Received: from sun2.nsfnet-relay.ac.uk (sun2.nsfnet-relay.ac.uk [128.86.8.45]) by montebello.soest.hawaii.edu (8.6.7/8.6.6) with SMTP id WAA01615 for ; Thu, 24 Mar 1994 22:20:39 -1000 Via: uk.ac.cambridge.mrc-applied-psychology; Fri, 25 Mar 1994 08:20:04 +0000 Received: from vega.mrc-apu.cam.ac.uk by sirius.mrc-apu.cam.ac.uk with UK-Sendmail (4.1/UK-2.1-APU); Fri, 25 Mar 94 08:15:33 GMT From: James Miller Date: Fri, 25 Mar 94 08:15:32 GMT Message-Id: <14874.9403250815@vega.mrc-apu.cam.ac.uk> To: joe@montebello.soest.hawaii.edu Subject: Re: GIF Status: R Joe - here it is. The associated text explanation is as "important" as the GIF: mail g3ruh@amsat.org MOONLIBR.GIF Moon's Optical Libration MOONLIBR.GIF -------------------------------------- by James Miller G3RUH 1994 Mar 21 The following data is a UUencoded GIF file MOONLIBR.GIF, 11302 bytes. It's a plot showing the locus of Earth's apparent movement as viewed from the Moon over a 36 month period. The Earth Moved --------------- When viewed from Earth, the Moon appears to rock a little. This is called Moon's Optical Libration; because of this, some 60% of the Moon's surface is visible to us. Conversely, when viewed fom the Moon, Earth appears not to be fixed in space, but wanders around slightly with a complex cyclic motion. The GIF ------- In the picture, the large circle indicates scale, and has a radius of 10 degrees. The black blob in the middle is the same size as the Earth, and moves around following the trail. The coordinates are ecliptic latitude and longitude. That is, the line across the middle is the ecliptic plane. The Sun glides across here from left to right, once a month, and when in the middle it's Full Moon. If the Moon's in the middle too, then there's an eclipse of the Moon. The average up/down latitude excursion is 5.13 degrees, corresponding to the Moon's orbital inclination with respect to the ecliptic. Influences from the Sun add up to +/- 0.9 degrees more to this. The left/right longitude excursions are caused mainly by the Moon's orbital eccentricity e. The major term is 2*e radians, or 6.29 degrees, where e=0.0549. Interaction with the Sun contributes a little more, bringing the longitude peak movement to some 8 degrees. The combined latitude and longitude movement is a maximum of 9.5 degrees, as can be seen at the "corners". Thus the plot represents pointing error vs. time for a fixed Moon-based antenna. Moon Downlink ------------- The maximum total excursion of 9.5 degrees is the same as the beamwidth of a 5 wavelength diameter dish antenna. This has a gain of some 20 dbi, and represents an upper limit for an unsteered Moon-based antenna. However the higher the frequency used, the smaller mechanically is the antenna, which makes 2.4 or 5.6 GHz a good choice. Five wavelengths is 60 cm and 26 cm diameter respectively; quite small. For a given TX e.i.r.p., signal strength received at Earth depends only on the mechanical size of the RX antenna; frequency is irrelevant. Noise level however is not, and S-band (2.4 GHz) is the sensible choice because very low noise performance is robustly obtainable "off the shelf" [1]. As an example, 1 watt transmitted from a 20 dbi gain dish on the Moon, received on a 1.2m dish at Earth with a system noise temperature of 100K results in a signal to noise ratio in 2.4 kHz bandwidth of 10.5 db. This would support one rather noisy SSB voice signal. Alternatively it would carry an error-free 2400 bps binary PSK data transmission without coding, 9600 bps with modest coding [2]. Algorithms ---------- All Moon tracking programs compute the Moon's ecliptic latitude and longitude, though it is not so obvious to the untutored eye. Low precision formulae that are adequate for graphical work can be found at the end of section D of the Astronomical Almanac, published annually by USGPO and HMSO. References ---------- [1] Miller J.R.; "Mode-S - Tomorrow's Downlink", Oscar News (GB) 1992 Oct No. 97 p20-22. Also: Amsat-DL Journal Dec 1992 Nr4. Jg.19. Also: Amsat Journal (USA) 1992 Sep. Also: Amsat-VK Newsletter (VK), No. 90, September 1992. Also: CQ-DL 9/93 1993 September, p.614-617. [2] Miller J.R.; "Shannon, Coding and the Radio Amateur", Oscar News (GB) 1990 Feb No.81 p.11-15. 73 de James g3ruh@amsat.org +-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-+ | 73 de James G3RUH | | g3ruh@amsat.org | | StarDate: 1994 Mar 21 [Mon] 1734 utc | +-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-+