◌  AGL 3080 notes

27 August 2009

A couple weeks ago I replaced the Globalsat DG-100 GPSr Nick got me for Xmas 2007, which had served me very well but was falling apart, with an Amod AGL 3080. They’re pretty similar: both SiRFstar III–based and designed for economy and durability short of waterproofing. The finer differences are interesting only to pathetic GPS weenies. Hi. I elide the stuff that’s interesting but covered in tech specs and other reviews (e.g., that the ’3080 provides DOP figures and the ’100 doesn’t).

• My ’3080 is somewhat more accurate than my ’100. This is just the kinds of accuracy I’ve happened to be able to look at (no urban canyons yet, for example), without real statistical analysis, and might be only because my ’100 is old and chronically water-damaged. But it’s safe to say the ’3080 is no worse than the ’100 for general use. (When I get around to it, I’ll post a clean comparison with both of them cold-started and then warm-started while resting on either side of a survey marker, etc.) So far the lowest HDOP I’ve seen is 0.8, sitting for several minutes in a backpack on top of some lenses but under a very clear sky at the beach at night.

• The ’3080’s interface is as simple as it sounds. This is especially great compared to the hackety-hack of the DG-100 (which I was using before gpsbabel support). It will be trivial to write a script that gets all the data into Postgres quickly and reliably. Not having the built-in USB cable and charger is a slight inconvenience, but it takes the same cable as my camera and external drive, so fine. Using an odd number of batteries is less forgivable.

• The NMEA looks good but not perfect. The first stanza of any session has only the GPRMC, GPGGA, and GPGSA sentences, which seems strange, and there are other absurd situations where, for example, lat and lon are defined but the satellite count is 0. I assume these are the fixes on either side of signal loss, and possibly gpsbabel lying a little, but again I haven’t tested carefully; I assume they’ll make sense once I have a big enough corpus. They’re rare, though, and near enough to deterministically findable as far I’ve seen – they’re either at the beginning of a session or have DOPs of 50 – so it’s not a serious problem. An even less worrying quirk is that sessions seem to start with fractional seconds in the time field, then round them off after a small random number of fixes. I hope to work these out, but it’s hard to be too upset considering the overall quality of the data for $70.

• I hit the widely discussed static navigation gotcha. This is what Amod calls the common practice of filtering out certain kinds of slowish motion. There are plenty of people who like this, but not me: I was getting horrible gaps when I strolled uphill after a long day. I borrowed a Windows® machine to upgrade from the 2.2 to the 2.3S firmware (S stands for without the feature that starts with an S) and it works fine now. But the thing is, it was giving nominal-Hz readings when I first tested it in with a long set on a stump, which you would think would all be filtered out. So I think either static navigation is doing something clever (only kicking in at high PDOP?) or something else is behind the gapping I saw. When I have the chance to spend a couple hours swapping firmwares out, I’ll come back to this.

So far the accuracy and extra data make up for the cable and battery inconveniences. My ideal $70 SiRFstar III–based GPSr would basically be the ’3080 with 2×AAs and perhaps a little more rainproofing by putting a rubber gasket around the battery door and the USB port under it, and of course a realtime NMEA-over-USB mode. And if it could bring the TTF down at the expense of DOP in the first few fixes, that would be great. But I was quite happy with the ’100 and now a little happier with the ’3080.