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2-D Operation
2-D operation requires a minimum of 3 satellites. It provides horizontal coordinates but will not provide elevation.
3-D Operation
3-D operation requires a minimum of 4 satellites. It provides both horizontal coordinates and elevation.
12 Channel Parallel Acquisition
The GPS receiver can simultaneously search for 12 of the 24 GPS satellites.
18 Channel Parallel Acquisition
The GPS receiver can simultaneously search for 18 of the 24 GPS satellites. This will allow the GPS receiver to find the available satellites faster.
24 Channel Parallel Acquisition
The GPS receiver can simultaneously search for 12 of the 24 GPS satellites by using two 12 Channel GPS receivers each tuned to either the L1 or L2 carrier frequencies. These GPS receivers are used for RealTime Kinematic (RTK) applications such as surveying and machine control that require 1cm real time accuracy.
Acquisition Time
How long it takes a GPS receiver to lock onto satellite signals.
Industry standard for all GPS systems to receive satellite signals from the sky based on antenna design.
Almanac Data
Each satellite transmits orbital data for all 24 satellites. This almanac data is accumulated by the GPS receiver, so that it can predict which satellites will be available at any given time. So the next time you turn on your GPS, it will know which satellites to search for.
When the almanac data is stored, it is based on your current location at that time. The almanac data may become invalid if you travel far and then turn on your GPS. This is because the GPS will not find the satellites it was expecting. The almanac data may also become invalid if it has been a long time since you used your GPS (more than 1-2 months). In these cases your GPS will take longer to acquire satellites as it accumulates new almanac data.
Altimeter
Not only does the GPS function provide position, it can also display elevation. Typical accuracy of a handhel GPS in height is half that of the horizontal accuracy, meaning, if your hz is 5 metres then your vertical is approximately 10 to 12 metres. Exception to this with handhelds is if it has a built-in barometer bringing the accuracy down to a few metres.
Anti-Spoofing
Anti-spoofing is a system in which the U.S. Military uses an encrypted satellite transmission (called the Y-code), which is a separate transmission from what civilian GPS uses. This affords 2 advantages:
# It protects the military signal from being intentionally manipulated (spoofed) by adversaries.
# It allows jamming the standard GPS signal within a desired region. The military can still use its encrypted Y-code, but adversaries are unable to use the GPS system.
The anti-spoofing system is more desirable than Selective Availability (SA), since SA degrades GPS accuracy world-wide. The anti-spoofing system can be limited to a specific region.
As Russia has it's own GPS system called GLONASS and Europe is on the way to building its own GPS system, GALILEO, anti-spoofing tends to be negated.
ASAP Technology (Accelerated Satellite Acquisition Protocol)
This is an exclusive technique used in Cobra GPS receivers that allows the GPS unit to quickly lock onto multiple satellites, allowing for faster acquisition.
Atomic Clock
An atomic clock is an extremely accurate timepiece, using the element cesium or rubidium to maintain an accuracy within one second in one million years. Each GPS satellite has multiple atomic clocks on-board to maintain accurate timing. When your GPS receiver locks onto satellites, it synchronizes to the atomic clocks. This, in effect, is like having an atomic clock built into your GPS! Your GPS will keep perfect time.
Automatic Locate (in regards to acquisition time)
In Automatic Locate mode, your GPS may not have any almanac data stored which would help it find satellites. So the GPS must spend extra time finding which satellites are available. The GPS will be in automatic locate mode when you use it for the first time, before it has stored any almanac data. The GPS will also default to automatic locate mode if the almanac data is no longer valid (see almanac data for details).
This presents the longest acquisition time, which takes approximately 50 seconds. The other startup modes are Cold Start and Warm Start.
Bearing
Bearing refers to compass direction. Starting with north at zero degrees, bearing is measured clockwise through 360 degrees. Hence north = 0°, east = 90°, south = 180°, and west = 270°.
Cold Start (in regards to acquisition time)
If it has been longer than 4 hours since you last used your GPS, the stored ephemeris data is no longer valid. The GPS will default to cold start mode, where it takes extra time to acquire new ephemeris data (see ephemeris data for details).
In cold start mode, it will take approximately 35 seconds to acquire satellites. The other startup modes are Automatic Locate and Warm Start.
Control Segment
The Control Segment of the GPS system consists of five ground-based control stations. The control stations monitor and control the GPS satellites, routinely uplinking correction information to maintain system accuracy.
The other segments of the GPS system are the Space Segment and the User Segment.
Coordinates
Coordinates refer to a specific position, typically in terms of latitude and longitude.
Datum - See Map Datum.
Differential GPS (DGPS)
Differential GPS is a special system that increases the precision of GPS receivers. It consists of a special transmitter in a precise location, which then transmits a correction signal. A special receiver is required, which connects to your GPS unit. The GPS unit can then make use of the correction signal to provide increased accuracy. DGPS transmitters are typically maintained by the government, and are placed in areas that have a special need for precision. As an example, the Australian Maritime Services maintains a system of several DGPS sites placed along waterways to provide improved marine navigation.
Current DGPS sites cover only a small fraction of Australia. --- limited to areas that have a special need for the increased precision. .
Important: Click on link below for coverage within Australia. Please note that the picture does not show coverage inland but if you complete the radius you will get an approximate guide to inland coverage. However, due to factors such as forest and variations in land topography you may have areas not covered by the transmitters with the radius of coverage.
Ephemeris Data
Each satellite transmits its own ephemeris data, which contains positioning information that your GPS uses to calculate your position.
Ephemeris data is good for about 4 hours. If it has been within 4 hours since you last used your GPS, it will be ready to navigate very quickly. If it has been longer than 4 hours, then it will take extra time to accumulate new ephemeris data.
Geostationary (Geosynchronous)
A geostationary satellite remains at a fixed position over the earth. For example the satellites used for satellite television. The receiving dish for satellite TV is aimed at a particular spot, which does not change.
The GPS satellites are not in such a geosynchronous orbit. Each GPS satellite orbits the earth twice within a 24 hour period.
Global Positioning System (GPS)
GPS is a satellite-based system that allows you to pinpoint your exact location anywhere on the planet. The system consists of 24 satellites orbiting above the Earth. Using a GPS receiver, the satellites allow you to determine your location with great precision. Originally developed for military use, the GPS satellite system has been opened to the general public. The GPS system is maintained by the U.S. government with U.S taxpayer dollars, and there is no charge to access the system with a GPS receiver.
The GPS system consists of three segments: the Control Segment, the Space Segment, and the User Segment.
Grid
A mapping system that projects the surface of the earth onto a flat surface (such as a paper map), using square zones for position measurements.
Ionosphere
The ionosphere is an upper part of the atmosphere extending from about 30 miles to about 620 miles above the earth's surface. Ultraviolet radiation from the sun ionizes gas molecules in the ionosphere. The level of ionization varies depending on solar activity. This ionized band of gas exerts an influence on the propagation of radio signals as they pass through.
Ionospheric Delay
As a satellite signal passes through the ionosphere, it slows down a very small amount. The resulting ionospheric delay is a source of error for GPS units. The amount of delay will vary with the level of solar activity, and is also dependant on the direction of the satellite (straight up vs. towards the horizon).
L-Band
The L-Band consists of frequencies between 390MHz and 1550MHz, typically used for satellite communications. The GPS satellite signals fall within the L-Band.
Magnetic North
A simple compass points to the magnetic north pole. However the magnetic north pole is not the same as the true geographic north pole. The magnetic north pole is located somewhere in the northernmost reaches of Canada. Actually, “somewhere” is a good term since the magnetic north pole moves over time.
If you are comparing your Cobra GPS to a magnetic compass or to magnetic compass headings, then set the North Reference to Magnetic. Otherwise, leave it on the default setting of True.
Map Datum
We all know the earth is not flat. But neither is it round. In reality, the earth is flattened at the poles, into a shape called an “ellipsoid”. A map datum is a mathematical model of this ellipsoid shape, which then allows a system for placement of latitude and longitude lines as well as for calculations (such as surface area). The idea is to represent the true shape of the earth’s surface as accurately as possible. Highly accurate navigational charts are based on map datums.
Over the generations, our knowledge of the earth’s shape has become more precise. New, more accurate datums have replaced older ones. Whenever a navigational chart is based on a new datum, every position on the map will be slightly shifted as compared to older charts.
In addition, different countries have created their own map datums. If you compare 2 maps that are based on different datums, you will find that they do not exactly match.
The map datum is important when comparing your GPS to such a navigational chart. For instance when entering latitude and longitude from the chart into your GPS. You must first make sure your GPS unit is set to the same datum as specified on the chart, otherwise there will be an error.
Please note that this does not apply to a street map or a road atlas. Map datums are only used with highly accurate navigational charts. On such charts the map datum will be specified in the legend.
Multi-Channel Receiver - See Parallel-Channel Receiver.
Signal multi-path occurs when the satellite signal reflects off a surface before reaching the GPS receiver. For example signal reflection off of tall buildings. This adds a small timing delay, which introduces a measurement error.
NAVSTAR (Navigation Satellite Timing and Ranging)
NAVSTAR is an alternate name for the GPS system. “NAVSTAR” and “GPS” refer to the same system.
Parallel-Channel Receiver (Multi-Channel Receiver/Parallel Tracking)
A parallel-channel GPS receiver incorporates dedicated receiver electronics for each channel, and can therefore simultaneously track multiple satellite signals. This is in contrast to a multiplexing receiver, which must receive signals one-at-a-time. A parallel receiver is therefore mush faster. GPS units incorporate 8/12/14/18 or 24 parallel channels, which provides exceptionally quick access time. Originally GPS engines had one channel and had to multiplex to find the minimum 3 satellites and acquire their almanacs and position data. This meant extremely slow first fix and extremely slow position updates. Cold starts typically were 15 minutes whilst cold starts in todays parallel channel receivers are typically 4 minutes.
POI (Point-Of-Interest)
Popular destinations (such as restaurants, museums, etc.) are called "Points-of-Interest" (POI). These points of interest are represented by icons in the mapping software. Clicking on one of these icons will allow you to download it into your mapping GPS.
Post-Processing
In post-processing, the GPS unit stores data which can then be downloaded for processing at a later time. The post-processing may involve applying correction information to obtain a more precise record of the locations that were visited. An example of this is when better accuracy than 5 metres is required. Often Post-Processing provides centimetre accuracy, however, specialised GPS units and not the standard Garmin, Magellan and Lowrance type units can do this. The idea is to place a GPS at a position for typically 20 minutes and store the position data to be taken back to the office where specialised PC software uses that data plus other but corrected data logged during the same period. The uncorrected data from the GPS in the field has the corrections applied and thus provide centimetre accuracy. Other methods of obtaining centimetre accuracy are available, either through alternative to the above or what is known as Real-Time Kinematic, or RTK for short.
Real-Time Kinematic (RTK)
RTK uses a base station GPS on a known point (if required to use a specific Map Datum) and a roving GPS. Usually a base station is a unit that sits on a tripod and placed above a marker used by surveyors. This marker is part of a national grid system such as AGD84 or GDA90. A rover is a GPS on a pole with the means to receive corrections from the base via radio link. The base being on a known point receives data from the satellites which are inherantly inaccurate 6 metres 95% of the time. It calculates the difference between the satellite data and its own fixed position and the difference is then used as the correcting data to the rover. Because the rover is receiving the same data from the same satellites as the base station the corrections allow for the centimetre accuracy. Typical accuracy in hz is 1cm whilst vertical is 2cm.
Route
A “route” is created from previously stored waypoints. It is nothing more than a list of waypoints. Your GPS will direct you to each waypoint in sequence. Once you reach a waypoint, the GPS unit will automatically direct you to the next waypoint in the route. The waypoints in a route are chosen by the user, thus, if you have waypoints from 1 to 10 you can program the GPS to only use odd number waypoints or miss just 1 waypoint. Most GPS units only allow 50 waypoints in every route and often allow up to 100 routes, depending on manufacturer and model.
Selective Availability (SA)
Selective Availability was incorporated into the GPS satellite system in March of 1990 to intentionally degrade the accuracy of GPS receivers. The purpose was to prevent hostile governments from using the high precision of the GPS system. Authorized branches of the U.S. government had special GPS receivers that could fully decode the satellite signals to allow full accuracy.
Since SA adversely affected GPS accuracy world-wide, the decision was made to deactivate SA beginning May 2000. This allowed higher accuracy for all GPS receivers, to better serve civilian needs.
The U.S. military has since pursued Anti-Spoofing technology, which allows accuracy degradation within a specific region
Space Segment
The Space Segment of the GPS system consists of the 24 satellites which orbit the earth, and also includes the signals they transmit.
The other segments of the GPS system are the Control Segment and the User Segment.
Track
A “track” saves your path from start to finish. You can then retrace your steps back to the starting point. This is in contrast to a waypoint, which would bring you back in a straight line. A track allows you to retrace your exact path. It is also known as backtrack. The track is usually automatically created by the GPS and can be set to stop when memory is full or over write the data progressively.
True North - See Magnetic North.
UTC (Coordinated Universal Time)
Previously known as GMT (Greenwich Mean Time), UTC is simply a government-maintained time standard. In the U.S., UTC is maintained by the National Institute of Standards and Technology (NIST). Be aware that UTC is not exactly in alignment of GMT, it is apparently shifted by only a few hundred metres. However, such difference doesn't affect GPS accuracy as paper maps have the GPS UTC format as standard and in some cases may show the GMT lats and longs.
User Segment
The User Segment of the GPS system consists of user equipment (GPS units and accessories), and any related software applications.
The other segments of the GPS system are the Control Segment and the Space Segment.
Waypoint
A “waypoint” is simply the coordinates of a specific location. You can save your current location as a waypoint, so that you can return to the same spot at a later date. You can also manually enter the coordinates of a waypoint and have your GPS take you there.
Example: You are camping, and plan to go hiking through the woods. You can mark your current location (marking a waypoint). It will then be easy to find your way back to the campsite. However, this will only guide you back as the crow flys whilst Track will guide you back via the same trail you took.
WAAS (Wide Area Augmentation System - not available in Australia, please see DGPS)
WAAS was implemented by the FAA to provide improved GPS navigation for aircraft. The WAAS system consists of 25 ground-based transmitters located throughout the U.S., and 2 satellites in geostationary orbit (one over the east coast and one over the west coast).
The precise location of each ground transmitter is maintained, and each location is compared to GPS distance measurements. Corrections are uplinked and broadcast via satellite. Compatible GPS units can receive this extra satellite signal, which provides significantly improved accuracy (down to 3 metres or 10 feet). All recreational GPS models are compatible with WAAS, however WAAS is mainly only available in USA, however a similar system may eventuate via a satellite system being produced by Japan. The only other option to improve accuracy (a free version is only along some coastline of Australia and partly inland near the transmitters) is DGPS, which provides approximately 1 to 3 metre accuracy. A special differential receiver is required for this. To cover the whole of Australia and or overseas is a paid subscription service.
Warm Start
(In regards to acquisition time) If it is within 4 hours since you last used your GPS, then all stored satellite data is still valid. In this case the GPS will default to Warm Start mode, where it will acquire satellites very quickly (approximately 10 seconds). The other startup modes are Automatic Locate and Cold Start.