Well, you can't just program the probe to look for the effect. It has to be designed a certain way. Basically, it needs to be spin-stabilized. The upshot of spin stabilization is that you don't have to use thrusters to correct your antenna's aim at Earth. Since every single thruster firing adds uncertainty to your spacecraft's true speed and position, this normally negligible effect becomes significant over a long period of time.<br /><br />So spin-stabilization shoudl also buy you reduced propellant consumption. This is why many commsats have been spin-stabilized as well. It's cheaper.<br /><br />But it has some significant disadvantages too. The biggest disadvantage by far is that you cannot slew the spacecraft. Pioneers 10 & 11 were limited in the kinds of pictures they could take because they were spin-stabilized. The Voyagers greatly outclassed the Pioneers because they could not only slew themselves, they also had science instruments (including the cameras) on a moveable arm -- the scan platform. The drawback to a scan platform is that it's got moving parts; Voyager 2 almost lost the use of its scan platform before the Neptune flyby, but engineers were luckily able to coax it back most of the way. (The lubricant was freezing.) Neither Voyager can use its scan platform today.<br /><br />Newer probes don't generally use scan platforms or spin stabilization. Instead, they use gyroscopes both for stabilization and for attitude control. Cassini can use its reaction wheel assemblies to slew itself around with remarkable delicacy, far beyond that acheivable with thrusters, but it has thrusters also as a backup. I don't believe they are sufficient to fully stabilize it; Cassini is an enormous probe, easily the most massive ever sent into the outer solar system. Galileo had an intriguing (but complicated) strategy: most of the probe was spin-stabilized, but the bottommost part was not. Many science instruments were placed onto the despun section. This <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em> -- The Tenth Doctor, "Blink"</font></p> </div>