FAQ localisation Argos

Argos can track platforms anywhere in the world, supplying positions to users around the globe. Platforms can be attached to practically any type of physical object, for example: an ocean buoy, a truck, a bear, a bird, or a sailboat. Argos platforms are located by using the Doppler Effect which gives an accuracy of up to 150 meters.

Doppler locations are good for compact, low-power transmitters and in difficult radio environments. The satellites receive the signals sent even in extreme conditions such as a platform transmitting from a dense rainforest or from transmitters attached to stacked containers.

As an option, GPS can be added to an Argos platform. Applications that require more accuracy can use GPS fixes in addition to Doppler locations. GPS allows an accuracy better than 100 meters and can provide regular positions.

Each time a satellite passes over a platform it collects messages transmitted by your platform and accurately measures the frequency of the received signals. Messages and measured frequencies are relayed to the Argos processing centers via ground stations. The centers then calculate the location of the platform for this given pass, accurate to within 150 meters.

Argos positions are calculated by computing the Doppler shift on the transmitter signals. This is the change in frequency of a sound wave or electromagnetic wave when a transmitter and a receiver are in motion relative to each other. The classic case is the change in the sound you notice when a train approaches and moves away. Similarly, when the satellite “approaches” a transmitter, the frequency of the signal measured by the satellite receiver is higher than the actual transmit frequency, and lower when it moves away.

You can interface a GPS receiver to your Argos platform. GPS fixes collected by this receiver at pre-set intervals will be coded in Argos messages and relayed to the Argos processing centers. Several fixes can be stored in a single Argos message; the total number depends on the coding and the required accuracy (up to a few meters for full GPS accuracy).

GPS fixes can be decoded, validated and distributed just like Argos locations.

Argos Doppler derived locations can still be used as a back-up in case GPS signals cannot be received by the platform.

Using GPS in addition to Doppler locations is useful if:

• you need locations at regular intervals, for example 24 times a day, calculated on the hour (Doppler locations are usually spaced irregularly through the day)
• you need extra or frequent locations
• you need better location accuracy
• the satellites receive only a few of your messages during each pass from your transmitter (if your platform is in an unfavorable position such as a deep valley, for example).

Most Argos manufacturers provide transmitters integrated with GPS receivers.

If your platform is transmitting in difficult conditions and if only a few messages are collected at each satellite pass, you should apply to Location Service Plus (Auxiliary Location Processing) service. This service will provide you with locations calculated with 3 or 2 messages (respectively class A or B), enabling you to get more locations.

We have developed a new algorithm that combines data from several overlapping or successive satellite passes and allows you to locate slow-moving or still transmitters that send just one message per satellite pass. This service is available upon request. Please contact your User Services for more information.

This happens when the error ellipse has a strong eccentricity (when the error radius ellipticity is greater than 10%). This mainly happens in two cases:

• When the distance between the sub-satellite track and the PTT is small.
• When that distance is very large.

It can also happen when frequency measurements are not homogeneous during the satellite pass.

No, because in some cases we cannot apply all plausibility tests and only two or less tests are left for the matching: 

• For class A or B locations we have no residual error value, so this test cannot be applied.
• The frequency continuity test is not applied if the dates of new and previous locations are too far apart (platform type dependant), as the transmission frequency may experience large drifts over longer periods. 
• The speed test does not make sense if the new and previous locations are separated by a long time period (the threshold is two days for drifters, birds and land animals, 4 days for marine animals, 11 days for an ARGO float).

As an example, a class A or B location just after a long transmit switch-off period cannot be validated at the first satellite pass because just one test is left: the minimum displacement from the previous location.

Two messages will provide, if the calculation is successful, a class B location. The location is the first guess obtained from the geometric initalization. The transmit frequency used is the previous transmit frequency computed at the previous successful location (with at least 3 messages). This explains why, when this previous location is old, the quality of the location is usually poor.

Three messages will provide, if the calculation is successful, a class A location. The 3 message processing method is similar to the 4 and more message method, but there are not enough messages to compute the residual error (3 equations can only resolve 3 unknowns: latitude, longitude and frequency).

Four or more messages are needed. Messages need to be well distributed along the satellite pass in order to enable proper determination of the Doppler curve, and in particular of the inflection point which corresponds to the actual transmit frequency.

The maximum speed is used to validate the locations. Note that this test is given special weight as it is the only one clearly related to the nature of the platform. When both tests (max. speed and minimum distance from previous location) fail, the validation fails and the location is given a class Z.

It is in your interest to provide a realistic maximum speed of your platform.

• If the maximum speed is underestimated, this will not suppress all the locations but may reduce their number. As an example, you should still receive some locations from your drifter when it is on its way for deployment on the ship deck though the speed limit is exceeded. This is the case when the three other tests are passed.
• Overestimation of the maximum speed will reduce or cancel the interest of this test, thus increasing the chances of releasing wrong locations.

Note: the speed test will fail only if your platform moves in a straight line at a speed higher than the indicated maximum speed between the two satellite passes that are being considered.

For each message collected, the satellite measures a frequency. The altitude of the platform is assumed to be known, the orbit of the satellite is known as well. The unknown parameters are the longitude, the latitude and the true transmission frequency of the platform. Each message collected by the satellite provides an equation and there are three unknown parameters. The estimation of the location accuracy requires at least one extra equation – i.e. an extra message – that’s why the full location process requires the collection of at least 4 messages in a satellite pass. With only two or three messages, it is not possible to determine all of these parameters; only latitude and longitude will be provided to the user.

Location calculations for land animals are extremely sensitive to altitude variations. An altitude error of 1,000 m can dramatically erode the accuracy of calculated locations, especially if a PTT is on or near the subsatellite ground track.

To enhance location accuracy, CLS applies a digital elevation model (DEM) from the USGS to all location calculations for terrestial mobiles and birds. Composed of basic squares of 30 seconds of an arc (900 meters at the equator) for which the altitude is known, the DEM is used to estimate PTT altitude and enhance location accuracy.

Another impact of topography is that fewer messages are collected as a result of screening by mountains, cliffs or other obstacles. As a result, you will get more class A and B locations when 2 or 3 messages are collected and poor or no locations at all with more messages just because they are concentrated on one side of the pass (Doppler curve cannot be assessed).

When 4 or more messages are received by the satellite, the location calculation process follows 5 steps:

Step 1 – Geometrical initialization:

Initial estimates of the platform position are computed from the first and last messages collected during a single satellite pass and the last computed frequency of the transmitter. The intersection of the cones for these two messages with the altitude sphere (whose radius is the local terrestrial radius) gives two possible locations.

Step 2 – Least mean-squares calculation:

Principle: in an ideal world, when the transmission frequency of the platform and its position are known, and the satellite positions are known as well, the frequencies measured by the satellite match with the frequency calculated using the Doppler Effect formula.

The least mean square estimation method is an iterative calculation which minimizes the difference between the frequencies measured by the satellite and the expected frequency values calculated from theory. At the end of the calculation, if it converges, the results are latitude, longitude, transmission frequency and residual error (the incompressible minimum difference).

For each of the two initial positions provided in Step 1, the least mean square estimation is run and one set of results (latitude, longitude, transmit frequency, residual error) is obtained for each. If the processing does not converge, the location calculation fails and no location is provided.

Step 3 – Choice between two possible locations:

The position with the minimal residual error is chosen as the best possible location, and its plausibility is checked.

Step 4 – Validation of the best possible location:

Four plausibility tests are used to validate the position:

• Minimum residual error,
• Transmission frequency continuity,
• Minimum displacement (shortest distance covered since latest location),
• Plausibility of velocity between positions.

Two tests must be positive for the location to be validated. If the position candidate doesn’t pass the tests then the alternative position is tested. If none of the positions pass two tests, then none is validated.

Step 5 – Accuracy estimation:

A location class (estimation of the location accuracy) is calculated using the residual error and the satellite pass characteristics.

If you have subscribed to the Location Service Plus (Auxiliary Location Processing – ALP), the location calculation is usually reduced to steps 1 and 4. No accuracy estimation can hence be provided. In some cases, for 3 messages, when a recent position is available, the calculation uses the previous transmission frequency and goes through step 2, 3, 4. In such cases an estimation of the accuracy is provided.

During the location calculation process (see above), the processing system occasionally selects the wrong solution (the “mirror” image location). If this occurs, the result is a bad location.

When the previous location is between 30 minutes and 3.5 hours, the speed of the mobile is estimated using the possible location and the previous one. The location calculation is done again taking into account the estimated speed of the platform, and if results are improved the new location result is kept.

Whenever appropriate, the software attempts to improve the solution by considering that the mobile has moved at a uniform speed since the previous location.

Generally yes, but not always:

• Transmitters and antennas should be tuned to enable the collection of 4 or more messages at each satellite pass so that accuracy estimation can be provided.
• 3 message locations are generally more accurate than 2 message ones.
• More messages poorly distributed along the pass – i.e. on the same side of the Doppler curve, will not help.
• Getting more than say 6 messages well distributed in a satellite pass has no significant impact. Oscillator stability is then the key parameter.

Location classes provide information on the location process and an indication of the location accuracy:

• classes 0, 1, 2, 3 indicate that the location was obtained with 4 messages or more and provides the accuracy estimation,
• class A indicates that the location was obtained with 3 messages,
• class B indicates that the location was obtained with 2 messages,
• class G indicates that the location is a GPS fix obtained by a GPS receiver attached to the platform. The accuracy is better than 100 meters.
• class Z indicates that the location process failed.

The accuracy cannot be estimated for classes A and B (not enough messages).

Location accuracy varies with the geometrical conditions of the satellite passes, 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 250 m radius
• Class 2: better than 500 m radius
• Class 1: better than 1500 m radius
• Class 0: over 1500 m radius

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.

Our experiments with different sets of transmitters at fixed positions or moving slowly have shown proper matching with the error estimations. Yet, in a few cases users have reported significant discrepancies. The error estimation process is not totally independent of the transmitter frequency stability, or of the platform motion which also translates into a frequency shift. It assumes that the frequency is “approximately” stable during the satellite pass. As a consequence, oscillator instability or a fast moving platform may lead to underestimation of the error.

The error estimation of a class 0 location is higher than 1.5 km – the error could be 50, 100, 500 km. The error estimate (radius) for a Class 0 is provided.

Class A and B locations 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 locations, 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.

The location calculation assumes that the platform is at a fixed pre-defined altitude. An error on platform altitude translates into an error varying between half and four times this error on longitude. To enhance location accuracy, CLS applies a digital elevation model (DEM) from the USGS to all location calculations for terrestial mobiles and birds. Composed of basic squares of 30 seconds of an arc (900 meters at the equator) for which the altitude is known, the DEM is used to estimate PTT altitude and enhance location accuracy.

Platform motion reduces the accuracy of Doppler locations. Platform motion, seen from the processing point of view, looks like frequency drift. The motion introduces an additional Doppler shift which is included in the frequency signal measured by the satellite. It tends to distort the ideal Doppler curve which would be obtained from a fixed platform with a stable oscillator. As a consequence, the residual error is larger and the accuracy is lower.

Short term instabilities in the transmitter oscillator distort the Doppler curve. The location calculation is rather robust to random instabilities, but is sensitive to frequency drift which produces the same effect as platform motion. As a consequence, location accuracy will be reduced and in the case of a highly unstable oscillator there will be no location at all.

COM and PRV commands provide you with the validated locations (see Plausibility test FAQ above). You can always find the two location candidates resulting from the location calculation by using PRV/C or DIAG command, unless the calculation failed (class Z). The best candidate is shown in the first position.

Note: to access these commands, you need to subscribe to the Location Service Plus (ALP) service.

Users can obtain further information on location process and transmitter performance by consulting DIAG command. Users can access DIAG information via ArgosWeb in the Consultation/Data Table section (display Diagnostic Data by clicking on this icon , then selecting ) or from the Data Download screen (display diagnostic data by clicking on  ). These parameters are available in a clearly identified table.

CLS has developed “in house” tools to provide some additional expert explanations on the Argos location calculation process. These tools may be used upon request in some dedicated cases.

If you haven’t done so, subscribe to the multisatellite service, which will provide you with additional locations resulting from the complete Argos constellation.

Ask your User Office to provide you with class 0 locations; you will get locations computed with 4 messages or more with an accuracy of less than 1000 m.

Subscribe to the Location Service Plus (ALP), if you haven’t done so yet. You will get additional locations computed with 2 or 3 messages.

If power and size constraints allow, use an Argos+GPS platform.

Increase the number of messages collected (and hence the locations) by working with your manufacturer and us to define proper transmitter power and frequency channel allocation. Note that there are three dedicated channels for low-power transmitters. In these channels low-power signals don’t compete with high power signals, so more messages can be collected.

If your platform does not transmit permanently, work with your manufacturer and us to adjust the transmitter duty cycle; some locations may be lost just because the silent mode is too long and transmission time too short.

If power and size constraints allow, use an Argos+GPS platform, this is particularly interesting if your platform travels at high speed.

If enough messages are collected (4 or more), the key issue is the oscillator stability. Select the most stable transmitter for your application.

If you are getting a lot of class A and B locations, your first target will be to increase the number of messages collected by the satellite to get more 0, 1, 2, 3 classes. Work with your manufacturer and us to tune the transmitter power, frequency allocation and duty cycle.

In parallel, try to go for stable transmitters to enhance your chances to get class 3 and 2 locations. Note that stability is harder to achieve on miniaturized transmitters.

Increasing the repetition rate should increase the number of messages collected, thus giving you better chances to get more locations and better accuracy. Yet, this has an impact on power consumption and is not always appropriate (see FAQ above and also ‘Does accuracy increase with the number of messages received’). You should consider in your approach all the parameters such as transmit power, duty cycle and especially the frequency allocation, all of which have a very significant impact on the overall performance.

Argos user community must choose between two location processing algorithms. Here are links to the documents that will help you make a decision regarding which algorithm is best for your application.

Technical paper :

• Improving Argos Doppler Location using Kalman Filtering : Lopez, Remy and Malardé, Jean-Pierre.
• Improving Argos Doppler Location with Kalman Filtering – Advantages for Argo Floats  : Bernard, Yann and Belbeoch, Mathieu.

KALMAN :

• Kalman location processing FAQ

Argos FLASH :

• #19 : Argos location at its best! (sepcial edition)
• #20 : Argos location algorithm It’s your choice !

User manual :

• Chapter 3.2: Argos location principle 
• Chapter 3.5 on Argos location principle (How to make a decision)