Details are presented of the H I Jodrell All Sky Survey ( H I JASS ) .
H I JASS is a blind neutral hydrogen ( H I ) survey of the northern sky ( \delta > 22° ) , being conducted using the multibeam receiver on the Lovell Telescope ( FWHM beamwidth 12 arcmin ) at Jodrell Bank .
H I JASS covers the velocity range –3500 km s ^ { -1 } to 10000 km s ^ { -1 } , with a velocity resolution of 18.1 km s ^ { -1 } and spatial positional accuracy of \sim 2.5 arcmin .
Thus far about 1115 deg ^ { 2 } of sky have been surveyed .
The average rms noise during the early part of the survey was around 16 mJy beam ^ { -1 } .
Following the first phase of the Lovell telescope upgrade ( in 2001 ) , the rms noise is now around 13 mJy beam ^ { -1 } .
We describe the methods of detecting galaxies within the H I JASS data and of measuring their H I parameters .
The properties of the resulting H I -selected sample of galaxies are described .
Of the 222 sources so far confirmed , 170 ( 77 per cent ) are clearly associated with a previously catalogued galaxy .
A further 23 sources ( 10 per cent ) lie close ( within 6 arcmin ) to a previously catalogued galaxy for which no previous redshift exists .
A further 29 sources ( 13 per cent ) do not appear to be associated with any previously catalogued galaxy .
The distributions of peak flux , integrated flux , H I mass and cz are discussed .
We show , using the H I JASS data , that H I self-absorption is a significant , but often overlooked , effect in galaxies with large inclination angles to the line of sight .
Properly accounting for it could increase the derived HI mass density of the local Universe by at least 25 per cent .
The effect this will have on the shape of the H I Mass Function ( H I MF ) will depend on how self-absorption affects galaxies of different morphological types and H I masses .
We also show that galaxies with small inclinations to the line of sight may also be excluded from HI-selected samples , since many such galaxies will have observed velocity-widths which are too narrow for them to be distinguished from narrow-band radio frequency interference .
This effect will become progressively more serious for galaxies with smaller intrinsic velocity-widths .
If , as we might expect , galaxies with smaller intrinsic velocity-widths have smaller H I masses , then compensating for this effect could significantly steepen the faint-end slope of the derived H I MF .