e b y t h e

Ap o l lo A s t ron au t i n t he fi na l s t age o f the l una rdescen t . I n the advanced conf igu ra t ion r a t e -o f -d e s c e n t r e s p o n s e is i m p r o v e d , a n d t o t h e h o r i z o n t a lchanne l i s added a s i t e des igna t ion sys t em which


8/8

250 DONALD EYLES

makes it easier to place the LM precisely in asuitable spot. It is suggested that the service ren-dered the Astron aut can be maximized by providinga high-frequency velocity computation to whichvarious guidance algorithms may be connected as

needed while allowing the frequency of the othernavigational computations notably calculation ofposition and incorporation of radar data to lan-guish during moments o f inner loop excitation.

A c k n o w l e d g e m e n t - - T h i sreport was prepared under DSRProject 55-23890, sponsored by the Manned SpacecraftCenter of the National Aeronautics and Space Administra-tion through Contract NAS9-4065.

The site designation scheme for Program 66 was suggestedby Astronaut John Young.

The publication of this report does not c onstitute approvalby the National Aer onautics and Space Administration of thefindings or the conclusions contained therein. It is publishedonly for the exchange and stimulation of ideas.

REFERENCES

[I] A. R. KLUMPP: Apol lo LM Descent Guidance, Att itudeControl and Trajectory Targeting--Fundamental Conceptsand Algorithms, Fourth lFAC Symposium on AutomaticControl in Space (1971).[2] A. R. KLUMPP: A ma nually retar geted auto mati clanding system for the lunar module. J . spacecraf t Roc kets5, 129-138 0968).

R6sum6--Dans tousles alunissages humains passes, le com-mandant du module lunaire a pris le contrSle manuel partialde v6hicule spatial au cours du stage final de la descente au-dessous d'envir on 150 m~tres d'alt itude . Du fait de l'irr6gu-larit6 de la surface lunaire et de l'in6vitabilit6 de certaineserreur s dans l'est imati on "3. bord de la position du v6hiculespatial ~. l'6gard du relief, les alunissages enti~rement auto-matiques avec hommes ~. bord sont impossibles rant que desterrains d'alunissage ne seront pas pr6vus sur la lune.

Ce rappor t d6crit les programmes d6velopp6s aux Labora-toires Charles Stark Draper, MIT, pour emploi dansl'ordinateur de guidage du module lunaire au cours de ladescente finale. Ac e moment les demandes en calculs surI'ordinateur de bord sont au maximum, et une interactionparticuli~rement serr6e est n6cessaire avec l'6quipage. Onsouligne l'importance de la conception du programmed'ordinateur plutft que la justification des algorithmes

particulie rs de guidage utilis6es. Apr~s I'intro ducti on del'ordinateur et de la mission, la configuration pr6sente desprogrammes d'alunissage final et une version avanc6ed6velopp6e exp6rimentalement par I'auteur sont d6crits.

Zusammenfassung Beiallen bisherigen bemannten Mond-landungen flbernahm das Lunar-Modul-Kommandogerfit(LM) eine teilweise manuelle Steuerung der Landef~ihrew~ihrend des Endabschnittes der Landung unter etwa 200 m.Wegen der Unregelm/if~igkeiten der Mondoberflfiche undder Unvermeidbarkeit yon Fehlern bei der bordgebundenenSchatzung der Raumschi ffsposition zu dem tirtlichenGel~inde, kSnnen vollautomatische Landungen mit Men-schen an Bord erst vorgenommen werdea, wenn auf demMond Langeplhtze vorbereitet sind.

Dieser Bericht beschriebt Programme, die am CharlesStark Draper Laboratorium, MIT, zur Benutzung im LM-Ftihrungsrechner w~ihrend der LandeschluBphase entwickeltwurden. In dieser Ziet erreichen die Rechenanforderung enan den Bordrechner ihr Maximum, wobei eine besonders engegegenseitige Einwir kung mit der Besat zung niStig ist. DerNachdruck liegt mehr auf dem Entwurf der Rechner-programme als auf der Rechtfertigung der angewandten

speziellen FiJhrungsalgorithmen. Nachde m der Rechner unddie Aufgabe vorgestellt sind, werden die laufende Konstel-lation der P rogramme ffir die Lande-Endpha se und eine vomVerfasser experimentell entwickelte verbesserte Versionbeschrieben.

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