THE NERC MST RADAR FACILITY AT ABERYSTWYTH
Click here to find out about the contents of other files.
Availability
MST radar spectral data are only available in this format up to 6th February 2007. Data for later dates are available in netCDF files.
File naming convention:
 |
YY | |
is a 2-digit year [00 - 99] |
| MM | is a 2-digit month [01 - 12] | ||
| DD | is a 2-digit day [01 - 31] | ||
| hh | is a 2-digit hour [00 - 23] | ||
| DD | is a 2-digit minute [00 - 59] | ||
| dd | is the integer number of minutes covered by the file |
File location: /badc/mst/data/mst-raw/spectra
Click here for the location of other files.
Archiving convention: YYYY/mon
Click here for a further explanation.
File availability
Doppler spectra data are available from 1990 onwards.
File format
The data are stored in non-standard format binary files, as described below. The following is a verbose description. Click here to see the original file format description. Note that some terms and abbreviations differ between the two descriptions.
Owing to the relatively limited file storage capacity available when the radar began operations in 1990, the data were stored in a compressed format (which is still being used today). Although this format is not particularly difficult to decode, it will be necessary for users to write a special reading routine. The description below should make more sense once you start to read a file.
The files contain 4 different data block types:
- This gives the number of bytes required to store
the data for each dwell, the number of dwells within each
cycle, and the number of cycles within the file.
- The file contents information is associated with only the first dwell of a file. An EB, which contains no information, is therefore substituted for the FCB for subsequent dwells.
- This gives the values of the radar parameters used to make the observations for the dwell. It is necessary to know some of these values in order to read in the spectral data correctly.
- This contains the range-gated spectra for the dwell.
- File Contents Block (FCB) - 64 bytes
- Empty Block (EB) - 64 bytes
- Parameter Block (PB) - 64 bytes
- Spectral Data Block (SDB) - variable number of bytes
|
PB | |
(Cycle 1, Dwell 1) |
| FCB | (Cycle 1, Dwell 1) | ||
| SDB | (Cycle 1, Dwell 1) | ||
| PB | (Cycle 1, Dwell 2) | ||
| EB | (Cycle 1, Dwell 2) | ||
| SDB | (Cycle 1, Dwell 2) | ||
| . | . | ||
| . | . | ||
| . | . | ||
| PB | (Cycle 1, Last dwell in cycle) | ||
| EB | (Cycle 1, Last dwell in cycle) | ||
| SDB | (Cycle 1, Last dwell in cycle) | ||
| PB | (Cycle 2, Dwell 1) | ||
| EB | (Cycle 2, Dwell 1) | ||
| SDB | (Cycle 2, Dwell 1) | ||
| PB | (Cycle 2, Dwell 2) | ||
| EB | (Cycle 2, Dwell 2) | ||
| SDB | (Cycle 2, Dwell 2) | ||
| . | . | ||
| . | . | ||
| . | . | ||
| . | . | ||
| . | . | ||
| . | . | ||
| PB | (Last cycle in file, last dwell in cycle) | ||
| EB | (Last cycle in file, last dwell in cycle) | ||
| SDB | (Last cycle in file, last dwell in cycle) |
The following abbreviations will be used to describe data types:
|
int8 | |
Signed 1 byte (8 bit) integer |
| uint8 | Unsigned 1 byte (8 bit) integer | ||
| uint16 | Unsigned 2 byte (16 bit) integer |
File Contents Block (FCB) Structure
The FCB is located within bytes 64 - 127 of the file (where the first byte in the file is byte 0). The number of bytes used (of the available 64) depends on the the number of dwells per cycle (nr_dwells).
|
uint16 | |
nr_dwells | |
Number of dwells per cycle |
| uint16 | nr_recs(1) | Number of records (of 64 bytes) from the beginning of the PB for the first dwell to the end of the SDP for the first dwell in a cycle. | |||
| uint16 | nr_recs(2) | Number of records (of 64 bytes) from the beginning of the PB for the first dwell to the end of the SDP for the second dwell in a cycle. | |||
| . | . | . | |||
| . | . | . | |||
| . | . | . | |||
| uint16 | nr_recs(nr_dwells) | Number of records (of 64 bytes) from the beginning of the PB for the first dwell to the end of the SDP for the last dwell in a cycle. |
The beginning of a PB for a given dwell (dwell_nr) of a given cycle (cycle_nr) within the file is therefore given by:
where nr_recs(0) = 0, and the number of cycles within the file is given by:
where total_nr_bytes is the total number of bytes in the file.
Parameter Block (PB) Structure
This uses 44 of the 64 available bytes.
|
uint8 | |
LTP | |
Length of transmitter pulse (μs) [1 | 2 | 4 | 8 | 16 | 32] |
| uint8 | PCT | Pulse coding type [0: uncoded | 1: 8 μs sub-pulse length | 2: 4 μs sub-pulse length | 3: 2 μs sub-pulse length | 4: 1 μs sub-pulse length] | |||
| uint16 | IPP | Inter pulse period (μs) [80 | 160 | 320 | 640] | |||
| uint16 | NCI | Number of coherent integrations | |||
| uint16 | DFT | Number of points in discrete Fourier transform [64, 128, 256 or 512] | |||
| uint16 | NII | Number of incoherent integrations | |||
| uint16 | RG1 | Lowest range gate number for ST mode | |||
| uint16 | RG2 | Highest range gate number for ST mode | |||
| uint16 | BDN | Beam direction number [0: Vertical | 1: N4.2° | 2: N8.5° | 3: S4.2° | 4: S8.5° | 5: E4.2° | 6: E8.5° | 7: W4.2° | 8: W8.5° | 9: NW6.0° | 10: NW12.0° | 11: NE6.0° | 12: NE12.0° | 13: SE6.0° | 14: SE12.0° | 15: SW6.0° | 16: SW12.0°, where the actual azimuths are offset 17.5 ° anticlockwise from the nominal directions] | |||
| uint16 | Year of start of dwell minus 1900, i.e. 99 corresponds to 1999 and 101 to 2001 | ||||
| uint16 | Month of start of dwell [1 - 12] | ||||
| uint16 | Day of start of dwell [1 - 31] | ||||
| uint16 | Hour of start of dwell [0 - 23] | ||||
| uint16 | Minute of start of dwell [0 - 59] | ||||
| uint16 | Second of start of dwell [0 - 59] | ||||
| uint16 | RG3 | Lowest range gate number for M mode | |||
| uint16 | RG4 | Highest range gate number for M mode | |||
| uint16 | Range interval [integer multiples of 150 m] | ||||
| uint8 | RFL | Receiver filter length (1/Bandwidth) [1, 2, 4, 8, 16, 32] | |||
| int8 | Raw data collection flag [-ve if raw data collected] | ||||
| uint16 | dwell_nr | Dwell number within cycle [0 - nr_dwells | |||
| uint16 | cycle_nr | Cycle number within file [0 - nr_cycles | |||
| uint16 | Run/file number (since start of year) | ||||
| uint16 | Number of right shifts incorporated in input data |
Spectral Data Block (SDB) Structure
The start of a SDB is 128 bytes on from the start of the corresponding PB. For each range gate, DFT Power spectral densities (PSDs) are stored in a coded format (in 8 bit signed integers) starting at the most negative Doppler frequency. They should be read with a routine of the form:
do range_gate_nr = RG1,RG2
do dft_pnt_nr = (-DFT/2),(DFT/2 - 1)
value(range_gate_nr,dft_pnt_nr) = int8
end do
end do
if (RG3 > 0) & (RG4 > 0)
do range_gate_nr = RG3,RG4
do dft_pnt_nr = (-DFT/2),(DFT/2 - 1)
value(range_gate_nr,dft_pnt_nr) = int8
end do
end do
end if
where the Doppler frequencies are given by:These must be multiplied by a factor of [-λ/2], where λ is the radar wavelength (6.45 m), in order to convert from Doppler frequency to Doppler velocity. Note that this implies reversing the order of spectral points. A positive Doppler velocity implies movemnet away from the radar.
The PSDs are normalised to the peak PSD (within each spectrum) and stored at 0.2 dB intervals [PSD_dB = 10 * log10(PSD_linear)] in an 8 bit signed integer, i.e. :
|
(value == 127) | |
corresponds to 0.0 dB |
| (value == 0) | corresponds to -25.4 dB | ||
| (value == -128) | corresponds to -51.0 dB |
This conversion applies to all but the zero Doppler frequency position, i.e. to the (DFT/2 + 1)th spectral point. This frequency bin contains the coded scaling factor (CSF) for the spectrum, which is coded at 0.5 dB intervals:
|
(value == 127) | |
corresponds to 95.5 dB |
| (value == 0) | corresponds to 32.0 dB | ||
| (value == -128) | corresponds to -32.0 dB |
In summary, the actual PSD (dB) is given by:
The PSD of the zero Doppler frequency bin is typically contaminated by ground clutter, and so little information is lost by removing it. Once the above decoding operations have been performed, the zero Doppler frequency PSD should be estimated as the mean of the PSDs for the two adjacent Doppler frequency bins.
The range of the centre of a range gate from the radar (m) is given by:
where range_gate_0 is the sea-level range gate, which is given by:
|
5.2 | |
if (LTP == 1 μs) |
| 5.7 | if (RFL == 1 μs) & (LTP > 1 μs) | ||
| 6.7 | if (RFL == 2 μs) | ||
| 8.7 | |
if (RFL == 4 μs) | |
| 12.7 | |
if (RFL == 8 μs) |
and δz is the range interval between gates: 150 m. Note that this must be multiplied by the cosine of the off-vertical beam pointing angle in order to calculate the altitude above the radar, i.e.:
|
149.6 m | |
if (BDN == 1) | (BDN == 3) | (BDN == 5) or (BDN == 7), i.e. for an off-vertical angle of 4.2° |
| 149.2 m | if (BDN == 9) | (BDN == 11) | (BDN == 13) or (BDN == 15), i.e. for an off-vertical angle of 6.0° | ||
| 148.4 m | if (BDN == 2) | (BDN == 4) | (BDN == 6) or (BDN == 8), i.e. for an off-vertical angle of 8.5° | ||
| 146.7 m | if (BDN == 10) | (BDN == 12) | (BDN == 14) or (BDN == 16), i.e. for an off-vertical angle of 12.0° |