1、 Rec. ITU-R SA.1742 1 RECOMMENDATION ITU-R SA.1742 Technical and operational characteristics of interplanetary and deep-space systems operating in the space-to-Earth direction around 283 THz (Question ITU-R 235/7) (2006) Scope This Recommendation specifies technical parameters (frequencies, link, si
2、gnal and data characteristics, antenna parameters, etc.) and operational characteristics of interplanetary and deep-space systems operating in the space-to-Earth direction around 283 THz, which could be used in sharing studies. The ITU Radiocommunication Assembly, considering a) that telecommunicati
3、on links are being planned for use on some satellite systems for deep-space and interplanetary radiocommunications at frequencies in the region of 283 THz; b) that, using recent technological developments, astronomers are making a concerted effort to build telescopes and make observation in this seg
4、ment of the spectrum; c) that this segment of the spectrum is also being used for other terrestrial and space services; d) that this segment of the spectrum is also being used for scientific and industrial purposes other than radiocommunication; e) that mechanisms of interference between satellites
5、operating in deep-space and passive systems such as astronomy operating above 20 THz may differ from those in the radio-frequency portion of the spectrum, recognizing 1 that No. 78 of Article 12 of the ITU Constitution states a function of the Radiocommunication Sector includes, “. carrying out stud
6、ies without limit of frequency range and adopting Recommendations .”; 2 that, under Note 2 of No. 1005 in the Annex to the ITU Convention, Study Groups may consider “radiocommunication” to include electromagnetic spectrum above 3 000 GHz propagated through space without artificial guide in the cours
7、e of their studies and in the creation of draft new Recommendations; 3 that use and sharing of this segment of the spectrum has not been thoroughly studied within the ITU-R, recommends 1 that sharing studies considering space research satellites operating in the space-to-Earth direction around 283 T
8、Hz in deep space should take into account the technical and operational parameters presented in Annexes 1 and 2. 2 Rec. ITU-R SA.1742 Annex 1 1 Introduction The increased pressure for use of the radio spectrum and the advancement of technology, there is more attention being given to the use of frequ
9、encies above 3 000 GHz for free space radiocommunications. Radiocommunication links have become a reality in the frequency bands above 3 000 GHz as a result of many recent technological developments in the field of optical fibre telecommunication especially in the area of lasers, modulation and rece
10、iver technology. Free space radiocommunication at frequencies above 3 000 GHz has the ability to support higher data rates with less mass than traditional radio-frequency systems as well as meet gain and directivity requirements of beams used for deep-space applications. 1.1 Frequency considerations
11、 Currently, most of the interest in free space radiocommunication links above 3 000 GHz is focused around the frequencies 200, 283, 311 and 353 THz1, whose corresponding wavelengths are approximately 1.5, 1.06, 0.965 and 0.850 m. These frequencies are the same as those most widely used for telecommu
12、nications in optical fibres. For interplanetary and deep-space radiocommunication in the space-to-Earth direction, attention is being focused on use of Ytterbium-doped (Yb) fibre-optical amplifier at 1.06 m in a Master-Oscillator-Power Amplifier (MOPA) configuration, Q-switched, Neodymium: Yttrium A
13、luminium Garnet (Nd:YAG) or Neodymium: Yttrium Vanadate (Nd:YVO4) lasers operating around 283 THz (1.06 m) though, depending on mission requirements, other frequencies are feasible. The use of Yb, Nd:YAG and Nd:YVO4are of primary use due to availability and reliability. 1.2 Generic mission parameter
14、s Technical parameters suitable for interference analyses should be based on generic interplanetary missions to Mars and Jupiter. For the purposes of minimizing weight and power consumption, the links must support radiocommunication requirements beginning shortly after launch and continuing througho
15、ut the duration of the mission negating the need for additional radiocommunication systems. Therefore, link distances will vary from a few thousand km to several Astronomical Units (AU2). Distances from the Earth to Mars or Jupiter vary from 0.5 to 6.2 AU. A summary of the fundamental technical para
16、meters of a 283 THz deep-space link operating in the space-to-Earth direction is provided in Table 1. 2 Link considerations Deep-space links operating at 283 THz in the space-to-Earth direction may utilize Yb, Nd:YAG or Nd:YVO4lasers. The beam would be transmitted from a 30 cm telescope on board the
17、 spacecraft and received by a telescope on the Earth with an effective diameter of 4.2 m to 10 m.311 THz = 1 000 GHz. 21 AU 149 597 870 km. 3For the purposes of telecommunication through free space at around 283 THz, a telescope is effectively an antenna. Rec. ITU-R SA.1742 3 TABLE 1 Technical param
18、eters of two reference deep-space missions operating at 283 THz in the space-to-Earth direction Parameter Mars Jupiter Transmitter power 5 W (average) Transmitter aperture 30 cm Transmitter frequency (wavelength) 283 THz (1.06 m) Modulation Pulse position modulation (PPM) (M = 64 to 256) with concat
19、enated coding Pointing accuracy 0.35 rad Range 0.5 to 2.5 AU 4.2 to 6.2 AU Data rate (during the day at Earth terminal)(1)3 to 30 Mbit/s (4.2 m Earth terminal) 1.5 to 3 Mbit/s (10 m terminal) Receiver aperture 4.2 to 10 m equivalent size Detector type An array of Geiger-mode InGaAsP/InP APDs Require
20、d link margin 2 to 3 dB (1)Night-time data rates on Earth are about 30% (1.13 dB) higher. 2.1 Link performance Like a deep-space system operating in the traditional radio-frequency spectrum, performance of a link operating at 283 THz is measured in terms of data rate and bit error rate (BER). Perfor
21、mance is calculated as a function of power, telescope quality, propagation considerations, noise and receiver sensitivity. Each of these parameters is function of additional variables. 2.1.1 Data rate Unlike a deep-space system operating in the traditional radio-frequency spectrum, when all other pa
22、rameters are held constant, the data rate is not exactly inversely proportional to the square of the propagation distance; however, it is a very close approximation for links operating from the vicinity of Mars and Jupiter and thus a good rule of thumb. Data rates from Mars will vary depending on ma
23、ny parameters including link range and geometry with the sun. Data rates from Mars will generally be around an order of magnitude higher than from Jupiter. 2.1.2 BER Frames of data must have a BER of less than 106after error correction in order to be retained. A link must retain 99% of data frames.
24、2.1.3 Margin requirement The typical margin requirement of a deep-space or interplanetary link operating at 283 THz in the space-to-Earth direction is on the order of 2 to 3 dB. Conditions are dependent on factors which include inter alia weather, time of day and elevation angle. 4 Rec. ITU-R SA.174
25、2 2.2 Modulation Deep-space and interplanetary links operating around 283 THz will utilize PPM. This modulation technique allows for direct detection (specifically photon counting) by the receiver rather than implementing coherent receivers. The PPM signal will be encoded with a concatenated code. P
26、PM uses a single pulse of energy within the time of a word. M bits of data may be transmitted with a single pulse of energy temporally located within 2Mtime slots of a word. A portion of the total word time is used for recharging the laser and will never contain a pulse. This recharging or “dead” ti
27、me often accounts for the majority of the word time at low data rates but becomes less of a factor at higher data rates. The temporal characteristics of a PPM signal and its relevant measures are illustrated in Fig. 1. 2.3 Deliverable power The Yb, Q-switched, Nd:YAG or Nd:YVO4transmitter will typic
28、ally produce 5 W of average power. Peak power will vary with data rate but may be on the order of 30 to 40 dBW. The following procedure calculates the peak power, Ppeak, of a PPM transmitter. The following parameters are required: M: modulation index Pave: average transmitter power (W) td: dead time
29、 (s) p: transmitter pulse time (s) ts: slot time (s). Step 1: Calculate the word time, tw, by: dswttMt += s (1) Rec. ITU-R SA.1742 5 Step 2: Calculate the energy per word, Eword, by: wavewordtPE =J (2) NOTE 1 As only one pulse occurs during each word time, the energy per pulse is equivalent to the e
30、nergy per word (i.e. Epulse= Eword). Step 3: Calculate the peak transmitter power, Ppeak, by: ppulsepeaktEP = W (3) 2.4 Received signal The general method for calculating the signal level at 283 THz received by the earth station is the same as that used with traditional radio-frequency systems. sapr
31、trttSLLLLLGGPP += dBW (4) where: PS: receiver signal power Pt: average laser output power (typical value is 4.7 to 7.0 dBW) Gt: transmitter antenna gain (typical value is 119 dB) is discussed in detail in 2.6.2 Gr: receiving antenna gain (typical value is 129 to 149 dB) is discussed in detail in 2.6
32、.3 Lt: transmitter losses Lr: receiver losses Lp: pointing losses La: atmospheric losses along the space-to-ground link Ls: free space loss. 2.5 Link losses There are five primary sources of link losses: internal transmitter losses, Lt, that include the effects of absorption, scattering and reflecti
33、on losses in the optical train of the transmitter; internal receiver losses, Lr, that include the effects of absorption, scattering and reflection losses in the optical train of the receiver; pointing losses, Lp, that include the effects of antenna or spacecraft jitter and mispointing of the transmi
34、tting antenna; atmospheric losses, La, that include the effects of atmospheric scatter and turbulence; free space loss, Ls, that is due to the physical separation between the transmitter and receiver. Values of each source of loss will vary with hardware design, hardware age, mission requirements an
35、d the phase of the mission. Suggested values of losses to be used in generic interference analyses are provided in Table 2. Atmospheric propagation in this region of the spectrum has been addressed in detail by Radiocommunication Study Group 3 through Recommendations ITU-R P.1621 and ITU-R P.1622. 6
36、 Rec. ITU-R SA.1742 TABLE 2 Technical parameters of two reference deep-space missions operating at 283 THz in the space-to-Earth direction Mechanism of loss Typical value Transmitter losses, Lt0.63 (= 2 dB) Receiver losses, Lr0.63 (= 2 dB) Pointing losses, Lp0.63 (= 2 dB) Atmospheric losses, La0.89
37、(= 0.5 dB) at 90 0.56 (= 2.5 dB) at 30 Free space loss, Ls, is calculated at 283 THz in the same manner as with traditional radio-frequency systems: 224 4=RfcRLs(5a) which, at 283 THz, reduces to: 21510 169.7RLs= (5b) where: R: distance between the transmitter and receiver (m). 2.6 Transmit/receive
38、telescope parameters Deep-space and interplanetary radiocommunication links operating at around 283 THz will utilize telescopes as transmitting and receiving antennas. The typical parameters of the transmitting and receiving telescopes will differ greatly from each other. These differences will effe
39、ct each telescopes respective gain pattern. The transmitter and receiver antenna patterns are also different since the transmitter optics are usually fed by a Gaussian distributed beam while the receiver optics have a planar detector. For an envelope of the antenna gain patterns of transmitting and
40、receiving antennae operating around 283 THz, refer to Annex 2. 2.6.1 Diameter For the purposes of interference analyses, the diameter of the transmitting antenna should be assumed to be 30 cm. The transmitting aperture will either be unobstructed or have a 3 cm obscuration. The effective diameter of
41、 the receiving antenna may vary between 1 and 10 m, but for most applications will be at least 4.2 m. For the purposes of interference analyses, antennas of 1, 4.2 and 10 m should be considered. The primary receiving aperture will have a secondary obscuration with a diameter of no more than 20% of t
42、he diameter of the primary aperture. 2.6.2 Transmitting gain pattern The transmitter utilizes a telescope that is fed by a laser. Such lasers normally operate only in the lowest cavity mode, TEM00, which results in a beam that has a Gaussian distribution of energy with a maximum intensity along its
43、axis of transmission. The beam pattern is tailored such that as the intensity of the beam falls off in amplitude with angular separation from the axis of transmission, no more than a few percent of the beam power is wasted. Two points of reference are the angles at Rec. ITU-R SA.1742 7 which the bea
44、m amplitude falls off to either 37% or 13% of the amplitude on axis. These points are called the 1/e and 1/e2points respectively and are referred to frequently in the characterization of emitted laser energy patterns. The full-angle beamwidth at the 1/e2point is approximated by: rad42e/1D= (6) where
45、: 2e/1 : beamwidth (rad) : wavelength (m) D: aperture diameter (m). In the case of a 283 THz Gaussian beam transmitted from a 30 cm aperture, the beamwidth at the 1/e2point is approximately 4.5 106rad. For the transmitting terminal, the following equations can be used to calculate the far field radi
46、ation pattern of a laser with a Gaussian amplitude plane wave feeding a telescope. Use of these equations makes the following basic assumptions: the laser source is characterized as single mode Gaussian emission; the antenna gain patterns are measured in the far field; the aperture is circular. The
47、gain pattern of a transmitting telescope of radius, a, fed with a Gaussian amplitude plane wave having a waist radius of , where is the distance from the central axis of the optical system to the 1/e2intensity point, and having a central obscuration of radius, b, is given by equation (7). The term,
48、G0, is the upper limit on antenna gain which is obtained for a uniformly illuminated unobscured circular aperture. The second term, gt (, , X), is a gain efficiency term which accounts for obscuration, truncation, off-axis intensity, and defocusing effects. dBi),(),(0gGGtt= (7) where: dBi2 4220=aAG
49、(8) 2102de)(2),(22uuXJgut= (9) ab= (10) A: area of the telescope aperture (m2) a: radius of the primary aperture (m) b: radius of the obscuration (m) J0: Bessel function of the first kind of order zero : the ratio, /, of the radius of the transmitter aperture, a, to the radius of the Gaussian feed beam waist, , at the 1/e2point u: the variable of integration 8 Rec. ITU-R SA.1742 X: ()sin2a : angle off the optical axis (rad). For the on-axis, X =
