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Remote upgrading of a space-borne instrument
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Advances in Space Research 42 (2008) 1180–1186 www.elsevier.com/locate/asr
Remote upgrading of a space-borne instrument B. Kirov
b
a,*
, K. Georgieva a, D. Batchvarov b, A. Boneva b, R. Krasteva b, G. Stainov b, S. Klimov c, T. Dachev a
a Solar-Terrestrial Influences Laboratory, Bulgarian Academy of Sciences, Sofia, Bulgaria Central Laboratory of Mechatronics and Instrumentation, Bulgarian Academy of Sciences, Sofia, Bulgaria c IKI-RAN, Moscow, Russia
Received 1 November 2006; received in revised form 23 October 2007; accepted 29 October 2007
Abstract The main purposes of experiment ‘‘Obstanovka’’ (‘‘Environment’’ in Russian) consisting of several instruments are to measure a set of electromagnetic and plasma phenomena characterizing the space weather conditions, and to evaluate how such a big and highly energy consuming body as the International Space Station disturbs the surrounding plasma, and how the station itself is charged due to the operation of so many instruments, solar batteries, life supporting devices, etc. Two identical Langmuir electrostatic probes are included in the experiment ‘‘Obstanovka’’. In this paper the Langmuir probes for ‘‘Obstanovka’’ experiment are described, including the choice of geometry (spherical or cylindrical), a more reliable method for the sweep voltage generation, an adaptive algorithm for the probe’s operation. Special attention is paid to the possibility for remote upgrading of the instrument from the ground using the standard communication channels. Ó 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: International Space Station; Thermal plasma; Cylindrical Langmuir Probe; Remote upgrading of software
1. Introduction The Langmuir probe is one of the classical instruments for plasma diagnostics (Mott-Smith and Langmuir, 1926) and among the first space-borne instruments. In principle, a Langmuir probe can be any conducting body introduced into the plasma. Voltage is applied to the probe, changing over a certain range, and the probe current is measured. The dependence of the probe current on the voltage is usually referred to as ‘‘probe characteristic’’ or ‘‘volt–ampere curve’’. The plasma parameters are derived from the form and amplitude of this volt–ampere curve. When the probe potential is negative enough to repel all the electrons, the probe current is due only to the ions which are accelerated towards the probe, and the current’s amplitude is proportional to the ion concentration. When the probe potential *
Corresponding author. Tel.: +359 2 979 34 32; fax: +359 2 870 01 78. E-mail address: [email protected] (B. Kirov).
is less negative, the more energetic electrons overcome it, and produce an electron current, exponentially increasing when the voltage is further decreased. The electron temperature is derived from the slope of the volt–ampere curve in this electron retardation region. When the probe is positive with respect to the plasma, the ion current is negligible while the electrons are accelerated, and the amplitude of the current is a function of the electrons concentration. Langmuir probes have been successfully used aboard a number of rockets and satellites for in situ measurements of thermal plasma parameters in the terrestrial ionosphere (Brace, 1998), at other planets (Krehbiel et al., 1980), and comets (Grard et al., 1989). The continuous monitoring in quiet and disturbed conditions of the thermal plasma parameters, together with other plasma and wave parameters, is essential for the understanding of the fundamental problem of the mechanisms of energy generation and transfer in the solar wind–magnetosphere–ionosphere–atmosphere–lithosphere system. These so called ‘‘space
0273-1177/$34.00 Ó 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2007.10.028
B. Kirov et al. / Advances in Space Research 42 (2008) 1180–1186
weather’’ effects are becoming critically important for our increasingly technological civilization. The International Space Station (ISS) provides the unique opportunity for long-term continuous measurements at low altitudes and low latitudes, which can give a new insight into the solar activity influences on the Earth. The Plasma–Wave Complex ‘‘Obstanovka’’ to be operated aboard the Russian segment of ISS, is intended to measure a set of plasma and electromagnetic phenomena, which may be a part of the space weather disturbances. But while all earlier experiments were conducted aboard relatively small and homogenous spacecraft, with the launch and gradual build-up of the ISS we face the problems of the interaction of a super-large body at a low orbit with its environment. For the first time we have a structure which is not only that large but also that much energy consuming and emitting. The surface floating potential can reach extremely high values. A special device has been constructed by NASA to keep the potential under 40 V. However, the station is a very complex structure which moreover is not fully metalized, so both the surface charging and the ambient disturbed plasma parameters are different in different points, and their special distribution is not known. An adaptive algorithm is used to detect the surface potential and to sweep the probe voltage relative to it. But it may not be sufficient to foresee all possible situations, so the modes of operation may need to be upgraded in real time from the ground. This paper describes some new features of the Langmuir probe used for the ISS, and the possibility for remote upgrading from the ground of its mode of operation using standard telemeter lines. 2. The Langmuir probes for ‘‘Obstanovka’’ experiment 2.1. General characteristics Two Langmuir probes are included in the Plasma–Wave Complex ‘‘Obstanovka’’ (‘‘Environment’’ in Russian) which is aimed at studying the role of the mechanisms of generation and the energetics of the electromagnetic emissions in the frequency range under 15 MHz in the processes of solar wind energy transfer and transformation in the terrestrial ionosphere and atmosphere, especially during magnetic storms; the correlation of the emissions registered in the ionosphere with extreme phenomena and processes on the surface and below (hurricanes, earthquakes, etc.); the disturbances introduced by the space station into the surrounding plasma, and the charging of the station’s structure (Klimov and Korepanov, 2004). The Plasma– Wave Complex consists of multiple units for measuring the following physical parameters: thermal plasma parameters (in two points): electron and ion temperature Te, Ti; electron and ion density, Ne, Ni; electromagnetic parameters (in two points): DC electric and magnetic fields and currents; AC electric and magnetic fields and currents;
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ISS potential Us (in two points); electron spectra with energy range 0.01–10 keV; spectra of VLF electromagnetic fluctuations. The main task of the two identical cylindrical Langmuir Probes LP1 and LP2 is to measure in two points the thermal plasma in the vicinity of the ISS and the potential of the ISS structure. The measured parameters are the electron and ion concentrations Ne and Ni in the range 1.109–1.1013 m3, the electron temperature Te from 1000 to 6000 K, and the space station potential Us in the range from 100 V to +100 V. There are two main modes of operation: ‘‘full’’, for measuring Te, Us, Ne, and Ni, with time resolution 1 s, and ‘‘fast’’ for measuring the fluctuations of the plasma concentration, with frequency 200 Hz. The presence of two identical instruments LP1 and LP2 mounted in two different points allows the determination of space variations of Ne, Ni, Te, and Us in the near surface zone. Each of the two probes contains an electronic block and a measuring monoblock consisting of a sensor and a multirange current meter. The electronic blocks provide the sweep voltages applied to the sensors and the measurement of the current collected by them, and include microcomputers for controlling and optimizing the measurement process, processing and preparing information for the telemetry, interfacing to the telemetry, telecommand, and the On-Board Data Handling Unit. Electrons and/or ions, depending on the voltage applied, are collected by the sensors, and the current generated by them is fed to the multirange current meter, converted there to voltage and, after the A/D converter, read out by the Data Processing Unit into the telemetry stream together with information about the voltage applied, at a rate dependent on instantaneous LP instrument mode. The multi-range current meter has six ranges – three for positive currents and three for negative currents: ±5.105, ±5.107, and ±5.109, with a resolution of 0.3%. 2.2. The probe geometry Many aspects of the theory and applications of the Langmuir probe have been treated in the literature, but the problem which probe is more suitable for ionospheric measurements, the spherical or the cylindrical one, is still discussional. The general equation (Laframboise, 1966) for the accelerated current to a moving spherical or cylindrical probe does not give an analytical solution, so some kind of approximation must be used. Two limiting cases are solved, determined by the ratio of a – the radius of the sheath formed around the probe by the applied voltage, to the probe radius r: (1) sheath area limited current – a ‘‘thin layer’’ approximation when the sheath radius is of the order of the probe radius, r/a 1; in this approximation it is
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assumed that all particles entering the sheath are collected by the probe, and the collecting area is the sheath area. The probe current depends on the sheath radius and doesn’t depend on the voltage applied to the probe; (2) orbital motion limited current – a ‘‘thick layer’’ approximation, r/a = 0. In this approximation the sheath spans to infinity, so the particles will be collected by the probe or not depending on the voltage distribution around the probe: the probe current depends on the voltage inside the sheath and not on the sheath radius. The sheath radius is proportional to the Debye length kd = (ekT/ne2)1/2, and therefore depends on the temperature and concentration of the ambient plasma, consequently on the orbital parameters of the spacecraft. In the planetary ionospheres in the Solar system kd varies between 103 and 2 m. For a cylindrical probe with r = 1 mm, r/kd 6 0.5 throughout the ionosphere, and for a spherical probe it varies over a wide range, from 102 to 10. Therefore, for the cylindrical probe the current is orbital motion limited in the whole ionosphere, and the error introduced by this approximation is less than 5% for any combination of plasma parameters and probe potential (Georgieva and Kirov, 1993). For the spherical probe the current can be both orbital motion limited and sheath area limited, and the differences between the general formula and any of the two approximations are significant (Laframboise, 1966). The error introduced by the approximation varies throughout the ionosphere; moreover, due to the strong dependence of the error on the potential applied to the probe, the error is different along the probe characteristic. Further, when the Langmuir probe is used for space research, the probe together with the spacecraft is moving with respect to the surrounding plasma. This results in high ion Mach numbers and much smaller but not negligible electron Mach numbers M = (mw2sin2h/2kT)1/2, where m is the ion mass, w is the probe velocity with respect to the stationary plasma, and h is the angle between probe axis and velocity vector for cylinder; for sphere h = p/2 (Wharton and Hoegy, 1971). The simplifications of the equations taking account of the probe movement relative to the plasma introduce additional errors of up to 5% in the cylindrical probe data, and much higher for the spherical probe. The examination shows that the spherical probe is practically unusable for the measurement of electron concentration, while for the ion concentration the results are unreliable for the lighter species at higher temperatures (Georgieva and Kirov, 1993). Therefore, though the design and the data processing of the spherical probe might be simpler, its results from ionospheric measurements are inherently loaded by significant errors due to the inevitable simplifications of the analytical formula, and cylindrical probes should be used for all ionospheric measurements.
2.3. Sweep voltage Recently, when sweep voltage is needed to be applied on a probe, it is generated in a digital way. But taking into account that the structure potential can vary in the range ±100 V, we chose analogous sweep voltage. Actually, we have two controllable current generators. They can be switched on simultaneously, switched off simultaneously, or one of them switched on and the other switched off, or vice versa. These two current generators charge a capacitance. Depending on which generator is switched on and which is switched off, and for how long, we will have positive or negative voltage fed to the probe, and with the desired value. The current generators are controlled by a microprocessor. The voltage generated by these current generators is continuously monitored through a 16-bit ADC by the microprocessor which, based on these voltage data and on the data for the measured current, makes the decision about the modes of operation. This scheme is highly reliable and enables us to always know the voltage applied while measuring the probe current. 2.4. Sweep voltage algorithm Four sweep voltage algorithms are foreseen for the Langmuir probe. They differ according to the measured parameters, the frequency of the measurements, and the allocated telemetry rate. The operation according to the basic algorithm, provided the situation is correctly interpreted, guarantees the derivation of the electron and ion concentrations, the electron temperature, and the structure’s potential. Because of the expected high and highly varying values of the structure’s potential relative to the plasma, it is extremely important to detect the structure’s potential and to sweep the probe voltage with respect to this voltage. But the detection of Us, the structure’s potential with respect to the plasma, at least as an initial task, is quite difficult. For this reason, instead of the detection of the structure’s potential with respect to the plasma, as an initial task the program is aimed at the detection of the floating potential Uf – the potential at which the currents due to positively and negatively charged particles to the structure are equal, so the net current is zero. Practically, we apply a sweep voltage to the probe and determine, with accuracy better than 3 mV, Uf – the voltage at which the current changes sign from positive to negative. For the expected temperature range at the altitudes where the International Space Station operates, the difference between the floating potential Uf and the station’s potential with respect to the plasma Us does not exceed 1.5 V. Further, we apply to the probe a sweep voltage in the range Uf U1 to Uf + U2, with U1 and U2 being functions of the slope of the probe characteristic dI/dU in the electron retardation region – that is, in the voltage range between Uf and Us where the current I is an exponential function of the applied voltage U. At low plasma temperatures the voltage difference between Uf
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and Us is too small, and if the data points are uniformly distributed in the measured range, very few points will be derived in the electron retardation region, and the electron temperature will not be correctly determined. To be sure we will have enough data points in the electron retardation region so that we could correctly derive the electron temperature, we divide each of the ranges from Uf U1 to Uf (the ion acceleration region), from Uf to U2 (the electron retardation region), and from U2 to U2 + U3 (the electron acceleration region) into 20 equidistant points, and measure the probe current in each of these voltage values. 3. Remote upgrading of the mode of operation 3.1. General principles The foreseen modes of operation and the adaptive sweep voltage algorithm may not be enough to account for all different situations that can occur due to the station’s charging and the disturbances introduced by the structure itself and by the operation of the numerous systems and instruments into the plasma in the near-surface zone of the station. For this reason, a possibility is foreseen for remote upgrading of the mode of operation of the Langmuir probe, which can be used for remote upgrading of any space-borne instrument in order to maintain the scientific experiments in real time. The communication between the station and the Earth is realized through the telemetry information channels based on radio-frequency connection. Apart from scientific data from the instruments, these channels will be also used for transmission of commands from the Earth (about the instruments’ modes of operation), as well as information blocks (containing object code) for changing the processing and control programs. The configuration of the complex – ground-based stations, orbital station, and scientific instruments – is presented in Fig. 1. It allows combining several functions: – deriving data (in real time) for the observed physical parameters and saving the measured parameters to the onboard computer; – maintaining an information channel between the ground-based stations and the onboard computer; – providing access to the derived information and adjusting the specialized software of the orbital scientific instruments using a virtual network (in Internet). Each scientific program is realized as a state machine and is controlled in real time by its state variables and by the values of the measured parameters. On the basis of the measurements it calculates scientific data and saves them as 8-byte elements in 512 K external memory. It is activated by a command from concentrators DACU (Data Acquisition and Control Unit) or it is active by default. The program realizes one out of four possible tasks (scientific experiments) and takes up a 16 K section of the
Fig. 1. Block-diagram of the measurement complex.
microprocessor’s address space. Using the communication channel between DACU and the instrument, a specialized protocol is realized permitting the program code of a given section to be replaced by another one. This change does not require the operation of the other experiments to be terminated. 3.2. Data organization and communication protocol The scientific instruments included in the two Plasma– Wave Complexes (Klimov and Korepanov, 2004) are connected to the respective concentrators (DACU1 and DACU2). The concentrators gather the information from the instruments, structure it and transmit it via the ETHERNET network (10 Mbit/s) to the Block of Storage of Telemetry Information (BSTM), located in the Russian segment of ISS. For this purpose exclusive busses (each instrument having its own communication bus for connection with DACU) and specialized communication protocol devised by Hungarian specialists are used. This protocol determines the minimum required number of commands guarantying the information exchange. The technology
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Table 1 Interface Commands MODE
Description
Action
1 2 4 8 10 20
Get house keeping Get science data Set command mode Set parameter values Load section code Service request
Time setting Send science package Send status information Send status information Send status information Send required information
presented here for updating the specialized program tools for the Langmuir Probe in real time is based on the extension to this set (Table 1). Four new types of commands (MODE 4, 8, 10, and 20) are added to the commands of MODEs 1 and 2 (Table 1), adding new functionality to the protocol. The basic communication commands (Table 1) have similar structure and are based on a sequence of requests from DACU and the respective replies from the instruments. Including commands from MODE 4 and 8 allows performing various scientific tasks and determining the respective parameters defining the specific modes of operation of the Langmuir probe. This type of applications are based on static (resident in the flash memory of the microcontroller used – ATmega128) program procedures (Atmel Corporation, 2004), with the possibility to switch between them and to change their input parameters. The remote upgrading of the mode of the operation is realized by executing a command of MODE 10 type. When such a command is executed, the probe operation is not terminated during the upgrading (the probe is working in the last activated mode). After the upgrade has been completed, the introduced code can be activated in the standard ways of control of static program procedures. The technology developed for upgrading is conformable to the requirements for authenticity and integrity of the transmitted information. The information blocks received by the onboard computer are transmitted to DACU which retranslated them to the Langmuir probe. These blocks contain built-in command sequences, interpreted according to the general protocol. With a view to improve the noise resistance in the basic software, an algorithm is realized for recovering individual damaged parts in the program code.
The microcontroller ATmega128 allows access to its resident energy-independent program memory (128 K flash) and its preprogramming in operational mode, and offers a large set of built-in devices, which makes it possible to realize the functions described above. The structure of the program memory used for the software of the probe is shown in Table 2. The whole address space is divided into sections 16 K each. An exception is the last section containing the resident software including: a driver realizing the chosen communication protocol; programs for processing external interruptions; a program for initialization, and a basic cycle of operation. The other sections contain blocks allowing reloading. They are used for saving procedures for the realization of the various scientific experiments. Section 0 (Table 2) plays the role of an intermediate buffer for reloading the sections. At a given moment a procedure is executed controlling one particular experiment. It can occupy only one section (16 K). The other sections are free for reloading. Each of the reloadable sections contains only one executable (scientific) procedure. The operation of such a procedure includes a sequence of actions described in Table 3. This sequence allows the setting of the initial parameters, measurement of the observed parameters, forming, and archiving elements containing scientific data, and completion of the operational cycle. The measurement values are formatted in blocks – elements with the structure shown in Table 4. Each of the elements contains information for the moment of the measurement, the type of the measured value, the scientific experiment during which the measurement has been made, Table 3 Sequence of actions executed in the procedure No.
Name
Remarks
1
3
Parameter number part Reading of parameter part Processing part
4
Termination part
Determinates how much parameters are expected Reading of the parameters and setting of the state variables Closed loop activities depending of the type of experiment. Saving the results into External memory Exit of experiment code
2
Table 2 Processor program space No. section
Description
Addresses
Remarks
0 1 2 3 4 5 6 Resident section Resident section
Section loader buffer Constants section Experiment 1 Experiment 2 Experiment 3 Experiment 4 Reserved Resident program off-line routines Resident section loader and on-line service routines
(0000-1FFF) (2000-3FFF) (4000-5FFF) (6000-7FFF) (8000-9FFF) (A000-BFFF) (C000-DFFF) (E000-F000) (F000-FFFF)
Scratch buffer Constants tables uploaded Uploaded codes Uploaded codes Uploaded codes Uploaded codes Not used Resident codes Resident codes
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Table 4 External memory element structure No.
Description of the data fields
0 1 2 3 4
The time (in 10 ms counts) from the start of the current year to the measurement moment, presented as 4 – byte integer
5 6 7
Status byte, giving current year, type of data, task number, number of experiment Two bytes signed integer presenting the measured value 8 bits CRC; calculated on the bytes from 0 to 6
the value of the measured character, and the control sum of the element. The elements are saved in a closed loop buffer kept by an external memory with a value of 512 K as demonstrated in Fig. 2. When a command of MODE2 type (‘‘Get Science Data’’) is executed, a set of the available elements is sent to DACU. Each element consists of 8 bytes as described in Table 4. The executable programs are upgraded in on-line mode by sending blocks of data with structure as shown in Fig. 3. Each block consists of a sequence of records, and each record consists of a title part, data, and a terminating byte announcing the end of the record. The data field consists of 22 bytes with the following meaning: Length of data Address in the section Program code CRC
2 bytes 2 bytes 16 bytes 2 bytes
The data field is repeated three times consecutively in order to avoid unwanted influences as noisiness, data loss, etc. The last record contains $4000 as the address in the section which distinguishes it from the other records, and this indicates it is the last one (Hipp, 1994). 3.3. Realization of noise resistant protocol for program block exchange The main goals of the protocol for data and programs exchange between the ground-based station and the probe are:
Fig. 3. Experiment section uploading file structure.
– guaranteed authenticity of the sent and received information; – possibility for partial recovery of information lost in the information channel (based on an algorithm included in the Langmuir probe software); – conforming to the requirements of the base protocol; – realization of a simplified processing algorithm for meeting the general time limitations; – possibility for a preliminary adjustment of the program modules intended to be sent in the framework of the communication blocks; – building in interface commands into the communication blocks to enable the realization of additional functions in the protocol used without having to change the existing software in the other systems (DACU and the onboard computer). The chosen scenario of operation is the following:
Fig. 2. External memory structure.
– compilation and adjustment of the program modules on ground; – processing of the initial file (saved in *.hex format) to extract the program code corresponding to the new scientific experiment in a new *.sec file. The latter contains the machine codes as well as special commands
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interpreted by the probe’s software. Security fields are also included to guarantee the authenticity and integrity of the sent (received) information (Fig. 3); – sending of the generated *.sec file by radiofrequency channel to the onboard computer. The actions described above can be done on research computers (’’High Privilege Client Station’’ – Fig. 1) connected via Internet to the ground-based station (‘‘Base Earth Station’’ – Fig. 1); – The on-board computer (Fig. 1) transmits the received *.sec file by the local network to the concentrator DACU which retransmits it (record by record) to the Langmuir probe via RS422. Each record contains formatting fields according to the base protocol. The command field contains code $10 which activates the command for loading and processing of the record and the included code. After processing, the program code is transmitted to the respective address from the flash of ATmega128; – The record processing includes the following stages: (1) verification and recovery of the information contained in the record; (2) control of the authenticity and the integrity of the recovered information by recalculating the CRC-16 sum and its verification; (3) saving the recovered information in the zero flash section according to the address contained in the record; and (4) after saving a record with the address $4000 (the last one of the block), section 0 is copied into the section with the desired number specified in the *.sec file. The noise-protection of the information included in the records is guaranteed by the algorithm chosen for coding and recovery of the data. The noise is supposed to have the nature of random influences with duration not exceeding the time for sending 22 bytes of useful information. During this time, random changes of the bits sent can occur. To guarantee the recovery, a representation of the data is chosen as three identical fields with 22 bytes each. After receiving the data by the probe, the different bits of the three fields are verified separately, and a resulting field of 22 bytes is created according to the following formula: di ¼ ðbi þ ci Þ:ai þ bi :ci where ai, bi, and ci are the values of the i-th bits of the three identical fields of the record, and di is the value of the i-th bit of the resulting field containing the recovered information. Subject to recovery is the CRC-16 sum contained in the last two bytes of each field. If the result is negative, a request for repeating the record is generated by DACU, the onboard computer, or the ground-based station. A record is only saved in the flash memory after the successful verification of the recovered record.
4. Summary The OBSTANOVKA-1 project unites the knowledge and skill of the space scientists and engineers from seven European countries and the Plasma–Wave Complex instrumentation developed by this team seems to be the most sophisticated device ever operated onboard the International Space Station for waves and plasma study. The realization of this project will allow answering a number of scientific and technological questions. This paper describes the Langmuir probes included in this experiment – the range of operation, the choice of geometry, the sweep voltage generation, and the adaptive algorithm for the probe operation. In view of the difficulties in the predictability of the environment in which the probes will have to operate, the possibility is foreseen for remote upgrading of the instrument from the ground. Moreover, this is accomplished using the standard telemetry, without sparing special physical and logical communication channels. Acknowledgement This work is supported by National Science Foundation Grant NZ-1509/05. References Atmel Corporation, Microcontroller with 128 K Bytes In-System Programmable Flash – ATmega128, Atmel Corporation, Rev. 2467L – AVR, 1–24, 2004. Brace, L.H. Langmuir probe measurements in the ionosphere, in: Pfaff, R.F., Borovsky, J.E., Young, D.T. (Eds.), Measurement Techniques in Space Plasmas – Particles: Geophysical Monograph 102. American Geophysical Union, Washington, DC USA, pp. 23–35, 1998. Georgieva, K., Kirov, B. The Langmuir probe for Earth and planetary ionospheric measurements: spherical or cylindrical? Bulg. Geophys. J. 19 (1), 28–37, 1993. Grard, R., Laakso, H., Pedersen, A., Trotignon, J.G., Mikhailov, Y. Observations of the plasma environment of Comet Halley during the VEGA flybys. Annales Geophysicae 7, 141–149, 1989. Hipp, R. Mktclapp: A toll for mixing C/C++ with Tcl/Tk, in: Ousterhout, J. (Ed.), Tcl/Tk Engineering Manual. Sun Microsystems Inc., Charlotte, NC, pp. 1–30, 1994. Klimov, S.I., Korepanov, V.Ye. The ‘‘Obstanovka’’ experiment aboard the International Space Station. Kosmichna Nauka i Tekhnologiya 10 (2/3), 81–86, In Russian, 2004. Krehbiel, J.P., Brace, L.H., Theis, R.F., Cutler, J.R., Pinkus, W.H., Kaplan, R.B. Pioneer Venus Orbiter electron temperature probe. J.Geophys. Res. GE-18, 49–54, 1980. Laframboise, J.G., 1966. Theory of spherical and cylindrical Langmuir probes in a collisionless, Maxwellian plasma at rest, University of Toronto UTIAS Report No. 100. Mott-Smith, L.R., Langmuir, I. The theory of collectors in gaseous discharges. Phys. Rev. 28 (4), 727–763, 1926. Wharton, L.E., Hoegy, W.R., 1971. Current to moving spherical and cylindrical electrostatic probes, Preprint X-621-71-276, Goddard Space Flight Center, 1971.
Advances in Space Research 42 (2008) 1180–1186 www.elsevier.com/locate/asr
Remote upgrading of a space-borne instrument B. Kirov
b
a,*
, K. Georgieva a, D. Batchvarov b, A. Boneva b, R. Krasteva b, G. Stainov b, S. Klimov c, T. Dachev a
a Solar-Terrestrial Influences Laboratory, Bulgarian Academy of Sciences, Sofia, Bulgaria Central Laboratory of Mechatronics and Instrumentation, Bulgarian Academy of Sciences, Sofia, Bulgaria c IKI-RAN, Moscow, Russia
Received 1 November 2006; received in revised form 23 October 2007; accepted 29 October 2007
Abstract The main purposes of experiment ‘‘Obstanovka’’ (‘‘Environment’’ in Russian) consisting of several instruments are to measure a set of electromagnetic and plasma phenomena characterizing the space weather conditions, and to evaluate how such a big and highly energy consuming body as the International Space Station disturbs the surrounding plasma, and how the station itself is charged due to the operation of so many instruments, solar batteries, life supporting devices, etc. Two identical Langmuir electrostatic probes are included in the experiment ‘‘Obstanovka’’. In this paper the Langmuir probes for ‘‘Obstanovka’’ experiment are described, including the choice of geometry (spherical or cylindrical), a more reliable method for the sweep voltage generation, an adaptive algorithm for the probe’s operation. Special attention is paid to the possibility for remote upgrading of the instrument from the ground using the standard communication channels. Ó 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: International Space Station; Thermal plasma; Cylindrical Langmuir Probe; Remote upgrading of software
1. Introduction The Langmuir probe is one of the classical instruments for plasma diagnostics (Mott-Smith and Langmuir, 1926) and among the first space-borne instruments. In principle, a Langmuir probe can be any conducting body introduced into the plasma. Voltage is applied to the probe, changing over a certain range, and the probe current is measured. The dependence of the probe current on the voltage is usually referred to as ‘‘probe characteristic’’ or ‘‘volt–ampere curve’’. The plasma parameters are derived from the form and amplitude of this volt–ampere curve. When the probe potential is negative enough to repel all the electrons, the probe current is due only to the ions which are accelerated towards the probe, and the current’s amplitude is proportional to the ion concentration. When the probe potential *
Corresponding author. Tel.: +359 2 979 34 32; fax: +359 2 870 01 78. E-mail address: [email protected] (B. Kirov).
is less negative, the more energetic electrons overcome it, and produce an electron current, exponentially increasing when the voltage is further decreased. The electron temperature is derived from the slope of the volt–ampere curve in this electron retardation region. When the probe is positive with respect to the plasma, the ion current is negligible while the electrons are accelerated, and the amplitude of the current is a function of the electrons concentration. Langmuir probes have been successfully used aboard a number of rockets and satellites for in situ measurements of thermal plasma parameters in the terrestrial ionosphere (Brace, 1998), at other planets (Krehbiel et al., 1980), and comets (Grard et al., 1989). The continuous monitoring in quiet and disturbed conditions of the thermal plasma parameters, together with other plasma and wave parameters, is essential for the understanding of the fundamental problem of the mechanisms of energy generation and transfer in the solar wind–magnetosphere–ionosphere–atmosphere–lithosphere system. These so called ‘‘space
0273-1177/$34.00 Ó 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2007.10.028
B. Kirov et al. / Advances in Space Research 42 (2008) 1180–1186
weather’’ effects are becoming critically important for our increasingly technological civilization. The International Space Station (ISS) provides the unique opportunity for long-term continuous measurements at low altitudes and low latitudes, which can give a new insight into the solar activity influences on the Earth. The Plasma–Wave Complex ‘‘Obstanovka’’ to be operated aboard the Russian segment of ISS, is intended to measure a set of plasma and electromagnetic phenomena, which may be a part of the space weather disturbances. But while all earlier experiments were conducted aboard relatively small and homogenous spacecraft, with the launch and gradual build-up of the ISS we face the problems of the interaction of a super-large body at a low orbit with its environment. For the first time we have a structure which is not only that large but also that much energy consuming and emitting. The surface floating potential can reach extremely high values. A special device has been constructed by NASA to keep the potential under 40 V. However, the station is a very complex structure which moreover is not fully metalized, so both the surface charging and the ambient disturbed plasma parameters are different in different points, and their special distribution is not known. An adaptive algorithm is used to detect the surface potential and to sweep the probe voltage relative to it. But it may not be sufficient to foresee all possible situations, so the modes of operation may need to be upgraded in real time from the ground. This paper describes some new features of the Langmuir probe used for the ISS, and the possibility for remote upgrading from the ground of its mode of operation using standard telemeter lines. 2. The Langmuir probes for ‘‘Obstanovka’’ experiment 2.1. General characteristics Two Langmuir probes are included in the Plasma–Wave Complex ‘‘Obstanovka’’ (‘‘Environment’’ in Russian) which is aimed at studying the role of the mechanisms of generation and the energetics of the electromagnetic emissions in the frequency range under 15 MHz in the processes of solar wind energy transfer and transformation in the terrestrial ionosphere and atmosphere, especially during magnetic storms; the correlation of the emissions registered in the ionosphere with extreme phenomena and processes on the surface and below (hurricanes, earthquakes, etc.); the disturbances introduced by the space station into the surrounding plasma, and the charging of the station’s structure (Klimov and Korepanov, 2004). The Plasma– Wave Complex consists of multiple units for measuring the following physical parameters: thermal plasma parameters (in two points): electron and ion temperature Te, Ti; electron and ion density, Ne, Ni; electromagnetic parameters (in two points): DC electric and magnetic fields and currents; AC electric and magnetic fields and currents;
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ISS potential Us (in two points); electron spectra with energy range 0.01–10 keV; spectra of VLF electromagnetic fluctuations. The main task of the two identical cylindrical Langmuir Probes LP1 and LP2 is to measure in two points the thermal plasma in the vicinity of the ISS and the potential of the ISS structure. The measured parameters are the electron and ion concentrations Ne and Ni in the range 1.109–1.1013 m3, the electron temperature Te from 1000 to 6000 K, and the space station potential Us in the range from 100 V to +100 V. There are two main modes of operation: ‘‘full’’, for measuring Te, Us, Ne, and Ni, with time resolution 1 s, and ‘‘fast’’ for measuring the fluctuations of the plasma concentration, with frequency 200 Hz. The presence of two identical instruments LP1 and LP2 mounted in two different points allows the determination of space variations of Ne, Ni, Te, and Us in the near surface zone. Each of the two probes contains an electronic block and a measuring monoblock consisting of a sensor and a multirange current meter. The electronic blocks provide the sweep voltages applied to the sensors and the measurement of the current collected by them, and include microcomputers for controlling and optimizing the measurement process, processing and preparing information for the telemetry, interfacing to the telemetry, telecommand, and the On-Board Data Handling Unit. Electrons and/or ions, depending on the voltage applied, are collected by the sensors, and the current generated by them is fed to the multirange current meter, converted there to voltage and, after the A/D converter, read out by the Data Processing Unit into the telemetry stream together with information about the voltage applied, at a rate dependent on instantaneous LP instrument mode. The multi-range current meter has six ranges – three for positive currents and three for negative currents: ±5.105, ±5.107, and ±5.109, with a resolution of 0.3%. 2.2. The probe geometry Many aspects of the theory and applications of the Langmuir probe have been treated in the literature, but the problem which probe is more suitable for ionospheric measurements, the spherical or the cylindrical one, is still discussional. The general equation (Laframboise, 1966) for the accelerated current to a moving spherical or cylindrical probe does not give an analytical solution, so some kind of approximation must be used. Two limiting cases are solved, determined by the ratio of a – the radius of the sheath formed around the probe by the applied voltage, to the probe radius r: (1) sheath area limited current – a ‘‘thin layer’’ approximation when the sheath radius is of the order of the probe radius, r/a 1; in this approximation it is
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assumed that all particles entering the sheath are collected by the probe, and the collecting area is the sheath area. The probe current depends on the sheath radius and doesn’t depend on the voltage applied to the probe; (2) orbital motion limited current – a ‘‘thick layer’’ approximation, r/a = 0. In this approximation the sheath spans to infinity, so the particles will be collected by the probe or not depending on the voltage distribution around the probe: the probe current depends on the voltage inside the sheath and not on the sheath radius. The sheath radius is proportional to the Debye length kd = (ekT/ne2)1/2, and therefore depends on the temperature and concentration of the ambient plasma, consequently on the orbital parameters of the spacecraft. In the planetary ionospheres in the Solar system kd varies between 103 and 2 m. For a cylindrical probe with r = 1 mm, r/kd 6 0.5 throughout the ionosphere, and for a spherical probe it varies over a wide range, from 102 to 10. Therefore, for the cylindrical probe the current is orbital motion limited in the whole ionosphere, and the error introduced by this approximation is less than 5% for any combination of plasma parameters and probe potential (Georgieva and Kirov, 1993). For the spherical probe the current can be both orbital motion limited and sheath area limited, and the differences between the general formula and any of the two approximations are significant (Laframboise, 1966). The error introduced by the approximation varies throughout the ionosphere; moreover, due to the strong dependence of the error on the potential applied to the probe, the error is different along the probe characteristic. Further, when the Langmuir probe is used for space research, the probe together with the spacecraft is moving with respect to the surrounding plasma. This results in high ion Mach numbers and much smaller but not negligible electron Mach numbers M = (mw2sin2h/2kT)1/2, where m is the ion mass, w is the probe velocity with respect to the stationary plasma, and h is the angle between probe axis and velocity vector for cylinder; for sphere h = p/2 (Wharton and Hoegy, 1971). The simplifications of the equations taking account of the probe movement relative to the plasma introduce additional errors of up to 5% in the cylindrical probe data, and much higher for the spherical probe. The examination shows that the spherical probe is practically unusable for the measurement of electron concentration, while for the ion concentration the results are unreliable for the lighter species at higher temperatures (Georgieva and Kirov, 1993). Therefore, though the design and the data processing of the spherical probe might be simpler, its results from ionospheric measurements are inherently loaded by significant errors due to the inevitable simplifications of the analytical formula, and cylindrical probes should be used for all ionospheric measurements.
2.3. Sweep voltage Recently, when sweep voltage is needed to be applied on a probe, it is generated in a digital way. But taking into account that the structure potential can vary in the range ±100 V, we chose analogous sweep voltage. Actually, we have two controllable current generators. They can be switched on simultaneously, switched off simultaneously, or one of them switched on and the other switched off, or vice versa. These two current generators charge a capacitance. Depending on which generator is switched on and which is switched off, and for how long, we will have positive or negative voltage fed to the probe, and with the desired value. The current generators are controlled by a microprocessor. The voltage generated by these current generators is continuously monitored through a 16-bit ADC by the microprocessor which, based on these voltage data and on the data for the measured current, makes the decision about the modes of operation. This scheme is highly reliable and enables us to always know the voltage applied while measuring the probe current. 2.4. Sweep voltage algorithm Four sweep voltage algorithms are foreseen for the Langmuir probe. They differ according to the measured parameters, the frequency of the measurements, and the allocated telemetry rate. The operation according to the basic algorithm, provided the situation is correctly interpreted, guarantees the derivation of the electron and ion concentrations, the electron temperature, and the structure’s potential. Because of the expected high and highly varying values of the structure’s potential relative to the plasma, it is extremely important to detect the structure’s potential and to sweep the probe voltage with respect to this voltage. But the detection of Us, the structure’s potential with respect to the plasma, at least as an initial task, is quite difficult. For this reason, instead of the detection of the structure’s potential with respect to the plasma, as an initial task the program is aimed at the detection of the floating potential Uf – the potential at which the currents due to positively and negatively charged particles to the structure are equal, so the net current is zero. Practically, we apply a sweep voltage to the probe and determine, with accuracy better than 3 mV, Uf – the voltage at which the current changes sign from positive to negative. For the expected temperature range at the altitudes where the International Space Station operates, the difference between the floating potential Uf and the station’s potential with respect to the plasma Us does not exceed 1.5 V. Further, we apply to the probe a sweep voltage in the range Uf U1 to Uf + U2, with U1 and U2 being functions of the slope of the probe characteristic dI/dU in the electron retardation region – that is, in the voltage range between Uf and Us where the current I is an exponential function of the applied voltage U. At low plasma temperatures the voltage difference between Uf
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and Us is too small, and if the data points are uniformly distributed in the measured range, very few points will be derived in the electron retardation region, and the electron temperature will not be correctly determined. To be sure we will have enough data points in the electron retardation region so that we could correctly derive the electron temperature, we divide each of the ranges from Uf U1 to Uf (the ion acceleration region), from Uf to U2 (the electron retardation region), and from U2 to U2 + U3 (the electron acceleration region) into 20 equidistant points, and measure the probe current in each of these voltage values. 3. Remote upgrading of the mode of operation 3.1. General principles The foreseen modes of operation and the adaptive sweep voltage algorithm may not be enough to account for all different situations that can occur due to the station’s charging and the disturbances introduced by the structure itself and by the operation of the numerous systems and instruments into the plasma in the near-surface zone of the station. For this reason, a possibility is foreseen for remote upgrading of the mode of operation of the Langmuir probe, which can be used for remote upgrading of any space-borne instrument in order to maintain the scientific experiments in real time. The communication between the station and the Earth is realized through the telemetry information channels based on radio-frequency connection. Apart from scientific data from the instruments, these channels will be also used for transmission of commands from the Earth (about the instruments’ modes of operation), as well as information blocks (containing object code) for changing the processing and control programs. The configuration of the complex – ground-based stations, orbital station, and scientific instruments – is presented in Fig. 1. It allows combining several functions: – deriving data (in real time) for the observed physical parameters and saving the measured parameters to the onboard computer; – maintaining an information channel between the ground-based stations and the onboard computer; – providing access to the derived information and adjusting the specialized software of the orbital scientific instruments using a virtual network (in Internet). Each scientific program is realized as a state machine and is controlled in real time by its state variables and by the values of the measured parameters. On the basis of the measurements it calculates scientific data and saves them as 8-byte elements in 512 K external memory. It is activated by a command from concentrators DACU (Data Acquisition and Control Unit) or it is active by default. The program realizes one out of four possible tasks (scientific experiments) and takes up a 16 K section of the
Fig. 1. Block-diagram of the measurement complex.
microprocessor’s address space. Using the communication channel between DACU and the instrument, a specialized protocol is realized permitting the program code of a given section to be replaced by another one. This change does not require the operation of the other experiments to be terminated. 3.2. Data organization and communication protocol The scientific instruments included in the two Plasma– Wave Complexes (Klimov and Korepanov, 2004) are connected to the respective concentrators (DACU1 and DACU2). The concentrators gather the information from the instruments, structure it and transmit it via the ETHERNET network (10 Mbit/s) to the Block of Storage of Telemetry Information (BSTM), located in the Russian segment of ISS. For this purpose exclusive busses (each instrument having its own communication bus for connection with DACU) and specialized communication protocol devised by Hungarian specialists are used. This protocol determines the minimum required number of commands guarantying the information exchange. The technology
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Table 1 Interface Commands MODE
Description
Action
1 2 4 8 10 20
Get house keeping Get science data Set command mode Set parameter values Load section code Service request
Time setting Send science package Send status information Send status information Send status information Send required information
presented here for updating the specialized program tools for the Langmuir Probe in real time is based on the extension to this set (Table 1). Four new types of commands (MODE 4, 8, 10, and 20) are added to the commands of MODEs 1 and 2 (Table 1), adding new functionality to the protocol. The basic communication commands (Table 1) have similar structure and are based on a sequence of requests from DACU and the respective replies from the instruments. Including commands from MODE 4 and 8 allows performing various scientific tasks and determining the respective parameters defining the specific modes of operation of the Langmuir probe. This type of applications are based on static (resident in the flash memory of the microcontroller used – ATmega128) program procedures (Atmel Corporation, 2004), with the possibility to switch between them and to change their input parameters. The remote upgrading of the mode of the operation is realized by executing a command of MODE 10 type. When such a command is executed, the probe operation is not terminated during the upgrading (the probe is working in the last activated mode). After the upgrade has been completed, the introduced code can be activated in the standard ways of control of static program procedures. The technology developed for upgrading is conformable to the requirements for authenticity and integrity of the transmitted information. The information blocks received by the onboard computer are transmitted to DACU which retranslated them to the Langmuir probe. These blocks contain built-in command sequences, interpreted according to the general protocol. With a view to improve the noise resistance in the basic software, an algorithm is realized for recovering individual damaged parts in the program code.
The microcontroller ATmega128 allows access to its resident energy-independent program memory (128 K flash) and its preprogramming in operational mode, and offers a large set of built-in devices, which makes it possible to realize the functions described above. The structure of the program memory used for the software of the probe is shown in Table 2. The whole address space is divided into sections 16 K each. An exception is the last section containing the resident software including: a driver realizing the chosen communication protocol; programs for processing external interruptions; a program for initialization, and a basic cycle of operation. The other sections contain blocks allowing reloading. They are used for saving procedures for the realization of the various scientific experiments. Section 0 (Table 2) plays the role of an intermediate buffer for reloading the sections. At a given moment a procedure is executed controlling one particular experiment. It can occupy only one section (16 K). The other sections are free for reloading. Each of the reloadable sections contains only one executable (scientific) procedure. The operation of such a procedure includes a sequence of actions described in Table 3. This sequence allows the setting of the initial parameters, measurement of the observed parameters, forming, and archiving elements containing scientific data, and completion of the operational cycle. The measurement values are formatted in blocks – elements with the structure shown in Table 4. Each of the elements contains information for the moment of the measurement, the type of the measured value, the scientific experiment during which the measurement has been made, Table 3 Sequence of actions executed in the procedure No.
Name
Remarks
1
3
Parameter number part Reading of parameter part Processing part
4
Termination part
Determinates how much parameters are expected Reading of the parameters and setting of the state variables Closed loop activities depending of the type of experiment. Saving the results into External memory Exit of experiment code
2
Table 2 Processor program space No. section
Description
Addresses
Remarks
0 1 2 3 4 5 6 Resident section Resident section
Section loader buffer Constants section Experiment 1 Experiment 2 Experiment 3 Experiment 4 Reserved Resident program off-line routines Resident section loader and on-line service routines
(0000-1FFF) (2000-3FFF) (4000-5FFF) (6000-7FFF) (8000-9FFF) (A000-BFFF) (C000-DFFF) (E000-F000) (F000-FFFF)
Scratch buffer Constants tables uploaded Uploaded codes Uploaded codes Uploaded codes Uploaded codes Not used Resident codes Resident codes
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Table 4 External memory element structure No.
Description of the data fields
0 1 2 3 4
The time (in 10 ms counts) from the start of the current year to the measurement moment, presented as 4 – byte integer
5 6 7
Status byte, giving current year, type of data, task number, number of experiment Two bytes signed integer presenting the measured value 8 bits CRC; calculated on the bytes from 0 to 6
the value of the measured character, and the control sum of the element. The elements are saved in a closed loop buffer kept by an external memory with a value of 512 K as demonstrated in Fig. 2. When a command of MODE2 type (‘‘Get Science Data’’) is executed, a set of the available elements is sent to DACU. Each element consists of 8 bytes as described in Table 4. The executable programs are upgraded in on-line mode by sending blocks of data with structure as shown in Fig. 3. Each block consists of a sequence of records, and each record consists of a title part, data, and a terminating byte announcing the end of the record. The data field consists of 22 bytes with the following meaning: Length of data Address in the section Program code CRC
2 bytes 2 bytes 16 bytes 2 bytes
The data field is repeated three times consecutively in order to avoid unwanted influences as noisiness, data loss, etc. The last record contains $4000 as the address in the section which distinguishes it from the other records, and this indicates it is the last one (Hipp, 1994). 3.3. Realization of noise resistant protocol for program block exchange The main goals of the protocol for data and programs exchange between the ground-based station and the probe are:
Fig. 3. Experiment section uploading file structure.
– guaranteed authenticity of the sent and received information; – possibility for partial recovery of information lost in the information channel (based on an algorithm included in the Langmuir probe software); – conforming to the requirements of the base protocol; – realization of a simplified processing algorithm for meeting the general time limitations; – possibility for a preliminary adjustment of the program modules intended to be sent in the framework of the communication blocks; – building in interface commands into the communication blocks to enable the realization of additional functions in the protocol used without having to change the existing software in the other systems (DACU and the onboard computer). The chosen scenario of operation is the following:
Fig. 2. External memory structure.
– compilation and adjustment of the program modules on ground; – processing of the initial file (saved in *.hex format) to extract the program code corresponding to the new scientific experiment in a new *.sec file. The latter contains the machine codes as well as special commands
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interpreted by the probe’s software. Security fields are also included to guarantee the authenticity and integrity of the sent (received) information (Fig. 3); – sending of the generated *.sec file by radiofrequency channel to the onboard computer. The actions described above can be done on research computers (’’High Privilege Client Station’’ – Fig. 1) connected via Internet to the ground-based station (‘‘Base Earth Station’’ – Fig. 1); – The on-board computer (Fig. 1) transmits the received *.sec file by the local network to the concentrator DACU which retransmits it (record by record) to the Langmuir probe via RS422. Each record contains formatting fields according to the base protocol. The command field contains code $10 which activates the command for loading and processing of the record and the included code. After processing, the program code is transmitted to the respective address from the flash of ATmega128; – The record processing includes the following stages: (1) verification and recovery of the information contained in the record; (2) control of the authenticity and the integrity of the recovered information by recalculating the CRC-16 sum and its verification; (3) saving the recovered information in the zero flash section according to the address contained in the record; and (4) after saving a record with the address $4000 (the last one of the block), section 0 is copied into the section with the desired number specified in the *.sec file. The noise-protection of the information included in the records is guaranteed by the algorithm chosen for coding and recovery of the data. The noise is supposed to have the nature of random influences with duration not exceeding the time for sending 22 bytes of useful information. During this time, random changes of the bits sent can occur. To guarantee the recovery, a representation of the data is chosen as three identical fields with 22 bytes each. After receiving the data by the probe, the different bits of the three fields are verified separately, and a resulting field of 22 bytes is created according to the following formula: di ¼ ðbi þ ci Þ:ai þ bi :ci where ai, bi, and ci are the values of the i-th bits of the three identical fields of the record, and di is the value of the i-th bit of the resulting field containing the recovered information. Subject to recovery is the CRC-16 sum contained in the last two bytes of each field. If the result is negative, a request for repeating the record is generated by DACU, the onboard computer, or the ground-based station. A record is only saved in the flash memory after the successful verification of the recovered record.
4. Summary The OBSTANOVKA-1 project unites the knowledge and skill of the space scientists and engineers from seven European countries and the Plasma–Wave Complex instrumentation developed by this team seems to be the most sophisticated device ever operated onboard the International Space Station for waves and plasma study. The realization of this project will allow answering a number of scientific and technological questions. This paper describes the Langmuir probes included in this experiment – the range of operation, the choice of geometry, the sweep voltage generation, and the adaptive algorithm for the probe operation. In view of the difficulties in the predictability of the environment in which the probes will have to operate, the possibility is foreseen for remote upgrading of the instrument from the ground. Moreover, this is accomplished using the standard telemetry, without sparing special physical and logical communication channels. Acknowledgement This work is supported by National Science Foundation Grant NZ-1509/05. References Atmel Corporation, Microcontroller with 128 K Bytes In-System Programmable Flash – ATmega128, Atmel Corporation, Rev. 2467L – AVR, 1–24, 2004. Brace, L.H. Langmuir probe measurements in the ionosphere, in: Pfaff, R.F., Borovsky, J.E., Young, D.T. (Eds.), Measurement Techniques in Space Plasmas – Particles: Geophysical Monograph 102. American Geophysical Union, Washington, DC USA, pp. 23–35, 1998. Georgieva, K., Kirov, B. The Langmuir probe for Earth and planetary ionospheric measurements: spherical or cylindrical? Bulg. Geophys. J. 19 (1), 28–37, 1993. Grard, R., Laakso, H., Pedersen, A., Trotignon, J.G., Mikhailov, Y. Observations of the plasma environment of Comet Halley during the VEGA flybys. Annales Geophysicae 7, 141–149, 1989. Hipp, R. Mktclapp: A toll for mixing C/C++ with Tcl/Tk, in: Ousterhout, J. (Ed.), Tcl/Tk Engineering Manual. Sun Microsystems Inc., Charlotte, NC, pp. 1–30, 1994. Klimov, S.I., Korepanov, V.Ye. The ‘‘Obstanovka’’ experiment aboard the International Space Station. Kosmichna Nauka i Tekhnologiya 10 (2/3), 81–86, In Russian, 2004. Krehbiel, J.P., Brace, L.H., Theis, R.F., Cutler, J.R., Pinkus, W.H., Kaplan, R.B. Pioneer Venus Orbiter electron temperature probe. J.Geophys. Res. GE-18, 49–54, 1980. Laframboise, J.G., 1966. Theory of spherical and cylindrical Langmuir probes in a collisionless, Maxwellian plasma at rest, University of Toronto UTIAS Report No. 100. Mott-Smith, L.R., Langmuir, I. The theory of collectors in gaseous discharges. Phys. Rev. 28 (4), 727–763, 1926. Wharton, L.E., Hoegy, W.R., 1971. Current to moving spherical and cylindrical electrostatic probes, Preprint X-621-71-276, Goddard Space Flight Center, 1971.