ARGOS USER STORIES
Recovery of a lost sea lion tag (in a residential housing area)
Notes from Ladd Irvine, Oregon State University Marine Mammal Institute
Background: An Argos satellite tag that was attached to a sea lion was recovered by someone but unfortunately they did not return the unit, and the team at OSU wanted to make sure they were able to find the tag before the battery power was depleted. Rather than going door-to-door they reached out to CLS America to lease an Argos Goniometer. Below are some notes from Ladd Irvine:
[It is extremely important to know the tag transmission times because if you do not go out when the tag is scheduled to transmit you won't be able to relocate it. In Ladd's situation the tag was only transmitting from 5 pm to 4 am.]
We had the general area narrowed down to a residential neighborhood in Richland, WA using the LC 3 Argos locations but there was still a pretty substantial spread, which was why we needed the Goniometer to help us focus in on some specific houses. We used the average lat/lon of the LC 3 locations as a starting point for our searching and continued on foot from there. The goniometer directed us toward two houses pretty quickly. After focusing in on the two houses we decided to move to the next street to make sure we weren’t picking up the signal from houses behind the one we were looking at. We again narrowed things down to a couple houses and these ones were directly behind the first two houses, so we were pretty sure we were close. There turned out to be a walking path that connected the two streets and happened to run right next to two of the suspect houses, so we were able to essentially stand at the back fence line of the houses and get a direction to see which street the tag was on. That narrowed it down to two houses. We knocked on the door of the one we thought was most likely and they had the tag in their garage! It took us about an hour all told (the tag’s rep rate was 89 s so that was a bit of a limiting factor) and we actually ended up having parked directly in front of the correct house, but one street over.
A couple miscellaneous points/thoughts:
This was definitely not a case of just pulling out the equipment and it pointed us right to the tag. We had what turned out to be reflected signals off some nearby apartment buildings (we were very happy when that didn’t end up being the final direction), and would not pick up a signal at all in other spots, which I’m assuming was due to interference/obstructions.
With all that said, the goniometer still worked very well! We generally moved in small increments (~ 50 m) and would wait, or make a small movement when we missed a transmission. We generally waited for two or three fixes before moving to get a general idea of the direction and would rotate the antenna to point in the direction of the first fix. If the second fix showed more or less straight ahead (relative to the antenna) we would then move accordingly. Reflected signals were a bit of a problem, but it seemed like the ‘true’ signal would come through after a move or two in the direction of the reflected signal. Also, once we had been going for 10 – 15 min, we had a general idea of where it should be, so the reflected signals were a little more obvious as strong diversions after we had moved a short distance.
I tested the system out prior to our trip by having a colleague hide one of our tags somewhere on the marine station campus where we work (Hatfield Marine Science Center). Using a similar methodology I was able to find the tag in about 30 min despite it being placed between two concrete buildings with other concrete buildings in between my starting point and the tag. I even walked past the tag initially and was turned around by the goniometer (after a few false starts from reflections) and found it.
So to summarize, the system works well, but you want to use an incremental approach and some patience. Reflected signals are a problem [in this type of environment, not so much in open areas], but they seemed to become apparent before too long as the direction would revert back to what was the ‘true’ direction after some movement. If you run into a spot where you lose the signal or it just isn’t making sense, go back to the last place you had a good signal and try again. You may have moved too far/at the wrong angle/who knows and caused the signal to be blocked.
The project was funded by the U.S. Navy and HDR, Inc., and carried out by researchers at Oregon State University with help from Navy biologists.