ESA - ESTB - ESTB - ESTB Architecture

3. Must the time indication on the maps correspond with the "last auto-update" indication?

8. Sometimes the performance map appears incomplete. What is the cause of that problem and how to solve it?

1. How this service works?

This service is based on:

The ESA SISNeT technology
The ESA ESPADA simulation Software

The three main components of the system are the following:

The SISNeT platform. Its mission is to provide access to the ESTB signals through the Internet (the SISNeT articles and presentations included in the publications section of this Website provide a global view of the SISNeT platform). The ESA SISNeT User Application Software (UAS) is able to connect to the SISNeT Data Server, and access the ESTB signal-in-space (SIS) in real time. The UAS includes a set of algorithms, able to calculate the ESTB SIS broadcast mode in real time, and send it as an icon to the ESA Web Server.
The ESA ESPADA Software is an EGNOS simulation tool, optimised for co-operating with the SISNeT UAS. ESPADA uses the real-time information provided by SISNeT to produce ESTB performance maps in real time.
The ESA Web Server. Stores the web pages related to this service and all the linked resources. Its main function is to allow users accessing the SISNeT/ESPADA information using a web browser.

System block diagram

2. What does "last auto-update" mean?

The main objective of the web page is to act as an online real-time monitoring tool. Therefore, the page must be refreshed automatically, in order to show the most recent information.

The "last auto-update" field indicates the instant of the last refresh. That instant is identified by date and time (using the local computer settings for date/time format), and just acts as a reference. Looking at that field, you can know when the page was updated the last time, and predict when the next refresh will take place. In addition, you can easily detect if the page is not properly auto-refreshing by observing that field.

"Last auto-update" field

3. Must the time indication on the maps correspond with the "last auto-update" indication

Absolutely no.

First of all, the "last auto-update" field (LAUF) is related to the web page, and is obtained from your system time. This is reasonable, since the objective is to provide a reference of the last automatic refresh…and what better reference than the local time for that purpose! Obviously, that means on a given instant users in different time zones will see different LAUF indications. And, of course, if the system clock is not on time, an incorrect indication will be shown.

On the other hand, the time reference stamped on the map indicates its time of production and, hence, it is exclusively related to the maps.

Since:

LAUF is exclusively related to the web page;
The time-stamps are exclusively related to the map production time;
The web page and the map update rates are different.

It can be concluded that there is no relationship between both time indications.

Time indications on the web page

4. Which are the update rates of the page components?

There are three auto-updated components in the web page:

The web page itself: it is refreshed each 5 minutes;
The broadcast mode icon: an updated version is generated each 5-6 minutes and sent to the ESA web server;
The performance map: it is sent each 10 minutes (approx.) to the ESA web server.

Those three components are independently updated, without any relationship between them. The choice of updating the web page each 5 minutes is reasonable, taking into account the icon/map update rates. Anyway, you can always force the page to be refreshed, by simply using the "reload" option of your web browser. You will appreciate that LAUF is updated accordingly.

Under certain anomalous situations, those rates might vary. For instance, if the system switches to "Info N/A" state, that information is immediately sent to the web server.

5. What does "Info N/A" mean?

The "Info N/A" status is set by the SISNeT system, and not by the ESTB. Hence, the Info N/A status does not necessary means the ESTB is not working properly. It must be understood as a status in which SISNeT is not able to provide information (the broadcast status and / or the performance maps). For instance if - due to a major reason - any SISNeT functional block is disconnected, the system will immediately detect it and activate Info N/A mode. The table bellow summarises some of those possible scenarios, indicating which are the elements affected by the "Info N/A" status. Note the cause of the error is not reported by the web page.

Note some transient statuses may be reached. For instance, suppose the system is working normally, and showing:

Suppose the broadcast status changes to mode 3. The following is a probable transient episode:

Now, say that the broadcast status returns again to mode 2. The transient occurs as follows:

From the user's point of view, the complete episode occurs as shown in the table bellow. SISNeT properly handles any possible transient state, so that the user will always see a coherent indication of the system status.

Step	Broadcast icon state	Performance map state
Initial status	Mode 2	Showing a map
1	Mode 3	Info N/A
2	Mode 2	Info N/A
3	Mode 2	Info N/A
4	Mode 2	Showing a map

Suggestions and bug reports are welcome. Please, send them to the ESTB Helpdesk.

The system test/debug tasks started in November 2001. The complexity of such activities is extremely high, since a big amount of aspects must be covered and the main goal is to provide a high-quality service. Since November 2001, more than 15 major improvements have been made. The following are some of the benefits derived from the test/debug activities:

The system will remain operational while finishing the testing/debugging activities. During that period, a warning is shown (both in the status bar of the web page and above the performance map), reminding the page is under test. That means the system can provide temporary incoherent information. Once the debugging activities are finished, the warning will be removed, and that limitation will disappear.

8. Sometimes the performance map appears incomplete. What is the cause of that problem and how to solve it?

The performance map must show a lot of information, using a limited space and high-quality graphics. That involves the transference of big amounts of data through the SISNeT network. The image files have been optimised to quickly load from a Web browser. However, the transference from the SISNeT UAS to the ESA Web server can be affected by network delays, making some data transferences longer than expected, in terms of time. If the web page refreshes just while the performance map image is being uploaded to the Web Server, the user may see an incomplete map. You can find the same effect in the World Wide Web, when accessing a website that is being updated with big image files.

This problem can be easily solved, by simply forcing a refresh of the web page (using the "reload" option of the Web browser).