Site: Kashima Space Research Center (KSRC)
Communications Research Laboratory
Ministry of Posts and Telecommunications
Ibaraki 314, Japan
Date Visited: October 19, 1992
Report Author: C. Bostian
Dr. Takashi Iida
Dr. Hajime Fukuchi
Dr. Hiromitsu Wakana
Dr. Tadashi Aruga
Kashima Space Research Center (KSRC) is a branch of the CRL of the MPT. Its mission is to develop satellite communications technology with the Space Communications Division. Related research activities include very long baseline interferometry (VLBI) and remote sensing.
CRL's total budget was approximately 5.3 billion yen in FY 1992. A breakdown is given below, in Table KSRC.1, of the number of researchers and budget for FY 1990 for CRL and related research centers in Japan.
Researchers and Budgets in Japanese Research Centers
COMETS is an R&D satellite whose mission objectives include investigation of a PCS system using mm-waves and Ka-band. Dr. Iida's group developed the mm-wave PCS concept in 1984. COMETS will be launched in February, 1997, by the H-II launch vehicle, with a mission life of three years.
The COMETS program has the following R&D objectives:
The COMETS spacecraft will carry a 3 m diameter reflector for Ka- and S-band ISLs, a 2.3 m reflector for advanced Ka-band broadcasting, a 2 m reflector for Ka-band ISL feeder links, and mm-wave and Ka-band mobile communications links. The spacecraft bus is based on ETS-VI (scheduled for 1994 launch). The body dimensions are approximately 2 x 3 m and the spacecraft carries 30 m solar panels.
For use in advanced mobile satellite communications experiments, COMETS satellite has three spot beams, two adjacent Ka-band beams covering the Tokyo and Nagoya areas, and one mm-wave beam for the Tokyo area.
The two Ka-band transponders have 20 and 10 W SSPAs. The mm-wave transponder consists of a Super HEMT LNA with an NF of only 3 dB and a 20 W TWTA. The IF filterbank and the Single Channel Per Carrier/Time Division Multiplexed (SCPC/TDM) regenerative modems are adapted to interconnect between 2 x 2 matrix spot beams. In this experiment, for example, a mobile terminal can establish a 24 kbits/sec channel using only a 20 cm antenna and a 0.9 W transmitter.
COMETS feeder links will operate at 27 GHz with downlinks at 20.7 GHz, into 100 MHz transponders. The spacecraft will provide 2 beams at any one time. Each transponder will have a 200 W end of life (EOL) TWT HPA.
The COMETS Ka-band broadcasting mission will study wide RF bandwidth HDTV distribution and high quality audio distribution. It will explore the NHK concept of ISDB (Integrated Services Digital Broadcasting), a broadcast analog to ISDN that will provide a flexible mix of voice, data, and video services in broadcasting. Other goals of the program are the development of small and inexpensive receiving terminals and a feasibility study of regional satellite broadcasting for Japan.
CRL has identified six different cultural or ethnic regions in Japan and thus envisions regional satellite broadcasting with six beams. The beams will use one of two downlink frequencies and depend on physical separation of the beams to keep the programs from interfering with each other. Isolation between two neighboring beams which share the same frequency must be greater than 35 dB. The COMETS advanced broadcasting experiment will test this concept with a single-frequency two-beam prototype system that will provide service to the Kanto and Kyushu regions.
Rain fading is potentially a serious problem for the COMETS broadcasting mission. The small size of most Japanese houses limits antenna diameters to 1 m maximum, and 50 cm is preferred. The margin for rain attenuation is necessarily very low. The system is expected to experience 8 to 10 dB of rain attenuation at the one percent level. Source and channel coding are necessities.
CRL wants to develop an intelligent array antenna for the COMETS broadcast subscribers. BSS and COMETS will have different look angles; the desired antenna would point a beam at one and discriminate against the other. Ideally this would be an active flat plate antenna.
Viewgraphs were presented describing some details of an uplink power control (UPC) system for 30 GHz based on receiving the 20 GHz beacon and 20 GHz downlink transmissions from a satellite. (This is probably obsolete now, since the 1992 WARC provided for 30 GHz satellite beacons for UPC applications.) Both open loop and closed loop systems were tested using the CS-2 spacecraft. The control error was stated as being within 0.3 dB when the downlink attenuation was less than 10 dB. The number of user earth stations that could be accommodated when UPC is used is 3.7 times larger than when UPC is not used. (Presumably this is because, with UPC, the individual links can operate with very low margin during clear weather, permitting the satellite's available EIRP to be divided between more downlinks.) KSRC is also studying adaptive data rate TDMA systems and developing what they call ADR/TDMA modems which keep track of downlink BERs and use these to adjust transmission speeds in response to rain fades.
In addition to direct measurement of rain-induced impairments to satellite signals, KSRC is studying rain scatter using an impressive dual-polarized 14.375 GHz meteorological radar with a 13 m dish for transmitting and a 60 cm dish for receiving.
CRL has started a small satellite project in which two technologies are being examined: (1) store and forward, and (2) intersatellite connectivity with links between many satellites. The frequency of the crosslinks hasn't been selected yet. CRL is now studying the use of S-band for this, but only on an experimental basis.
CRL began a small satellite project last year. The CRL staff is looking into the possibility of launching two 50 kg class satellites carrying store and forward and ISL experiments in 1998 and 2000. Their concept includes gravity-gradient stabilized, dumbbell-shaped configurations, offering VHF or UHF store and forward links to the ground at 9.6 kbits/sec and an S-band (perhaps) ISL at 1.2 kbits/sec.
GEO satellites BS-3a and BS-3b are co-located at 110 degrees E. In the future there may be other satellites co-located there under a decision of the WARC-BS, and thus KSRC sees clusters of satellites as an area for future research. A cluster would look like a single satellite to ground users. One hypothetical cluster system would involve four satellites no more than 20 km apart. Needed are schemes to control orbits efficiently (to minimize fuel use) and avoid collisions, and techniques for using clusters efficiently. One motivation is to develop failure-free satellite systems by distributing transponders over two or more satellites.
KSRC is studying a differential interferometer type system for precise tracking and orbital control. It would require three antennas on the ground. A simulation based on a moving mechanical model was shown, but it wasn't clear what kind of sensors the simulation system used.
KSRC is investigating two vehicular antenna technologies: a mechanically steered array and a very-low-profile phased array. Both apparently use an open loop tracking method that combines a geomagnetic sensor and an optical fiber gyroscope. The geomagnetic sensor serves to reduce the cumulative error in the gyro.
Experimental results were presented comparing power received by the mechanically steered array and power received by a reference omnidirectional antenna. The antenna tracks only in azimuth, which is all that is required at L-band. The next challenge will be to include both azimuth and elevation tracking. Ka-band operation will require elevation tracking.
The very-low-profile phased array antenna offers 14 dBi gain at 90 degrees elevation and 11 dBi at 45 degrees. Its half power beamwidth (HPBW) is 28 degrees and its size is 60 cm diameter by 3 cm thick.
Published work for aeronautical communications describing the vehicular phased array states that it compensates for Doppler. Our hosts said that the Doppler shift could be as much as 1.2 kHz but that its rate of change is low. The method by which the Doppler compensation is accomplished was not described.
Several other prototype antennas were displayed, including those for hand-held, aircraft, and maritime units. The aircraft unit can track electronically. Our hosts were asked to estimate the cost of a commercial version of the aircraft unit, but could not. The maritime unit includes a multipath fading reduction system (described in the literature) which takes advantage of differential fading on the cross-polarized and co-polarized waves components of the incoming signal to compensate for multipath effects.
KSRC is working on a hybrid position location system that uses ETS-V and an Inmarsat spacecraft in combination. The system is more accurate at high latitudes than at low latitudes. Its accuracy compares favorably with the Global Positioning Satellite System (GPSS).
PARTNERS is a C- and L-band communications experiment for the Pacific. KSRC has developed PARTNERS terminals which can operate in the 64 kbits/sec range for audio and slow-scan TV.
ETS-VI experiments include an S-band Intersatellite Communications (SIC) link, an O-band communications experiment, a fixed and mobile communications experiment, a space laser communications experiment, and a K-band single access communications experiment.
The purpose of the SIC effort is to establish the basic technology of S-band multiple data relay. It has interoperability with Japan's data relay system for the space station era.
SIC includes a 19 sub-array phased array multibeam antenna made by Mitsubishi. All are used on receive and 16 sub-arrays are used on transmit. Each sub-array consists of seven elements and has its own LNA; those also used to transmit also have HPAs and diplexers. The array transmits at 2.1 GHz and receives at 2.3 GHz, developing one transmit beam and two receive beams. Both are fully steerable over a 20 degrees cone. This covers the surface of the earth plus anything in orbit up to an altitude of 1000 km. The minimum gain is 27.1 dBi over the full field of view, and the total EIRP is 35.8 dBW. The gain of the individual sub-arrays is 14.3 dBi. The phased array is connected to the 20/30 GHz feeder link, to relay data collected from LEOSATs.
The ETS mm-wave equipment (OCE) uses a 40 cm dish which is mechanically steerable (by moving the entire OCE platform) 18.6 degrees. The OCE transponder has two 38 GHz SSPAs. One, manufactured by NEC, uses four GaAs FETs in parallel; the other, manufactured by Fujitsu, uses two GaAs MMICs in parallel. The NEC unit develops 0.79 W at saturation, the Fujitsu 0.5 W. Typical power consumption is 16.8 W at a -10 dBm input and 14.2 W with no RF input. The transponder LNAs will operate at 43 GHz. Each LNA has three HEMT stages and provides 13.3 dB gain with a 4.8 dB NF. The overall receiver NF is 5.1 dB. NEC integrated the OCE transponder. The transponder can operate in a loopback mode or in a cross-strap mode.
The ETS-VI laser communications equipment (LCE) uplink operates at 0.51 micrometers and the downlink at 0.83 micrometers. The LCE includes a 7.5 cm diameter telescope, a silicon avalanche photodiode receiver, and a 13.8 mw transmitter incorporating two AlGaAs laser diodes. The data rate is 1.024 Mbits/sec. The downlink can be used to retransmit uplink data or transmit telemetry or test data generated onboard the spacecraft. The dual link optical communications experiment will be performed between the ETS-VI and the ground optical station at CRL, Koganai.
Dr. Iida described work on robot assembly of large antennas in space. This is not yet an approved Space Station experiment, but work has been under way at CRL since 1986. The antenna is assembled from a central section and petals using a CCD camera for remote control. They have tested the concept by constructing a 2 m model whose measured pattern at 43 GHz looked very good. They will conduct an experiment on ETS-VII. The goal is to move through the two to five meter stage to the 10 m stage, and ultimately to construct a 100 m antenna in space.
CRL performs R&D on optical ISLs and large space antennas for mobile communications.
CRL representatives explain their aggressive pursuit of Ka-band technology as follows: "It is our obligation to develop higher frequencies at this laboratory ... At the time that much of our development work started, Ka-band was available for satellite communications in Japan but Ku-band was only for terrestrial use." When Ku-band satellite service was introduced in Japan, terrestrial interference problems were anticipated.
Japan has launched an impressive number of developmental communications satellites in comparison to the U.S. In the interval between CTS (joint Canadian-U.S. satellite launched in 1977) and ACTS, our hosts quickly counted 10 or 11 Japanese satellites, and the exact number is probably higher.
CRL is conducting an ambitious and comprehensive research program that includes in ETS-V, ETS-VI and COMETS the world's most sophisticated experimental communications satellites.