Data Delivery Method



QUESTION:  The SAM-Manual mentions transmitting priority mode for SeaTag-3D. Can something similar be done with the ST-MOD; e.g. first all position data, followed by surface times, depth, temperature, … and how does the tag detects successful delivery of this data before switching to the next group?

Also, does the tag automatically switch to the battery of the MRPS section for all its functions once the stored solar power is low and reverse in case the stored solar power has been recharged?

ANSWER: As with the SeaTag-SAM tags, the SeaTag-MOD tags use the priority data delivery scheme, in which the prioritized packets are transmitted first. For MOD, you can select which packet types are considered prioritized, such as only the daily summaries for positions, or both daily summaries and depth histograms for example.

The Argos transmission system is one-way, so the tag does not know which or how many packets have been delivered. So instead, all priority data is transmitted for the specified number of loops, such as for example 10 loops.  The percentage of packets received grows as more loops are selected, depending on the Argos satellite coverage at the site.  Argos coverage ranges from around 12% of the time at the equator, to over 20% near the poles where the orbits of all satellites converge.  Let's say, you are in an area with 15% coverage. In this case, you expect about 15% of packets to reach the satellite during the first loop. In the second loop, 15% are received again, but some of them are now the same as already received during the first loop. So the expected percentage  of unique packets you have received now is 0.15 + (1-0.15) * 0.15 = 0.2775 (27.75%) and so on. So, the more loops you specify, the higher percentage of the priority packets will be received, asymptotic approaching 100%. But, the more transmissions are necessary and therefore less ultimately available for the routine data that follows.  Note that the above equation is only a first approximation, and other factors need to be considered.  Some packets received by the satellite will have corrupted data. In an ideal case such as when we test the tags on the roof of our building, or for one of our customers who works in the flat waters of the Bahamas, about 80% of received packets will be good and 20% corrupted.  Looking at another case from the Indian Ocean recently, 58% were good; close to what we consider the nominal average for ocean conditions of 50%.   Further, even as a satellite is overhead, it does not always receive each transmission. For example, the signal path may be blocked by the crest of a wave near the tag if the satellite is close to the horizon.  And, a given tag may transmit at the same time and on the same frequency as another tag in view of the satellite, resulting in a data colission and no data received.  

So, SeaTag-MOD can for some degree counter all this because unlike battery tags, the solar panel keeps running. But still, it is a tough battle. However, the typically long endurance of transmission also makes it more likely that you can physically recover a tag, because its position will be known via Argos following separation from the shark for a long time (five months on average observed).