A Boresig ht Adjustment Mechanism For Use on Laser Altimeters 

Claef Hakun*, Jason Budin off*, Gary Brown*, Fil Paro ng* and Arma ndo Mo rell* 

Abstract 

This paper describe s the developme nt of the Boresi ght Adjustm ent Mechani sm (BAM) for th e Geoscience  Laser Altimeter System (GLAS) Instru ment. The BAM was dev elope d late in the integra tion and test   phase of the GLAS instru ment flight progra m. Therma l vacu um tests of the GLAS instru ment indi cated  that the instrument boresi ght alignment stability over temperat ure may be marginal. To reduce the ri sk  that GLAS may not be abl e to meet the  bore sight ali gnment requi reme nts, an i ntensive effort was starte d  to develop a BAM. Obse rvatory-level testing a nd further evalu ation of the boresight ali gnment data  indicated th at suffici ent m argin could  be obtaine d utilizing exi sting i nstrum ent resources a nd th erefore the  BAM was n ever integrate d onto the G LAS Instrum ent. However, the BAM was de signed f abricated and  fully qualified over a  4 month timeframe  to be capable  of preci sely steeri ng (< 1 arcse c over ±300 arcsec )  the output of three ind ependent lasers to  ensu re the al ignment bet ween the tra nsmit and re ceive paths of  the GLAS in strum ent. Th e sh ort timel ine for the developme nt of the mech anism resulted in severa l  intere sting d esign solutions. Thi s paper di scusses the requi rement definit ion, de sign,  and testin g  processes of  the BAM d evelopme nt effort, how the de sign was affected by the extre mely tight  developm ent sched ule, an d the lesson s learne d thro ugho ut the proce ss. 

Introduc tion 

The GLAS in strum ent wa s succe ssfully launched a board th e Ice,  Cloud, a nd Land Elevati on Satellite  (ICESat) into a near polar orbit on January 12, 2003.  NASA’s ICESat will accura tely measure the height   of the e arths pola r ice masses, land and ocean surfaces u sing the a dvance d preci sion 1064-nm la ser  altimetry technolo gy of the GLAS instrument1. The instru ment  also uses 5 32-nm LIDA R to measure   clouds and aerosols in the  atmosph ere2. ICESat was desi gned to operate fo r 3 to 5 years. 

The G LAS re ceive p ath (F igure  1) consists of a 1-meter-diamete r beryllium tel escop e, and an aft-o ptics  assembly whi ch splits the incomi ng reflected la ser signal into 2 be ams by wave length, 532 n m and 106 4  nm. The 5 32-nm LIDA R be am is di rected onto a fa st steeri ng mi rror, thro ugh an etalon filter,  and into a n  array of 8 si ngle-photo n counting mo dules. Th e 1064-nm altime try chan nel is directe d into the prim ary  altimeter det ector asse mbly or shunt ed via a flip  mirro r me chani sm, (the  Altimeter Detector S elect  Mech anism o r ADSM3) into  the redun dant altimeter a ssembly.  

A view of the laser transmit path i s shown in Fig ure 1. The la ser transmit p ath uses one of three Nd:YAG   4-watt lasers,  each producing a 25-mm x 25-mm  square beam. The lasers are used one at a time, and  all  fire alo ng a common  beam path2. The firs t laser, Las er 1, fires  until it  fails. T he on-orbit laser lifetime i s  proje cted to be app roxim ately 18 mon ths. La ser 1 reflect s off a  fixed fold mirro r and is d irecte d to a   second fixed fold mirro r. The laser be am then passe s thro ugh a pick-off mirro r and exits th e instru ment  along the tra nsmit path bo resight. The p ick-off mirro r re-directs a small portion o f the laser en ergy into the  Laser Refe rence System, which monito rs laser outp ut and provides pre cision beam pointin g knowledge.  Lasers 2 a nd 3 ca n be selected via p recision flip mirror s, which are deploye d into the optical path by th e  Laser Sel ect Mechani sms (LSM’ s)3. In the ca se of a laser being  powere d on out of sequ ence, beam   dump s provided safety for the inst rume nt detecto rs, personn el, and  the lasers th emselve s. The individual   optical alig nment and sta bility require ments pla ced on these tra nsmit path compon ents were extrem ely  tight, with <5 arcsec stability ov er a 30 ºC temperature range. 

                                                  * NASA Goddard Spac e Flight Center, Greenbelt, MD 

Procee dings of the 37th Aeros pace Mec hanisms Sy mpos ium, Joh nson S pace Ce nter, May 19-2 1, 200 4  45