290 JOURNAL OF COMMUNICATIONS, VOL. 7, NO. 4, APRIL 2012 A Survey of Communication Sub-systems for Intersatellite Linked Systems and CubeSat Missions Paul Muri, Janise McNair Department of Electrical and Computer Engineering, University of Florida P.O. Box 116130, Gainesville, FL 32611 USA Email: pmuri@ufl.edu, [email protected]fl.edu Abstract— Intersatellite links or crosslinks provide direct section III. In section IV, open research issues such as connectivity between two or more satellites, thus eliminating internet protocol layering, delay tolerant networking lay- the need for intermediate ground stations when sending ering, radio frequency allocation, optical communications, data. Intersatellite links have been considered for satellite constellation missions involving earth observation and com- applications, and orbital properties are analyzed. munications. Historically, a large satellite system has needed an extremely high financial budget. However, the advent of II. INTERSATELLITE LINKING COMMUNICATION the successful CubeSat platform allows for small satellites of less than one kilogram. This low-mass pico-satellite class SUB-SYSTEMS platform could provide financially feasible support for large Intersatellite links have appeared in certain large plat- platform satellite constellations. This article surveys past form satellite constellations for over 30 years. Exam- and planned large intersatellite linking systems. Then, the article chronicles CubeSat communication subsystems used ples of these large communication relay constellations historically and in the near future. Finally, we examine the include TDRSS and Iridium. More recently, CubeSats history of inter-networking protocols in space and open have changed the communication philosophy from long- research issues with the goal of moving towards the next range point-to-point propagations to a multi-hop network generation intersatellite-linking constellation supported by of small orbiting nodes [2]. Separating system tasks CubeSat platform satellites. into many dispersed sensor nodes can increase overall Index Terms— Satellites, Earth Observing Systems, Satellite constellation coverage, system survivability, and versatil- Communication, Satellite Constellations, Wireless Networks ity [3]. However, to understand the challenges involved in satellite networking, large-platform intersatellite linking I. INTRODUCTION systems were chronicled and listed in table I. Details of the frequency, protocol, application, and orbit of all the In 1954, the United States Navy started relaying signals intersatellite linked satellite systems are presented in this off the moon. This simple reflection experiment led to the section. world’s first space communications satellite system: Com- munication Moon Relay (CMR). CMR allowed Hawaii to TABLE I. communicate with Washington, DC [1]. For many years HISTORY OF INTERSATELLITE LINKING (ISL) SYSTEMS after, satellite systems were simple relays consisting of a ground station to satellite uplink channel, and a satellite Launch Year(s) Satellite(s) ISL Frequency to ground station downlink channel. Following simple 1972-1978 OSCARs 6, 7, 8 146 MHz relay systems, a constellation of satellites that commu- 1976 LES-8 and 9 36, 38 Ghz nicated directly with one another was designed. Instead 1983-2013 TDRSS C, Ku, Ka of simple uplinks and downlinks between a satellite and 1985-1995 Luch UHF, Ka its ground station, these systems would use satellite-to- 1994 ETS-6 2, 23, 32 GHz, Optical satellite relays known as crosslinks. Crosslinks allowed 1997 Navstar Block IIR UHF for less ground stations. Also, frequencies that are quickly 1997 Iridium 23 GHz attenuated in the atmosphere could be used, making the 1998 Comets (ETS-7) 2 GHz link undetectable and unjammable from the ground. In 1994-2003 MilSTaR I/II 60 GHz 1998 Spot-4 Optical section II, this paper surveys the frequency, protocol, 2001 Artemis S, Ka Optical application, and orbit of satellite systems launched that 2002 Envissat S-band featured a crosslink. With CubeSats seen as a viable 2002 Adeos-II 2 GHz, 26 GHz platform for intersatellite communications, the report 2005 OICETS Optical details frequency allocation, protocol, application, orbit, 2010 AEHF SV-1 60 GHz and communication subsystems used for all CubeSats in 2015 Iridium Next 23 GHz Manuscript received Jan 12, 2012; accepted March 25, 2012 © 2012 ACADEMY PUBLISHER doi:10.4304/jcm.7.4.290-308 JOURNAL OF COMMUNICATIONS, VOL. 7, NO. 4, APRIL 2012 291 A. 1972, 74, 78 OSCARs 6, 7, 8 absorbs the microwave band. However, with the technol- The Radio Amateur Satellite Corporation’s (AMSAT’s) ogy in 1971, little test equipment was available beyond first generation of Orbiting Satellites Carrying Amateur 40 GHz. As a result, LES-8 and LES-9 demonstrated Radio (OSCARs) were launched in the early 1960s. Soon crosslinks at the Ka-band (36 GHz and 38 GHz) [6]. after, AMSAT began work on a second-generation known 60 GHz crosslinks came later in MilSTAR constellation as OSCARs 6, 7, and 8. These amateur satellites were described in section II-J. Intersatellite antennas were ◦ characterized by an intersatellite repeater capability [4]. either a 24 db gain horn with 10 beamwidth or a more ◦ OSCAR 6 received at VHF frequency of 146 MHz and directional 42.6 db gain dish with 1.15 beamwidth. ◦ transmitted at 29.5 MHz with a repeater bandwidth of The satellites were steerable +/-10 by a gimbaled flat 100 kHz. OSCAR 7 had two repeaters. The first repeater plate [7]. was similar to OSCAR 6 except for a slight frequency As for a power budget, LES-8 and LES-9 had no solar change and increased output transmission power. OS- cells or batteries. Instead, radioisotope thermoelectric gen- CAR 7’s other repeater received at UHF 432 MHz and erators (RTGs) powered the satellites. RTGs are the same transmitted at VHF, 146 MHz. An on-board timer on sources that power the 1977 Voyager 1 and 2 spacecraft OSCAR 7 automatically switched from one repeater to that have explored beyond our solar system. the other daily. Also, satellite communication control An advantage of the Lincoln Lab’s space communica- circuitry automatically switched one repeater on in a tions program was that each LES was developed in the low-power mode when the battery was discharged to a same facility. Lincoln Labs was able to experiment exten- certain point. On several occasions, OSCAR 6 and 7 sively for end-to-end communication testing including the were used together with a 432 MHz uplink to OSCAR 7, ground terminals that Lincoln developed for the Air Force a 146 MHz intersatellite link, and a 29 MHz downlink and Navy. The smooth testing following the launching of from OSCAR 6. OSCAR 8 also had two repeaters. the satellites could be attributed to this. OSCAR 8’s first repeater was the same as OSCAR 7, Optical crosslink communication for LES-8 and LES- operating at a 146 MHz crosslink and 29 MHz downlink. 9 was also considered. However, the idea was later OSCAR 8’s second repeater received at 146 MHz and dismissed in 1971 because it was considered infeasible transmitted at 435 MHz. Only one repeater was on at a with the current technology and beyond the resources of time. So, OSCARs 7 and 8 could relay to OSCAR 6 or the project [5]. Optical crosslinks were accomplished with themselves. As a result, AMSAT demonstrated the first the Artemis satellite described in section II-K. So, one can simple omni-directional intersatellite repeating system. conclude that LES-8 and LES-9 set the groundwork for AMSAT’s OSCAR 7 still remains semi-operational. many future satellite communication systems. C. 1983-2013, TDRSS Starting in 1983, NASA launched a series of geosta- tionary satellites to relay low earth orbit (LEO), 300 km to 1,000 km, spacecraft communications. This Tracking and Data Relay Satellite System (TDRSS) satellite communi- cation system increases the time window that spacecraft are in communication with ground terminals from ten minute intervals to virtually round the clock. Another advantage of TDRSS is during atmospheric re- entry of manned missions. During re-entry of spacecraft during Mercury, Gemini, and Apollo missions, communi- cation was lost due to the tremendous heating experienced by the craft and is termed ”re-entry blackout”. However during the space shuttle missions, communication was not Figure 1. OSCARS 6 to OSCAR 7 and 8 carrier frequency bands lost because TDRSS relayed transmission from the cooler conditioned antennas on top of the shuttle. Previously launched from space shuttle missions, the B. 1976, LES-8 and 9 most recent generation of TDRSS are launched from The Lincoln Laboratory experimental satellites 8 and rockets. This generation of TDRS provides downlinking 9, known as LES-8 and LES-9, demonstrated long-range datarates of 300 Mbps and 800 Mbps in Ku/Ka and S- digital transmissions in many bands between themselves bands respectively with five meter diameter dishes [8]. and ground terminals. Once launched, LES-8 and LES-9 Also, a C-band phased array antenna can receive from five thrust 90◦ apart to a coplanar, inclined, circular, geosyn- different spacecraft while transmitting to another location chronous orbit [5]. simultaneously [9]. Engineers at Lincoln desired crosslinks at a 60 GHz TDRSS supports some of NASA’s most famous mis- frequency band because of how oxygen molecules in the sions. LEO spacecraft such as the space shuttle, Hubble atmosphere absorb 60 GHz. This is much like how water telescope, LANDSAT, ISS all have their signals relayed © 2012 ACADEMY PUBLISHER 292 JOURNAL OF COMMUNICATIONS, VOL. 7, NO. 4, APRIL 2012 from TDRSS to NASA ground terminals [10]. TDRSS E. 1994, ETS-6 has three main command and control ground terminal lo- The Experimental Test Satellite (ETS-6) was developed cations at White Sands (WSGT) in New Mexico, Goddard and launched by NASDA, Japan’s National Space De- (GRGT) in Greenbelt, Maryland, and Guam (STGT) as velopment Agency, renamed to JAXA, Japan Aerospace shown in figure 2. Exploration Agency after 2003.