Japan kind-of completes its sovereign satnav constellation Japan has successfully launched the last of the current generation of the satellite navigation orbiters it will use to operate a sovereign positioning system. The satellite, known as “Michibiki No. 7”, aka QZS-7, made it into orbit early on Tuesday, local time, aboard an H3 rocket. The H3 is the Japan Aerospace Exploration …
The problem with national solutions (eg mobile phone bands/TV standards/Japan's FM radio band) is that technology can fail to work when you cross a border. Great for politicians, lousy for tourists. And lousy for consumers when it pushes up prices.
My understanding is that the satellites are GPS compatible. They look to GPS users just like any other GPS satellite. GPS users in Korea, Australia, etc who happen to lock onto one presumably can use it without difficulty. The "national" part is that their somewhat unusual orbit keeps them over Japan longer than over other places.
The problem in cities is twofold: First, you need to see at least four GPS satellites to establish a position and buildings may block some signals that would be visible on level ground without obstructions. Second, Even if you think you can "see" a satellite, you may actually be seeing a reflection off one or more structures which effectively lengthens the observed path. That length is used to compute your position. And yes that can actually happen. I still recall trying to navigate a city (downtown Hartford Connecticut) that I wasn't familiar with using a car GPS that had us mislocated by hundreds of meters.
This system also reduces Time To First Fix, a GPS cold start can take 5-12mins.
Now your phone will (almost) never do a cold start since the mobile towers are transmitting assisted GPS (A-GPS) signals telling mobile devices which satellites are above which speeds up getting a lock.
But for other devices it can take a while to get a lock, this constellation helps that, if the GPS receiver sees one of these new satellites it can immediately narrow down where on the planet it is.
Say you are hiking and get into an accident and need help, you activate your Personal Locator Beacon (which of course you have purchased since you are a sensible person), it does a GPS cold start, and then once it gets a fix it starts transmitting your location over 406 MHz emergency radio and if it is a newer one it can send it to satellites as well.
