by Josh Karnes
Chapter 38
Isla Roca, Puerto Rico
“Is it just because it's too deep?” Mark Grady asked Aaron, who was tinkering with the color touch screen on the side-scan sonar device's display. Mark thought it reminded him of a high-power scientific version of a fish finder like he'd seen in fancy fishing boats back in Houston. Only this one had a towfish, a five-foot-long torpedo-looking thing attached to a cable that they dropped into the water. This allowed it to more accurately measure at the extreme depth of El Pliegue. They were using it in some sort of 3-D mode to show objects and forms on the bottom of El Pliegue. Unfortunately, it was not giving a coherent image, and Aaron had been adjusting it and trying to get the image to clear up for the better part of an hour. Meanwhile, Melissa and Mark were just wasting time out here, peering into the abyss below, seeing nothing but deep blue.
“No, we can map way deeper than this sinkhole. The problem is just incoherent readings. Like distortion. I've never seen anything like it before with this equipment. It's like...” Aaron trailed off as his mind picked up the thought, it's like the echo is arriving before the tone is sent.
He didn't like this thought at all. He and Carl had been right out here in this area measuring extreme time errors even in clocks they had posted in these buoys. Larry suggested they might actually be experiencing time distortion; time was passing faster for the phones than it was for other things. If the sonar tone were to be affected by the same time distortion, then it would be heard by the sensor earlier than it should, which would indicate an object closer than it really was. But for the tone to arrive before it was sent? He tried not to think about it. It must be a calculation error, he thought. Maybe if the tone arrived too early, the device couldn't correctly calculate it, and it had a math error that resulted in a negative number when it was just a very small positive number. Or maybe the thing was programmed with an offset. Who knows, besides the engineers that made the sonar rig? Whatever the cause, these readings he was getting were useless. They were not going to be able to map this.
“Well, can you fix it?” Melissa asked, impatiently. Aaron didn't blame her. They'd been out here over an hour and he had nothing to show for it.
“I have been trying, but I am beginning to think it's just not going to work. The side-scan must be damaged, or maybe there's a software bug. One way or the other, it's not getting meaningful images.” Aaron explained without revealing his true thoughts.
“Could this be because of the tests you are running? Or your other equipment?” Mark said, pointing out the buoys, the closest of which floated only about twenty yards away from their boat.
“No, I don't think so,” Aaron lied. “Those are just clocks in the buoys. They couldn't affect the sonar.” At least that much was true.
“Clocks?” Melissa asked, confused. “I thought you guys were doing GPS testing.”
“Yes, we are. But GPS uses precise time measurements to calculate location, so some GPS errors are really just time errors. One way to measure time errors is with a precise clock.”
“Why not just measure it with a GPS receiver? That would seem to make more sense,” Mark said. Mark was too clever for his own good. He was curious and sharp. This was going to be a problem for Aaron, firstly because he liked the kid, and really did want to talk to him. And secondly, because Mark was clever enough to see through Aaron's crummy attempt at deception. Aaron was not a dishonest person. It was uncomfortable for him to try to avoid speaking freely about this problem. And he thought Mark was likely to find a way to crack his flimsy defense of the facts.
Aaron thought of another way to explain it that might not reveal too much. “Yeah, we do that too. But these clocks on the buoys are much more accurate than the clocks in GPS receivers. You see, the clocks in a GPS receiver synchronize themselves with the GPS satellite clock every second, so if they're wrong, like if they are running too fast or too slow, then the amount of error that can stack up is just however much can happen in one second, which is not much. But the precise clocks we are using for testing can stay accurate for a long time without having to synchronize with the GPS. It helps us to measure errors that may stack up over a long period of time, like minutes or hours.” What Aaron said was true, and not at all related to the thing they were measuring. He hoped it was also far enough over the heads of Mark and Melissa that it would discourage additional questions. It didn't work.
“So these GPS, umm, problems, you are testing. They would have to be time problems then, right?” Mark said. “Since GPS uses time to figure out location, right?” Aaron nodded, and thought, well, crap. “So, did you measure any time problems? Because that's how sonar works too, right? It sends a ping or something and then starts counting time when it sends it, then when it echoes off of something and comes back, you stop the clock and calculate the distance to the object based on how long the ping took to bounce off. Isn't that right? If that's how this sonar works, then wouldn't a time problem screw it up?”
Aaron considered his options. He could try to shut down the discussion using misdirection and being cagy. This wasn't working at all so far. Or he could come clean with Mark and his mother. Larry would probably fire him if he found out. Maybe there was a middle path.
“You're right. It's possible that timing problems could throw off the sonar, even this side-scan. And we have been measuring time problems out here.” Aaron thought for a moment of how to say this honestly, while not telling the truth. “The equipment we are using out here can measure extremely tiny time errors, and extremely tiny time errors are enough to cause major problems with GPS, but that's because the signals that GPS receivers are using to determine position travel at the speed of light, and that's about a million times faster than the sound waves that sonar uses.” Aaron didn't bother mentioning the fact that the signal from the GPS also has to travel about a million times further than the sonar signal does. Maybe Mark wouldn't figure that out.
Aaron continued, “Look, it really doesn't matter what is causing the problem. One way or the other, the side-scan sonar is not working. We're going to have to come up with another way to—”
Suddenly the Zodiac they were in lurched and the tow cable that was attached to the sonar's towfish down below the water briefly became taught before the lightweight carabiner holding it to the tie-down on the boat's floor snapped open. The cable went overboard, and pulled along with it the side-scan sonar unit, which went under without a splash. This all happened in a fraction of a second. All three of the boat's occupants stared overboard in vain effort to see the sonar unit rocket into the abyss below, but it was gone before they could even get a glance.
“What the...” Mark began, aghast.
“We need to get out of here, now.” Aaron strode to the helm, fired up the engine and pinned the throttle forward as Mark and Melissa instinctively hung on for dear life.
Dear God, Melissa thought. And then, Joey...