Augmenting GPS

By 2018 a centimeter-resolution positioning system is scheduled to be in operation covering Japan and locations at the same longitude.  The system will augment the US GPS system. 

The satellites will be in geosynchronous (not geostationary) orbit that will cause them to trace a figure eight in the sky over Japan. 

The high elevation in the sky increases the number of satellites that will have a direct path to receivers in Japan’s densely populated cities. 

The US GPS system uses 1575.42 MHz and 1227.60 MHz.  The centimeter level augmentation signal (CLAS) will be on 1278.75 MHz.  Several ground stations will monitor the location calculated from fixed receivers.  A control center compares the error due to the variable propagation rate of radio waves passing through the ionosphere.  It computes parameters that receivers can use to compensate for this variation and send them to the satellites to be transmitted to QZSS receivers.  The data is compressed and sent at 2kbps to make it more robust.  The process of computing the parameters, compressing them, and getting them to the satellites and back takes six to ten seconds.

The system also has a provision for sending text messages to receivers during an emergency when terrestrial communications systems are down.  All references I can find say this information will be carried in the 2 GHz S-band.  If this is in the high-end of 2 GHz, I wonder if receiver manufacturers will choose to omit this feature. 

In the 80s and 90s some amateur radio operators secretly hoped for a disaster to take out the phone lines to give amateur operators a chance to provide emergency communications.  I do not know if those people are still around, but it’s hard to imagine amateurs being able to provide communications today in a disaster.  (I didn’t research this claim.  I would love to hear from amateurs involved in emergency preparedness in the comments.)  A simple messaging system that receives messages directly from satellites on mobile phones would be very useful.  My only concern about the system is whether it being on the 2GHz band will make it too costly for high-volume GPS receivers in mobile phones.

Conclusion

QZSS approaches the problem of increasing accuracy and coverage by having satellites high in its users’ sky.  This increases the chance of getting a direct path to more than one satellite in an urban environment. 

I wonder if anyone has looked into a GPS-augmentation approach based solely on terrestrial transmitters.  The transmitters would be on the users’ horizon instead of in the sky.  Multipath reflections would be much worse, but if the pulses were very short, a good receiver could work out the time of the shortest path, i.e. the first “tap” of the channel function.  The pulse being short, would mean it would have a higher bandwidth and therefore require more power to attain a good SNR at the receiver.  This would not be a problem, though, for a transmitter and antenna located with other repeaters and broadcast stations at a radio tower.

Further Reading

• A nice page on the QZSS website explaining error due to multipath and variable delay in the ionosphere
• Recent Spectrum article on QZSS
• A brief comparison of QZSS with other satellite positioning systems