Hi,
I'm trying to subsample GPS collar data to replicate the position frequency of older collars and have been running into an error when using subsample() on a trajectory.  I'm hoping someone might be able to provide some guidance.

The original GPS data set looks like this:
> junk[1:10, c(1:3,14:15, 24) ]
    RecIDNew BrID     Date       x               y             IDBrYrSsn
297      297 X12156  13/12/2007 5:00:00 1188846 13247936 X12156_2008_Winter
298      298 X12156  13/12/2007 9:00:00 1193456 13250036 X12156_2008_Winter
299      299 X12156 13/12/2007 13:00:00 1196286 13259320 X12156_2008_Winter
300      300 X12156 13/12/2007 17:00:00 1196592 13262720 X12156_2008_Winter
301      301 X12156  14/12/2007 1:00:00 1192298 13273493 X12156_2008_Winter
302      302 X12156  14/12/2007 5:00:00 1190678 13285359 X12156_2008_Winter
303      303 X12156  14/12/2007 9:01:00 1190813 13293345 X12156_2008_Winter
304      304 X12156 14/12/2007 13:00:00 1192753 13289709 X12156_2008_Winter
305      305 X12156 14/12/2007 17:01:00 1194743 13287344 X12156_2008_Winter
306      306 X12156  15/12/2007 5:00:00 1197650 13318058 X12156_2008_Winter


I have created the trajectory from the GPS data using the following code
           da<- as.POSIXct(strptime(as.character(junk$Date), "%d/%m/%Y %X"))
           tr <- as.ltraj(xy, da, id=junk$IDBrYrSsn, typeII = TRUE,  slsp="missing")
           # Fill in missing values and make the trajectory regular
           tr2 <- setNA(tr, tr[[1]]$date[1], dt=4, units="hour")
           is.regular(tr2)
           > FALSE
          refda <- tr2[[1]]$date[1]
          tr3 <- sett0(tr2, refda, 4, units="hour")
          is.regular(tr3)
          > TRUE

the output trajectory looks like this:
>head(tr3[[1]])
        x        y                date       dx       dy      dist    dt       R2n abs.angle rel.angle
1 1188846 13247936 2007-12-13 05:00:00  4609.74  2099.89  5065.495 14400         0        0.4274459        NA
2 1193456 13250036 2007-12-13 09:00:00  2829.68  9284.29  9705.933 14400  25659241   1.2749586 0.8475127
3 1196286 13259320 2007-12-13 13:00:00   306.75  3399.73  3413.541  14400 184944524  1.4808122 0.2058536
4 1196592 13262720 2007-12-13 17:00:00       NA       NA        NA        14400 278567145        NA        NA
5      NA       NA       2007-12-13 21:00:00       NA       NA        NA        14400        NA             NA        NA
6 1192298 13273493 2007-12-14 01:00:00 -1619.99 11865.87 11975.944 14400 665047612  1.7064826        NA


When I use subsample I encounter the following:
tr3.sub <- subsample(tr3, dt = 3600*24*6, units="sec") # to subsample one location every 6 days

Error in res[, names(res) %in% which, drop = FALSE] :
  incorrect number of dimensions

If anybody has encountered this error before and can provide some advice on what step(s) i might be missing or any other thoughts on the error I would greatly appreciate your help.

Thanks very much,
Steven