Space and ground segments

Svalbard station

Design trade-offs have led to a medium-Earth orbit as the optimal altitude for navigation satellite constellations, commencing with the US GPS and Russian Glonass. There are solid practical reasons for this: medium-Earth orbits are relatively stable and the satellites move across the sky relatively slowly. Lower orbits would require more satellites to maintain the same coverage while higher orbits would reduce coverage extent.

In addition an extensive ground infrastructure distributed worldwide is required to uplink the navigation signals, keep the different clocks of the constellation synchronised and correct any onboard timing or positioning deviation.

User receivers

In-phone satnav receiver

Satellite navigation receivers also do a great deal of work, containing ‘ephemerides’ or electronic almanacs recording the expected locations of the constellation, to reduce the time taken to acquire signal locks from minutes to a matter of seconds.

The satellite navigation signals are very faint, equivalent to car headlights shone from one end of Europe to another. The signals are based around pseudo-random number codes that identify each satellite in a constellation. The receiver has records of each of these complex codes, so a full-power replica can be generated within the receiver from the faint signal received and used for the calculations deriving the final navigational data displayed to the user.

Ionosphere map

Interference can be a problem: the ionosphere, the uppermost electrically-charged layers of Earth’s atmosphere can cause signal delay equivalent to several metres in the worst case. Signals reflecting off local surroundings, known as ‘multipath’ can cause spurious signal locks and decrease the overall accuracy.

Larger dual-frequency receivers can remove most ‘iono-interference’ while smaller receivers of the type fitted into phones or car dashboards rely only on single frequency signals, but employ software models to remove up to half the error.