Radio Science Bulletin 313 - June 2005 - URSI

Figure 9. UCIRA-2 in stowed configuration.We have some concern about the use of nylon in space;however, it appears to work better than expected so we usedthis material for the UCIRA-2. We also continued to use thepolypyrrole treated polyester for the load resistors as wehave done in the past. This material has a surface resistanceof approximately 200 Ω /sq so it is easy to obtain the correctload resistance.For the ribs or stays that support the reflector we usedgraphite reinforced composite rods. The hinges in the staysare aluminum with stainless steel torsion springs.The UCIRA-2 is shown in Figure 9 in the stowedconfiguration and in the fully deployed configuration inFigure 10. The UCIRA-2 has a hyperboloidal reflector tobroaden the beamwidth and has dual polarization capability.The hyperboloidal shape should increase the beamwidth byabout 20° [4, 5]. The diameter of the reflector is 1.22 m (48inches) and the F/D is 0.3 as with the UCIRA-1 and 1B. Thefeed arms have the outer edge inline with the rim of the dish(non-floppy) and are evenly spaced around the reflector tofacilitate the dual-polarized feed. The antenna weighsapproximately 2.6 kg (5.75 lb.). Most of the mass is in thedeployment mechanism. This mass could be reducedsomewhat by additional mechanical analysis and reducingthe thickness of parts where possible.Figure 10. UCIRA-2 in deployed configurationFigure 11. The conversion of a paraboloidal reflector toa hyperboloidal reflector.Now we will describe the modification required tobroaden the beamwidth of the UCIRA, we defocus theaperture by converting the paraboloidal reflector into ahyperboloid. In doing so, we must maintain the samereflector diameter and focal length. We specify an angle,φ O , which is the angle at which the extreme ray at the edgeof the reflector deviates from the focal direction (y), asshown in Figure 11. When φ O = 0, the hyperboloid revertsback to the paraboloid.It is unclear how the parameter φ O affects thebeamwidth. At extremely high frequencies, the beamwidthis 2φO , which is the optical limit. In practice, of course, thisis never achieved. As proof of this, a focused paraboloidwith φ O = 0 does not have a beamwidth of 0° at anyfrequency. At lower frequencies, a reasonable guess is thatthe beamwidth will broaden the beamwidth by an extra2φ O beyond the beamwidth of the focused (paraboloidal)case. J. Scott Tyo has investigated this in somewhat moredetail in [4, 5]We revisited several issues concerning the mechanicaldeployment system used on the UCIRA-1 and made severalchanges. First we found that the manual release pin on theUCIRA-1 was much too difficult to pull. An automatic pinpuller capable of holding the cam in place during launch andthen releasing the cam on command was out of the question.Therefore, we replaced the release pin with an EjectorRelease Mechanism (ERM) from TiNi Aerospace, Inc.Release is accomplished on command (9 VDC at 5 A for20 ms), by retracting five detent balls that hold a coupler tothe actuator, thereby ejecting the coupler and releasing thecam to which it is attached.Next, we replaced the gas spring used in the UCIRA-1 with 9 stainless steel conventional springs arranged in acircle around the ERM. The gas spring had the advantage ofsmall size and nearly constant force over the length of thestroke. However, we learned that gas springs cannot be usedbelow -40°C because they tend to lose the high pressurenitrogen charge at such low temperatures. The 9 conventionalsprings have a total force of 2.67 kN (600 lb.) in thecompressed or launch configuration and 1.33 kN (300 lb.)TheRadio Science Bulletin No 313 (June, 2005) 43