1. Shifting the frequency
The
first of the different actions to encourage manufacturers
to spread frequencies over the entire Argos-1 bandwidth
started two years ago. Since the beginning of 1999,
representatives from CLS, SAI and Cubic-I visited all of
the major transmitter manufacturers. These visits were
followed by a global meeting in September 1999 where once
again the importance of frequency spreading was stressed.
This communication process was followed by the Argos
International Users Conference in April 2001 with a
complete presentation of results regarding the improvement
of data due to the shift of transmission
frequency.
The processing of the frequencies
received from PTT's over the past two and a half
years shows that, little by little, the manufacturers are
spreading the frequencies out. Some manufacturers such as
Microwave Telemetry (USA) or Serpe IESM (FR) are already
taking advantage of these results either to track animals
over Europe even with very low output power (250 mWatt or
even lower), or to track vessels when they are fishing in
large groups close together.
2. The Argos-2 generation satellites
Since May
1998, NOAA has launched satellites with the second generation
of Argos instrument. Advantages of using the Argos-2 system
include:
- Quality of the transmission link is globally enhanced
because of the greater sensitivity of the Argos receiver (an
increase of 3 dBm, higher number of Data Processing Units
(8 vs. 4 for Argos-1) and larger receiver bandwidth
(80 kHz instead of 24).
- Because there will be fewer (none today) transmitters in
these Argos-2 channels the competition between messages
arriving at the antenna is dramatically reduced. Hence many
more messages are received with a better quality.
- This benefit will even be greater for low-power
transmitters which are significantly affected by competition
with higher-power transmissions.
To sum up the situation, the graph below
compares the elementary probability of reception by Argos-1 and
2 satellites vs. the platform density.
This figure
shows that for a defined population of beacons, Argos-2
satellites have a higher probability of reception than Argos-1
satellites .
Today 2 satellites from this second
generation (NOAA 15 and 16) are flown and used by
NOAA to make the "operational" system. Three more satellites
complete this constellation for those who are working in
"Multisat" service (NOAA 14 launched in December 1994
with only real time service, NOAA 12 launched in May 1991 and
part of NOAA 11 launched in September 1988 with only global
processing).
3. The evolution of the
constellation in 2002
A new NOAA satellite, NOAA M (17), will be
launched in Mid 2002. It will carry the second generation of
Argos instruments. At that time, and due to the great age of
NOAA 11, it is likely that NOAA modify the constellation as
described below:
- NOAA 16 and NOAA 17 will become the
"operational" system,
- NOAA 15, NOAA 14 and NOAA 12 will be included in
Multisat service.
In other words, by mid 2002, the full
constellation will consist of 3 satellites from the second
generation and 2 satellites from the first one.
In addition to this evolution, the system
will be improved, by November 2002, with a new Japanese
satellite (ADEOS II) with Argos downlink capability onboard the
payload. This satellite will have the same characteristics as
the Argos-2 generation plus it will have the downlink function.
As a consequence, by the end of this year our system will
include four (4) satellites from the second generation (NOAA
16, 17, 15 and ADEOS II) plus 2 satellites from
the first generation. Thus, transmitters working on the upper
end of the frequency band (reserved for low output power
beacons) can be received by 4 "new" satellites.
With this information along with the graph
below showing the current Argos PTT frequency distribution, a
decision to transmit outside the central frequency band is
clearly interesting for both users and manufacturers.
To complete
this information note the launch of NOAA N (NOAA 18) carrying
an Argos-2 instrument will occur in June 2004. After this
launch there will be 5 Argos-2 satellites.
4. Some test results obtained on the Argos-2 band
Some test
results obtained in 2001 are illustrated below,. The main
conclusions of these tests were that in all conditions of
output power and geographical area, the shift of frequency
improves the:
- A. Bit Error Rate on long or short messages
(results presented are obtained from a low=power Microwave
Telemetry transmitter located in the US)
- B. the number of received messages, i.e. the number of
locations (results presented obtained from a transmitter
located in Europe)
(*Nota : No
Test has been done at +10 kHz)
Data from
additional testing is summarized in the graph below. The
conclusion from this data is: ARGOS PERFORMANCE IS BETTER
WHEN YOU TRANSMIT DATA AT 0.5 WATT WITH A SHIFT OF 10 KHZ THAN
WHEN YOU TRANSMIT AT 1 WATT WITHIN THE CENTRAL FREQUENCY
BAND.
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