[HH] cheap navigation unit

[Tom Metro]

Kurt Keville wrote:
> I'm still looking for a way to build a cheap navigation unit which
> could get "voicemail service"...

Tell us more about this device. What would it be used for?


> Some of that functionality you can already find in a number of
> smartphones already;

I've found that only a tiny minority of navigation apps for smartphones
can work without a live data connection. Google's flagship navigation
app, for example, will let you go offline and follow a route you
established while connected to the Internet before you depart on your
trip, but if anything comes up along the way that requires new data, you
are out of luck.

(Recently I've wondered if anyone makes an offline database version of
the yellow pages. Sort of a throwback to the early 90's CDROM-based
products, but packaged as a modern smartphone app.)


> ...save you on your Verizon minutes for instance.

Another option to make wireless data services more economical on
ancillary devices are new data plans that let you support unlimited
number of devices on a single phone plan, while only paying for
minutes/data used. See:
http://www.pcmag.com/article2/0,2817,2399807,00.asp


> I'm pretty sure you can use cell towers for navigation even if you 
> don't have a service plan because of the requirement placed on CellCos
> to offer emergency phone calls. So you should be able to get AGPS for 
> free, right?

According to:
http://en.wikipedia.org/wiki/Assisted_GPS

AGPS involves:

   1. Information used to more quickly acquire satellites
     * It can supply orbital data or almanac for the GPS satellites to
       the GPS receiver, enabling the GPS receiver to lock to the
       satellites more rapidly in some cases.
     * The network can provide precise time.
   2. Calculation of position by the server using information from the
      GPS receiver
     * The device captures a snapshot of the GPS signal, with
       approximate time, for the server to later process into a
       position.
     * The assistance server has a good satellite signal, and plentiful
       computation power, so it can compare fragmentary signals relayed
       to it
     * Accurate, surveyed coordinates for the cell site towers allow
       better knowledge of local ionospheric conditions and other
       conditions affecting the GPS signal than the GPS receiver alone,
       enabling more precise calculation of position. (See also Wide
       Area Augmentation System)

  A typical A-GPS-enabled receiver will use a data connection (Internet
  or other) to contact the assistance server for aGPS information.
  ...
  In AGPS, the Network Operator deploys an AGPS server. These AGPS
  servers download the orbital information from the satellite and store
  it in the database. An AGPS capable device can connect to these
  servers and download this information using Mobile Network radio
  bearers such as GSM, CDMA, WCDMA, LTE or even using other wireless
  radio bearers such as Wi-Fi. Usually the data rate of these bearers is
  high, hence downloading orbital information takes less time.
  ...
  Many mobile phones combine A-GPS and other location services including
  Wi-Fi Positioning System and cell-site triangulation and sometimes a
  hybrid positioning system.

I'm pretty sure the emergency phone call capability does not include
data access, and thus you are out of luck as far as AGPS is concerned.

Your best bet would be to get location hints using WiFi and cell-site
triangulation.

I'm not getting why "orbital information" needs to be downloaded at the
time of need. It sure sounds like something that wouldn't change often,
and should be something that can be periodically cached (monthly?) if
not algorithmically extrapolated, given a known starting point and the
current time.

I guess one of the problems you run into is a catch-22 over precise
time. Knowing it is helpful in predicting where the satellites are, but
if you don't have network access, the GPS satellites are the next best
source of precise time info.

 -Tom