REPORT ITU-R SA Telecommunication characteristics and requirements for space VLBI systems

Transcription

1 Rep. ITU-R SA REPORT ITU-R SA.2132 Telecounication characteristics and requireents for space VLBI systes (2008) This Report describes the characteristics of the space VLBI systes. These characteristics for a technical basis for Recoendations related to space VLBI systes. The contents of this Report were originally included in the Recoendation ITU-R SA.1344 Preferred frequency bands and bandwidths for the transission of space VLBI data, as an annex. That annex aterial has been reoved fro the Recoendation and is aintained in this Report. In preparing this Report, any revisions have been ade to the aterial forerly in the Recoendation and the topics have been rearranged to iprove clarity. The Report includes a general description of the space VLBI systes and detailed descriptions of teleetry link for science data and of phase transfer link for tie and frequency synchronization. Also included are the explicit equations for cross-correlation SNR and its degradation due to interference, required carrier frequencies and teleetry bandwidths, an interference criterion for the teleetry channel, effects of noise on the phase-transfer link, the characteristics of existing and planned space VLBI systes, and the characteristics of earth stations.

2 2 Rep. ITU-R SA.2132 Contents Page 1 Introduction Description of the space VLBI syste Telecounication links for space VLBI Earth-to-space (E-s) telecoand link E-s phase transfer link for tie and frequency synchronization Space-to-Earth (s-e) teleetry link for science data S-E phase transfer link Technical characteristics Teleetry link Space VLBI cross correlation function Cross-correlation SNR degradation Required interference criterion for the teleetry link Required bandwidths for the teleetry channel Preferred space-to-earth teleetry carrier frequencies Phase-transfer link Phase noise introduced in propagation Phase noise introduced in carrier recovery Preferred frequency bands and bandwidths within the space research service (SRS) allocated bands Characteristics of existing and planned space VLBI systes Characteristics of earth stations Bibliography... 16

3 Rep. ITU-R SA Introduction Very long baseline interferoetry (VLBI) allows experienters to observe radio sources with angular resolutions that cannot be approached by other ethods. In addition, VLBI has other scientific and engineering uses. Observations of distant radio sources with two or ore VLBI stations can be cobined to deterine the structure and positions of extra-galactic radio sources, to deterine the geodynaical characteristics of the Earth, to study the Moon s libration and tidal response, to deterine orientation of the solar syste with respect to the extra-galactic inertial frae, to deterine the vector separation between antenna sites, and to provide navigation and tracking of spacecraft. 2 Description of the space VLBI syste Space very long baseline interferoetry (SVLBI) is a highly useful extension of very long baseline interferoetry (VLBI), which in turn is a developent fro conventional radio interferoetry. In all three cases, a specified bandwidth of cosic or other radio eission is received siultaneously at two or ore antennas that are distributed over distances uch larger than the size of individual antennas. These bands, which can be described as tie-varying spectra, are downconverted to a lower frequency so that they can be further aplified and then cross-correlated. In conventional interferoetry this processing is done in real tie. To preserve the aplitude and relative phases of the spectral coponents the downconversion has to be based upon a coon local oscillator or frequency reference. The attraction of interferoetry is that the angular resolution of the interferoeter is related to the separations between the antennas rather than their physical size. However, there is a practical liit to how far antennas can be separated and still use real-tie signal transfer, and that the largest conventional interferoeters lack the angular resolution needed for the investigation of any types of cosic radio source or deterination of the position of distant space probes. The developent of ultrastable oscillators, accurate clocks and large-bandwidth data recording systes (using discs or agnetic tape) ade it unnecessary to connect the antennas or use coon local oscillator references, so the antennas could be oved further apart and the data taken after the experient to a processing station where they could be synchronized and correlated, yielding a ap of the source region. Antenna separations (interferoeter baselines) of thousands of kiloetres have been used successfully. However the diaeter of the Earth sets a hard liit to the usable antenna spacings. Source visibility above the horizon in ost cases liits the spacing even further. Space very long baseline interferoetry (SVLBI) reoves this liitation by putting one of the interferoeter antennas in space. Although in essence the process is still that of conventional ground-based VLBI there are soe additional coplications. Firstly the spacecraft carrying the spaceborne antenna eleent is oving at orbital velocity, and the otion has to be known quite accurately, and secondly the data have to be downlinked to a ground station for recording. Maintaining accurate tie tagging of the data is uch ore coplicated. The configuration of a typical SVLBI experient is shown in Fig Telecounication links for space VLBI The telecounication links of the space VLBI syste are represented in Fig. 1 by the four dashed lines between the space VLBI spacecraft telecounication antenna and the space VLBI earth station. A description of the radio links follows.

4 4 Rep. ITU-R SA Earth-to-space (E-s) telecoand link This radio link is used for reliable transission of telecoands required for operation and correction of possible spacecraft alfunctions E-s phase transfer link for tie and frequency synchronization In VLBI accurate knowledge of the tie, the signal frequency, and the signal phase is needed for post-real-tie cross-correlation. This requireent is et by using high-stability oscillators, often referred to as atoic clocks, at every station and also by utilizing the Global Positioning Syste (GPS). At present an Earth-to-space phase-transfer link is used to ipart the required tie/phase reference to the spacecraft on-board clock and local oscillators. In the future the space VLBI spacecraft ay have a space-qualified atoic clock. However, the distant space VLBI station ay not be able to utilize the GPS syste for tie synchronization. Hence, the E-s phase transfer link will still be needed for tie synchronization.

5 Rep. ITU-R SA Space-to-Earth (s-e) teleetry link for science data The space VLBI spacecraft observes the radio source over a selected bandwidth. This observed spectru is transitted to the space VLBI earth station using this s-e teleetry link for science data for recording and subsequent cross-correlation with the spectru observed by the VLBI earth stations S-E phase transfer link This radio link will be a coherent frequency translation of the Earth-to-space phase transfer link described above and will be used to calibrate the phase errors introduced in the Earth-to-space phase transfer link by different causes. This radio link ay be dedicated to this phase transfer operation or ay be cobined with the s-e teleetry link for science data to transfer the observed spectra fro the spacecraft as described in Technical characteristics A detailed characterization of the space VLBI teleetry link for science data and the phase-transfer link for tie and frequency synchronization is given below. In a space VLBI syste, we need to consider two issues: firstly the observing syste itself, which is siilar to a terrestrial VLBI syste except that at least one antenna is oving at a high and varying velocity copared with the rest of the network, and consequently establishing the velocity and position vectors is ore coplicated. This requires the transfer of tiing and frequency standard signals between the ground and the spacecraft. Secondly, the data have to be transferred to the ground receiving station by a teleetry link. There could be quantization effects and also phase scintillation produced by the ionosphere. The space VLBI spacecraft receives the radio source frequency spectru containated with background and syste noise in the observation bandwidth. This observed spectru is transitted to the space VLBI earth station where it is recorded and cross-correlated with the frequency spectru observed at the earth station of the sae radio source. The transission of the observed spectru fro the space station to the earth station ay be analogue or digital. In digital transission, the observed analogue signal is first converted to a digital forat and then transitted to the space VLBI earth station for recording. The transission of a teleetry signal through space degrades the signal before it is detected at the earth station receiver. In digital transissions, this degradation increases the errors in detecting the inforation bits. These degradations, thus, affect the final cross-correlation process of the space VLBI experient by lowering the cross-correlation signal-to-noise ratio (SNR). 3.1 Teleetry link Space VLBI cross correlation function The basic observables in radio interferoetry are the aplitude and relative phase of the cross-correlation of the two observed spectra. This cross-correlation process is usually perfored in non-real tie and ay be expressed as: R τ ) = < x( t) y( t τ ) > (1) xy ( g g where R xy is the cross-correlation function, < > is the estiated ean for the observation over tie, x(t) and y(t) are the recorded signals at sites 1 and 2, and τ g is the wave front tie delay.

6 6 Rep. ITU-R SA.2132 In the cross-correlation function of equation (1), the pre-recorded signals will be containated with noise fro the receiving systes. For each receiving station, we can define an observer signal-tonoise ratio (ξ) as: 1 2 S A B S A ξ = = = 1,2 (2) kt B 2kT where: S 1 : spectral flux density of observed source at antenna 1 (W/(Hz 2 )) A 1 : effective area of receive antenna 1 ( 2 ) T 1 : syste noise teperature of receiver 1 (K) S 2 : spectral flux density of observed source at antenna 2 (W/(Hz 2 )) A 2 : effective area of receive antenna 2 ( 2 ) T 2 : syste noise teperature of receiver 2 (K) k: Boltzann s constant (= W/(Hz K)) B: observation bandwidth (Hz). Note that equation (2) has a factor 1/2, since the spectral flux densities refer to the total eission fro the source, which is alost always largely unpolarized, and any practical antenna syste can only receive half of this total. The effective area of a receive antenna can be written as: 2 D A = ηπ = 1, 2 (3) 4 where η is the aperture efficiency and D is the diaeter of the antenna (in etres). Using the effective area of the antenna, we can define a syste noise equivalent flux density (S*) for each receiver as: * 2kT 8kT S = = = 1,2 (4) A 2 η πd Now, using the syste noise equivalent flux density for the receivers, we can express the observer signal-to-noise ratios as: S S ξ = = = 1,2 (5) 2 kt / A * S It has been shown that the cross-correlation signal-to-noise ratio (X 1,2 ) ay be expressed as a function of the two observing signal-to-noise ratios ξ 1 and ξ 2 as: X1,2 = ξ1 ξ2 2Bτ (6)

7 Rep. ITU-R SA where B is the observation bandwidth and τ is the integration tie of each observation. This cross-correlation SNR can also be expressed in ters of the ean flux density S1,2 = S1 S2 of the observed source and the syste noise equivalent flux densities of the observing stations as: X 1,2 2Bτ = S1,2 (7) S S * 1 * 2 The cross-correlation SNR should be aintained as large as possible to decrease the easureent error of the τ g in equation (1). Note that for this space VLBI syste with two eleents, if we define the noise flux density threshold (S th ) of the cross-correlation as: we can then write the cross-correlation SNR as: * * S1 S2 S th = (8) 2Bτ S X1,2 = (9) This equation does not account for the ionospheric scintillation, that is, for the rapidly fluctuating ionospheric delays. The phase error introduced lowers the aplitude of the cross-correlation of a space VLBI syste. Thus, the actual, easured cross-correlation SNR with ionospheric scintillation is ore accurately odelled as: X 1,2 1,2 S th g S1,2 S1,2 = = (10) S S th where g is called the coherence factor and S 0 = S th /g is the effective sensitivity threshold of the space VLBI syste. Note that when coherence factor is one, the VLBI sensitivity threshold equals the cross-correlation noise flux density threshold, an ideal situation. Norally coherence factor will be less than one, and the sensitivity threshold will rise above the noise density threshold. The coherence factor used in equation (10) is deterined experientally. The aount of ionospheric delay fluctuation is represented by an experientally deterined scintillation index. Low scintillation index eans less ionospheric delay fluctuation, whereas high scintillation index eans a large ionospheric delay fluctuation. Equation (9) shows that, in order to increase cross-correlation SNR, we need to decrease the space VLBI noise flux density threshold. This we can accoplish by using wider observation bandwidths and longer integration ties, and by having stations with lower equivalent flux densities, which in turn eans having larger antennas with low syste noise teperatures Cross-correlation SNR degradation The space VLBI spacecraft receives the radio source frequency spectru containated with noise (background, syste, etc.) in a selected observing bandwidth, B, at a given observing SNR, ONR 1. This observed spectru has to be transitted to the space VLBI earth station to be recorded and further processed (cross-correlated). This transission ay be an analogue transission or the observed analogue signal ay be converted to a digital forat and transitted to the space VLBI earth station for recording. 0

8 8 Rep. ITU-R SA.2132 The transission of a teleetry signal through space iplies soe signal degradation when detected at the intended receiver. In digital transissions, this degradation is due to the probability of inforation bits being in error and is dependent on the received sybol signal-to-noise ratio (SSNR). This link degradation will affect the final process of the space VLBI experient, i.e. the cross-correlation function of equation (1). For an analogue teleetry link, the cross-correlation SNR that includes the teleetry link losses is shown to be: X ξtl = ξ1 ξ2 Bτ 1+ ξ + ζ a 2 1 tl (11) where ζ tl is the teleetry-link SNR. The first factor is the cross-correlation SNR degradation for analogue teleetry, i.e.: X a degradation (analogue) = X X a = 1,2 1 + ξ ζ 1 tl + ζ tl (12) For the digital teleetry link with 1-bit quantization (see Report ITU-R SA.2065 Protection of the space VLBI teleetry link), the cross-correlation SNR is written as: X 2 ( Erfc ξ ) 2 ξ ξ τ = 1 tl π 1 2 B q 2 (13) where ξ tl is the teleetry link sybol SNR, and Erfc is the copleentary error function. The factor in brackets is the cross-correlation SNR degradation for digital teleetry, i.e.: X q X d degradation (digital) = = ( 1 Erfc ξ ) X 1,2 tl 2 2 π (14) Note the inherent degradation introduced by the quantization of the norally distributed observed source, i.e. when the teleetry sybol SNR (ξ tl ) is large, the cross-correlation SNR degradation approaches to 2/π. Figure 2 shows the degradation of cross-correlation SNR for analogue and digital teleetry links, when the observer signal signal-to-noise ratio ONR 1 = 20 db. The analogue and digital crosscorrelation SNR degradations are plotted in the sae graph by using SNR = SSNR. tl 2 tl In Fig. 2, the curve for digital teleetry transission clearly shows an inherent degradation of the cross-correlation SNR introduced by the quantization of the noise signal the observed source; i.e., even with a very high teleetry sybol SNR, the cross-correlation SNR degradation approaches to 1.96 db (=10 log(2/π)).

9 Rep. ITU-R SA Required interference criterion for the teleetry link Since the interference in the space VLBI teleetry link causes data errors and hence degrades the SNR of the final VLBI cross-correlation, an interference criterion has to be specified. The degradation of the cross-correlation SNR caused by the theral noise and external interference in the space VLBI teleetry link is analyzed in detail in Report ITU-R SA.2065 for the GHz band. The tolerable interference level is recoended to be 0.02 db degradation in the crosscorrelation SNR. For the GHz band, the results indicate that, for a cross-correlation SNR degradation tolerable to the VLBI easureent, the interference power received fro any interfering syste should not exceed db(w) per GHz at the input of the low-noise aplifier of an SRS earth station. In addition, Recoendation ITU-R RA.1513 Levels of data loss to radio astronoy observations and percentage-of-tie criteria resulting fro degradation by interference for frequency bands allocated to the radio astronoy on a priary basis, presents data loss criterion for the VLBI observations to be no ore than 2% of the tie per interfering syste, and no ore than 5% of the tie fro all interfering systes. For the GHz band, since the syste noise teperatures are coparable with the GHz band, the interference received in any 1-GHz band should not exceed db(w) for ore than 2% of the tie. For the GHz band, the interference criterion will have to be deterined depending on the syste noise teperatures in this band Required bandwidths for the teleetry channel Phase odulation has been shown to attain optiu perforance on satellite telecounications links. Therefore binary phase shift keying (BPSK) or quadri-phase shift keying (QPSK) will be considered as the preferred digital odulation schees.

10 10 Rep. ITU-R SA.2132 When digitizing the observation bandwidth of B Hz, the required Nyquist sapling rate will be twice the bandwidth or 2B saples per second. Each observed voltage saple is quantized either at two levels (1-bit representation), four levels (2-bit representation), eight levels (3-bit representation), etc. The total teleetering channel sybol rate, R s, required will therefore follow the equation: ( L) Bn R s = 2B log2 = 2 (15) where L is the total nuber of quantization levels and n is the nuber of bits. The radio-frequency bandwidth (W) required for the transission of BPSK with teleetry losses less than 0.3 db has been recoended (see Recoendation ITU-R SA.1015 Bandwidth requireents for deep-space research) to be: W = 4 Rs = 8Bn (16) If QPSK is used, the sae bandwidth can accoodate twice the sybol rate with approxiately the sae perforance as the BPSK case, or for the sae sybol rate the required bandwidth is half the bandwidth for BPSK. Table 1 is a suary of the above considerations and shows the required radio-frequency (RF) bandwidths as a function of observation bandwidth, B. Note that saller bandwidths than those recoended ay be used with a reduced link perforance (saller data rate). TABLE 1 Required radio-frequency (RF) bandwidth Observation bandwidth Hz B Analogue teleetry bandwidth Hz 2B Digital teleetry Sapling rate saples/s 2B Nuber of bits/saple Quantization levels Sybol rate sy/s 2B 4B 6B BPSK bandwidth Hz 8B 16B 24B QPSK bandwidth Hz 4B 8B 12B Preferred space-to-earth teleetry carrier frequencies One old and one planned space VLBI syste require a axiu RF transission bandwidth of less than 500 MHz. These systes would require high carrier frequencies with enough allocated bandwidth. In this respect, the SRS GHz band is not suitable since it does not have enough bandwidth allocation. Future RF bandwidth requireents (1 GHz to 10 GHz), however, indicate the need for carrier frequencies larger than 20 GHz. 3.2 Phase-transfer link The phase transfer link is used to derive a stable on-board frequency reference fro a clock on the ground. The frequency of the on-board reference ust be precisely known to support science data processing.

11 Rep. ITU-R SA A prie requireent for an on-board local oscillator of a space VLBI spacecraft is that its frequency/phase stability be nearly as good as that of a VLBI earth station s local oscillator driven by the atoic standard. No space-qualified atoic standard exists today; therefore, the required stability will be transferred to the space VLBI spacecraft via an Earth-to-space radio link. The carrier frequency of this radio link, f up, is recovered at the spacecraft to generate the on-board reference frequencies to be used in the radio source observing process. In order to calibrate all the unknown phase errors introduced in this Earth-to-space phase-transfer radio link, this carrier frequency is coherently down-converted and transitted back to the space VLBI earth station, f down. In this two-way phase calibration, phase errors are ainly introduced by the propagation ediu, spacecraft receiver, and space VLBI earth station receiver. These phase errors will contribute to the uncertainty in the deterination of the aplitude and relative phase of the non-real-tie cross-correlation process of equation (1) effectively lowering the cross-correlation SNR of equation (6) Phase noise introduced in propagation There are three priary sources of error in deterining the on-board frequency: error in the odel value of the uplink Doppler shift due to spacecraft orbit uncertainty, unodelled uplink path delay changes due to troposphere, and unodelled uplink path delay changes due to ionosphere. To calibrate these errors the round-trip phase is easured and used to derive an estiate of the uplink phase error. At the beginning of the analysis, it is assued that errors are reciprocal, eaning that the uplink and downlink delay errors are the sae. The phase, φ up, of the on-board reference frequency, f up, is retrieved fro the easured round-trip phase, φ round, easured on the ground station through the equation (17): ϕ up f = 2 f up down Equation (17) gives an accurate value of the uplink phase error assuing that all error sources are non-dispersive and reciprocal. The spacecraft orbit uncertainty and the tropospheric path delay are non-dispersive and nearly reciprocal, so the equation (17) does a good job calibrating these error sources. However, the ionospheric path delay change is dispersive, so an error is ade when using this forula when the transponder turnaround ratio, f down /f up, is not equal to unity. If the transponder turnaround ratio is nearly equal to unity, then the equation (17) also does a good job of calibrating the ionosphere. There exists frequency dependent path delay, τ i, in the propagation of an electroagnetic wave through the ionosphere. Therefore, equation (17) should be odified to: ϕ round [ τ ( f ) τ ( f )] (17) fup ϕ up = ϕround + fup π i up i down (18) 2 f down where: 40.3 τ i ( f ) = TEC 2 i s (19) c f c: velocity of light (/s) f: propagation frequency (Hz) TEC i : total electron content (electrons/ 2 ).

12 12 Rep. ITU-R SA.2132 Now the phase error can be written as: [ τ ( f ) τ ( f )] fup ϕ error = ϕup ϕround = fup π i up i down (20) 2 f down This phase error is due to the frequency dependent ionospheric delay. Unless additional inforation about the total electron content (TEC i ) in the ionosphere is provided, a proper correction for this error cannot be ade. Nevertheless, this error becoes saller if frequencies of both f up and f down are ade higher and closer to each other. Table 2 gives the calculated results of the ionospheric tie delay error in units of picoseconds (psec) for two frequency pairs ( GHz and GHz). A total electron content of electrons/ 2 has been assued. It is clear that the phase transfer errors at higher frequencies is uch saller than at lower frequencies. TABLE 2 Ionospheric propagation effects Link frequencies f up GHz f down GHz Period psec Absolute value of ionospheric error (TEC = electrons/ 2 ) psec Note that in these cases, the phase error introduced due to ionosphere is ore than twice the wavelength of the lower frequency pair (7.2 GHz, 8.46 GHz), but only about half the wavelength of the higher frequency pair (15.3 GHz, 14.2 GHz). The contribution of the ionospheric delay to the phase of the received signal is inversely proportional to the frequency used. Therefore, the phase fluctuation induced by the ionosphere is approxiately twice as low at the (15.3 GHz, 14.2 GHz) bands than at the (7.2 GHz, 8.46 GHz) bands. Thus, the expected coherency losses due to ionosphere are lower at the higher frequencies Phase noise introduced in carrier recovery At the space VLBI spacecraft receiver of the Earth-to-space radio link as well as at the space VLBI earth station s receiver, the carrier recovery process considered ay be the result of any cobination of the following odulation schees: an unodulated carrier, or a binary phase shift keying (BPSK), or a quadri-phase odulation (QPSK). It has been shown that the phase error variance, σ 2, for carrier recovery processes ay be expressed as a function of the phase lock loop receiver closed-loop bandwidth (B L ), one-sided noise spectral density (N 0 ), and carrier power (P). For unodulated carrier, the phase error variance is: B N σ 2 = L 0 (21) P For BPSK odulation, it is given as: σ 2 = B L N P SSNR (22)

13 Rep. ITU-R SA where P is the suppressed-carrier power and SSNR is the sybol signal-to-noise ratio, i.e. SSNR = PT s /N 0 with T s being the sybol period. For QPSK odulation, the phase error variance is given as: 2 σ = B L N P 2 3 SSNR SSNR SSNR (23) where SSNR is the binary sybol SNR, i.e. SSNR = PTb N 0 with T b being the binary sybol period. Note that quaternary sybol period T s = 2T b. For very strong SSNR, the three cases converge to: B N σ 2 = L 0 (24) P It is clear that to reduce the phase noise introduced in carrier recovery, narrower loop bandwidth, lower syste noise density, and higher signal power are needed. 4 Preferred frequency bands and bandwidths within the space research service (SRS) allocated bands The list of frequency bands given in Table 3 is intended to identify those frequency ranges that are preferred for the operation of the science data teleetry and phase-transfer links with iniu degradation in the space VLBI easureents. The use of these frequency bands for space VLBI systes, however, is also guided by frequency sharing considerations, technical equipent liitations, and necessary bandwidth. The RF bandwidths indicated could be assigned anywhere within the frequency band and could be increased without exceeding the frequency band liits. Also, the nuber of such RF links could be increased to ake use of the allocated frequency band to support any single or ulti-spacecraft syste. TABLE 3 Preferred frequency bands and bandwidths for space VLBI within the space research service (SRS) allocated bands Frequency band RF bandwidth Direction Typical use GHz MHz space-to-earth Earth-to-space Phase transfer Teleetry and phase transfer Phase transfer and coand

14 14 Rep. ITU-R SA Characteristics of existing and planned space VLBI systes Existing and planned space VLBI systes are shown in Table 4. VSOP systes typically use data rates in the order of 128 Mbit/s and QPSK odulation. The Radioastron will use a data rate of 144 Mbit/s with QPSK odulation. The axiu RF bandwidth required would therefore be in the order of 128 MHz for VSOP and 144 MHz for Radioastron. For VSOP-2 the sybol rate will be about 1 GSybol/s, which will require RF bandwidths greater than 1 GHz. Theoretical studies of propagation effects on wide bandwidth transissions have indicated that the atosphere can support several gigahertz of bandwidth at carrier frequencies above 10 GHz. Therefore, transission bandwidths in the order of 3 GHz to 4 GHz ay very well be envisioned in future space VLBI systes. TABLE 4 Characteristics of existing and planned space VLBI systes Paraeter Radioastron VSOP ASTRO-G (VSOP-2) Observing antenna diaeter M ; ; ; ; 8.5 Observing frequency and 1.6; ; ; 30 15; 23 GHz; K syste teperature 5.0; 70 20; ; 40 42; ; ; Noinal integration tie s Space-to-Earth Frequency GHz (phase transfer only)* Transitting power W 8.5 IVS Modulation type QPSK QPSK QPSK --- Maxiu bit rate Mbit/s Quantization levels 2 2, 4 2, RF bandwidth MHz Miniu E b /N 0 db Teleetry link interference criteria Earth-to-space (phase transfer) Received interference power db(w) in any 1-GHz band (not to be exceeded by ore than 2% of the tie) Frequency GHz Modulation type None None None --- Maxiu bit rate Mbit/s RF bandwidth MHz PLL bandwidth Hz Miniu E b /N 0 db

15 Rep. ITU-R SA TABLE 4 (end) Paraeter Radioastron VSOP Orbital characteristics ASTRO-G (VSOP-2) Inclination degrees Height at perigee k Height at apogee k Period h NOTE 1 Radioastron will continue to use the frequency 8.4 GHz for phase transfer under existing ITU-R publication API/A/3957. IVS Table 4 above suarizes the salient radio link and orbital characteristics of the current and planned space VLBI systes. The space VLBI spacecraft for the VLBI Space Observatory Project (VSOP, Japan) was launched in The VSOP-2 and the International VLBI Satellite (IVS) systes are being considered as the next generation space VLBI issions. 6 Characteristics of earth stations In space VLBI systes any teleetry receiving stations around the Earth are used; for exaple the Deep Space Network Orbiting VLBI Subnet (United States). The ain characteristics of these earth stations are suarized in Table 5. Paraeter Receive frequency band TABLE 5 Suary of space VLBI earth station characteristics GHz Frequency bands db receive bandwidths MHz Receive zenith G/T dbi/k Transit frequency band GHz Transit antenna gain dbi Transit power levels W db transit bandwidths MHz Receive or transit polarizations RHCP or LHCP RHCP or LHCP RHCP or LHCP Teleetry receiver capability Mbit/s Antenna diaeter