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ISSN 0010 9525, Cosmic Research, 2015, Vol. 53, No. 3, pp. 182­185. © Pleiades Publishing, Ltd., 2015. Original Russian Text © V.V. Andreyanov, 2015, published in Kosmicheskie Issledovaniya, 2015, Vol. 53, No. 3, pp. 195­198.

Data Format and Observational Modes for the RadioAstron Interferometer
V. V. Andreyanov
Astro Space Center, Lebedev Physical Institute, Russian Academy of Sciences, Moscow, 119991 Russia e mail: andre@asc.rssi.ru
Received December 16, 2013

Abstract--To transmit radio astronomy and auxiliary data from a space radio telescope to a ground tracking station (and then to a correlator) via radio link it is necessary to use a special data format and coding and decoding procedures. Here, the format developed and successfully implemented in the RadioAstron ground­space radio interferometer is considered in detail. The goal of the paper is to present the character istics necessary for astronomers, observers, designers of tracking stations, and management and planning workgroups, as well as for testing for compatibility of the space radio telescope and tracking stations. DOI: 10.1134/S0010952515030016

INTRODUCTION The output data of the Space Radio Telescope (SRT) are transmitted to the tracking station by two streams simultaneously via radio link in the Q band (15 GHz) using a special data format and QPSK (dou ble relative phase manipulation, DRPM) transmitter modulation. Before modulating, both formatted and "packed" data stream are additionally differentially coded in order to separate streams in phase. Each stream is divided into frames and contains radio astro nomical data obtained after clipping and 1 bit quanti zation (in sign of a signal). At the beginning of each frame a header is introduced, which contains a syn chronized code and the technical parameters of SRT and the service module. The rate of transmission streams is equal to 2 â 72 or 2 â 18 Mbit/s depending on the video band width and the number of bands used simultaneously, i.e., on the chosen observational mode. Indicated operations in SRT are performed by the For matter device. Formatter flight models are manufac tured in SINP MSU and SKB IRE (Fryazino). The tracking station (TS) receives, demodulates, and removes the differential coding in the signals from SRT. Then, streams on TS are "unpacked," i.e., frame head ers are distinguished, and the astronomical data are transformed into several parallel streams compatible with the data of the ground based radio telescopes (GRT). Figure 1 illustrates these procedures. During flight tests only TS in Pushchino was used, later TS in USA (Green Bank) was put into operation. OBSERVATIONAL MODES RadioAstron can provide for observations 2 or 4 video signals with a band width of 4 or 16 MHz and 1 bit quantization (in sign of a signal). Two input sig

nals come either from two SRT different receivers, but with a single polarization, or from one receiver, but with two circular polarizations. Figure 2 shows the mutual position of the SRT operating frequencies and spectra, which can be processed for each polarization. Here, lines of deuterium D, hydroxyl OH, formalde hyde H2CO, and water H2O are marked by an asterisk to select the narrow band observations. The mode is indicated by four parameters: the used frequencies of the second heterodyne for video convert ers (F1...F4 for two intermediate frequency signals); a width of one (lateral) video band, MHz; the total used band, MHz; the transmission rate of the binary data to the Earth, Mbit/s. For example, the mode F2F2 ­16­ 32­72 means: F2 = 508 MHz for both channels, video band is 16 MHz, full band is 16 â 2 = 32 MHz, the transmission rate is (32 â 2) â (9/8) = 72 Mbit/s for both channels simultaneously. We provide a choice of 9 combinations of 2 hetero dynes: F2 F2, F3 F3 for video bands of 16 MHz (see Table 1), as well as F1F2, F2F3, F3F4 and any pair of identical heterodynes for 4 MHz bands (see Table 2). During flight tests of the RadioAstron SRTVLB for simplicity's sake only part of the modes was used, mainly that with a video band of 16 MHz. In further observations, modes with video band of 16 MHz were thus also successfully implemented. THE SRT DATA FORMAT By applying the QPSK modulation of the 15 GHz transmitter the data stream 1 and the data stream 2 can be transmitted simultaneously. In this case, the order of the device connection in SRT (from the L and R antenna polarizers) is as follows:

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DATA FORMAT AND OBSERVATIONAL MODES Operations in SRT ­ Transformation in video spectrum ­ Quantization ­ Generation of synchronized code ­ Introducing the TM data from SRT and satellite ­ Formatting data (formation of serial data stream) ­ Differential coding Operations on TS ­ Radio receiving ­ DRPM demodulating ­ Differential decoding ­ Detecting synchronized code selecting, and decoding of all headers ­ Dividing received stream into initial substreams ­ Measuring current Doppler effect and Doppler phase into account performed prediction

183

Radio channel with DRPM

Input data

TM

Clock frequency

Output register data

TM

Data for determining residual and F and local time

Fig. 1. Basic operations for forming, transmitting, and allocating data from SRT.

L ­ the first receiver channel ­ the first Formatter channel ­ the I input of the QPSK modulator; R ­ the second receiver channel ­ the second For matter channel­the Q input the QPSK modulator. Figure 3 illustrates the structure of frames and their headers for both packed streams. Each frame in the streams contains 20 000 bytes: 30 bytes for the header and 19 970 bytes for the radio astronomy data. Each frame is numbered from 1 to 400 (the number is placed in the header); 400 frames occupy a time of 1 or 4 seconds depending on the selected observational mode (the data transmission rate). To distinguish streams 1 and 2 the position of synchronized codes in the headers are shifted by 8 bits. Each byte in the frames contains 9 bits; the 9th bit is a parity bit (always in synchronized code, each byte always contains an even number of "1" bits). This measure reduces the number of mistaken bytes (due to noise of the radio channel) and avoids failures or, which is the most dangerous, the formation of falsely synchronized code. The structure of the astronomical data. The posi tion of bits in bytes depends on byte packing. When observing sources with a wide spectrum, both lateral video bands can be used: the bottom (BLB) and the upper (ULB) in each channel. In this case, this byte packing takes the form:
Astronomical byte Bit 1 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7 Bit 8 ULB BLB ULB BLB ULB BLB ULB BLB
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This feature should be taken into account before correlation with the GRT data in order to use identical video bands. When a source with a narrow spectrum (for example, 4 MHz in ULB) is observed, the other output of the converter (BLB) will be filled with useless data (receiver noise). To check SRT and TS without observing the source, there are two test modes: Test 1, when Formatter continuously repeats synchronized code without the header, and Test 2, when the binary sequence 0101010101... is generated instead of astro nomical data. The rule of differential coding of streams before transmitter modulation is shown in Table 3.
Table 1 Band 327 MHz

Possible mode
F4 ­ 4­8­18, F3F3 ­ 16­32­72

1665 MHz For both F2F2 ­ 16­32­72 4830 MHz polarizations 22 235 MHz simultaneously F3F3 ­ 16­32­72 F1F2 ­ 4­16­18 F2F3 ­ 4­16­18 F3F4 ­ 4­16­18

Bit 9 Parity

FiFi ­ 4­16­18

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184 Middle frequency of SRT receivers Line frequencies (*) 324 * 327.4 D 1664

ANDREYANOV 4832 * 4829.6 H2CO 22232 * 22235 H2O MHz

** 1665.4; 1667.3 OH

200 Frequencies

1152

4320

21720 MHz

of first heterodynes

4 â 4 MHz 2 â 16 MHz Used part of IF spectrum for continuum sources

Conditional middle

Only for 327 MHz 2 â 4 MHz

2 â 16 MHz

* H2CO Frequencies of second heterodynes (for video converters)

**

*

* D FIF MHz

OH H2O 512

F1 = 500

F2 = 508

F3 = 516

F4 = 524

Fig. 2. Relative position of the SRT working ranges and bands.

Detailed structure of the header. Bytes 1­10 con tain technical telemetry from the TM satellite system. Bytes 11­12 contain data on the output power level (8 bit ADC) of video channels 1 and 2, including noise calibration with the period of second frame (even­odd). Byte 13 is reserve (000000001).
Table 2 Band 327 1665 4830 22 235 MHz MHz MHz MHz F4 F3 F2 F1 F2 F3 F4 ­ ­ ­ ­ ­ ­ ­ Possible mode 4­8­18, F3 ­16­32­72 4­8­18 4­8­18 16­32­72 Selected depending on predicted source 16­32­72 velocity 16­32­72 (up to 1500 km/s) 16­32­72

Byte 14 shows the receiver mode. Byte 15 shows the Formatter mode. Bytes 16­22 for stream 1 and 17­23 for stream 2 are anti interference synchronized code representing pseudonoise 63 bit sequence with an even number of units in bytes: 111011001 111000001 010001110 010010110. 000011000 101001111

Byte 23 in the stream 1 and byte 16 in stream 2 are reserve. Byte 24 is the same as byte 14. Byte 25 is the receiver and Formatter modes. Bytes 26­27 are the frame number (1­400) in 9 bit binary code. Bytes 28­30 are reserve in both streams.
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DATA FORMAT AND OBSERVATIONAL MODES Table 3 Previous bits of streams 1 and 2 00 Current bits of streams 1 and 2 00 01 10 11 00 01 10 11 01 11 00 10 10 00 11 01 11 10 01 00 01 10 11

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THE RESULTS For observations of celestial sources with different spectral widths in different bands recommended obser vational modes are composed. The necessity to transfer the data from SRT to the Earth requires the conversion of the received SRT data into a form compatible with the capabilities of the radio link of the SRT­TS com munication. For this, in the case of RadioAstron, a par ticular format is selected. All this requires the restora tion on TS of the data form from SRT and the reliability of their compatibility when correlating with the GRT data. The transmission in the scientific data stream of a part of technical information eliminates the loss of the most important service telemetry during scientific ses sions. The knowledge of observational modes and the
Header 1th frame Data
(i + 1)th frame (i + 2)th frame

SRT data format after decoding on TS allow to users of this radio interferometer to ensure the identity with the GRT data format. ACKNOWLEDGMENTS The RadioAstron project is carried out by the Astro Space Center of Lebedev Physical Institute and Lav ochkin Association under contract with the Russian Space Agency together with many scientific and tech nical organizations in Russia and other countries. REFERENCES
1. Andreyanov, V.V., Compatibility problems of tron, VSOP, VLBI, and VLBA in Frontiers Hirobayashi, H., Inoue, M., Kobayashi, Tokyo: Universal Academy Press, Inc., 1991, 167. Radioas of VLBI, H, Eds., pp. 163­

Header for stream 1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

2. Andreyanov, V.V., Radio telescope in the sky, Science in Russia, 1993, nos. 3­4, pp. 8­13. 3. Kardashev, N.S., Radioastron--A radio telescope much greater than the Earth, Experimental Astronomy, 1997, vol. 7, pp. 329­343. 4. Andreyanov, V.V., Strategy and scenario for Radioas tron space VLBI observations, Experimental Astronomy, 1999, vol. 9, pp. 103­117. 5. Andreyanov V.V., Borisov A.A., and Knorin I.A., Radioastron data format & observation modes, Preprint of Lebedev Physical Inst., Russ. Acad. Sci., Mosccow, 1997, no. 55.

Service TM

Synchronized code

Header for stream 2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Service TM

Synchronized code

Fig. 3. Structure of frame and its header. COSMIC RESEARCH Vol. 53 No. 3 2015

Translated by N. Topchiev