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# 5.8 Expected Telemetry Rate

As it was mentioned at various times, the amount of data one would ideally like the GAIA satellite to transmit to the ground is far larger than the available telemetry rate. Any observing proposal must therefore provide an estimate of the necessary effort in terms of telemetry rate which is implied by the suggested observations.

Generally speaking, we would like to observe galaxies in all sky areas that trigger the detection and from a suitable surrounding area, and to send the corresponding data to the ground. Such a surrounding area could be defined as composed by all the square areas of a given side that are adjacent to the areas where an excess surface brightness has been detected, as it is shown in Figure 5.3, where a side of 2 arcsec was chosen for illustrative purposes.

The outlined observation strategy appears to satisfactorily cover the galaxy regions whose surface brightness is just below the detection limit, thus allowing to study the galaxy morphology in greater detail. In the following, however, in order to estimate the telemetry rate required for galaxy observations, a few simplifying assumptions will be made. It is assumed that data are transmitted from circular areas centered on the galaxy center, and that the radius of this areas can be written as

 (5.7)

where is a positive constant. The overall sky area within a radius for all galaxies brighter than could then be used as a rough estimate of the overall sky area from which data should be transmitted to the ground in order to observe all galaxies down to this magnitude. Neglecting the possible superposition on the sky between different galaxies, which is correct as far as the sky area in consideration is reasonably small, can be written as

 (5.8)

 2 4 6 8 10 10 5.9559699 6.4369017 6.9178336 7.3987655 7.8796973 11 13.015628 14.507042 15.998455 17.489869 18.981282 12 28.234900 32.617360 36.999820 41.382281 45.764741 13 62.711973 74.930181 87.148388 99.366598 111.58480 14 147.90940 180.29082 212.67225 245.05368 277.43510 15 379.93125 461.71573 543.50020 625.28469 707.06916 16 1059.6399 1257.0964 1454.5528 1652.0093 1849.4657 17 3118.8339 3576.2198 4033.6057 4490.9917 4948.3775 18 9354.6744 10375.459 11396.243 12417.028 13437.812 19 27814.818 30020.274 32225.728 34431.184 36636.639 20 80550.210 85187.870 89825.529 94463.189 99100.849

Numerical values of for different values of and are given in Table 5.7. From these it can be concluded that, with arcsec, all galaxies brighter than the detection limit would cover about of the sky. The telemetry rate required to cover such a sky area can be derived as function of the adopted sample size under the assumptions that each sample value is coded into 16 bits and that a loss-less compression factor of 16/5 can be applied before transmission. The assumption of such a relatively high compression factor is deemed realistic, since most sample values will be low, thus allowing a very efficient compression.

Under these assumptions, the required telemetry rate after compression for galaxy observations can be estimated as

 (5.9)

where is the chosen lower limit in Galactic latitude for galaxy observation ( is the fraction of sky where the absolute value of the Galactic latitude is greater than ), the number of photometric bands in which observations will be carried out, the scan velocity of the satellite, the height of the field of view, the sample size and indicated the number of bits per sample after compression. Values of are given in Table 5.8 for the presently assumed values of the relevant parameters.

 108 217 433 650

As mentioned in Section 2.6, a total telemetry rate of about 1 Mbit/s after compression is presently foreseen for GAIA. From Table 5.8, it appears that the observation of galaxies, carried out with or pixels/sample, as suggested in Section 5.7, would require a significant, but probably not unreasonable, part of the total telemetry. On the other hand, observing the assumed fraction of the sky with a smaller sample size would require a prohibitively high telemetry rate.

Next: 6. Simulation and Stacking Up: 5. Detection and Observation Previous: 5.7 Expected Accuracy in   Contents
Mattia Vaccari 2000-12-05