S

SA

Selective Availability. A policy adopted by the Department of Defense to introduce some intentional clock noise into the GPS satellite signals thereby degrading their accuracy for civilian users. This policy was discontinued as of May 1, 2000 and now SA is turned off

Satellite constellation

The arrangement in space of a set of satellites

Scale

The distance between two points on a map as they relate to the distance between those same points on the earth

Secular Motion

That portion of crustal motion which is continuous and at a constant velocity. Secular motion is uniformly predictable over time and is independent of any seismic events

Selective Availability (SA)

A policy adopted by the Department of Defense to introduce some intentional clock noise into the GPS satellite signals thereby degrading their accuracy for civilian users. This policy was discontinued as of May 1, 2000 and now SA is turned off

Signal to Noise Ratio (SNR)

The ratio of incoming signal strength to the amount of interfering noise as measured in decibels on a logarithmic scale

Single Difference

A GPS observable formed by arithmetically differencing carrier phases that are simultaneously measured by a pair of receivers tracking the same satellite, or by a single receiver tracking a pair of satellites. The between-receiver's-single-difference procedure essentially removes all satellite clock errors

Slow switching channel

A sequencing GPS receiver channel that switches too slowly to allow the continuous recovery of the data message

SMG

Speed Made Good. Marine term Giving speed to waypoint taking into effect the course you are steering

SNAFU

Situation Normal, All Fouled UP. Military term

SOG

Speed Over Ground. Marine term to tell you to ignore tidal and current effects

SOIT

FAA's Satellite Operational Implementation Team

Space segment

The part of the whole GPS system that is in space, i.e. the satellites.

Spatial Decorrelation

The distance between the user and the reference station. When calculating differential corrections, the greater the distance between the two, the greater the error of corrections

Speed Made Good (SMG)

Marine term Giving speed to waypoint taking into effect the course you are steering

Spherical Error Probable (SEP).

A measure of accuracy in navigation. SEP is the radius of the sphere inside of which the true three-dimensional coordinates of a position have a 50-percent probability of being located

Spoofing

The deliberate transmission of fake signals to skew the position calculations of a GPS receiver. The spoofer mimics a GPS satellite, rather like a pseudolite, but with disruptive intent

Spread spectrum

A system in which the transmitted signal is spread over a frequency band much wider than the minimum bandwidth needed to transmit the information being sent. This is done by modulating with a pseudo random code, for GPS.

SPS

Standard Positioning Service (civilian GPS)

Squelch Control

This control on a radio is used to silence the inherent background noise in a radio receiver so that only signals that appear above this background will be heard. To set a squelch control, turn it until the background noise is heard and then rotate the control until the noise just stops

Standard Positioning Service (SPS)

The normal civilian positioning accuracy obtained by using the single frequency C/A code

Static GPS

Carrier phase differencing technique where the integer ambiguities are resolved from an extended observation period through a change in satellite geometry

Static positioning

Location determination when the receiver's antenna is presumed to be stationary in the earth. This allows the use of various averaging techiques that improve the accuracy by factors of over 1000

Statistic accuracy

The industry standard definition of stataic accuracy is the standard deviation of error if a point set by a receiver was tested in a fix position for 24 hours.

Statute Mile

A distance of 1609 meters

Stop-and-Go Surveying

A precision DGPS surveying technique, also referred to as semikinematic, by which the rover is used to collect data a point for a few seconds or minutes before moving on