GPS Units

How GPS Works in a Nutshell

• Pseudo-random Code – A unique I.D. code in a high FM frequency which resembles noise signals. This code is used to identify which satellite is currently transmitting and is also used to synchronise the atomic clock on the satellite to the quartz clock on the GPS receiver.


• Almanac Data – This data is periodically transmitted to the GPS receiver. It tells the receiver the approximate locations of the satellites, the status and the current date and time. Using almanac data saves time by letting the receiver skip looking for satellites that are below the horizon. GPS satellites include almanac data in the signals they transmit to GPS receivers. Although variations in satellite orbits can accumulate with time, almanac data doesn't need to be highly accurate to be useful. The data that is collected before your receiver was last switched off may remain usable for weeks or months.


• Ephemeris Data – This positioning data is much more accurate and is monitored and uploaded to the satellites by the US Department of Defence’s ground stations. This in turn comes back down to the GPS receiver and provides an updated position of each GPS satellite and where it should be at any time throughout the day. To accurately calculate your location, ephemeris data is only usable for a limited time (a few hours or less). Up-to-date data is needed to minimize error that results from minor variations in a satellite's orbit.

Basic GPS Handheld Units

• Position in the format you choose


• Altitude above sea level (although, altitude will not be as accurate as position)


• Compass heading


• Accurate time in 12 and 24 hour mode


• Satellite status, location and signal strength


• Provide calculations such as the total distance traveled


• Functionalities such as: record path as set of waypoints, navigation of routes and step retracing

Map-based GPS Handheld Units

• They are great for hiking, bushwalking and other outdoor activities


• Plan your route on a computer or laptop at home and then upload the waypoints and plots to the handheld unit.


• They are extremely portable and usually ruggedly built


• Many hold SD (Secure Digital) cards and Compact Flash (CF) cards for additional memory.

External Antenna

PC Cable with cigarette lighter plug

In-car mounting bracket

SD Memory Cards

GPS:- Global Positioning System. A satellite-based navigation system consisting of a network of orbiting satellites in space. These satellites are consistently moving. One of the biggest advantages is that GPS receivers provide accurate world wide position fixes 24hrs a day in all weather conditions. For many people, simply knowing position is sufficient, but the real power of GPS technology is the ability to direct you TO a destination.

Waypoint:- The term for a co-ordinate identifying a known position. GPS users can enter a Waypoint into their GPS unit and the GPS will tell you the distance and direction to it, time to go until you reach your destination and other functions depending on your unit. Most, but not all GPS units, allow you to store the Waypoint as a name instead of a number, some allow you different colour and shape symbols.

Route:- A route is a series of waypoints that allow you to navigate from one Waypoint to another in that particular order. When you arrive at your first destination, the GPS automatically advises your course and distance to your next destination. The ability to store routes is very helpful. Most GPS units 20 routes and some even have reversible routes, great for when you are ready to come home along the same route.

Geodetic Datum:- If your Waypoint positions are not where you think they should be it is most likely caused by a datum mismatch. Datums are complicated so I shan't go into it here. Putting it simply, a Datum is essentially a measurement system - it describes the size and shape of the earth. Coordinate systems are relative to the datum. Both the GPS receiver and the map in use can be set to use various datum types, however a datum mismatch can result in position errors. Typical map datums for GPS users are the World Geodetic System 1984 (WGS-84), Australian Geodetic Datum 1966 (AGD66), and the Geodetic Datum of Australia 94.

Georeferencing:- Refers to the manner by which locations in digital map images are related to actual earth-surface positions. Digital maps with georeferencing means they are calibrated to enable pointing to any map position with a cursor and obtaining an accurate grid coordinate. Maps with georeferencing are of great benefit to drivers because they can be automatically interfaced to a GPS receiver to set Waypoints, Routes, etc.