Rawarch plugin

2. The scan parameters (items 2, 4, 6, ...)

The even numbered items in the biglist are the parameters for scans 1, 2, 3, and so on. Scans parameters are stored in the order dictated by the table of contents.  For the file in the example, the parameter sets for the 711 scans from the first run are in biglist items 2, 4, 6, ..., 1422; these are followed by parameter sets for the 261 scans from the second run in biglist items 1424, 1426, 1428, ..., 1944; and so on.

For example, the parameters for the first scan are:

> bl[[2]]
$pulses
[1] 1024              ## the number of pulses in the first scan

$samples.per.pulse    ## the number of samples per pulse (i.e. range cells)
[1] 2048

$bits.per.sample      ## the bit resolution of each sample
[1] 12

$timestamp
[1] "2006-03-06 14:41:31 EST"    ## this is an integer of class "POSIXct"

$time.offset                     ## for more precise timing, this is the
[1] 0.2430000                    ## fractional seconds after timestamp of the
                                 ## start of the scan

$duration
[1] 2609                         ## duration of the scan, in milliseconds

$sample.dist                     ## distance covered by one sample, in metres
[1] 4.996542                     ## (i.e. the size of a "range cell")

$first.sample.dist               ## distance from the radar of the first sample
[1] 0                            ## (i.e. range of the first "range cell")

$bearing                         ## degrees clockwise from north of the first pulse
[1] 0

$orientation                     ## radar rotation direction: +1 = clockwise, -1=counterclockwise
[1] 1

$latitude                        ## geographic location of the radar antenna(optional)
[1] 45.08933

$longitude
[1] -64.36908

3. The raw scan data (items 3, 5, 7, ...)

The odd numbered items in the biglist (except for item 1) are string vectors with single elements containing the raw scan data in binary form.  Scans are stored in the order dictated by the table of contents.  For the file in the example, the 711 scans from the first run are in biglist items 3, 5, 7, ..., 1423; these are followed by the 261 scans from the second run in biglist items 1425, 1427, 1429, ..., 1945; and so on.

Unlike in C, strings in R can have embedded NULL (0x00) characters .  The only reason we don't use an R raw vector is that (surprisingly) R handles raw vectors much less efficiently than character vectors when reading/writing them from/to files.

Samples are packed as tightly as possible into the R string.  Samples occupying more than one byte have their data stored in order from least significant to most significant (i.e. little-endian), as is standard for i386-family computers.

> class(bl[[3]])
[1] "character"

> length(bl[[3]])
[1] 1

> nchar(bl[[3]])
[1] 3145728   ## 12-bit data is packed so 3 bytes holds two samples: 3145728 = 2048 * 1024 * 3 / 2

> charToRaw(substr(bl[[3]],1,16))
[1] 67 ba 8d 4b cc 09 4b ab 6e 1f 9a 88 46 99 b5 f7       ## the first two 12-bit values are:  0x0a67, 0x08db

The the nibble-packing order for 12-bit data is as follows:

     input:     byte0    byte1    byte2
     nibble:    A   B    C   D    E   F
                lo hi    lo hi    lo hi    

     output:    short0           short1
                A   B   C   0    D   E   F   0
                lo         hi    lo         hi

i.e. two 12 bit words: 0x0CBA, 0x0FED  <--> 0xAB, 0xCD, 0xEF (3 bytes)