COMETS

The COMETS project is sponsored jointly by the National Space Development Agency (NASDA) and MPT/CRL.

This GEO three-axis stabilized satellite, having an initial in-orbit mass of about 2,000 kg and an end-of-life power of 5.4 kW, will be placed at 121 degrees E by the NASDA H-II rocket in 1997 with an expected mission life of three years. Its bus characteristics are shown in Table6.3. As shown in Figure 6.8, COMETS has three deployable antennas and a 32 m long solar paddle. It has three mission payloads. The first is an advanced mobile satellite communications system using millimeter wave (mm wave) and Ka-band, which is being developed by CRL. The second is a 21 GHz band advanced broadcasting system developed by CRL and NASDA. The third is an intersatellite communications system developed by NASDA using the S- and Ka-bands.

The objective of the first mission is to study the feasibility of advanced mobile satellite communications at mm wave (47/44 GHz) and Ka-band (31/21 GHz). The payload consists of a multibeam antenna, mm-wave and Ka transmitters and receivers, an intermediate frequency (IF) filter bank and a regenerative repeater which enables the use of a 2 x 2 matrix switch for beam interconnections.

The 2 m diameter antenna has one mm wave spot beam (maximum gain 55 dB) and two Ka spots (Tokyo and Nagoya) with a receive gain ranging from 45 to 48 dB. These two beams are located close to each other for the purpose of making interbeam interference measurements. The 3 dB contour diameter of each beam is about 300 km. The antenna has a pointing mechanism which tracks a beacon signal transmitted by an earth station. The footprints of the mm-wave and the Ka-band receive antennas are shown in Figures 6.9 and 6.10 respectively. A block diagram of the payload is shown in Figure 6.11. The two Ka-band transponders use 20 W and 10 W solid-state power amplifiers. The mm wave transponder consists of a 20 W traveling wave tube amplifier and high electron mobility transistor low noise amplifier with a noise figure (NF) of 3 dB. The IF filter bank and the single channel per carrier time division multiplexed (SCPC/TDM) regenerative modems are used to interconnect for the 2 x 2 matrix beams. In the regenerative transponder, eight SCPC signals (six for continuous signals and two channels for packet signals), at 24 or 4.8 kbits/sec rate, are received, demodulated and multiplexed into a single TDM signal for binary phase shift keyed downlink transmission. This TDM signal is then demodulated and demultiplexed by a digital polyphase fast Fourier transform filtering technique. The forward error correction codec makes use of rate-1/2 convolution coding with Viterbi decoding. For a transmission rate of 24 kbits/sec at a 20 cm diameter antenna receive terminal, the expected bit error rate (BER) is 10(superscript -4.)

Table 6.3
Major Characteristics of COMETS Satellite System


Figure 6.8. Japan's COMETS


Figure 6.9. Footprints of COMETS Receive Antenna -- at mm Wave (47 Ghz)


Figure 6.10. Footprints of COMETS Receive Antenna -- at Ka-band (31 GHz)

Key technologies and devices are used to test the following:

The objective of the second mission is to carry out direct satellite broadcasting experiments in the 27/21 GHz band, as shown in Figure 6.12, including the following:


Figure 6.11. Block Diagram of COMETS Payload


Figure 6.12. Outline of Experiment on 21 GHz Band Satellite Broadcasting by COMETS

The impact of rain fade on system performance will be investigated and the design of a wideband, high-power TWTA and a high-gain multibeam antenna verified. Key features of this experimental broadcasting system and the parameters of the ground segment are summarized in Tables 6.4 and 6.5, respectively.

A block diagram of the payload and the antenna coverage are given in Figures 6.13 and 6.14, respectively. An extremely reliable 200 W class TWTA has been developed for this experiment. The overall efficiency of this design is expected to be about 50 percent due to the use of a high thermal capability coupled-cavity slow-wave structure and the velocity-tapered slow-wave circuits.


Figure 6.13. Block Diagram of Transponder for COMETS


Figure 6.14. Antenna Pattern in 20.7 GHz band

Table 6.4
Features of Advanced Satellite Broadcasting Equipment

Table 6.5
Parameters of Earth Stations

The experimental HDTV broadcasting system is expected to operate at 100 Mbits/sec with an BER performance of 10(superscript -10) for 99 percent of the worst month.

The objective of the third mission is to perform the S- and Ka-band intersatellite communications experiment. Signals from other satellites will be received at S-band while the return signal transmission channel will be carried at Ka-band. CRL is currently investigating the feasibility of using these ISLs in conjunction with the operation of small satellites to improve the in-orbit reliability of operating satellites.


Published: July 1993; WTEC Hyper-Librarian