Tuesday, May 14, 2013

Adding GPS to a GOBI 3000 cellular module

GOBI 3000 Cellular Module

Digi products such as the ConnectPort X4 or WAN 3G which support the GOBI 3000 cellular module include the ability for direct GPS support. However, without a special antenna, you will find the signal weak and the GPS only semi-reliable. This may be good enough for a static device which never moves, because as long as the device can get a signal once within the first few days, then you'll know where it is.

However, most of Digi's GOBI-enabled products include two SMA connectors for two cellular antenna. CDMA can directly use two antenna, but GSM only uses one, ignoring the second. You can remove one of the two antenna, installing a PASSIVE GPS antenna with a male SMA connector. You'll still need a clear view of the sky.

Manual Position

Don't have a clear view of the sky? Don't want to use a special antenna? Most Digi gateway allow you to manually enter a static position. For example on the Digi ConnectPort products, you can enter the latitude and longitude manually on the Configuration > Position web page.

You can obtain the information from a cell phone - sadly, most mapping apps show you a Google Map, but not the coordinates. On my Andorid devices, I use Yigiter's "Sensor Data Monitor" app (github.com/yigiter) as it gives me the raw GPS information in real-time.

USB / Serial Modules

Alternatively, many of the gateway (for sure any of the ConnectPort with a host USB port) can support some USB-based GPS units such as the USGlobalStat BU-353 (available at amazon.com).

Some of these gateways also support RS-232 models, which is listed under the Serial Profiles as GPS.

Friday, March 29, 2013

Be Careful with 'Assisted GPS'

Many cellular modules have GPS capability, and support up to 3 modes:
  1. Standalone (no network assistance): the hardware tries its best with GPS satellite signals captured through the cellular antenna path. Depending on your antenna, the signal could be difficult to obtain and hold, but it won't add anything to your cellular data costs.  A fix might take as long as 12 minutes after power up or gaining clear sky view.
  2. Mobile-based (network assisted) or MSB: again, the local hardware is calculating the position, but obtains various data files by billable network traffic. In MSB, the hardware must detect viable satellite packets and the network help is primary to speed up the initial fix, plus help when fewer satellites are visible to the device.
  3. Mobile-assisted (network calculated) or MSA: is like MSB, but will likely have more billable network traffic. In this case, the mobile device literally forwards corrupt GPS satellite packets and other information to a network-based server, which (over-simplified) can use pattern-matching with good packets supplied by other GSP-MSA devices in your neighborhood. The network server literally offers error-correction with the very worst of satellite conditions.
The problem is that the billable network traffic amount is pretty weakly defined. Some of the network traffic is free, while other traffic is billable.

Now an example: I had a Digi ConnectPort X4 IA with a Gobi3000 cell module. An early firmware incorrectly allowed the cell module to default to GPS active in MSB mode, which wasn't a huge problem with I had the Gobi module configured for GSM/AT&T, as I had a good cell signal in my odd inner-corner office. I have been told the Gobi 3000 'ephemeris' data file & related data is about 40K, and is loaded after a cell module restart, and then refreshed once a day (I was told by word of mouth only!) 

However, after 6 months of happy GSM operation, I reconfigured that unit for CDMA/Verizon to test some CDMA issues, and at my desk the CDMA signal is a much worse than GSM. A few days later I started getting 'usage alerts' from my carrier warning that my daily limit of 100K was being breached. Some investigation lead me to discover that my 10MB per MONTH cell data plan was now seeing nearly 8MB per DAY!

Fortunately, I asked a fellow engineer about this, and he'd seen the problem at a large customer site.  Apparently, the AGPS subsystem in my unit was failing to fetch the ephemeris data file from Qualcomm's web site, and retrying quite aggressively.  This retry was creating the 8MB of unwanted data every day for enough days that I began seeing the alerts!

Solution? Well, technically I didn't even need GPS, plus even if I did, since the unit runs 24/7, the faster initial fix wasn't of much value. My solution was to set GPS to standalone mode. In the end, the overage cost me about $34. Without the alert, I would not have detected the overage until my bill arrived, and then likely would have seen (at my $2/MB overage charge) about $500. Image if I had 100 units doing this for the same month!

Moral of the story: Be very deliberate about GPS use within a cell module; make sure the mode selected is correct, or even turn it off if you have no need for it.  Of course, also make sure your usage alerts are enabled at your carrier!

Wednesday, February 06, 2013

The future of GPRS (update)

In May 2012 I created a blog entry on the future of GPRS and Industrial Automation, so here is an update for February 2013.

First, AT&T has officially announced its shut down of 2G / GPRS will be complete by the end of 2016.
  • Why? To free up radio bandwidth for 4G technologies, which makes much better use of that scare resource, so more customer-bang for less-bucks
  • What does this mean? Between now and then, customers using GPRS will increasingly suffer more and more problems. AT&T officially says GPRS customers with problems can contact AT&T and they will try to sustain suitable GPRS support, however I've already heard from customers who say the answer from such contact is "Sorry, in your situation, you'll need to switch to 3G".
Second, even if AT&T is trying to maintain existing GPRS service, they are unlikely (or certainly NOT?) adding new GPRS hardware to towers. For example we had one customer in northern Minnesota who checked with AT&T, confirmed AT&T had home-network service in an area, but upon equipment installation, it turned out there was no GPRS support - the AT&T towers in that area where NEW, so did not have any GPRS hardware. This customer could not use their six existing GPRS-only products, instead they had to buy six new sets of 3G cellular hardware.