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Training

Currently, registrations are requested via the member's listserve up to 8 weeks prior to an event. To register, reply to the Wellington Land SAR training co-ordinators.

However, by the end of May 2006, members should be able to register through this website.

If members identify training needs which have not been catered for in the current training schedule, then this should be discussed with the Wellington Land SAR training co-ordinator .

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GPS

Intro

GPS stands for Global Positioning System. As the name suggests, a GPS can provide you with a position reference. WLSAR provides its team members with Garmin 12's, and Garmin 76's. Use the links below to obtain user information for these machines.

Required skill sets

WLSAR team members must have the following minimum GPS skill-set:

Read a grid co-ordinate and communicate your position back to base,
Enter waypoints (co-ordinates and description),
Use the 'GoTo' function,
Use the 'Mark' function,
Know how to clear old track route information,
Know how to clear old Waypoint information.

Standard operational procedures

Upon receipt of GPS from logistics, GPS users must:
1. Check that GPS has fresh batteries and can acquire satellites, and
2. Clear old track route information, and
3. Check waypoint information. If GPS contains old waypoint info, clear it.
Unless told otherwise, keep GPS on at all times.
Attach GPS to pack strap or sturdy part of clothing. This should prevent unit from being lost in bush or water.
Use notebook to record relevant information associated with clues or other points recorded in waypoint list.
Upon return to base from tasking, ensure that GPS info is downloaded. Keep notebook handy for debriefing.
When using a non-WLSAR type of GPS, ensure that you can provide a compatible download cable for debriefing and intel/planning purposes.

More Information

Links to the Garmin web site:

PDF Documents from Garmin:

What is GPS and how does it work?

System introduced in the 70s by the US military.
Comprises 24 satellites in 12-hour orbits at about 20,000 km altitude.
Arranged in 6 orbital planes, each with 4 satellites.
Aims to provide visibility of 6 to 11 satellites from anywhere on the earth.
Each satellite transmits a unique code signal.
Every satellite's exact position is known by the system at all times. This data (the almanac) is built into each GPS receiver.
Each satellite has an atomic clock for precise measurement of time it transmits signals.
When your GPS locks on to a satellite, it compares the code signal received against one generated in the receiver, and calculates the exact distance from the satellite by the length of time taken to receive the signal.
Can pin-point your position to somewhere on a sphere whose surface is that distance from the satellite
When it locks onto a second satellite, it similarly knows you are somewhere on a sphere centred on the 2nd satellite.
Can now narrow down your position to the places where the two spheres intersect.
The intersection of two spheres traces out a circle, so your position is somewhere on that circle.
When the GPS lock onto a third satellite, it traces another sphere of your possible locations.
Can now narrow down your position to the places where the circle and the 3rd sphere intersect.
There are only two possible locations.
When you lock onto a fourth satellite, it eliminates one of those locations.
Has now pin-pointed your position, at least to within 100m, more probably to within a few metres, and possibly to within a few centimetres.
Therefore you need a minimum of four satellites located by your GPS before you can use it to determine where you are and navigate with it.
More satellites enable greater accuracy, and 3-dimensional navigation (determination of altitude)

The GPS Receiver

The principle components of a GPS receiver system are:

antenna
receiver
processor
I/O device

The basic operations of a GPS receiver are:

Satellite selection:: Selects the best four satellites. Those that are below the horizon, or soon will be, are excluded and, if possible, any that form a poor geometrical solution are also excluded.
Signal acquisition:: Matches the code sequence, in turn, for each of the satellites it can hear, and identifies individual satellites.
Tracking:: Compares data received from different positions (waypoints) and plots the position and movement of the receiver.
Measurement:: Measures the distance between waypoints, speed of travel, estimated arrival time at destination waypoint, etc.
Data recovery:: If the data in the GPS is old, the receiver collects new data from the satellites that it can 'hear'. This may delay its selection of satellites.
Corrections:: Accounts for the slight discrepancies between satellite signals; reviews the best selection of satellites as either the satellite or the receiver moves.

Receiving Satellite Signals

The opening page of the GPS shows the locations of satellites that would be visible to your GPS if you were on flat open ground. Topography, vegetation, and structures may hide some satellites. Moving your position may enable you to hear particular satellites. Face north to avoid body shielding GPS from satellites (There are no satellites in the south). Satellites directly overhead give best reading of altitude. Satellites near the horizon give best reading of position. Satellites nearer than 15o to the horizon give distortion.

Datum, Projections and Maps

A Datum mathematically defines the shape, size, and position of an ellipsoid used to represent the surface of the earth (approximately Mean Sea Level). The universal global datum is WGS84. Regional areas may have a specialised local datum, to account for local aberrations in the shape of the earth. Currently, New Zealand is transitioning from an old datum, NZGD49, to a new datum, NZGD2000. The current topographical maps (NZMS260) are based on NZGD49. New series of maps will be based on NZGD2000. Click this link for more information from LINZ.

Maps are representations of a curved surface (the earth) as a flat surface. A Projection is a mathematical formula that projects points from the curved surface to the flat surface. Some distortion occurs. A range of projection types are used to reduce distortion depending on shape of the country and the position on the globe. Main types are Mercator, Transverse Mercator, and Conic projections.

NZ topographical maps use NZMG, a mathematical formula that reduces distortion by making a constant scale factor along the country's backbone. The correct projection and datum, as stated on your map, are critical in GPS use. GPS data input and calculation is always in terms of WGS84. Need output (information on the screen) to be in terms of local datum (usually NZGD49) to relate to local maps.

In NZ, a position given by WGS84 is approximately 200m away from the position given by NZGD49. Positions on a datum are given by latitude and longitude; Positions on a projection are given by eastings and northings (as grid references).

Glossary

A glossary of Terms and abbreviations often used in relation to GPS, or by GPS receivers.

ALT: Altitude: Height above sea level.
BRG: Bearing: Direction to a given point.
CDI: Course Deviation Indicator: A bar graph indicator of how far you are on or off course.
DATUM: A surveying model of the elliptical nature of the earth for a specific geographical area.
DGPS: Differential Global Positioning System.
DOP: Dilution of Precision: A measure of inaccuracy caused by position of satellites relative to your position (Also known as GDOP or Geometric Dilution of Precision).
DIS: Distance: Distance to a given point.
DNT: Count Down Timer.
ETA: Estimated Time of Arrival.
ETE: Estimated Time En-route.
FIX: Sometimes referred to as current position. Sometimes a reference to the receivers position quality.
GPS: Global Positioning System.
GS: Ground Speed: Actual Speed over ground.
PCF: Position Correction Factor: A correction offset specific to a map or chart.
PDOP: Position Dilution of Precision: An indicator of the overall inaccuracy of a position due to a number if factors.
POSITION: Present Location as determined by the GPS receiver.
PROJECTION: A surveying model to project the curved earth onto a map for a specific geographical area.
ROUTE: A series of waypoints linked together in a sequence.
SA: Selective Availability: The deliberate error introduced into the GPS signal by the US Military.
TRK: Track: The direction that you are travelling in at this instant.
TTG: Time To Go: Time remaining at your present speed until you reach a given point.
UPT: Count Up Timer.
VMG: Velocity Made Good: Your ground speed toward a given point.
WPT: Waypoint: A stored position.
XTK: Cross Track Error: The distance left or right off of desired line of travel.

History

GPS was introduced in the 70s by the US military and comprises 24 satellites in 12-hour orbits around the earth at an altitude of about 20,000 km. These satellites are arranged in 6 orbital planes, each with 4 satellites. This aims to provide visibility of 6 to 11 satellites from anywhere on the earth at any given moment in time.

Every satellite's exact position is known by the system at all times. They all have an atomic clock for precise measurement of time it receives / transmits signals. When your GPS locks on to a satellite, it calculates the exact distance from the satellite by the length of time taken to receive the signal. This pin-point your position to somewhere on a sphere whose surface is that distance from the satellite.

When you lock onto a second satellite, it similarly knows you are somewhere on a sphere centered on the 2nd satellite. You can now narrow down your position to the places where the two spheres intersect. The intersection of two spheres traces out a circle, so your position is somewhere on that circle.

When you lock onto a third satellite, it traces another sphere of your possible locations. You can now narrow down your position to the places where the circle and the 3rd sphere intersect. There are now only two possible locations.

When you lock onto a fourth satellite, it eliminates one of those locations. The GPS has now pin-pointed your position, at least to within 100m, more probably to within a few meters, and possibly to within a few centimeters. Therefore you need a minimum of four satellites located by your GPS before you can use it to determine where you are and navigate with it.

More satellites enable greater accuracy, and 3-dimensional navigation (determination of altitude). The opening page of the GPS shows the locations of satellites that would be visible to your GPS if you were on flat open ground. Topography, vegetation, and structures may hide some satellites so moving your position may enable you to 'see' particular satellites.

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This page last modified December 7, 2007