Terminology

The following terms are used throughout this document:

• SoH - State of Health. This term is used to refer to information about the status of an instrument, such as its mass positions, and information about the status of a digitiser, such as whether its sample clock is locked or not.
• GNSS - Global Navigation Satellite System. This is a generic term for any of a number of systems which use constellations of satellites, each fitted with an extremely accurate atomic clock, to provide signals to earth-based receivers. This allows the receivers to use the signal propogation delays to compute position to an accuracy of a few metres and time to an accuracy of a few microseconds.
• GPS - Global Positioning System. Although this term properly refers only to the US-operatated GNSS (formerly called "NavStar"), it is often used as a synonym for GNSS. Timing systems for Güralp digitisers were initially developed for use only with the GPS system and many of the commands and messages reflect this fact. The connectors are labelled GPS even on equipment which supports multiple GNSS constellations. Within this document, if there is any doubt, the context should make it clear to which system we refer.
• Constellation - This is the formal term for the collection of active satellites that form the space-based sector of a GNSS.
• SV - This is an abbreviation for Space Vehicle, the formal term for a satellite in a GNSS constellation. Each satellite in a constellation has a unique identification number, referred to as its "SV#". These are allocated in ranges as follows:

  1 — 32	Navstar/GPS
  65 — 88	GLONASS
  89 — 96	GLONASS (future extensions)
  301 — 336	Galileo
  401 — 437	BeiDou

• NMEA - This initialism for the National Marine Electronics Association is used (in this context) as an abbreviation for the NMEA 0183 standard for marine communications. It is the most common standard used for transmitting time, position and other information from GNSS receivers. It consists of a series of formatted text messages called "sentences" which provide operational and status information. The NMEA from a GNSS receiver contains information such as the time, date, location, fix status (see below), number of satellites visible and accuracy of calculations.
• PPS - An abbreviation for one-Pulse-Per-Second, this term describes a pulse train sent by the GNSS receiver to the digitiser. The leading edge of each pulse accurately identifies the start of each second. Subsequent NMEA sentences make explicit the time and date of the second thus indicated.
• FIX - When a GNSS receiver has computed a position, it is described as having obtained a "fix". When satellite visibility is limited, a first approximation can be made by assuming that the receiver is near sea-level (actually: on the surface of the WGS84 reference elipsoid). This is referred to as a two-dimensional or "2D" fix. When signals from a sufficient number of satellites are being received, a full three-dimensional fix, which includes elevation is achieved. This is known as a "3D" fix.
• GNSS power-cycling - This is a Güralp-specific firmware feature intended to reduce power consumption in power-critical applications. The user can specify a time period, n of between one and twenty-four hours. The GNSS receiver is powered up at boot and every n hours thereafter. Each time it is powered up, the system resynchronises, if necessary, and then waits until the offset and drift have fallen to acceptable levels. It then powers the receiver down again before repeating the cycle.
• Status stream - For CD24 and DM24 digitisers, along with all equipment containing them, this is a textual stream which contains state-of-health information about the digitiser, including the GNSS and sample-clock. The term is also used loosely to refer to status information available from the console of the Minimus and Fortis.