Results from two simulated Huygens mission profiles
The sawtooth-type blue line divides the plot into two areas. Above the line, the Huygens receiver on board Cassini is not able to achieve synchronisation, indicating that Probe data would be lost. Below the curve, the probe data can be retrieved without errors.
It was the objective of the recent Probe-Relay Test to simulate new Probe mission profiles in terms of corresponding frequency offset (in parts per million) and signal strength (in decibels) during the Huygens descent (as seen by the receiver on board Cassini). The profiles were simulated by varying the transmit power and data rate of a signal sent by Goldstone ground station to the Probe-relay link receiver on board Cassini. The first figure represents the results produced during a nominal mission simulation. The horizontal distance between each pair of blue crosses shows the shifts in signal strength caused by the Probe spin, while the triangles represent the average signal strength readings at 10-minute intervals. The triangles below the solid curve are green, indicating that - as predicted by computer models - the receiver on Cassini was able to detect the Huygens data correctly during the 3-hour long test
The results produced during a non-nominal mission simulation are shown in the second figure. In this simulation the frequency has been increased by 4 ppm and the signal level reduced by 1 dB with respect to the expected nominal values. The receiver on Cassini was able to detect the Huygens data correctly from the beginning of the simulation until about the last 10 min. . At the end of the simulated profile (well after Probe landing) two red triangles show that the signal strength and corresponding frequency offset are above the solid line. This indicates that, as expected, the receiver on Cassini was unable to detect the data for a few minutes under these conditions. If the actual mission is carried out under those conditions, some Probe data would be lost during the last 10 min on the surface.