We have been tracking woodcocks via satellite with PTTs developed by Microwave Telemetry Inc (MTI) since 2006.The first two projects fixed the beginning of a new era in the knowledge of that migration. It was the first time that such experiments were made in Europe with this kind of birds, due to their small weight and their very special behavior.
In these projects (first with 12 gram Solar PPTs in 2006; second with 9.5 gram Solar PTTs in 2007) we have real problems with interference. (As a result, we thought that one possible way to overcome the interference could be to increase the charge of the PTTs.)
In March 2008, MTI gave us two new prototypes (9.5 grams) equipped with a new technology, to try and prove them.
The final result was net: the European interferences were overcome with both PTTs, during all the time the tracking lasted and in all the different atmospheric circumstances [1].
Location accuracy
Location accuracy of emissions gathered via satellite is the matter. We dealt with these kinds of accuracies in a previous paper [2].
Let’s try a new approach taking into account the new data given by Argos.
Argos
According to Argos [3]:
“(a) What is the accuracy of Doppler locations?
Location accuracy varies with the geometrical conditions of the satellite passes [4], the stability of the transmitter oscillator, the number of messages collected and their distribution in the pass. This means in particular that a given transmitter can have locations distributed over several classes during its lifetime.
Classes for which accuracy is estimated and related values:
- Class 3: better than 150 m on both axes [5], 250 m radius
- Class 2: better than 350 m, 500 m radius
- Class 1: better than 1000 m, 1500 m radius
- Class 0: over 1000 m, over 1500 m radius
- Class 2: better than 350 m, 500 m radius
- Class 1: better than 1000 m, 1500 m radius
- Class 0: over 1000 m, over 1500 m radius
These are estimations at one sigma.
The error is assumed to be isotropic and hence characterized by a single number called the radius of error. It corresponds to one standard deviation (sigma) of the estimated location error. The location class is attributed based on the radius of error. The location class and associated error are sufficient for many applications.
(b) How accurate are class A & B locations (empirical data on class A & B)?
Class A & B location may be accurate. We cannot specify the accuracy since more messages are needed to estimate the error. We can just say that class A locations are usually more accurate than class B because the transmit frequency has been computed thus enhancing the process. Some users have experimentally tried to answer this question and provided interesting hints. We are confident that approaches of this type will help provide interesting guidelines to enhance practical use of this type of locations.”
Our experiments
Let’s take a look at our experiment in 2007 [6]:
Methodology
Here the accuracy of LC 2 and LC 1 locations given by Argos:
- Class 2: better than 350 m, 500 m radius
- Class 1: better than 1000 m, 1500 m radius
- Class 1: better than 1000 m, 1500 m radius
Taking as good the LC 2 and LC 1 [7] locations given by our bird (Navarre), we have calculated the discrepancy of LC B locations and LC A locations from the good ones (LC 2 and LC 1) in the following way:
Assuming as on March 5th that we have a LC 2 location followed by a LC B location, in a very short period of time when the bird is supposed not to move, the accuracy of LC B location is, more or less:
0.500 km + distance between LC 2 and LC B (measured through Google Earth): 0.500 + 0.6 = 1.100 km
Studying all the data given by our birds, we have found that in the worst cases taking into account locations in very short periods of time, where it is supposed that the bird doesn't move:
(1) The accuracy of the LC A and LC B locations depend on the pair PTT/bird.
(2) For Navarre, ID 73388, the LC A locations, in the studied cases, have an error between 5.46 and 9.37 km [8].
(3) For Navarre, ID 73388, the LC B locations, in the studies cases, have an error between 1.10 and 10.82 Km.
(2) For Navarre, ID 73388, the LC A locations, in the studied cases, have an error between 5.46 and 9.37 km [8].
(3) For Navarre, ID 73388, the LC B locations, in the studies cases, have an error between 1.10 and 10.82 Km.
We have very few samples of data from the Asturian bird to compare LC A and LC B locations; even worse, we do not have any emission with LC 2 or LC 1 location to measure the discrepancies. We can only say the following:
(4) For the Asturian woodcock, ID 72452, the distance between LC A locations, in very short periods of time, has an error between 0.42 and 3.66 km.
(5) For the Asturian woodcock, ID 72452, the distance between LC B locations, in very short periods of time, has an error between 3.12 and 4.36 Km.
In this case, we needed at least a LC 3, a LC 2 or a LC 1 location to measure the error of these locations.
All in all, one can say that, whatever the real errors of LC A locations and LC B locations for this bird, the LC A locations do not have a big difference among their errors (0.42-3.66), neither LC B locations among theirs (3.12-4.36).
(6) Using the same methodology, our LC 0 locations for Navarre can be considered as pretty goods. For Navarre, ID 73388, the LC 0 locations, in the studied cases, have an error between 5.18 and 5.90 km.
(7) In our experiment one LC B location has more accuracy than LC 1 locations and two LC A locations had equal accuracy than LC 0 locations, in line with the works of Hays et al. and Vincent et al. [9].
Now, let’s see the data in 2008:
(8) In 2008 Navarre, ID 73388, has given very few good positions to make such kind of calculus.
We have only a datum, on May 16th, when we have a LC 2 location (at 3:27 UTC) followed by an LC A location (at 3:22). Five minutes between both locations.
If we apply the above mentioned methodology (as an exercise, since, the time between both locations is too long):
Distance (through Google Earth) between LC 2 and LC A locations: 4.5 km.
Accuracy of LC 2 location according to Argos: 500 meters.
So, in the worst of all cases, accuracy of that LC A location: 4.5 + 0.50 = 5.00 km.
But, five minutes is too long. So, let’s try a new methodology.
When Araba was flying at night, above the sea, we calculated his velocity: 66km/h, in tune with the literature.
Assuming a velocity of 60 km/h during the day, high enough, we can perform the following calculus.
1m/s = 3.6 Km/h, so 60 km/h = 16 m/s, but let’s take this very extreme hypothesis as good.
(9) Let’s use this new methodology with Navarre:
As mentioned above, in the worst of all cases, accuracy of Navarre’s LC A location, on May 16th: 4.5 + 0.50 = 5.00 km.
Now we have to add the possible space traveled in those five (5) minutes (a very strong hypothesis).
5 x 60 x 16 ---> 4,800 meters ---> 4.8 km.
So the accuracy of the LC A location given by Navarre is 4.8 + 5.00 = 9.85 km.
(Again, the hypothesis is really strong.)
(10) With Laguna (ID 83297) and Araba (ID 83300) we don’t have the possibility to know the accuracy of LC A and LC B locations, since the locations given in a single emission are not very near in time.
Assuming, as on March 16th, that we have, for Araba (ID 83300), a LC 1 [11] location followed by a LC A location, in a period of time of 4.5 minutes, the accuracy of LC A location is, more or less:
Accuracy of LC 1 location (according to Kaatz) 3.19 km + distance between LC 1 and LC A locations (measured through Google Earth) + distance traveled by the bird (at 16 m/s) in those 4,5 minutes:
3.19 + 0.5 + 4.32 = 8.01 km.
We don’t have any more data to apply this new methodology.
Upshot
The point to be underlined is that in order to track the Eurasian woodcocks these LC B and LC A locations are very important: furthermore, they are real, objective and with a little work one can get their accuracy.
Bibliography
Hays, G. C., S. Åkesson, B. J. Godley, P. Luschi and P. Santidrian (2001)
The implications of location accuracy for the interpretation of satellite-tacking data, Animal Behaviour 61; 1035-1040.
Kaatz, M. (2004) Mit Prinzesschen unterwegs, 25 Jahre Storchenhof Loburg, 25 Jahre NABU-Bundesarbeitsgruppe Weißstorchschutz. Thesis’ title: Der Zug des Weißstorchs Ciconia ciconia auf der europäischen Ostroute über den Nahen Osten nach Afrika.
Vincent, C., B. J. McConnel, M. A. Fedak and V. Ridoux (2002) Assessment of Argos location accuracy from satellite tags deployed on captive grey seals, Marine Mammal Science 18:301–322 (see
For more information, see http://es.scribd.com/doc/58456473/Accuracy-of-A-and-B-class-locations.
[1] See Scolopax rusticola without frontiers: http://www.euskonews.com/0522zbk/gaia52202en.html.
[2] See Location accuracy of emissions gathered via satellite: http://www.euskonews.com/0484zbk/gaia48404en.html.(Christian Ortega from Argos wrote us to take into account the GDOP phenomenon and the fact that the accuracies given by Argos are inside ellipses.)
[3] See http://www.argos-system.org/html/system/faq_en.html and http://www.clsamerica.com/argos-system/faq.html.
[4] The so-called GDOP, Geometric Dilution of Precision, is a measure of the effect of the geometry of the satellite-beacon configuration on location accuracy. The actual Argos location accuracy depends on both the GDOP value and the quality of the transmitter (frequency stability). As a general rule, a smaller GDOP value yields a more accurate position. See http://www.sevin.ru/seminary/argos/Argos%20location.pdf.
[5] The errors 150m/ 350 m/ 1000m are in each axis of the ellipse. The radius of the circle of error is greater.
[6] In 2006, we had no possibility to make this kind of calculus. The periods of time between the different emissions given in the same day were not short at all. (About our different projects, see Scolopax rusticola without frontiers: http://www.euskonews.com/0522zbk/gaia52202en.html.
[7] One should take into account what Kaatz says in his thesis about class 1 locations. Michael Kaatz (2004) has taken into account the real positions following the storks and the positions given by Argos. When dealing with the accuracy of the emissions, Kaatz underlines the following: Using the 68th percentile to define the accuracy of locations estimates, observed accuracy was 3.19 km for location class (LC) 1. With more percentile the accuracy is even less. According to Kaatz, the accuracy depends on the radiated power and on oscillators: lower oscillators minimize the frequency drift.
[8] We have taken into account Kaatz’s thesis.
[9] As to the accuracy of class A and B locations, here what can be read in Hays et al. (2001): “In our trials the accuracy of LC A was comparable to that of LC 1. LC B had poorer accuracy than LC A, but the worst level of accuracy was found in LC 0.”
According to C. Vincent et al. (2002), "Both filtered and unfiltered LC A locations were of a similar accuracy to LC 1 locations, and considerably better than LC 0 locations."
[10] See Scolopax rusticola without frontiers: http://www.euskonews.com/0522zbk/gaia52202en.html..
[11] See what is said in note 8.
[2] See Location accuracy of emissions gathered via satellite: http://www.euskonews.com/0484zbk/gaia48404en.html.(Christian Ortega from Argos wrote us to take into account the GDOP phenomenon and the fact that the accuracies given by Argos are inside ellipses.)
[3] See http://www.argos-system.org/html/system/faq_en.html and http://www.clsamerica.com/argos-system/faq.html.
[4] The so-called GDOP, Geometric Dilution of Precision, is a measure of the effect of the geometry of the satellite-beacon configuration on location accuracy. The actual Argos location accuracy depends on both the GDOP value and the quality of the transmitter (frequency stability). As a general rule, a smaller GDOP value yields a more accurate position. See http://www.sevin.ru/seminary/argos/Argos%20location.pdf.
[5] The errors 150m/ 350 m/ 1000m are in each axis of the ellipse. The radius of the circle of error is greater.
[6] In 2006, we had no possibility to make this kind of calculus. The periods of time between the different emissions given in the same day were not short at all. (About our different projects, see Scolopax rusticola without frontiers: http://www.euskonews.com/0522zbk/gaia52202en.html.
[7] One should take into account what Kaatz says in his thesis about class 1 locations. Michael Kaatz (2004) has taken into account the real positions following the storks and the positions given by Argos. When dealing with the accuracy of the emissions, Kaatz underlines the following: Using the 68th percentile to define the accuracy of locations estimates, observed accuracy was 3.19 km for location class (LC) 1. With more percentile the accuracy is even less. According to Kaatz, the accuracy depends on the radiated power and on oscillators: lower oscillators minimize the frequency drift.
[8] We have taken into account Kaatz’s thesis.
[9] As to the accuracy of class A and B locations, here what can be read in Hays et al. (2001): “In our trials the accuracy of LC A was comparable to that of LC 1. LC B had poorer accuracy than LC A, but the worst level of accuracy was found in LC 0.”
According to C. Vincent et al. (2002), "Both filtered and unfiltered LC A locations were of a similar accuracy to LC 1 locations, and considerably better than LC 0 locations."
[10] See Scolopax rusticola without frontiers: http://www.euskonews.com/0522zbk/gaia52202en.html..
[11] See what is said in note 8.
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