1 Glossary and GPS Case Studies

2 Anywhere: The ability of a receiver to start position calculations without being given an approximate location and approximate time. Bandwidth: The range of frequencies in a signal. C/A code: The standard (Course/Acquisition) GPS code. A sequence of 1023 pseudo-random, binary, biphase modulations on the GPS carrier at a chip rate of MHz. Also known as the "civilian code." Carrier: A signal that can be varied from a known reference by modulation. Carrier-aided Tracking: A signal processing strategy that uses the GPS carrier signal to achieve an exact lock on the pseudo random code.

3 Carrier frequency: The frequency of the unmodulated fundamental output of a radio transmitter. Carrier phase GPS: GPS measurements based on the L1 or L2 carrier signal. Channel: Channel of a GPS receiver consists of the circuitry necessary to receive the signal from a single GPS satellite. Chip: The transition time for individual bits in the pseudo-random sequence. Also, an integrated circuit. Also a snack food. Also a betting marker. Clock bias: The difference between the clock’s indicated time and true universal time. Code phase GPS: GPS measurements based on the pseudo random code (C/A or P) as opposed to the carrier of that code.

4 Control segment: A world-wide network of GPS monitor and control stations that ensure the accuracy of satellite positions and their clocks. Cycle slip: A discontinuity in the measured carrier beat phase resulting from a temporary loss of lock in the carrier tracking loop of a GPS receiver. Data message: A message included in the GPS signal which reports the satellite's location, clock corrections and health. Included is rough information on the other satellites in the constellation. Differential positioning: Accurate measurement of the relative positions of two receivers tracking the same GPS signals.

5 Dilution of Precision: The multiplicative factor that modifies ranging error. It is caused solely by the geometry between the user and his set of satellites. Dithering: The introduction of digital noise. This is the process the DoD uses to add inaccuracy to GPS signals to induce Selective Availability. Doppler-aiding: A signal processing strategy that uses a measured doppler shift to help the receiver smoothly track the GPS signal. Allows more precise velocity and position measurement. Doppler shift: The apparent change in the frequency of a signal caused by the relative motion of the transmitter and receiver. Ephemeris: The predictions of current satellite position that are transmitted to the user in the data message.

6 Fast switching channel: A single channel which rapidly samples a number of satellite ranges. "Fast" means that the switching time is sufficiently fast (2 to 5 milliseconds) to recover the data message. Frequency band:A particular range of frequencies. Frequency spectrum: The distribution of signal amplitudes as a function of frequency. Hardover word: The word in the GPS message that contains synchronization information for the transfer of tracking from the C/A to P code. Ionosphere: The band of charged particles 80 to 120 miles above the Earth’s surface.

7 Ionospheric refraction: The change in the propagation speed of a signal as it passes through the ionosphere. L-band: The group of radio frequencies extending from 390 MHz to 1550 MHz. The GPS carrier frequencies ( MHz and MHz) are in the L band. Multipath error: Errors caused by the interference of a signal that has reached the receiver antenna by two or more different paths. Usually caused by one path being bounced or reflected. Multi-channel receiver: A GPS receiver that can simultaneously track more than one satellite signal. Multiplexing channel: A channel of a GPS receiver that can be sequenced through a number of satellite signals.

8 P-code: The Precise code. A very long sequence of pseudo random binary biphase modulations on the GPS carrier at a chip rate of MHz which repeats about every 267 days. Each one week segment of this code is unique to one GPS satellite and is reset each week. Precise Positioning Service (PPS): The most accurate dynamic positioning possible with standard GPS, based on the dual frequency P-code and no SA. Pseudolite: A ground-based differential GPS receiver which transmits a signal like that of an actual GPS satellite, and can be used for ranging. Pseudo random code: A signal with random noise-like properties. It is a very complicated but repeating pattern of 1's and O's.

9 Pseudorange: A distance measurement based on the correlation of a satellite transmitted code and the local receiver's reference code, that has not been corrected for errors in synchronization between the transmitter's clock and the receiver’s clock. Real-time differential GPS: A base station which computes, formats, and transmits corrections usually through some sort of data link (e.g. VHF radio or cellular telephone) with each new GPS observation. The roving unit requires some sort of data link receiving equipment to receive the transmitted GPS corrections and get them into the GPS receiver so they can be applied to its current observations. Real-Time Kinematic (RTK): The DGPS procedure whereby carrier-phase corrections are transmitted in real time from a reference receiver to the user's receiver. RTK is often used for the carrier-phase integer ambiguity resolution approach.

10 Receiver Autonomous Integrity Monitoring (RAIM): A technique by which a GPS receiver system detects incorrect satellite signals by comparing solutions with different sets of satellites. Reference Station: A ground station at a known location used to derive differential corrections. The reference station receiver tracks all satellites in view, computes their pseudoranges, corrects these for errors, and then transmits the corrections to users. Relative Accuracy: The accuracy with which a user can measure position relative to that of another user of the same navigation system at the same time. Relative Positioning: The determination of relative positions between two or more receivers which are simultaneously tracking the same GPS signals.

11 Relative precision: Precision is defined as a measure of the tendency of a set of numbers to cluster about a number determined by the set (e.g. the mean). The usual measure is the standard deviation with respect to the mean. Relative precision denotes the tendency for the various components (X, Y, Z) between one station and other stations in the network to be clustered about the adjusted values. Current custom is to express relative precision at the two- standard deviation (95% confidence) level. This may be stated in terms of a relative error ellipse or as a proportion of the separation distance (e.g. 10 ppm or 1:100,000). Reliability: The ability to perform a specific function without failure under specified conditions for a given length of time.

12 Satellite constellation: The arrangement in space of a set of satellites. 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 Slow switching channel: A sequencing GPS receiver channel that switches too slowly to allow the continuous recovery of the data message. Space segment: The part of the whole GPS system that is in space, i.e. the satellites.

13 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. Standard Positioning Service (SPS): The normal civilian positioning accuracy obtained by using the single frequency C/A code. Static positioning: Location determination when the receiver's antenna is presumed to be stationary on the Earth. This allows the use of various averaging techniques that improve accuracy by factors of over 1000.

14 Three-Dimensional Coverage: The number of hours-per-day when four or more satellites are available with acceptable positioning geometry. Four visible satellites are required to determine location and altitude. Three-Dimensional (3D) Navigation: Navigation mode in which altitude and horizontal position are determined from satellite range measurements. Time-To-First-Fix (TTFF): The actual time required by a GPS receiver to achieve a position solution. This specification will vary with the operating state of the receiver, the length of time since the last position fix, the location of the last fix, and the specific receiver design. Track: A planned or intended horizontal path of travel with respect to the Earth rather than the air or water. The track is expressed in degrees from 000° clockwise through 360° (true, magnetic, or grid).

15 User interface: The way a receiver conveys information to the person using it. The controls and displays. User segment: The part of the whole GPS system that includes the receivers of GPS signals. Update Rate: The GPS receiver specification which indicates the solution rate provided by the receiver when operating normally. Universal Time Coordinated (UTC): Solar time in the Greenwich Mean Time (GMT) time zone but only ever adjusted in one-second increments. Because the GPS time is different to UTC a correction factor is applied to the received time that is reported by GPS receivers in UTC.

16 World Geodetic System 1984 (WGS84): An ellipsoid designed to fit the shape of the entire Earth as well as possible with a single ellipsoid. It is often used as a reference on a worldwide basis, while other ellipsoids are used locally to provide a better fit to the Earth in a local region. GPS uses the centre of the WGS-84 ellipsoid as the centre of the GPS ECEF reference frame. Waypoint: A reference point on a track. Wide Lane: A particular integer ambiguity value on one carrier phase range measurement or double difference carrier phase observation when the difference of the L1 and L2 measurements is used. It is a carrier phase observable formed by subtracting L2 from L1 carrier phase data: Φ' = Φ1 - Φ2. The corresponding wavelength is 86.2 cm Y-Code: An encrypted form of P-Code. Satellites transmit Y-Code in replace of P-Code when Anti-Spoofing is in effect.

17 DGPS for Precision Farming Describe how GPS technology helps farmers examine fields for crop yield, crop damage and how it assists in mapping soil conditions. See:

18 DGPS for Weed Control Describe how GPS technology is used for weed control?

19 DGPS for Train Control

20 DGPS for Aviation

21 DGPS for Car Navigation

22 GPS for Animal Migration and Population Studies

23 GPS for Construction: Tunnels, Golf Courses, Roads, etc.