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How does NAVIGATION work?
In order to be able to travel from point A to point B, first of all it is essential to know what direction you should move in and for how long or for how many metres, but prior to this one has to accurately determine:
- where you are
- where you are going
To do this, you need a road book, a trip master (ICO/IMO), maps, a compass and a GPS.
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SCALE BEARING Global Positioning System |
Using a GPS doesn’t mean you don’t need to think or to check an itinerary using other media, such as maps and the road book, or to question the information provided!
GPS... Global Positioning System
HISTORY:
The development of this navigation equipment was directly linked to space exploration and the competition in this field.
On October 4th 1957, the Soviets launched SPUTNIK-I which emitted the famous « Bip-Bip ». American researchers recorded these signals and realised that there was a delay in their frequency due to the DOPPLER effect. After studying this, astrophysicists found the formula that allowed them to make use of the degradation in the frequency of the « Bip-Bip ». They then turned the problem around: if one knows the exact dimensions of the satellite’s orbit, one can deduce the geographical position of a mobile fitted with adequate radio receivers.
In 1973, the Department of Defense (DoD) decided to develop GPS, a new positioning system (NAVigation System with Time And Ranging – Global Positioning System) with the following improvements :
- Position in 3 Dimensions : Latitude, Longitude and Altitude;
- Considerably improved accuracy;
- The system is separate from the speed of the receiver;
- Permanent, continuous, worldwide coverage.
HOW THE NAVSTAR-GPS WORKS:
4 satellites are needed to obtain a position in 3 Dimensions.
The user’s (or the receiver’s) movements are based on the measurement of the variation index between the distances measured. It is therefore the DOPPLER effect that is measured as it affects the signals emitted by the satellites.
The is no INDICATION of the BEARING
or it is completely wrong.
This specificity occurs on all GPS receivers. The minimum speed for obtaining a valid bearing is 0.2 knots, i.e. 370 metres per hour.
To obtain a position in 2 Dimensions (Latitude and Longitude),
at least 3 satellites are needed.
This means once the GPS is switched on, one has to wait at least:
3 x 30 s = 1 min 30 s
before a VALID position is displayed on the screen
whatever brand of GPS is used.
OPERATIONAL POSSIBILITIES OF GPS RECEIVERS:
There are more than 50 GPS receivers on the market and before making such an important purchase, it is a good idea to take a general look at the operational possibilities of the devices, such as:
A – the number of waypoints...
Without being over-impressed by the number quoted, it is an advantage for the GPS receiver to have A LARGE NUMBER OF WAYPOINTS because this allows for greater flexibility in “manufacturing” an itinerary, in particular for land travel. Indeed, depending on the difficulty of the terrain, the itinerary can be marked with waypoints that can be closer together or further apart.
B – the number of itineraries...
Certain GPS units also offer the possibility of creating parallel routes or completely different directions which the user is completely free to choose. All this depends on the total number of waypoints available: in no event can the equipment’s operational possibilities (number of memories) be exceeded.
C - Set Up or Initialisation...
This function allows one to establish the receiver’s initial settings by selecting certain essential reference parameters. The 2 main parameters are as follows:
1 – Measurement units...
- NM = Nautical Mile = approximately 1 852 metres;
- SM = Statute Mile = 1 609 mètres;
- KM = Kilometre. It is preferable to use this unit to have measurements coherent with the mileage in kilometres and/or the TRIPMASTER or TERRATRIP.
2 – Reference North...
Most GPS units allow the choice between two Norths:
Magnetic North. Thanks to the fact that all the magnetic possibilities in the world have been memorised by all satellites and that these are automatically taken into account, magnetic directions can be obtained (bearings, plottings).
True North. This allows for a rapid verification of the road book or any route on a map since the directions are TRUE. The use of true north has the advantage of allowing for TRUE directions indicated by the GPS (itineraries and/or plottings) to be transferred directly to maps without having to use the famous conversion formula.
Road Book...
A road book is a collection of notes that indicates changes in direction, hazards, bearings to follow and WPs… These notes are made by the organiser during his reconnaissance of the course and they must be followed to the letter by competitors. The road book for the following day is issued by the organiser at the overnight camp.
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It comes in the form of a booklet (pages bound at the top) for cars or in a roll (pages glued one after the other) for motorcycles. Each page in the road book comprises several lines (5 or 6) divided into three columns: mileage, direction and observations. A drawing (a diagram) in the direction column indicates any hazards or changes in direction. A dot shows the position of the vehicle in relation to the diagram (in general this is also the mileage marker) and an arrow shows the direction to be taken. A road is shown as two parallel lines; a track is shown as a thick line and off-road is shown as a short dotted line. |
The observation column is used for comments, bearings or GPS coordinates. Several abbreviations are used in this column: DSS (Departure point for Special Stage), CP (Checkpoint),TDSPP (Straight on along the main track), MVS (Bad), HP (Off road), NBX (Numerous), PP// (parallel tracks)... Not easy to begin with!
Preparation...
Before setting off on a special stage it is essential that the road book is properly prepared : check the number of pages and partial distances, the list of any modifications, the colour coding and proof-reading.
This preparation must be carried out in the evening, just after the briefing.
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Checking the number of pages ensures you don’t have a nasty surprise in the middle of a special stage: it occasionally happens that pages are missing, the photocopy is not clear or they are upside-down. If you find a defect in your road book, ask for another one from race headquarters! Markers, partial mileage calculations and part of the layout are done by computer: mistakes in calculating partial distances are very rare; it is not compulsory to check all the partial distances. |
An “opening car”, which is 2 or 3 days ahead of the ride, checks the road book and carries out any last-minute alterations which are passed on
to the race headquarters.
These alterations are displayed at the same time as the road book is distributed and they are explained during the briefing. It is essential to note all the modifications: it sometimes happens that sections of the road book are completely re-worked following a storm, a ford bursting its banks, works…
Trip...
The TripMaster is a re-adjustable mileage meter that can be calibrated. A tripmaster comprises an electronic box, one or two screens (also called displays), a keyboard (to enter data), a re-set button (switch controlled by foot, from the handlebars or manually) and a mileage captor (also known as a probe).
The box...
The box houses the electronic apparatus, the screen(s) and the tripmaster controls. The electronic parts are responsible for converting the signals emitted by the probe into mileage data and displaying them on the Tripmaster screen.
The mileage probe...
The mileage probe sends electric impulses to the tripmaster ; these impulses are converted into distances after the calibration phase.