Indian Journal o f Radi o & Space Phys ics Vo l. 32, Fehruary 2003, pp. 5- 15

RPA aeronomy experiment onboard the Indian satellite SROSS-C2 - Some important aspects of the payload and satellite

S C Garg, J R Anand, M Bahl , P Subrahmanyam, S S Rajput, H K Maini , P Chopra, T John , S K Singhal, Vishram Singh & Dhan Singh

National Phy sical Laboratory, New Delhi 11 00 12 and

U N Das & S M Bedekar

ISRO Sate llite Centre, Bangalore 560 017 and

P Soma, Venkteshwarlu & D P Gael

ISRO Satellite Te lemetry & Tracking Centre, Banga lore 560 058

Received 5 November 2002; accepted 15 December 2002

An aerono my pay load of re tarding po te ntial analyser (RPA ) consisting of an e lectron RPA. ion RPA and a potc ntial probe (PP), designed and developed a t the Natio nal Physical Laboratory, New Delh i. was fl own onboard the Indi an sate lli te SROSS-C2 o n 4 May 1994, fo r making ill silll measurements of F-regio n ionospheri c plasma. The objecti ve of the mi ss ion was to investi ga te the char;)cteris ti cs and energe tics of the equato ri a l and low latitude ionosphere I thermosphere over the Indi an rcgion. The SROSS-C2 was a spin -stabili zed o rbit ing sate llite placed in an e lli pt ica l orbit hav ing orbit inclinatio n of

46.3°. The payload made s imu ltaneous sampling o f e lectron and io n plasma in the aititude range of 420-020 km for more than half a solar cycle from minima to max ima o f the 23'd sola r cycle act ivity. During initi a l phase o f the miss ion . measurements were made in a higher orbit fo r a li mited peri od o f two months. whi le the sa te llite apogee was at (riO kill kceping the pe rigee same. T he total ion density. e lec tron and ion te mperatures. ion com positi on. supra-the rmal e lec tron Ilu \ and irregulariti es in e lec tron and ion de nsities a long the sate llite o rbit can be derived from the measure me nts. In thi s paper. some important aspec ts o f the sc ient ific pay loads, the sate llite and its o rbital confi gurati on. w hi ch ;.I re importa nt ill understanding the pay load opera tion in space for data collectio n inc luding the analys is o f data and the ir inte rprctations. a rc descri bed.

1 Introduction The retarding potential analyser (R PA) probes

onboard several satellites and rockets have been successfull y used in the past for characteri zation of ea rth 's ionosphere (e.g, Atmospheric Explorers, TA IYO, Dynamic EAplo rer and DMPS F8 th rough FlO satellites l -4). With the development of 100 kg class 'S ROSS ' seri es of low-earth-orbiting sate llites by the Indian Space Research Organisation (ISRO) in mid 80s, it was decided to launch ae ro nomy pay loads onboard these satellites for study ing the equatorial and low latitude ionosphere. The first set o f aeronomy payloads, consisting of e lectron and ion RPAs and a potenti al probe (PP) des igned at the Nationa l Physical Laboratory (NPL), New Delhi , was fl own onboard SROSS-C satellite in May 1992. The other sc ientific payload on the sate llite was a gammn ray burst detector (G RI3 ) from IS RO for astronom ical studies . Thi s mi ss io n yielded payload data5 for a limited period of S6 days on ly, at lower a ltitudes ranging from 3S0 km

down to 200 km over the Indian region, primarily due to the pl acement o f the satellite in a much lower orhit than antic ipated. Fo llow ing thi s parti a l failure. anothcr set of identical pay loads6.7 was flown o nboard SROSS-C2 sate llite on 4 May 1994 from Shriharikota rall!,'c . The sate llite was placed initia ll y into an e llipti cal orhil

of 938x437 km, which was brought down to 63()x-l3() km after two months of operation in a hi gher o rbit. T he payloads, si nce then, had been operated regul arl y ttl y ie ld data while cross ing over the Indi an region covering the equatoria l and low latitudes. The SROSS-C2 miss ion ended on 12 July 200 I after seven years of acti ve life of the sate llite covering periods from minima to max ima of the 23'd solar cycle and recording data for more than 3800 orbits over the Indi an region.

2 RPA payload

2.1 RPA sensors The two independent RPA sensors were used for

making simultaneous measurements of e lectron and ion

6 INDIAN J RADIO & SPACE PHYS. FEBRUARY 2003

plasma paramete rs. Both the sensors utili zed planar geometry simil a r to that used by Hanson ' for the ion sensor. They had multi-grided cylindrical structure li ke a Faraday cup, having an open aperture, number of grids and a so lid collector electrode plate (Fig . I). The sensors are ana logous to a pentode vacuum tube without having a cathode, where the ionospheric plasma entering the o pen aperture acts as the source of plasma. Diffe re nt grids in the sensor are referred to as the entrance grid, retarding grid, suppressor grid and a co ll ector shi e ld g rid . They were made from very

fi ne I OOx I 00 and SOxSO count gold-plated tungsten wire mesh (w ire diameter 0.025 mm) having optical transparency as high as 90-95%. The inter-grid spacing was kept quite small and varied from 0 .8 to 2 mm . All the sensor parts were gold-plated with a dull finish. The entrance and reta rding grids were made double grids , and suppressor and shi e ld grids were sing le grids in both the sensors. Both the sensors were

/ ' .... / ....

/ .... / \

I ...:::t: c;:, \

/ / \ : I , _ \ I I r \

I I Pr-l-l- . .;.: I ~ U i . 1; \

\ \ t! :::: . + / \ '\ -:--- - I

~""'------_//

12cm

5cm

== = ======~~~= ======== = Gl_ -----------------------________ _ 9~~ _________ _

G1 • ENTRANCE GRID

G2 · RETARDING GRID

G3· SUPRESSER GRID

G4· COLLECTOR SHIELD GRID

C • COLLECTOR

Fi g. I - Cross-section of RPA sensors

3cm

made identi cal in mechanica l design , bu t diffe red in voltages applied o n the ir grids.

The entrance g rid isolates the free-s pace plasma from the influence of the e lectric fi elds generated ins ide the sensor, due to changing biases o n va ri ous grids. The e ntrance g rid in io n RPA was kept at the sate llite potential , which is the potential of sensor outer casing also. In case of e lectron RPA , provision was made to apply a small positive vo ltage to the entrance grid in order to counter the e ffect of any negative potential acquired by the sate llite in space. This voltage can be varied from 0 to +5 V in stcps of 0.5 V with the he lp of ground telecommand for ensuring the smooth flow of io nospheric e lectro n tlu x into the sensor. The level of thi s vo ltage was optimized for su nlit and no n-sun lit passes based on the potential acquired by the sate llite . The d iameter of thi s grid was 5 cm in both the sensors .

The retarding grid , which acts as energy fi Iter for the charged particles, was applied with a time va rying or fixed potential for retarding the e lectrons or ions of different energy range in different modes of pay load operations as described later. With the appli cation of thi s re tardation voltage, the retarding gri d allows only those ions (in io n sensor) or e lectrons ( in e lec tron sensor) to pass through, whose energies arc greater than the applied bias , and smalle r ellergy particles arc repe lled back. Thus, RPA sensors act as energy fi !tel's.

The suppressor grid prevents opposi te polarit y charges, i.e. e lectrons in ion sensor or ions in e lectron sensor that have passed through the re tarding grid . from reaching the collector e lectrode. Potential on thi ~ grid was kept pos itive (+1 8V) in e lectron sensor for discarding a ll the ions having energies less than 18 e V which covers a lmost a ll the ions that are norma ll y present in the ionosphere. In case of ion sensor the suppressor grid was kept negat i ve (-24 V) for discarding supra-thermal e lectrons hav i ng energic :--less than 24 eV. Presence of higher energy e lectron flux could contaminate the ion RPA data.

Collector shie ld grid shields the sensit i vc e lectrometer connected to the collector e lec trode from e lectrica l transients which a re generated due to vary ing potentials on other g rids. It wa . . the refore. held at the sensor case potential in both the sensors.

The collector electrode is a gold-plated so li d metal pla te and was kept high ly insul ated from othcr g ricl\ and also from the body of the sensor. It v. a :--e lectrically connected to the input of a sensitive auto-gain rang ing e lectrometer ampli fier (EA) which

. )..

GARG el al. :RPA AERONOMY EXPERIMENT ON BOARD SROSS-C2 7

measures currents collected by the collector e lec trode. In ion sensor, the collector bias was kept at o V with respect to the sensor ground . But, in electron sensor, provision was made to apply a small positive bias on the collector e lectrode for compensating the effect of sate llite charging. Thi s ensures definite co llectio n of all the e lectron flux that reaches in the vicinity of thi s e lectrode after pass ing through the screen grid . This bias can be varied from 0 to +5 V in 3 discrete steps selectable by ground te lecommand.

Thus, in RPA sensors, by apply ing appropriate bias vo ltages on vari ous grids , on ly those e lectro ns (in e lectron sensor) or ions (in ion sensor) whose energies are greater th an the applied vo ltages on the retarding grid , are a llowed to reach the solid collector plate to cause co ll ector current. A varying bias o n the retarding gri d thus results in a characte ri stic curve(/- V curve) of the co llector current (l) versus retarding grid bias vo ltage (V), which is then used to derive information about the plasma characteri stics like e lec tron and ion densiti es , their temperatures , ion composition and other parameters. Variations and fluctua ti ons in densiti es a long the orbital path can be recorded by app ly ing fixed bias. When the retarding grid vo ltage is swept highly negative in the electron RPA, the instrument g ives information about the supra-thermal e lectron !lux also. In the present experiment, the supra-thermal electrons up to and above 30 eV were measured . Electron density (Ne), e lectron temperature (Te), io n density (Ni) , ion temperature (Ti), irregul ariti es in ion and electron densities along the sate ll ite path, and ion composition (H+, He+, 0 +, 0 2+/N0+, Fe+) are derived from the I-V char3cteri sti c curves. With thi s grid structure the transparency factor for ion collection is 0.5 for ion sensor and 0.7 for e lectron collection in e lectron sensor.

2.2 PI' sensor

The PP sensor is basically a spherical Langmuir probe of 2 cm di ameter with a guard ring and can be oper(Jted in current or voltage mode through ground te1ecommand. In vo!t3ge mode of operation, it gives a measure of difference be tween the probe floating potential and the potential acquired by the satellite. In current mode, it works like a normal Langmuir probe and generate 1- V characteristic curves of ambient plasma independent of RPAs.

2.3 RPA electronics

The curre nts from RPA and PP sensors were measured by the auto-gain ranging electrometer

amplifiers capable of measuring large dynamic range (5-6 decades) on a linear scale. The I-bit level of ion electrometer is IpA and that of e lec tron RPA is JpA. Detailed description of RPA e lectron ics consi sting mainly of bas ic RPA electro meter amplifi c r". derivative amplifiers , g rid bias generato rs, e tc.. a rc given by Rajput and Garg 8· 10 . The retarding g rid bi as voltage is a co mpl ex stepped-wave in thc form of staircase hav ing 64 steps. The dwe ll time of cach step is 22 ms. The e lectron and io n RPA and PP measure currents corresponding to each step and generate o ne data se t. every 22 ms . Each data sct has number of analog and di g ita l words (Tab le I) which were arranged in a complex format of 9 word s of 16 bits each (Tabl e 2).

3 RPA sensors operational modes The operatio nal mode of a RPA sensor depe nd s

on the shape of the bias vo ltage app li ed on it " retarding g rid . Correspondingly , the e lec tron and inn RPAs had several modes of operati o n. Fixed hi a~ modes are for density and irregul arity meas urcmcnts and swept bi as modes for temperature and all other meas ure me nts. Measurements in each mode hav ing 64 steps are completed in 1.408 s .

The io n sensor could be operated in thrce type" of modes, namely the rm a l ion mode (T IM ), the rma!

ion mode derivative (TIM~) and duct ion mock (DIM). The retarding grid was ~ wep t from 0 to +20 V in 64 non- linear steps in TIM and TIM~ Ill uc! e:-- . a nd was kept at a fixed potential of 0 V in DIM mode. In o rde r to ge t a bette r resoluti on ill measure ments, the re tardin g vo ltage steps \\iCIT kept low in the reg io n of 0 - 7 V. where th e re tardation was slow and beyond the 7 V pot en tial. the step size was increased to mo re than a vo lt where the re ta rdation was f

8 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2003

Table I - Details of analog and digital output from RPA payload

Description Symbol Nature of signal Electron current Ie -5 to +5 V analog Gai n range Ie GR Ie 4-bits digital Derivative/duct of Ie Meld Ie -5 to +5 V analog Gai n range !'J. /e GRMe 3-bits digital Gain range dIe GR dIe I-bit di gi tal Duct cont rol Ie dIe conI. I-bit digital RDA con trol Ie le RDA I-b it digital Retarding grid bias electron RPA Vrc -5 to +5 V analog Mode seq uence electron RPA M;e 3-bit digital STEM control STEM conI. I-bit digital TEM control TEM conI. I-bit di gital Ion current I; -5 to +5 V analog Gain range I; GR I; 4-bit digital T IM control TIM conI. I Bit digital OBT sync. OBTS I-bit digital Derivative/duct I; M;ld/; -5 to +5 V analog Gai n rang!'! M, GRM; 3-bit digital Gain range dl, GR d/; I-bit di gital Duct control I; d/; conI. I-bit digital RDA control I; I;RDA I-bit digital Retarding grid bias ion RPA V, I; - 5 to + 5 V analog Mode seq. ion RPA M;I 3-bit digital Prom monitor bit I-bit digital PI' ou tpu t I pp - 5 to +5 V analog Gai n range I p GR I pp 2-bi t digital PI' bias Vpp - 5 to +5 V analog Electron staircase counter 8-bit digital Ion staircase counter 8-bit digital

Table 2 - R I' A data format

Word Description DI6 -7 D7 D6 D5 D4 03 D2 DI

I Frame sync. EI3 90 (HEX)

2 Onboard time 16- bits LSB (OBT) weight = 8 ms

:; Step counter D16-7D9 ion D8-7D I electron staircase data stai rC

GARG el al. :RPA AERONOMY EX PER IM ENT ON BOARD SROSS-C2 l)

Electro n sensor could a lso be o perated in 4 d ifferen t modes by vary ing retarding g rid vo ltage waveform . In thermal electron mode CTEM) which is used for est imating the energy d istributio n of thermal e lectrons (/- V curve), the retarding grid vo ltage was

varied from +2 .76 V to -3 V in 64 very fine steps. In

TEM "" the same steps were modulated with a wiggle vo ltage. The supra-the rmal electron mode (STEM) is used for estimating the flu x of supra-thermal electrons in addition to the thermal e lectron flux and so the vo ltage was vari ed from +2.76 V to -32 V. In duct electron mode (DEM) the voltage was kept fixed for the estimati on of irregularities in e lectron density along the sate llite o rbit.

Both e lectron and ion RPA could be operated in 8 pre-programmed combi nations of the bas ic modes described above. They are called mode sequence and each mode sequence contain s 4 basic modes and thereby takes 5 .632s to complete. Thus, RPA measurements repeat themselves every 5.632 s. The se lected mode seq uence information was set by ground data telecommand. A provis ion was also made to shift the retarding grid-b ias wave forms of electron and ion sensors in all modes bodily both in positive and negati ve directions by fixed pre-determined

vo ltages varyIng from -5 to +5 v. Thi s provision facilitates the nullifi cation o f the effect of satellite potentiaL il any, on the RPA pay load performance. The most commonl y used mode seq uences in SROSS-C2 operat ion are shown in Fig.2.

The electron and ion modes were synchronized in such a wa that when one sensor was operational in sweep mode, the o the r remained in duct mode (fi xed bias mode) /"or avoiding interfere nce among them du e to changes in the satellite potenti al, if any, during sweepi ng bias vo ltages .

4 SRO -C2 satellite

·tt S::: tellitc configuration

The SROSS-C2 satellite was of octagonal pri smoid shape with eight body-mounted solar panels on the eight sides of the prismoid . The top deck acco mmodated the payload sensors and the telemetry antenn ae (Fig.3). The mounting configurat ion of RPA and GRB sensors can also be seen in Fig.3. The locat ion of RPA sensors was such that e3ch sensor

had a minimum of 1400 cone fi e ld of view clear from a ll nearoy projec tions on the top deck. The PP ~ensor was mOlll1ted o n a go ld-plated cy iindri cal boom (which was electri cally insulated from the probe) at

the centre of the top deck near the io n R PA se nsor ~11ll1 projecti ng 2 em above the ground plane. The e lectronic subsystems of the sate llite and the payloads were mou nted o n the cent ral frame and covered by the

B~---1-.9-1-S -V------------------~

! -: ",no" I .

~ -16 c a: ~ ~ -24

-32 +--------.--------..,.--------,

2S ..,.-----------------·---------------1

>, 20-w

" ~ 15 o > c i:i1 10 Z

8 S a: ;:: w a: 0

21.103 V

-S +1--------..-----o 64 128

STEP NUMBER 192 256

• ig. 2 - Mode sequence electron and ion RPA

.1. Ion RPA 2. Electron RPA

3. PP Sensor 4 GRB Sensor

Fig. 3 - Top deck iaYOl!1 of Iii ..: "alf'l iill"

10 INDI A J RAD IO & SPACE PHYS, FEBRUARY 2003

solar panels. There were no deployabl e solar panel s in the satellite. The satellite was spin stabili zed with its spin ax is orien ted normal to the orbi tal pl ane. The spin ax is was parallel to one of the lateral axes and its longitudinal axis rotated in the orbital plane. Thus, the top deck of the sa teliite, carrying payload sensors and the grollnd plane, faced the plus ro ll axis and the s,ttellite velocity vector once in every spin ro tation. The sate llite in thi s confi guration moves in cartwhee l mode keeping its sp in ax is perpendi cu lar to the orb ital plane.

The RPA sensors were mounted on the top deck over the cylindrical shaped sensor mounts. The front-end electronics of respective electrometer amp I i fiers were mounted just at the backside of the sensors, c lose to the colleclor electrode, and housed inside the sensor mount. The electronic box containing rest of the electronics for RPA payload was mounted below the top deck on the central frame or tile satellite. The electron and ion RPA sensors were mounted on the octagon-shaped top deck in such a way that other objects/sensors over the top deck had minimum possible shadow/projection over them and their axes were paral lel to the longitudinal ax is of the satellite and perpendicul ar to the top deck plane. In the present spin configurati on of the satellite, the angle between the sensor normal and the satellite velocity vector (whi ch is defi ned as the sensor look angle (8) kept on varying continuously between 0 and 360° at the satellite spin ra te of 5 rpm. The dev iation of the satellite spin ax is from the orbit normal , denoted by y, was also taken in to account for applying correct ion in 8.

It may be mentioned here that e lectron and ion sensors behave quite differently for the coll ect ion of charges from the ionospheric plasma. This is due to the large differences in the velociti es of thermal ions and electrons in F- region pl as ma with respect to the satellite motion. Thermal ion velociti es in the F-region are of the order of 0.5 kmls, which is much smaller than the satellite velocity of 8 km/s. But, on the contrary, thermal electrons move with much Im-ger velocities of more th an 100 km/s. Thus, while moving, the satellite sweeps the ions, but the electrons sweep the satellite. So the ion sensor mounted on the satellite becomes sensitive to the orientation of the sensor with respect to the satellite velocity vector. It collects maximum plasma when entrance aperture faces the velocity vector and gets modulated as the r. ngle between velocity vector and sensor axis (8) increases. This is treated as cosine 1l10dulation for planar

geometry of the RPA sensor. There is no collection or ions when the sensor comes in the satellite wake. i.e. for '/alues of 8 greater than 90°. In SROSS-C2. ion RPA data in a single spin period was analysed for a limited range of 8 within ±30° by appl ying cosO correction. For larger va lues of 8. the modulati on wa" much larger and so required additional correction. The sensor grid transparency factor (0.5) is assumed to remain constant within ±30° of 8.

In electron RPA, ideally, no spin modu lation shoulcl occur in sensor current as electrons impinge on the sensor from all sides. But, in SROSS-C2, some sp in modulation was still observed in th e elec tron RPA elata. probably, due to the shape and location of the grouncl plane and the satellite configuration in the prc~c nt mission . The projected area of the ground plane to the satellite velocity vector, and 'vvhi ch act, as the retul'll path for the currents collected by the sensors. kepI on changing during the satellite spin. Th is perhaps res ulted in changing satellite poten ti al during spi nning or the sa tellite. The electron RPA measurement s are still v~li id for a variation of 8 for ±60° in SROSS-C2. In ihe satellite wake, currents were relati vely much small er.

The solar panels generated 14- 18 V of raw power tn charge the on board battery. The bus vol tages required to operate vari ous satellite subsystems were generated by separate DC-DC converters. The RPA payload h ~ l d its own DC-DC converter for generating various hu s voltages whi ch were from the DC supply directl y from the onboard battery. During RPA payload operati on. the battery remained cut off from the solar panel s. Thi s means that during RPA operations, the so lar ce ll s did not charge the onboard battery . With th is arrange ment the solar array could be made positi ve or negative with respect to satellite ground by connecting the negati ve or positive voltage end of the solar array to the satellite/payload ground . In the default mode. the "olar array remained in pos itive bus configuration and a provision was made to operate the payload in negati ve bus configuration using ground telecommand. This was done for avoiding any excessive negative voltage acquisition by the satellite that could occur due to excessive electron collection through interconnects or solar panels, as had been observed by other~ someti mes in satellite missions.

The communication system on SROSS-C2 Clln s i ~ ted of both S-band systf' r.. and a VHF systemh The do,-, 'Il -link frequencies for S-band and VHF systems were 2245.68 MHz and 137.4 MHz, respec ti vely. and the corresponding up-link frequencies were 2067.897 and

. }

GARG el al. :RPA AERONOMY EXPERIMENT ONBOARD SROSS·C2 II

149.522 MHz. The S-band antenna system, which was des igned to be used both for up-link and down-link, consisted of an array of 24 c ircularly po larized mi crostri p antenna elements mounted c ircumferentially on the top deck o f the satellite. The VHF sys te m consisted of an array of four monopoles fed in ph ase quadrature and mo unted on the top deck of the sate ll ite .

The satel lite was eq uipped with reaction control sys tem (RCS), which had the capability to correct the orb it on a ro utine basis and a lso to inc rease or decrease the orbita l he ight, if required . Using this RCS, the satellite orb it was changed twice during the mlSS lon.

After lau nch, the sate llite was placed in e lliptical orbit hav ing apogee at 938 km and pe ri gee at 437 km .

The orbit inclinatio n was 46.3°. But, after two

month s, the orbit was brought down to 620x420 km fo r mak ing the o rbit less e llipti ca l. The spin rate of the sate llite was maintained at around 5 rpm. The angle between sate llite spin ax is and the orbit plane normal

was maintai ned w ithin ± 10°.

4.2 Ground plane for the sensors

T he ground pl ane, which forms an important e lement for a ll ionization pay loads and acts as the return path for the charges collected by the sensors, was provided in the form of an annular gold-plated aluminium ring of 17 cm diameter fixed and flu shed wi th the RPA sensors entrance aperture. In addition, an e lectricall y conducting multi-layer insulatio n (MU ) sheet was spread over the top deck (leav ing the entrance grid and the annular ground plane of the sensors exposed to the ambient) for creating add itional ground plane. The exposed conducting layer of MU , with a conducti vity of 104 mho/m and connected to the sate llite gro und, provided additional ground plane for the payloads . The area of to tal ground plane including MLI was kept more than 500 times the area of sensors entrance aperture. Thi s MU also provided thermal insul ation to the electronic packages mounted on the top deck and inside of the satelli te.

5 In-orbit operations and data collection Bangalore(l2.5° N, 77 .3° E) , Lucknow (26.8°N,

80 .8°E) and Mauritius(200S, 56°E) ground tele metry and track ing stations were used fot the SROSS-C2 sate llite operations and payload data collection. As mentioned earli er, SROSS-C2 orbit was inclined

46.3° with the equatorial plane. So the satellite

covered a latitude be lt of 46° S-46° N (geographi c).

The sate llite orbiting in cartwheel mode in 630x430 km orbit had orbita l peri od of 90 min . Thus, it made approximate ly 16 o rl:>its in 24 h. out of whi ch two hi gh e levati on passes in day time conditions and two during night were visi ble from a single te lemetry station . Bangalore, being the main telemetry stat ion. had been used for continuous data logging for RPA payload . The other two stations were used for ex tended range of data co llection in the campa ign modes only . As viewed from the Bangalore ground station, during a high elevation pass whi ch lasted for a durati on of about 10 minutes, the latitud inal coverage

was from 5° S to 30° N and the sate llite could be

tracked in the lo ngitude range vary ing from 50° to

100° E. Extended data coverage beyond 30° N was made poss ible by down-linking the data at Lucknow

(26 .8°N, 80.8°E) ground station . The RPA data were collected only in those orbi ts wh ich had sufficiently high e levation from the track ing stat ions fo r having larger data coverage. In general , RPA data were collected on an average du ri ng one daytime and one nighttime pass over the Ind ian region dail y. T he other two passes were used fo r G RB pay load operati on.

The pay load data for each orbit were made avai lable in the fo rm of a computer compatib le flopp y (CCF) conta ining two files , namely , the *.RPA and * .OPT files. The * .RPA file header contai ns information re lated to station identi ficat ion. orbit number, several payload biases and h alth paramete rs re lated to the basic RPA instrument, satell ite on-board time (OBT) and matched ground reference time (GRT) and the rest of the f il e conta ins RPA main data collec ted every 22 ms for the e ntire duration 0 1" the pass. The "'.OPT fil e conta ins satellite orbital parameters at I s intervr.l for the pass duration . Its header contains information re lated to stat ion identification , orbit Humber, spin rate includ ing the angle between orbit normal and the spi n ax is of the satellite. The orbital paramete rs include the sate llite long itude, latitude, altitude, satellite ve loc ity . angle between RPA sensors normal and the satellite ve locity vector, sun ang le and magnetic ang le and sunlit or non sun-lit at 1 s interval during a pass . The format 01" raw telemetry data availab le in *. RPA and * OPT files are g iven in Table 3.

The *.RPA and "'.OPT files are processed to get the basic data in the form of /- V curves for electro n and ion RPAs by incorporating the pay load constants. scaling factors , corrections, etc. and by interpo lating

12 INOIAN J RAOIO & SPACE PHYS. FEBRUARY 2003

Table 3 - Raw pay load data from * RPA ;1I1d ".OPT file

*.RPA file CRT OBT CNTR Electron Ion pp Sync

6ELJ8 61UO IA54 iYJ90 5.+9F FFCO 7FCF 137C4 72 10 OOOF SOF7 0013 7 E /3')() 86LJLJ 6iOO IA57 DI91 5C5i3 EEOO 7FCF B784 2300 OOOF 80F7 0077 EW)O IE9LJ 6100 IA5A 0 292 8899 FFCO 7F8F B794 13 10 OOOF SOF7 OOB7 E I3L)()

3699 6100 1J\5D 0393 78 12 FFCO 7F4F B644 1300 OOOF XOF7 OOB 7 Eil')() .1699 6 !DO IA5F D.+9.+ 3701 0000 7F8F B684 1300 OOOF 8137 oom En'H) '+Cljl) 610() IA62 0595 4851 51'-"40 7BCF B584 2000 OOOF XI37 0077 EI3'J() 6299 6100 IA65 D696 4501 FFCO 7134F B4C4 8590 OOOF 8177 -i FF7 EBl)()

789lJ 6100 IA6S 1)797 4051 FFOO 7F8F B404 7A I0 OOOF 8177 7FF7 EIV)()

8E99 6100 I A6A 0898 589 1 E200 70CF B404 4E90 OOOF 8 i 77 If'P7 EH')O ,\-199 6100 IA60 0999 5201 BCOO 708F B304 9550 OOOF XI 77 7FF7 EW)()

l33N) 610() IA78 OA9A 5.+01 132C8 70-lF 13304 7FOO OOOF 8177 7 1F7 E/ll)()

D099 61()() IA73 01398 5501 9FOO 700F 13204 68 10 OOOF 8177 7FF7 EBl)O

*.OPT file

III' Mill Sec inS Long. Lai.

I .+) 6 0 56.80 25.59 -15 7 0 56.85 25.63 .+5 8 0 56.90 25.67

-15 9 0 56.95 25.7 1

.+5 10 0 57 .00 25.75

-15 II () 57. O.'i 25.79

.+5 12 0 57.10 25.83

-15 12 0 57. 15 25.87

.+5 1-1 0 57.20 25.91

-15 15 0 57.25 25.95

the orb ital parameters corresponding to each sample of measuremen ts. Figure 4 shows typical /- V curves for electron and ion RPAs. These charac teris tic curves yie ld qualltitatin; information regarding the plasma parameters of ion densi ty, electron and ton tempcratl1res. ion composition, etc.

6 Dedvation of ion parameters from ion RPA l-l' curves

The I-V characterist ic curves or ion RPA require non-linear curve fi tting becau se of their composite nature re

GARG el al. :RPA AERONOMY EXPER IMENT ONBOARD SROSS-C2

0. E

14 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2003

and the supra-thermal region (C), when larger negat ive potential appears on the g rid . The current reaching the co ll ector decreases exponentially with increas ing retardation in the re tardation region and shows a linear relationship as governed by the equation given in Langmuir probe theory .

The slope of this straight line gives information about the electron temperature. A software programme has been developed to selec t the linear region in the retardation region of the / - V curve and then using the stra ight line fit, the slope of the region is derived which g ives info rmation abou t the electron temperature. Figure 6 shows a fitted /- V curve of electron RPA .

In SROSS-C2 mission , the saturation region of the , - V curve has been fo und heavily contaminated due to modulation of current during spin of the satellite, probably due to changing potenti al of the satellite. Th is mi ght have happened as the projected area of the grou nd plane which was confined only to the top deck of the satellite, kept on changing wi th respect to the satellite ve locity vector. Accordingly, the values of electron density derived from the saturation region were not fo und to remain valid . But the info rmati on about the nuctuation in e lectron current y ielding irregul ariti es could still be derived from thi s reg ion. The supra- thermal region beyond 3 V and up to 30 V had been used for deriving the supra-thermal electron nux in diffe rent energy ranges.

Io-'r---------------------,

a. E

.J.

J

GARG el al. :RPA AERONOMY EXPERIMENT ON BOARD SROSS-C2 15

ionization, the sate llite potential also gets modified with spin. Hence, immense care is needed for analysis and interpretation of sunlit orbits. In SROSS-C2 mission the e lectron RPA current was found to be modulated heavily when sunlight fell on the sensor. The effect was found to be maximum for sun angles

within ±20° from the sensor normal and tapered off with increase in sun angle and was nil for sun angles

greater than ± 100°. Thus, it is safe to analyse data for

sun angle more than 90°. For lower angles the e lectron current gets modulated heav ily by the photoelectron current and needs pecial attention , if analysed . In e lec tron I-V curve, the saturati on and supra-therm31 regions are found to be affected most by the sun . But, the e lectron temperatures derived from the retardation region are not found to change much. They show 10%-20% increase above the ambient va lue, if derived without any compensation.

The performance o f ion RPA sensor, on the other hand, is not found to be affected to that exten t during sunlit orbits. The effect is not seen in determination of 0 + densiti es and ion temperatures. But, the con tamination is fo und in the retardation region when ion currents go below I nA , i.e. the region where determination of ion species heavier than O2+ is done. ThIs is again due to local photoelectron emission by the sunlight at the sensor and is found to be a fun cti on of sun angle from sensor normal. This region is a lso found to be contaminated by the inco ming higher energy supra- thermal e lectron flux collected by the ion sensor. Hence, great care is needed while analysing the ion RPA data for heav ier ions than O2+,

Acknowledgements The authors wish to acknowledge Dr A P Mitra, the

then DG, CS IR, for providing adequate finances for

implerr.enting the SROSS project and also for hi s keen interest in the project. They are deeply indebted to late Dr Y V Somayaj ulu for providing guidance right from the conceptual stage of the project ti II the paper design. The authors owe their gratitude to Dr B C N Ran for the successful completion of the SROSS project. The support provided by the staff of a ll ISRO test faci l ili e~ during various stages of the project is thankfull\' acknowledged.

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