Archive for the 'Guia de Inicio' Category

Datos GNSS libres y abiertos de España

Wednesday, May 21st, 2008

Un gran problema de los datos GNSS en España es que la mayoria no son libres y abiertos. Depende mucho de la zona, de la idiosincracia de cada sitio y de la “novedad” de las instalaciones. Las regiones con mas tradiccion y capacidad tienden a dar accesso libre a sus datos, otros no. Creo que en mi entrada anterior está claro que todos tenemos que tener una “Politica de datos” y darlos libre y abiertamente lo antes posible sin cortapisas a todos los usuarios en igualdad de condiciones.

datos GNSS abiertos de España

Les incluyo el siguiente documento que me han hecho llegar sobre los datos GNSS abiertos en España. Me encontraba yo precisamente haciendo algo parecido por mi cuenta cuando un conocido me mando este ejemplar. Como creo que es un documento de valor para muchos lo pongo aqui publicamente para que todos nos podamos beneficiar.

Tambien debeis saber ya que el Centro GNSS Canarias pone a vuestra disposición datos abiertos de la zona de Canarias (Macaronesia, Peninsula iberica y Africa Occidental) para que los podais usar sin restricciones. Actualmente estoy añadiendo los datos de 1Hz disponibles en nuestra region en el servidor del CGC para vuestro uso.

Atentamente,

Ignacio Romero Trujillo
Director

Data Policy – Política de Datos

Saturday, April 26th, 2008

Política de Datos es la especificación clara y por escrito por la institución que instala una estación permanente GNSS para que los datos recabados sean de dominio público en algun momento. Especificando un tiempo determinado para acceder a los datos es fundamental para que los datos, que normalmente se han pagado por todos los contribuyentes, sean accessibles a todos en igualdad de condiciones.

Actualmente existen muchas estaciones GPS permanentes en Canarias y Africa que simplemente NUNCA ofrecen sus datos a la comunidad científico/técnica. Esto no es positivo para el avance de los estudios de nuestra región y sólo sirve para que varios organismos terminen duplicando esfuerzos con recursos limitados lo cual es un despilfarro.

Lo más importante para cualquier organización que instale un equipo permanente GNSS (GPS o GPS+GLONASS, etc), o cualquier otro equipo científico, es institutir una “Política de Datos” que sea pública y clara para que todos sepamos cuanto tenemos que esperar por esos datos (1 seg, 1 dia, 1 semana, 1 mes, 1 año), pero NUNCA es inaceptable. Esos datos serán beneficiosos para estudios y analisis a largo plazo y si nunca se hacen públicos corren el serio riesgo de perderse por olvido o error.

Una vez que se tenga una Política de Datos” hay que cumplirla! y dar los datos libremente a todos sin ningún tipo de traba ni cortapisas. Para esto lo mejor es publicarlos en servidores de datos globales y regionales como SIO, CDDIS, CGC, EUREF, etc, y anuncuar este echo en comunicaciones oficiales y en las webs pertinentes.

Si tiene cualquier duda al leer esto o le sugiere alguna idea adicional por favor contacteme y ayudenme a cambiar la forma de pensar de tantos investigadores que trabajan en nuestra region! Gracias.


Data Policy with respect to permanent GNSS installations is a very critical issue. I generally work in the IGS context which has an “open data policy” and also an “open product policy”.

It is easy to assume that since the IGS has 300+ permanent GNSS stations from dozens of organizations we have “won this battle” but nothing is further from the truth. In fact more data is hidden than open especially in areas with difficulties for permanent installations. We have recently discovered many GPS stations in the Canary Islands which never make their data public, the same is true for many stations in the African continent. This seems unacceptable and should change.

Considering that all these stations belong to public institutions which are paid with tax payer’s money this hidding of data seems incorrect. All organisations that decide to invest in GNSS stations should have a publicly available “Data Policy” so that after a certain amount of time the data is “opened” for free access to all. This could be 1 second, 1 hour, 1 week, 1 month, 1 year or whatever. What is reasonable is to have a Data Policy so that all the data is available “at some point” for long term scientific studies to all members of the scientific community in an open and uncomplicated download method (CDDIS, SIO, EUREF, CGC, etc).


Sincerely,

Ignacio Romero
Director

Estaciones GNSS en Canarias

Tuesday, March 25th, 2008

Recientemente desde SAC y el CGC hemos escrito un documento general sobre el posible desarrollo de la red de estaciones GNSS Canarias, por favor descargeselo y haganme llegar sus comentarios y opiniones:

Estaciones GPS Canarias 3.0

La idea fundamental del documento (white paper) es explicar los beneficios de tener una red de estaciones de financiación “mixta” … en otras palabras no todas las estaciones tienen que ser instaladas por el mismo organismo. Al contrario tiene mas sentido que cada organismo instale sus propias estaciones para satisfacer sus necesidades pero que todos compartan los datos como un bien público. Las estaciones públicas actuales en nuestro archipielago son las siguientes:

Estaciones GNSS Canarias


Como se puede ver solo dos islas tienen estaciones con datos públicos (Gran Canaria y La Palma) las estaciones son; MAS1 (de la Agencia Espacial Europea), GMAS (de la Agencia Espacial Japonesa), PLUZ (Instituto Español de Oceanografía), LPAL (Instituto Geográfrico Nacional) y todas distribuyen sus datos libremente. (Perdón por dejar fuera de la imagen a Lanzarote y Fuerteventura pero no tiene estaciones GNSS públicas abiertas) (Mas información sobre el estado actual de las estaciones regionales en el CGC)

Es muy posible que los datos de las estaciones se consideren “propiedad” de cada organismo pero esto en prespectiva es un error y desde SAC y el CGC queremos que cada organismo e institucion que financie una estacion GNSS vea que los datos derivados de esta instalación tendrán una cierta vida útil (minutos, dias, semanas, meses) dentro de su organizacion y una ciertya finalidad. Pero que pasado un tiempo los datos se pueden hacer públicos sin ningún problema. Estos datos pasan entonces a ser accesibles a todo el mundo y se pueden usar para importantes estudios científicos, de estabilidad regional, etc. Esta “politica de datos” es muy importante, cada organismo que instale estaciones tiene que definir su propia politica de datos y cuando sea posible los datos se puedan publicar en servidores públicos como el CGC, EUREF, IGS, etc.

Próximamente publicaremos en estas páginas las recomendaciones para una buena instalación GNSS, para que las estaciones puedan reportar los máximos beneficios posibles a todos sus usuarios.

Atentamente,

Ignacio Romero
Director

Satellite Navigation current status

Tuesday, January 15th, 2008

The situation with the Global Navigation Satellite Systems is not easy to predict so for now just explaining the current situation may illuminate what could happen in the near future.

We currently have only free signals. This is sometimes lost on regular users, the two flying constellations GPS and GLONASS offer services that are essentially free. I say essentially because obviously you have to buy a receiver, but there is a huge range of GNSS receivers so the good news is that there is a receiver out there for everyone! In any case the signals any receiver uses to calculate position and to ‘navigate’ are completely free.

The only difference between services in GPS and GLONASS is ‘open’ or ‘restricted’, the latter is only available to authorised users which can buy athorised receivers (military users, etc) so they will not be discussed further. In any case both services are free as the signals emmitted from the satellites can be received by any GNSS receiver.

For the future we assume that Galileo (Europe) and Compass (China) will also fly satellite constellations to provide navigation services. All of these systems should be interoperable but so far only GPS and Galileo have done the difficult work of properly coordinating the signals so as to provide interoperability.

Such interoperability is difficult at the moment since GPS and GLONASS do not interoperate very well. Of course the systems were designed during the Cold War so interoperability was never considered, but the decision of GLONASS to continue using FDMA instead of CDMA as GPS and Galileo use means that GLONASS could be relegated. The complexity of including FDMA and CDMA technology in a receiver would not be cost effective for the mass market. For high accuracy applications, where more signals means increased accuracy, as soon as an option exists most high-end receivers will use only the CDMA systems.

Therefore I predict that multiple system receivers (GPS, GLONASS, Galileo, Compass) will be very limited. On the other hand if GLONASS and Compass agree to interoperability terms with GPS and Galileo more 4-system receivers will be available in the future, otherwise GPS+Galileo receivers will dominate the high-end market and GLONASS receivers will become even more difficult to find!

Of course I could be wrong, what do you think?

GLONASS and GPS differences

Saturday, April 14th, 2007

You have probably already heard that there are actually TWO satellite constellations providing positioning signals available to everyone. the first is the well-known American GPS and the second is the Russian system GLONASS.

Both system do exactly the same thing. They provide a signal-in-space beamed down to earth from so-called Medium Earth Orbit (MEO) using dedicated satellites. Both systems provide a Civilian service and a “restricted” (Military) service.

The main differences as of now are:

  1. GLONASS is not complete. Whereas GPS has 32 satellites transmitting signals GLONASS only has 14 which does not guarantee world-wide coverage.
  2. GLONASS uses FDMA for its transmissions whereas GPS uses CDMA. in FDMA each satellite uses a different frequecy to transmit the same ranging code. in CDMA each satellite uses the same frequency with different ranging codes per satellite.
  3. GLONASS is providing a second Civilian signal on L2 which helps civilian users to properly eliminate the ionospheric error.
  4. GLONASS satellites are upgraded more often. This is due to a russian satellite design constraint or due to cost-saving measures but the GLONASS satellites have to be replaced much more often than GPS ones (every 5 to 7 yrs). This appears as a problem but it allows GLONASS to introduce new capabilities much faster than GPS.

The IGS provides precise orbit products and clocks for both constelations GLONASS and GPS as a public service. These orbits can be used together with precise GLONASS observations to perform the same type of calculations as with GPS observations. Many researchers are also supplementing GPS observations with GLONASS observations to increase precision and robustness of derived products.

Best Regards to all and Happy positioning.

What is GNSS about …

Wednesday, September 27th, 2006

For general information to the beginner I want to stress in this post the fundamentals.
I feel like they are often forgotten, and this is the most fundamental concept; ” It is all a matter of time!” With that phrase I mean to say that GNSS position is only possible because we measure the time ‘t’ that a signal generated in point X (the source) takes to travel to point Y (the receiver) and this time ‘t’ times the speed of light (300000 km/s ; 187500 mi/s) gives us a distance between points X and Y.

As long as we have 4 independent distances and we know the positions of all the sources we can calculate the 3D position of point Y. This is important; we need 4 distances because the emitter and receiver clocks are never synchronised properly by having 4 measurements we can solve the 3D + dT (receiver clock correction).

This is even more important; we must know the positions of the emitters and their clock corrections to a high degree of accuracy at the moment of the signal being emitted so that we can calculate the position of Y correctly. The GNSS constellations emitt each satellite’s positions and “clock bias” as part of the ‘Navigation Message’ encoded in the timing signal. It is also possible to use external sources for that information such as the IGS (IGS, etc).

In future blogs I will explain some of those fundamentals elements in more detail.

Happy Positioning!

Ignacio Romero
Director