Worst cases for a one-hop high frequency link

ANNALS OF GEOPHYSICS, VOL. 45, N. 1, Feb ua y 2002 201 Sho No e Wo s cases o a one-hop high equency link Glo ia Mi ó (1), Beni o A. de la Mo ena (1), Sand o Ma ia Radicella (2)and Miguel He aiz (3) (1) A mosphe ic Sounding S a ion «El A enosillo»,Na ional Ins i u e o Ae ospace Technology, Magazón,Huel a,Spain (2) Ae onomy and Radiop opaga ion Labo a o y,Abdus Salam ICTP,T ies e,I aly (3) Depa men o Geophysics and Me eo ology,Facul y o Physics,Complu ense Uni e si y,Mad id,Spain Abs ac The cha ac e isa ion o a HF channel by means o mon hly elec on densi y p o iles can be complemen ed wi h a de ailed s udy o adio p opaga ion «wo s cases» on si ua ions wi h ex emes condi ions o adiop opaga ion o a gi en pe iod. These «wo s cases» co espond o condi ions ha can be iden i ied by means o cumula i e dis ibu ions o he key pa ame e 0F2. This pape analyses he main pa ame e s o he HF channel: ime delay, apogee, ele a ion angle and ansmission equency wi h mean and ex eme condi ions. The me hod used o cha ac e ise he ionosphe ic channel is based on ay- acing echniques. 1. In oduc ion An HF communica ion link is cha ac e ised by he Maximum Usable F equency ha is he limi abo e which he signal will no be e lec ed by he ionosphe e unde medium condi ions. Howe e , i is con enien o s udy all possible condi ions. Fo his eason, an analysis o wo s cases aimed a ex ending he esul s ob ained by means o mon hly median condi ions o ex eme ionosphe ic si ua ions. The concep o wo s cases was in oduced in ay acing echniques (Mi ó e al., 2000) o ex end he channel cha ac e isa ion o ex eme ionosphe ic si ua ions a om median condi ions o HF links. Wo s cases selec ion was pe o med using 0F2 h ough hou ly cumula i e equency dis ibu ions. The 2D ay acing echnique used (called abc ay03), is based on he exp essions in oduced by C o and Hoogasian (1968) and C o (1969) and i has been conside ed in p e ious ionosphe ic s udies (Moo head and Radicella, 1998; Mi ó, 2000). The ou pu s ob ained wi h his compu ing echnique a e he main adio p opaga ion pa ame e s: ime delay o he signal a elling om he ansmi e o he ecei e , e lec ion heigh o apogee, ele a ion angle and ansmission equency. Mailing add ess: D . Glo ia Mi ó, A mosphe ic Sounding S a ion «El A enosillo», Na ional Ins i u e o Ae ospace Tech- nology, 21130 Magazón, Huel a, Spain; e-mail: mi [email p o ec ed] Key wo ds ionosphe e − adio p opaga ion − wo s cases − ay- acing 202 Glo ia Mi ó, Beni o A. de la Mo ena, Sand o Ma ia Radicella and Miguel He aiz A pa icula ionosphe ic condi ion is in oduce in he ay acing p og am using model o expe- imen al elec on densi y p o iles. 2. Da a As p e iously indica ed, he pa ame e 0F2 was used o pe o m he wo s cases selec ion h ough hou ly cumula i e equency dis i- bu ions. This s udy conside ed hou ly e ised 0F2 alues om he digisonde DGS256 si ua ed a El A enosillo A mosphe ic S a ion and he 1993- 1997 pe iod. An a e age o 1826 alues was a ailable o each 24 h and cu -o equencies limi ing 1% o hese alues wi h ei he he lowes o he highes equencies ha e been chosen as wo s cases condi ions. Fo example, a 00 UT hou s 1% o cases had 0F2 lowe han 2.4 MHz and highe han 7.3 MHz. Once he wo s cases selec ion has been done, he co esponding elec on densi y p o iles a e in oduced in he ay acing echnique. 3. Me hodology A each hou , p o iles belonging o hese uppe /lowe condi ions a e g ouped o calcula e a mean uppe /lowe ex eme p o ile. This p o- Fig. 1. Hou ly mon hly elec on densi y p o iles and he co esponding o uppe and lowe selec ion a El A enosillo A mosphe ic Sounding S a ion. 203 Wo s cases o a one-hop high equency link cedu e p o ides h ee mean p o iles pe hou : uppe ex eme, lowe ex eme and hou ly mean p o ile o all cases. These mean p o iles we e ob ained ollowing he me hodology desc ibed in Huang and Reinisch (1996). Figu e 1 shows an example o such p o iles. As has been no ed, he ionosphe ic condi ions in he e lec ion poin necessa y o cha ac e ise he HF channel a e in oduced by elec on densi y p o iles. I is impo an o poin ou ha he p o iles ob ained a e he wo s case selec ion belong o one o he link ex emes, El A enosillo S a ion. Howe e , he pa icula link be ween El A enosillo and Eb o Obse a o io is si ua ed a Fig. 2. Oblique ionog ams ob ained by abc ay03 p og am co esponding o uppe , lowe and mean condi- ions. midla i udes. The e o e, communica ions wi h a ange close o 785 km co espond o one iono- sphe ic hop wi h E (E mode) o F (Fmode). Mo eo e , he ionosphe ic condi ions a he e lec ion poin which is loca ed a ound he mid- dle poin in his speci ic link, can be app oxima e o da a om one o he ex emes, ansmi e o ecei e (Mi ó, 2000). Using he h ee mean p o iles a each hou ( ig. 1), ionosphe ic condi ions a e in oduced in he abc ay03 p og am o de i e a each 24 h he adio p opaga ion pa ame e s: ansmis- sion equencies, ele a ion angles, ime delays and apogees unde ex eme and s anda d si ua ions. 204 Glo ia Mi ó, Beni o A. de la Mo ena, Sand o Ma ia Radicella and Miguel He aiz 4. Resul s The dis ibu ion o he g oup pa h P’ (p o- po ional o ime delay) e sus he ansmission equency gi es wha is known as an oblique ionog am. Figu e 2 displays syn he ic oblique ionog ams ob ained wi h he ay acing echni- que co esponding o some o he 24 h s udied. Di e ences be ween he h ee condi ions (uppe ex eme, lowe ex eme and mean) can be clea ly seen mainly a F2 heigh s. F om he HF communica ion sys ems poin o iew, he main p oblem is o ind ou he ex- is ence o low 0F2 on low elec on densi y a- lues. This si ua ion is c i ical because his phenom- enon educes he Maximum Usable F equency and he communica ion equency band becomes na owe han ha om mean and uppe condi- ions ( ig. 2). Fo his eason, special a en ion has been paid o he numbe o cases in which he signal a i es a he ecei e in spi e o he na owing o he equency ange o igina ed by low elec on densi y condi ions. Resul s a e summa ised in able I. As can be seen, only dayligh s hou s (8:00 UT - 18:00 UT) p o ide an accep able pe cen age o si ua ions in which a link is es ablished unde all condi ions. As expec ed, mos o hese signals belong o E modes ha do no appea du ing he es o he hou s. In gene al, i he daily ansmission is done wi h a equency be ween 6 and 8 MHz, he signal will each he ecei e unde ex emes and mean si ua ions. This s udy indica es ha he cha ac e isa ion ob ained using ionosphe ic mean condi ions can be applied in mos si ua ions. Only some cases Table I. Numbe o lowe condi ions cases which a i e a he ecei e . Hou (UT) To al Lowe % E Mode % E Mode F Mode % F Mode 0:00 4 1 25 0 0 1 100 1:00 3 1 33 0 0 1 100 2:00 4 1 25 0 0 1 100 3:00 4 1 25 0 0 1 100 4:00 4 1 25 0 0 1 100 5:00 7 1 14 0 0 1 100 6:00 7 0 0 0000 7:00 16 2 13 1 50 1 50 8:00 23 15 65 11 73 4 27 9:00 24 18 75 13 72 5 28 10:00 26 20 77 17 85 3 15 11:00 28 19 68 17 89 2 11 12:00 30 21 70 16 76 5 24 13:00 28 20 71 17 85 3 15 14:00 27 20 74 16 80 4 20 15:00 27 18 67 15 83 3 17 16:00 28 16 57 11 69 5 31 17:00 27 12 44 5 42 7 58 18:00 24 6 25 2 33 4 67 19:00 19 2 11 0 0 2 100 20:00 15 2 13 0 0 2 100 21:00 11 2 18 0 0 2 100 22:00 6 1 17 0 0 1 100 23:00 6 1 17 0 0 1 100 205 Wo s cases o a one-hop high equency link du ing nigh hou s can p esen p oblems i he ansmission equency used is nea he Maxi-mum Usable F equency because in hese condi ions, he signal will no be e lec ed by he low ionosphe ic elec on densi y p esen in i s ay pa h. 5. Conclusions The main conclusion eached in his pape can be summa ised as ollows: – In gene al, he cha ac e isa ion ob ained by means o mon hly elec on densi y p o iles (Mi ó, 2000) could be applied e en wi h ex eme ionosphe ic condi ions. The main p oblem can appea du ing nigh ime o ansmission equencies close o he Maximum Usable F equency. Acknowledgemen s The au ho s a e g a e ul o D . Moo head o Nep une Rada L d., Glouces e U.K., o his help in unde s anding he ay acing echnique, abc ay03. This wo k has been possible hanks o inancial suppo g an ed by he Na ional Ins i u e o Ae ospace Technology (INTA). REFERENCES C ROFT , T.A. (1969): Me hods and applica ions o compu e ay acing, S an o d Elec onics Labs Tech., Repo . SU- SEL- 69-007. C ROFT , T.A. and H. H OOGASIAN (1968): Exac ay calcula ions in a quasipa abolic ionosphe e, Radio Sci., 3, 69-74. H UANG , X. and B.W. R EINISCH (1996): Ve ical elec on densi y p o iles om digisonde ionog ams - he a e age ep esen a i e p o ile, Ann. 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