Hvorfor sendes AIS på to kanaler

[2nd Trikker'n]

En AIS Frame = 1 minutt og inneholder 2250 slots. En slot er en melding. Det er plass til mange båter på en frekvens det.

 

Så - hvorfor to frekvenser?

[TrondT]

Lytter på ene mens sender på andre, dermed øker man sikerheten for at alle hører alle uten utjamming...?

Vet jeg har lest forklaringen et eller anet sted, men finne den igjen

 

For å få inn all informasjon fra alle transpondere må en AIS lytte i 6 minutter, så slot'ene er nok i bruk alle sammen

 

Fra WIKI:

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Broadcast information

 

AIS transceiver sends the following data every 2 to 10 seconds depending on vessels speed while underway, and every 3 minutes while vessel is at anchor. This data includes:

 

* The vessel's Maritime Mobile Service Identity (MMSI) - a unique nine digit identification number.

* Navigation status - "at anchor", "under way using engine(s)", "not under command", etc

* Rate of turn - right or left, 0 to 720 degrees per minute

* Speed over ground - 0.1-knot (0.19 km/h) resolution from 0 to 102 knots (189 km/h)

* Position accuracy:

o Longitude - to 1/10000 minute

o Latitude - to 1/10000 minute

* Course over ground - relative to true north to 0.1 degree

* True Heading - 0 to 359 degrees from eg. gyro compass

* Time stamp - UTC time accurate to nearest second when this data was generated

 

In addition, the following data is broadcast every 6 minutes:

 

* IMO ship identification number - a seven digit number that remains unchanged upon transfer of the ship's registration to another country

* Radio call sign - international radio call sign, up to seven characters, assigned to the vessel by its country of registry

* Name - 20 characters to represent the name of the vessel

* Type of ship/cargo

* Dimensions of ship - to nearest meter

* Location of positioning system's (eg. GPS) antenna onboard the vessel

* Type of positioning system - such as GPS, DGPS or LORAN-C

* Draught of ship - 0.1 meter to 25.5 meters

* Destination - max 20 characters

* ETA (estimated time of arrival) at destination - UTC month/date hour:minute

>

Hvis man så legger inn at man skal få all denne informasjonen fra alle andre innenfor feks en radius på 30NM så er det mye informasjon som skal ut.

 

TrondT

[2nd Trikker'n]

Takk for svaret, men det hjelpermeg ikke.

Det jeg finner av info er:

 

5 Frequency band

The AIS mobile station should be designed for operation in the VHF maritime mobile band,etc. etc.....

 

The system should be able to operate on two parallel VHF channels. When the designated AIS

channels are not available the system should be able to select alternative channels using channel

management methods in accordance with this Recommendation.

 

2.1.5 Dual Channel Operation

The transponder should be capable of operating on two parallel channels in accordance with § 4.1. Two separate TDMA receivers should be used to simultaneously receive information on two independent frequency channels. One TDMA transmitter should be used to alternate TDMA transmissions on two independent frequency channels.

Jeg forsøker å finne ut hva "Bånd B" tilfører systemet.

 

 

Vedr. 6 minutter for å få all data:

 

4.2.1 Reporting Rate

The different information types are valid for a different time period and thus need a different update rate.

 

Static information: Every 6 min or, when data has been amended, on request.

Dynamic information: Dependent on speed and course alteration according to Tables 1A and B.

Voyage related information: Every 6 min or, when data has been amended, on request.

Safety related message: As required.

 

Statisk data sendes hvert 6. minutt.

[TrondT]

Fra Shipplotter FAQ:

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What channels does AIS use?

 

AIS uses two frequencies in the Marine VHF band: 161.975 and 162.025 MHz. All ships use both frequencies and successive message transmissions are radiated on alternate channels. Professional AIS equipment monitors both channels simultaneously. This doubles the channel capacity and also combats interference. Monitoring a single channel with ShipPlotter means that you only hear half of the transmissions. Since the transmissions are often made at intervals of only a few seconds, this does not significantly prejudice the topicality of the view obtained. Messages are transmitted at 9600 bits per second using GMSK modulation. Each message is typically only 30ms (1/30 second) long – scarcely more than a click.

>

 

TrondT

[2nd Trikker'n]

Bingo og takker og bukker.

 

Litt flaut at jeg overså dette:

One TDMA transmitter should be used to alternate TDMA transmissions on two independent frequency channels.

Spesiellt ille var det at jeg faktisk inkluderte det i det ene sitatet mitt.