timcodes

Hi everyone,

I posted a message regarding what would be a great gps for 100-150 us dollars, I found that Delomre to be one of my highest ranked ones from the survey.

However, now I'm concerned: what is the best chipset and featureset (accuracy, acquisition, realiability of the signal) for a gps?

Thanks again!

Jigsaw

Originally Posted by timcodes

Hi everyone, However, now I'm concerned: what is the best chipset and featureset (accuracy, acquisition, realiability of the signal) for a gps? Thanks again!

Best for what, a chipset that is fine for driving may not suit geocachers and ramblers.

From the research I've done the new SIRF III chipset seems to be viewed as the one to go for it's better in terms of reception in poor signal areas, warm start etc. Have a look at www.gpspassion.com and www.pocketgps.co.uk there are many comments in the forums. The only thing to be aware of is that the first batch of Globalsat receiver was withdrawn because of issues in the cold, the problem has been identified and fixed, new devices due to ship soon.

Gordon

BKF

I can't really compare, since the SiRF III chipset is my first GPS unit, but all I can say is "Wow!"

Where I've read that old GPS units need a clear LoS through the front winshield, or have trouble indoors, I see none of that with the SiRF III. Inside my house I typically get 6-7 satellites. Outside, with the GPS inside a nylon case, placed on a seat (front, back, or floorboard... doesn't matter) I seem to get a lock on 10+.

Also, the GlobalSat BT-338 unit I have has an awesome battery life as well! And it even came with a car charger which outputs the same voltage the Axim (at least the X50v) needs. So if you have that little travel charger plug adaptor, you can plug the GPS charger into it and charge (and use!) your Axim in the car.

JakeRich

Chipset doesn't necessarily mean functionality. The GlobalSat BT-338, for example, doesn't support WAAS, even though the chipset does. So, if you read the specs on the chipset, you'd expect the BT-338 to be WAAS capable, but it isn't. Look at the features of the particular receiver instead of the chipset.

So, taking your Featureset:

Accuracy - the most accurate receivers will be the WAAS/EGNOS capable ones WHEN THEY HAVE A DIFFERENTIAL GPS LOCK (yep, emphasis was intended). Otherwise, they're all about the same accuracy. Accuracy depends on the satellite constellation and the number of satellites being used, but the general limits are 12-15m 95% of the time, non-WAAS and 3-5M, 95% of the time, WAAS locked.

Acquisition-If you mean number of satellites, the more sensitive the receiver the more you can receive and use. If you mean speed of signal acquisition, you'll have to look at the receiver specs, but most of the newer units are under a minute for cold, under 10 for warm, under 1 for hot. If you mean weak signals, the SirfStarIII (GlobalSat BT-338, etc) units are very sensitive, as are the Xtrac2 models (lots of them around). Antenna type and orientation also can affect signal use and reception, as can windshield and wall composition.

Reliability of the signal--not sure what this means. The absolute reliability of the signal depends on the satellite. Any unreliable satellite is taken off line until it can be analyzed and fixed. If not, a backup will be used. If you meant the ability to use weak signals, I talked about that under acquisition. If you mean the accuracy of the fix, it depends on WAAS capability and availability as well as the DOP factors (geometry, ephemeris, almanac, atmospherics, reflections and relativity).

So, bottom line, look for a unit that supports what you want to do with it.

BKF

From GlobalSat's webpage (Bold by me):

------------------------------

Products no.: BT-338
BT-338 contains a built-in SiFRIII Low Power Chipset for high sensitivity. It communicates with Host Platform (PDA, Laptop, and etc.) via Bluetooth Serial Port Profile. It also contains a replaceable battery with a large capacity (1700mAh), which enables BT-338 to run up to 15 hours. A slide battery lid makes it easy for battery removal. With the petite and sophisticated design, it would definitely be your best choice!
Feature:
1.SiRF Star III high performance and low power consumption chipset
2.Communication with Host platform Via Bluetooth Serial Port Profile
3.Built-in ceramic patch antenna
4.Superior sensitivity for urban canyon and foliage environment
5.3 LED to show the rechargeable Li-ion battery
6.Operation time:15 hours, in continuous mode
7.Bluetooth operation range:10 Meter

Specification:
˙Chipset：SiRF Star III
˙Frequency：L1, 1575.42 MHz
˙C/A code：1.023 MHz chip rate
˙Channels：20 channel all-in-view tracking
˙Antenna Type：Built-in Ceramic patch antenna
˙Position：10 meters, 2D RMS
　　　　　 5 meters 2D RMS, WAAS enable ˙Velocity：0.1 m/s
˙Time：1 us synchronized to GPS time
˙Default：WGS-84 Acquisition Time (Open sky, stationary)
˙Reacquisition：0.1 sec., average
˙Hot start：1 sec., average
˙Warm start：38 sec., average
˙Cold start：42 sec., average
˙Altitude：18,000 meters (60,000 feet) max.
˙Velocity：515 meters/second (1000 knots) max.
˙Acceleration：Less than 4g
˙Jerk：20 m/sec3
˙Voltage：Rechargeable and removable battery with 5V DC input charging circuit
˙Operation Time：Default 15 hours after fully recharged, in continuous mode
Programmable >20 hours, in trickle power mode
˙Connection：Communication with host platform via Bluetooth Serial Port Profile
˙Protocol messages：NMEA-0183 output protocol
˙Output format：GGA(1sec), GSA(5sec), GSV(5sec), RMC(1sec),VTG(1sec)
˙Operating temperature：-20 ° to 60 ° C
˙Humidity：5% to 95% Non-condensing
˙Dimension：72.5mm × 40.4mm × 23mm

----------------------------------------

They advertise, giving specs with WAAS enabled, and their unit doesn't have it?

JakeRich

That is correct, it does not have it enabled. Those specs are for the chipset, not the unit itself. Note that the "feature" list does not include WAAS. From www.gpspassion.com, " SiRFstartIII currently does not have WAAS enabled, not that this matters unless you intend on relying on it to operate a plane." There is a very long thread there about the BT-338, including the cold temp problem and the static navigation issue that is being addressed by GlobalSat.

From my own testing, it never got a WAAS lock, even when another gps right next to it did.

BKF

Interesting. Why have WAAS specs if it's not there? That makes no sense.

That's like advertising Pontiac cars all having superchargers... But they don't actually work.

Granted, I did some reading, and it seems the WAAS doesn't seem to help much in field testing, but still.... You don't advertise chipset features you don't support.

JakeRich

If only that were really true! Unfortunately, I've seen specs all over the place that aren't true. In this case, GlobalSat just copied the chipset specs from Sirf, then didn't activate the WAAS function. I haven't tried to force it with SirfDemo, for example, because I didn't want to end up with a useless brick, but I may do that when I have the review finished, just to see if it works. I'm not optimistic.

Actually the car example is very close--cars have options, not all of which are available on any given model. So, while you CAN get a Pontiac with a supercharger, it's not functional on all of them.

WAAS in practice is not really that critical for most general use. When driving at 60 mph, being 10m more accurate only means about 33 feet, the distance you cover in just under 1/3 second. Walking around and geocaching it might make a difference, but only if the maps are that accurate (most aren't) and if the geocache creator used WAAS to locate it in the first place.

BKF

Here's some info taken straight out of the PDF user manual on the CD-ROM:

You'll notice this time they're not saying it's the chipset specifications, but the System specification.

They also shipped a program called GPSInfo with the unit, which has a toggle box for enabling WAAS.

If this thing doesn't have WAAS, they're sure doing their darndest to make you think it does. :)

Is there a way to check the data stream and see if there's any WAAS data?
------------------

3. Specification
3.1 System Specification
Electrical Characteristics (Receiver)
Frequency L1, 1575.42 MHz
C/A Code 1.023 MHz chip rate
Channels 20 all-in-view tracking
Tracking Sensitivity -159 dBm
Accuracy
Position Horizontal 10m 2D RMS (SA off)
WAAS enabled 5m 2D RMS (SA off)
Time 1 micro-second synchronized to GPS time
Velocity 0.1m/sec 95% (SA off)
Datum
Datum WGS-84
Acquisition Rate
Hot start 8 sec., average (with ephemeris and almanac valid)
Warm start 38 sec., average (with almanac but not ephemeris)
Cold start 42 sec., average (neither almanac nor ephemeris)
Reacquisition 0.1 sec. average (interruption recovery time)
Protocol
GPS Output Data NMEA 0183 protocol, and supports command:
GGA(1sec), GSA(1 sec), GSV(5 sec), RMC(1sec)
( VTG and GLL are optional)
GPS transfer rate 38400,N,8,1
Dynamic Condition
Acceleration Limit Less than 4g
Altitude Limit 18,000 meters (60,000 feet) max.
Velocity Limit 515 meters/sec. (1,000 knots) max.
Jerk Limit 20 m/sec**3
Power
Voltage Built-in rechargeable battery(1650 mAh) and 5V DC
input charging circuit
Operation Time 15 hr. After fully recharged, in continuous mode
>20 hr in trickle power mode
Physical Characteristics
Dimension 72.5mm x 40.4mm x 26mm
Weight 75g
Temperature
Operating -20°~ 60°C
Storage -30°~ 80°C
Humidity Up to 95% non-condensing

JakeRich

Yes, you can look at the data stream in NMEA for the statement

$--GGA,hhmmss.ss,llll.ll,a,yyyyy.yy,a,x,xx,x.x,x.x,M,x.x,M,x.x,xxxx*hh<CR><LF>

Field Number:
1) Universal Time Coordinated (UTC)
2) Latitude
3) N or S (North or South)
4) Longitude
5) E or W (East or West)
6) GPS Quality Indicator,
0 - fix not available,
1 - GPS fix,
2 - Differential GPS fix
7) Number of satellites in view, 00 - 12
8) Horizontal Dilution of precision
9) Antenna Altitude above/below mean-sea-level (geoid)
10) Units of antenna altitude, meters
11) Geoidal separation, the difference between the WGS-84 earth
ellipsoid and mean-sea-level (geoid), "-" means mean-sea-level
below ellipsoid
12) Units of geoidal separation, meters
13) Age of differential GPS data, time in seconds since last SC104
type 1 or 9 update, null field when DGPS is not used
14) Differential reference station ID, 0000-1023
15) Checksum

When that is "2" you have a DGPS fix, which is WAAS. I've never seen that with the BT-338, as I said.

JakeRich

GPSPassion.com has said three times that WAAS is NOT enabled. The GPSInfo software has had that WAAS checkbox forever, it's not just for the 338, it's for all GPS units.

I'll use the 338 tomorrow and keep a log file to search for any DGPS fix, but in a few weeks of use, I've never seen a DGPS fix, ever.

BKF

I don't doubt you. I'm just a little upset that GlobalSat provides WAAS data specs on their unit (regardless if it's generic chipset data or not), having you believe their unit is WAAS capable, only to find out it's not. Looking at a stream for awhile through GPSInfo, it appears there is no WAAS lock, as you assumed.

I still think it's an amazing unit, but I would have liked a WAAS capable unit if it actually provided more resolution. Especially since I DO use it for geocaching. In my tests, though, it brought me within a few feet of the target cache in a few locations. I guess that's what really matters.

JakeRich

Well, you need to know about Static Navigation, too, if you do geocaching. The early units have SN turned on, which means that you have to be moving at more than 3mph, or move more than 50 meters before it registers. GlobalSat built that in to reduce the "jitter" that you see when stopped in traffic. Unfortunately, that makes geocaching with it very hard to do. They plan to turn SN off with the new ones that are due to come out in April. You might want to contact the vendor who sold you yours to see about getting a replacement with the cold fix and SN turned off.

tralalala

JakeRich - I think you just answered my questions but let me clarify that again - GlobalSat BT-338 doesn't perform well while walking? Is there any BT GPS unit right now on the market with SIRF III chipset that has SN off? Thanks.

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JakeRich

The GlobalSat plan is to have SN off when they start shipping again in early April. They stopped shipping because of the cold temperature performance problem, and while fixing that agreed to turn off SN as well. There is a 20 page thread at www.gpspassion.com, if you want to read the history back to last year. The rep from GlobalSat, Karen, has confirmed the changes. Some beta versions are out for review and testers have reported that the cold temp problem is fixed, but I haven't seen any confirmation that SN is off on those models.

As for other SSIII models, Leadtek has one out, but battery life is under three hours, by reports. I don't know if SN is on or off.