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The Forefront of Space Science

Development of Next Generation X-band Digital Transponder
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“Larva” of next generation model

Picture of Figure 1 is the new X-band transponder, which is expected to play a major role in the next generation. This is the trial model produced to verify the newly-added basic functions. This is in the early stage of development, so not qualified for use on satellite or explorer in space. But its function and performance are the most advanced. Everything to communicate is installed inside the box looking like four-tiered food box. The picture in the front cover shows the testing scene with the cover opened and cables connected. Table 1 shows the comparison of specification for three types of transponders. The trial model is 15cm x 15cm x 9.45cm in size, is made of aluminum housing, and weighs 3.4kg. Downsizing and lightweight are more emphasized than the past transponders. Nevertheless, it is difficult to design in the way to produce the cellular phone gathering the best of modern technologies. For transferring the technology from right to left models, specialty of space technology is naturally barrier for cost and, among others, reliability becomes the biggest issue.

The new transponder consists of four sections. From top they are: the base band section to demodulate the receiving radio wave and to synchronize and regenerate the signal (1st stage); the high frequency section to modulate and demodulate the radio wave transmitted and received through antenna (2nd stage); the frequency synthesis section to generate the radio wave with frequencies necessary for modulation and demodulation (3rd stage); and the power supply section to distribute the power necessary for the operation of transponder (4th stage). (Hereinafter, the term of signal transmission or reception is used from the side of transponder.) The base band section has also the function to regenerate the in-phase, or coherent, radio wave that has frequency with the proper fraction ratio (880: 749) to the received radio wave.

Feature of the new transponder is summarized as follows.

  • Supporting X-band frequency channel for deep space exploration
  • EImprovement of ranging (means “measuring distance”) link quality in deep space exploration
  • EImprovement of receiver sensitivity
  • EDigital signal processing in the base band section

Considering these features, we call the new transponder the “X-band digital transponder.”

In past, ISAS has had no transponder using X-band for transmission/reception, except for that aboard the deep space explorer HAYABUSA launched in May 2003. Frequency channel approved for the future deep space explorations is decided to be higher frequency than X-band. We are, therefore, requested to introduce the new transponder and use the X-band channel in the future. Well, as I use the term of “X-band” without explanation in above, we generally call the radio wave with 8GHz/7GHz frequency the “X-band.”

The X-band transponder was employed in HAYABUSA. But it is recognized that ranging link in the deep space beyond 2AU (AU is astronomical unit. 1AU is the distance between the Sun and earth, i.e., 150 million km) becomes a bottleneck. In other word, we have difficulty in determining the trajectory of explorer. In order to measure distance to explorers in space, first we send the radio wave carrying the ranging signal from ground. The radio is returned to the ground, like boomerang, via transponder aboard explorers. We measure the time from sending the signal to receiving it to calculate the distance. In the past ranging, transponder aboard explorers simply returned the radio wave. But, we adopt the new method that transponder in space regenerates the signal, restores its quality, and re-modulates the signal. And, then, it sends the ranging signal to the ground. In the conventional method, quality of the ranging signal on the radio wave is deteriorated over its round trip, up and down links. In the new method, the signal degradation occurs only in one way. Figure 2 shows the experiment result of the two methods. In the conventional method (upper), the ranging signal pattern is obscure due to interference by noise. The new method (middle) shows that the original signal is finely regenerated (Please see the correspondence in height level between the transmitted signal pattern (bottom) and positions of the regenerated signal’s peaks.) Thanks to the new method, it is expected that ranging link will be secured up to 5AU distance, not sparing time in thinking about directivity of antenna or attitude control of explorer. We believe that the new transponder has capability to enable the exploration of Jupiter.

The digital signal processing must be the demands of the times, though, a little belated. At long last, it becomes possible to obtain electrical components usable in the space environment, while meeting the requirements of power consumption and cost. The digitalization has the advantage, in addition to improvement of receiver sensitivity, to enable elimination of the electrical adjustment process peculiar to the microwave circuits, eventually leading to cost down. After improvement of ranging link by the said ranging signal regeneration method, the communication link between explorer and ground station becomes a new bottleneck. For this problem, improvement of receiver sensitivity is very effective. By combination with the new error-correcting code, the communication link can be also achieved to the extent of 5AU distance. These performances have been confirmed by the experiments using the trial model for functional evaluation shown in Figure 1.

Figure 2. ISTP Program
Figure 1 Trial Model of X-band Digital Transponder for Functional Evaluation

Figure 2. ISTP Program
Figure 2 Effect of Ranging Signal Regeneration Method
Measurement result of the received ranging signal patterns under the same condition.
(Upper: conventional ranging method, Middle: regenerative ranging method, Bottom: transmitted signal pattern)

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