Communication from space: radio and optical.

The importance of the development of laser telemetry for reception of scientific data from space is pointed out by J. Bland-Hawthorn et al. in their Policy Forum “Laser telemetry from space” (26 July, p. 523). The authors consider the limitations to data communication in the microwave range due to spectrum allocations, but they do not consider the fundamental limitations for both radio and optical communications due to the signal-to-noise average power ratio as expressed by Shannon ([1][1], [2][2]). The spectrum allocation limit is relevant for near-Earth applications, but for the important case of communication at distances of 0.1 astronomical units (AU) and beyond, the signal-to-noise limit is dominant. At large distances, the radio and optical communication rates feasible in the next 15 years are comparable. This situation is illustrated in the figure, which plots data rate as a function of distance for three cases: (i) NASA's current radio Deep Space Network (DSN), (ii) optical technology expected to be available by 2010, and (iii) a future large radio array, such as discussed recently in Science (“Tuning in the radio sky,” R. Irion, 3 May 2002, p. [830][3]). ![Figure][4] Comparison of current and future radio and optical communication rates at astronomical distances. Data rate ( 3 ) versus distance for DSN (blue), expected optical technology (purple), and a future large radio array (red). The leveling off of the radio data rate at distances less than 0.2 AU (about 30 million km) is due to spectrum allocations of 50 MHz for the current DSN at 8.4 GHz and 500 MHz at 32 GHz for a future large array. The data rate in bits/second can exceed the bandwidth in hertz by using multilevel coding but has a flat logarithmic dependence on signal-to-noise ratio and, hence, distance. At the distance of Jupiter, the current radio data rate is limited by signal-to-noise ratio on the order of 50 kbits/s, which is three orders of magnitude lower than the spectrum allocation. An important goal is video data rates from the outer planets, which requires on the order of 5000 kbits/s. This rate is feasible with SKA-class large radio arrays that are under development for radio astronomy and are under consideration by NASA for space communications. Other factors such as the proven reliability of current spacecraft radio transmitters, sensitivity to location of the sun, spacecraft vibration and pointing requirements, and availability due to weather conditions (clouds) all need to be considered. Radio spectrum allocations of several GHz in the 100-GHz frequency range have yet to be explored. Both optical and radio communications need to be developed for future space communications of scientific data. 1. 1.[↵][5]1. C. Shannon , Bell System Tech. J. 27, 379 (1948)623. [OpenUrl][6][CrossRef][7][Web of Science][8] 2. 2.[↵][9]The Shannon limit is given by C = B .log2 (1 + S / N ), where C is the data rate in bits/s, B is the channel bandwidth in Hz, and S and N are the average signal and noise powers. It is important to note that N is proportional to B and refers to additive noise due to background sources and detectors and not the fluctuations due to the Poisson distribution of signal photons. 3. 3.The data rates in the figure are all for 50 W of DC power consumption and similar communication equipment mass of 18 kg at the spacecraft and assume 9 dB of link margin to account for atmospheric variations, error-correction coding efficiency, and component degradations. The 2010 optical link assumes a 10-m receiving antenna on Earth, a 0.3-m-diameter transmit antenna, a background noise level (daytime) of 10 quanta, and a data rate increase of of 1/ln 2 = 1.442 allowed by the Shannon limit for an optimum bandwidth much greater than data rate. The current radio link assumes a 70-m DSN antenna, 1.5-m-diameter transmit antenna, and total system noise of 54 quanta ( T sys = 22 K at 8.4 GHz). The future large array assumes 3600-by 12-m-diameter antennas, 1.5-m transmit antenna, and total system noise of 26 quanta ( T sys = 40K at 32 GHz). # Response {#article-title-2} Radio communication technology has been around for a very long time and is now so nearly optimized that little improvement can be anticipated unless increasingly large and correspondingly costly antenna arrays are constructed. Optical/near-infrared communications are still in their infancy. But, in many applications, the high inherent bandwidth of optical frequencies should ultimately make them far more powerful than radio waves. Our Policy Forum anticipated that near-infrared telemetry techniques would take the lead in communication across space sometime in the next decade. Weinreb correctly points out the importance of Shannon's limit on data transmission rates in telemetry across vast stretches of space. Our Policy Forum took this limit into full account, albeit without specific mention. For transmission from the astronomically important Lagrangian point L2 at 0.01 AU, Weinreb's plot clearly illustrates the large advantage that optical telemetry has even over the most ambitious currently projected radio techniques. Over greater interplanetary distances, Weinreb's plot shows the future radio array winning out at radio frequencies, but this is only because he compares laser transmission with a relatively small 0.3-m transmitting antenna to radio transmission with a 1.5-m-diameter antenna. With comparable transmitting dishes, the two systems would perform equally well. For regular access from space, ground receiving stations need to be distributed around the globe to assure communication at all times of day. The anticipated ground stations for near-infrared telemetry are expected to be affordable and more easily installed at widely distributed sites. The billion-dollar Square Kilometer Array will be a wonderfully powerful astronomical instrument, but it should, and most probably will, be used primarily for exquisitely sensitive radio astronomical observations rather than for spacecraft telemetry. [1]: #ref-1 [2]: #ref-2 [3]: /lookup/doi/10.1126/science.296.5569.830 [4]: pending:yes [5]: #xref-ref-1-1 "View reference 1. in text" [6]: {openurl}?query=rft.jtitle%253DBell%2BSystem%2BTech.%2BJ.%26rft.volume%253D27%26rft.spage%253D379%26rft_id%253Dinfo%253Adoi%252F10.1002%252Fj.1538-7305.1948.tb01338.x%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [7]: /lookup/external-ref?access_num=10.1002/j.1538-7305.1948.tb01338.x&link_type=DOI [8]: /lookup/external-ref?access_num=A1948UH03900001&link_type=ISI [9]: #xref-ref-2-1 "View reference 2. in text"