Technical Terms

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GPS

The NAVSTAR Global Positioning System. GPS satellites broadcast carrier signals modulated with ranging codes and ephemeris. The ephemeris is information describing the satellite's orbit. A GPS receiver tracks the signals of selected satellites, and uses the signals to calculate the receiver's position and velocity.

Pseudorange

Pseudorange is the receiver's measurement of distance to satellite, plus an offset of the receiver's clock. The receiver obtains the measurements by tracking satellite-ranging codes. Pseudorange is measured by generating a replica of satellite transmission code within the receiver, and adjusting time offset until the replica and received waveforms match.

Carrier Phase

There are two types of receiver measurements based on tracking satellite carrier signals. Delta-range is the accumulated signal Doppler over a short interval, typically one second. It is primarily a measurement of relative velocity. Some systems also use the carrier as a ranging signal by resolving carrier cycle ambiguities.

Reference Station

Also called a base station, a reference station places a GPS receiver at a known location. Instead of using the receiver to navigate, the station uses the receiver's measurements to determine error in satellite signals. Those errors correlate over tens or hundreds of miles. Consequently, other receivers in the area can use the information to navigate more accurately.

Augmentation

Also called differential correction, augmentation is the technique of adjusting pseudorange and carrier phase measurements using data from a reference station.

Inertial Navigation

GPS navigators sense signals broadcast by satellites. An entirely different means of navigation is to sense linear acceleration and angular rotation relative to inertial space, and to dead reckon by accumulating the sensor's data in a computer.

IMU

Inertial Measurement Unit. An IMU is a cluster of instruments (gyroscopes and accelerometers) that measure 3-axis angular rate and 3-axis linear acceleration. An IMU contains sensor hardware, electronics, and usually a processor (a computer) that manages calibration and sensor compensation.

INS

Inertial Navigation System. An INS contains an IMU. It also includes a computer that can dead reckon using the IMU's measurements. In general usage, the term "INS" also means anything having to do with inertial navigation.

Baro-Inertial

A baro-inertial system adds a barometric altimeter to an INS to improve the vertical component of navigation.

GPS/INS, or INS/GPS

This means navigation that combines the data of a GPS receiver and an INS. The term GPS/INS or INS/GPS is often a misnomer, because there may only be an IMU within the system.

Strapdown Navigation

Strapdown navigation reduces the angular rates and accelerations sensed by an IMU instrument cluster to geodetic navigation in position, velocity, and attitude. In strapdown navigation, the sensors move with the vehicle instead of being mounted on a stabilized platform.

Receiver Aiding

Receiver aiding further reduces angular rates and accelerations sensed by an IMU to GPS frequency shifts, enabling the GPS receiver to track through greater dynamics and noise.

Satellite Orbital Computations

These solve for the distances to GPS satellites, using ephemeris data modulated onto satellite broadcasts.

Kalman Filtering

The Kalman filter compares strapdown navigation to pseudorange and carrier phase measurements, and watches the discrepancy to recognize the signatures of various error mechanisms. The filter identifies sensor and navigation errors, and applies corrections to keep the system in continual alignment. The Kalman filter runs in real-time, and makes best use of all sensor data accumulated to the present time.

Recursive Smoothing

The recursive smoother runs in post-processing. It extends the Kalman filter by making best use of all recorded sensor data within a mission to solve for each trajectory point within the mission. It is a common mistake to replay a mission by simply reading back a recording of sensor data, and re-running the Kalman filter. That imposes an artificial limitation, because in replay, all the data is in the past. Smoothed trajectories are substantially more accurate.

Floating Versus Steered Receiver Clock

Pseudorange is the time difference between the receiver's clock and the observed satellite clock, times the speed of light. Since the satellite signal takes about 70 milliseconds to travel from satellite to receiver, the satellite clock usually appears delayed, as detected by the receiver. Pseudorange is generally understood to be a measurement of distance, but it is normal and acceptable for pseudorange to be negative if the receiver's clock happens to be running more than 70 milliseconds slow.

It is cost effective to put atomic clocks in GPS satellites, but to use less expensive crystal oscillator clocks in all the thousands of receivers. Since those less-expensive clocks can drift, it is standard practice to combine the data of four or more satellites to solve for the receiver's position, and clock drift. The clock drift is a nuisance parameter, but as a byproduct of the navigation solution, clock drift is known to within about a hundred nanoseconds.

Since clock drift is known, there are two things that can be done using the information. One approach is to let the clock float; Its offset can become large (in milliseconds) but the clock drift is known and predictable. The system may occasionally reset when offset reaches a threshold. The other approach is to steer the receiver's clock by driving the offset to zero. In that case the clock error is small but jittery.

GINI requires that the receiver's clock be allowed to float. That enables the Kalman filter to use the clock as a sensor, and so make best use of all the available GPS measurements. Tightly-coupled systems can operate in poor signal environments where GPS signals are only momentarily and infrequently available, and yet perform almost as well as if the signals were all available continuously.

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