Archive for the 'Equipos' Category

GPS-GLONASS biases … a receiver issue

Tuesday, June 30th, 2009

GNSS receivers decode all the signals ‘more or less’ at the same time, the time is really set by the GPS but the other systems (Glonass) have a bias so that apart from the clock bias per satellite there is an inter-system bias between the constellations (at least that is what we thought!! here is a paper I wrote sometime ago on the subject: GLONASS_POD

The plot below shows the state of the art when I was still in charge of glonass processing at ESOC some years ago!! you can see that we estimated one bias per receiver and per arc (2 days of data) and the biases are very stable each day and they are ‘grouped’ per receiver type. all the Javads tend to behave ‘similar’ and all the Ashtech Z-18 the same … but the estimated clocks were still not very good so then we estimated ‘one bias per satellite per data arc per receiver’ this is more reasonable, as the previous was an average over all the glonass satellites. The average (in the plot) is enough to get reasonable orbits.

InterSystem Bias

Now at the ESA/ESOC IGS Analysis Center we estimate the intersystem biases per receiver and satellite pair, and we have excellent orbits and clocks for glonass, the size of the intersystem biases is similar to what you see in the plot for each receiver type except each satellite/receiver pair varies a bit around those initial averages per site.

The explanation is that internally the receiver takes longer to decode the glonass satellite signals and since glonass works in different frequencies for each satellite (FDMA and not CDMA) then the bias must be frequency dependent.

Certain ‘batches’ of receivers using the same electronics produce very similar biases per satellites but it still has to be estimated all the time.

For Galileo I would assume a general intersystem bias with GPS will work but this is still unclear to me at this time, maybe many biases are need per satellite and per measurement type?? … not sure yet!!

Happy positioning!
Ignacio Romero (Nacho of the IGS!)

SAC helping the ESA/ESOC GPS Reprocessing

Saturday, November 29th, 2008

The IGS Reprocessing is a fantastic activity which I have highlighted before in this blog.

The IGS reprocessing involves recalculating all the GPS products (Station coordinate files, Satellite orbits, Station and Satellite clocks and Earth Rotation Parameters). Most of the IGS Analysis Centers are participating in this effort to take all the IGS GPS data from 1994 to 2008 and recalculating everything with the current state-of-the-art programs and techniques. This will produce more precise and continuous GPS products for the lifetime of the IGS. All the details can be found at the IGS Analysis Center Coordinator Website on Reprocessing.

At ESA/ESOC/OPS-GN (Navigation Support Office) we are one of the original IGS Analysis Centers so we wanted to participate in the Reprocessing. Unfortunately we had no spare processing capacity, and it was not clear what new machines should be bought to take on this challenge.

To help resolve this situation our company, SAC s.l., provided access to the ESA/ESOC Navigation support engineers an Intel Quad Core PC with Ubuntu 64bit, named SAC01. On this new machine we could start doing compilation tests, short runs, speed tests, data handling tests, etc. The machine was bought and configured by SAC s.l. in February 2008 and located in Las Palmas, Spain, at the professional housing services of a local Internet Service Provider (ISP). The machine was accessible via ssh and via FreeNX which allows for graphical display export easily to any other internet-able computer. The machine, SAC01, has installed the ESA/ESOC software Napeos POD software:

SAC01 Ubuntu QuadCore PC for POD ESA/ESOC Napeos POD SW on SAC01

SAC01: Ubuntu QuadCore PC for POD

By the end of 2008 we will have reprocessed and delivered to the IGS all the days from year 2000 to 2007, using 150 stations per day. This has involved downloading and checking about 200Gb of GNSS data from the CDDIS. The processing was executed in parallel, leaving one core free for Operating System processes, so that 3 days are run at once. This raises the temperature of the PC considerably but there is enough cooling power at the ISP to handle it!!

The reprocessing has involved 150 stations per day (green crosses), it has generally taken 50 minutes per day (red crosses) using 30/31 satellites (blue stars), summaries of four of the years are shown below:

2000-2003 reprocessing statistics

2000-2003 reprocessing statistics

Also noted are the total number of GNSS stations downloaded from the IGS (blue squares), and the stations rejected from the processing (pink squares).

At the start of 2009 the ESOC reprocessing results from SAC for 2000 to 2007 will have been checked, submitted and validated at the IGS making them the official ESA/ESOC contribution for those 8 years. The excellent results obtained finally convinced ESA/ESOC to buy two similar machines which will be used to produce the 1994-1999 reprocessing internally.

We are very happy at SAC s.l. to have supported ESA/ESOC in this innovative and successful way,

Ignacio Romero

Vehicle management using GPS … my experience so far.

Wednesday, February 13th, 2008

I have recently added vehicle management to my own car! and let me tell you about it … it is an interesting experience but the system has some way to go before it is truly useful.

I was investigating representing a new fleet management service from mainland Spain in the Canary Islands. The islands are a strange place for fleet management services and I knew the market would be difficult. The territory is obviously fractured and the islands are not very big so most commercial vehicles have a relatively small area of operations compared to mainland based vehicles.

On the other hand the Canary Islands have excellent mobile phone coverage (fundamental for fleet management) which extends into the ocean surrounding the islands so that both land based and water based vehicles could use such a system, and given the number of vehicles of both kinds a fleet service can start to make sense.

The installation in my 1978 Triumph Spitfire was relatively simple. The approved installer found a suitable connection point by splicing the cable from one of the dashboard illumination lights and we stuck the two antennas (GPS and GSM) on the dahsboard where they are hardly noticeable. The device is not very large and we stuck it out of view inside the car close to the passenger side.

These are the kinds of information you can pull up from the fleet management system online; positions over digital maps and detailed reports of vehicle movements (right-click to view larger image):

Position of a vehicle Vehicle navigation report

My conclusions so far are positive but there are significant posible improvements:

  1. 1 minute position updates are not enough for the islands where the roads are very winding so it should be possible to select the time between updates. Some of my trips do not make any sense because the 1 minute update times skip many turns, exits, etc.
  2. The response is too slow, for some reason this version of the on-line system is too slow and this discourages its use! I am not sure if the problem are the service provider resources (PC, connection speed, etc), or the number of users, or the application speed.
  3. It needs to be possible to stop the display of positions in the online application, and thus to save those “messages” from the mobile device for later. If an authorised users decides to not track a vehicle for certain periods (like outside working hours) it should be possible to do so and to save the contracted messages as a result.

I would suggest that there should be a way to select the update time at least down to 30 sec. Since there are a limited number of positioning messages agreed per month between customer and provider then if the messages run out so be it … Of course this means that the customer should be able to save messages so that the device can be set to record positions more often as the customer needs. It is not the same driving to germany on the highways than driving in the mountains in Gran Canaria! and the level of detail needed here is much higher.

Those are my thoughs for now, the system continues to work fine and i am generally happy but unfortunately there have been no takers yet!

Happy positioning!
Ignacio Romero