2. The EPN format

The underlying principles of the format result from a number of requirements. This was essentially a balance between the need for efficient data storage and providing sufficient information about the data for potential users. Specifically, the following requirements had to be met:

• Operating system independence: To make the data format as portable as possible between present and future operating systems, we have opted to use only ASCII-data throughout. We have arrange these data so that words are aligned over 80-byte boundaries, this simplifies inspection and printing of the files.
• Completeness: The data should contain all information for the identification of the source and the observing circumstances useful for further analyses of the data by others.
• Compactness: Descriptive information should not dominate the format. The measured values that form the bulk of a block of data are given as scaled four-character hexadecimal numbers, giving a dynamic range of up to 65536:1.
• Versatility: The format should be suitable for continuously sampled multi-channel filterbank search data, synchronous integrated and single-pulse data as well as processed data. In addition, we have designed the format, so that it can be used for observations of pulsars outside the radio regime i.e. variable units for the observing frequency and bandwidth, as well as topocentric telescope coordinates which are time variable for satellite observatories. Space is left for more descriptors, future adaptations and expansions.
• Simplicity and ease of access: We describe a data format consisting of a standardised fixed-length header with a variable length data structure attached to it. The header fully describes the structure of the data, which is not changed within one file but can vary between files. In this way, it is possible to calculate the length of a data block within each file after reading its header. The file can then be opened for random access with fixed block length, faster than a sequential read.

Many of the above mentioned requirements were already met by a format in use at Jodrell Bank to which we made suitable modifications and extensions to make it more flexible.

  
Figure 1: Schematic representation of an EPN data block

Each EPN file consists of one or more EPN blocks. The basic structure of an EPN block is shown in Fig. 1 (click here). Each file has a common fixed length header followed by a number of individual data streams of equal length. The header describes the data, containing information on the pulsar itself, the observing system used to make the observation as well as some free-form information about the processing history of the data. The onus is on the site-specific conversion process to ensure correct conversion to the standardised entries and reference to common catalogues (e.g. the Taylor et al. 1993, catalogue of pulsar parameters). The full list of header variables is given in Tables 1 (click here) and 2 (click here).

The data streams themselves may be outputs of different polarisation channels, or individual channels (bands) of a filterbank or a combination thereof. In total, there may be data streams of i.e. different frequencies for each polarisation. Each data stream starts with a small, fixed length sub-header in front of the actual data values. The number of data streams and their length may vary between different EPN files, but is constant within each file. A character field and an ordinal number is provided for each stream for its identification.

 

Position Name Format Unit Comment

1 version A8 EPN + version of format (presently EPN05.00)

9 counter I4 No. of records contained in this data block

13 history A68 comments and history of the data

81 jname A12 pulsar jname

93 name A12 common name

105 F16.12 s current barycentric period

121 DM F8.3 pc cm^-3 dispersion measure

129 RM F10.3 rad m^-2 rotation measure

139 CATREF A6 pulsar parameter catalogue in use

145 BIBREF A8 bibliographical reference key (or observer's name)

153 8X blank space free for future expansion

161 I2,I2,F6.3 hhmmss right ascension of source

171 I3,I2,F6.3 ddmmss declination of source

182 telname A8 name of the observing telescope (site)

190 EPOCH F10.3 day modified Julian date of observation

200 OPOS F8.3 degrees position angle of telescope

208 PAFLAG A1 A = absolute polarisation position angle, else undefined

209 TIMFLAG A1 A = absolute time stamps (UTC), else undefined

210 31X blank space free for future expansion

241 F17.5 m topocentric X rectangular position of telescope

258 F17.5 m topocentric Y rectangular position of telescope

275 F17.5 m topocentric Z rectangular position of telescope

292 29X blank space free for future expansion

321 CDATE I2,I2,I4 d m y creation/modification date of the dataset

329 SCANNO I4 sequence number of the observation

333 SUBSCAN I4 sub-sequence number of the observation

337 I2 number of polarisations observed

339 I4 number of frequency bands per polarisation

343 I4 number of phase bins per frequency (1-9999)

347 F12.6 duration (sampling interval) of a phase bin

359 F12.6 temporal resolution of the data

371 I6 number of integrated pulses per block of data

377 I4 bin number for start of calibration signal

381 I4 length of calibration signal

385 FLUXFLAG A1 F = data are flux calibrated in mJy, else undefined

386 15X blank space free for future expansion

401 80X blank space free for future expansion

 

 

Position Name Format Unit Comment

481 IDfield A8 type of data stream (I, Q, U, V etc.)

489 I4 ordinal number of current stream

493 I4 number of streams averaged into the current one

497 f₀ F12.8 effective centre sky frequency of this stream

509 U_f A8 unit of f₀

517 F12.6 effective band width

529 A8

537 F17.5 time of first phase bin wrt EPOCH

554 7X blank space free for future expansion

561 SCALE E12.6 scale factor for the data

573 OFFSET E12.6 offset to be added to the data

585 RMS E12.6 rms for this data stream

597 F16.12 s apparent period at time of first phase bin

613 28X blank space free for future expansion

641 Data(1) I4 scaled data for first bin

I4 data for last bin of stream end of first stream,