How OS Net works

OS Net works by receiving raw data from GPS satellites. Data collected by the base stations is sent in real time via dedicated communication channels to Ordnance Survey®. The data is then transmitted within tenths of a second to partners.

Partners can then generate a national GPS correction model. Correction services can be transmitted through many different kinds of media, such as GSM/GPRS, Internet, DAB and radio. The corrections are combined with GPS signals at the user’s receiver to improve the computed position, either in real time or at a post-processing stage. OS Net is designed to enable partners to offer such error correction in real time on a nationwide scale.

GPS provides a widely accepted technique to identify location, offering an efficient solution to the positioning problem. Raw GPS positions have an accuracy of around 5-10 m in plan, 10-20 m in height. This inaccuracy is caused by the errors in the GPS, summarised below.

Error source	Description	Potential magnitude of positional error (m)
Orbit Errors	Our inexact knowledge of where the satellites are in their orbit.	3
Satellite Clocks	The small inaccuracy in the satellite clocks means the distance measurement is not precise.	1.5
Ionosphere	The atmospheric layer from 50 to 500 km disturbs the GPS signal, leading to inexact range measurements.	6.0
Troposphere	The atmospheric layer which includes the Earth’s weather disturbs the GPS signal, leading to inexact range measurements.	0.5
Receiver Noise	Internal receiver errors.	0.1
Multipath	The effect of indirect GPS signals arriving at the GPS antenna.	0.6

OS Net products will correct for the orbital, atmospheric and satellite clock errors.

As well as partner services, Ordnance Survey’s own free GPS service for non-commercial post-processing and coordinate transformation, www.gps.gov.uk, will continue to be supported and improved for GPS users.

[1] Some parts of north-west Scotland are outside of the OS Net network at November 2005. Refer to www.ordnancesurvey.co.uk/gps for further information.

How OS Net™ works