Archive for the 'Beginner’s Guide' Category

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?

what GNSS signals do we have …

Wednesday, June 13th, 2007

It is interesting sometimes to take a step back and remember what signals we actually have as they limit completely what we can accomplish. It has been amazing what engineers worldwide have been doing with these basic and limited signals. Noone at the start of GPS could have imagined the number of applications and how well the brilliant world-wide engineers have made them work.

So to take the step back we go back to the basic GPS and GLONASS signals. There are two frequencies being used by each GNSS satellite on the L-band. GPS uses the same frequencies for each satellite 1575 MHz (L1) and 1227 MHz (L2) and encodes ranging messages on both. GLONASS uses two distinct frequencies for each satellite (although antipode satellites may use the same frequencies) the GLONASS frequency ranges are 1610.6-1613.8 MHz (L1) and 1240-1260 MHz (L2).

The ranging messages encoded on the L1 signal give the following ranging information:

  • C1 ; Ranging to the satellite based on the Civil message.
  • P1 ; Ranging to the satellite based on the Encrypted (high-security) message.
  • L1 ; Number of cycles since satellite acquisition.

on L2 we have:

  • P2 ; Ranging to the satellite based on the Encrypted (high-security) message.
  • L2 ; Number of cycles since satellite acquisition.

a limited number of satellites are transmitting C2 now but not many receivers are providing this signal regularly. until a significant number of satellites have C2 capabilities it will remain an oddity in the constellation. Preliminary studies suggest the signal is good but for receivers that provide P2 this one is preferred. The issue of the P2-C2 calibration has also to be addressed as it has been for the P1-C1 bias in case of mixing the measurements in any calculations.

Good luck!

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