TTFF issues

[matt_field]

Wonder if people can help

Kit

iPAQ HP5450

Pharos iGPS-CF (with minor cosmetic trim to get it to fit over the WiFi antenna)

CF adapter

Compaq battery / PCMCIA sleeve.

Tomtom 3
Winfast Navigator
GPS Dash

Having recently swapped from wired TT3 antenna do to a faulty serial port, ive been experiencing a dramatic downturn in TTF times when using TT3 (to the point where a 2 hour drive today failed to get a lock). I know its not the GPS as using Winfast Nav, I get 530 seconds TTFF from factory reset, and 180 seconds from Hot start.

The issues I get are the following

1. If i reset ipaq with GPS loaded I get "unrecognised card" so I have to remove card and reboot.

2. If I run TT3, it says the card is in "MEW-CFGPS1.0" port, and i use either "pharos iGPS-180" or "NMEA 0183v2 4800" as GPS

Very occasionally I get TTFF of under 10 mins, but normally well over 20. During this time I see sat status moving, and get UTC time streamed so I know receiver is working

3. If I run Navigator it sets to NMEA Modem Com4 4800. TTF worst case is around 580 seconds factory, or 200 Hot. (I've gone through the settings from the help page on here, and all seems ok)

4. GPS Dash. Having finally got it to work with TT3 Hard wired GPS, without destroying TT3 GPS program, i now find it cant connect.

Does anyone have any idea why TT3 is behaving sooooo badly (there is no comm4 available in TT3.

Apart from that the receiver is superb, HDOP from Navigator was at a solid 1.0 for over 50 miles...

Thanks in advance

Matt

[ckuethe]

not knowing exactly what these cosmetic alterations are, they could very well have affected your receiver performance. I have an iGPS as well, and it's pretty quick about time to first fix even starting cold in an urban canyon. If you mangled the plastic that could very well be bad for the antenna, as the shape, position and thickness of the radome has a fairly significant impact on an antenna's performance. See the paper below
GPS Antenna Considerations for Automotive Applications:
http://www.macom.com/FileMapServlet/redirect.red?o=GPS01&t=0

Also, you mentioned you're using a wireless card. While I haven't done any tests on this myself (I will if people are curious) other people who know what they are talking about have tested the jamming effects of 2.4GHz on GPS receivers. An increase of 2.3 dBm in 2.4GHz interference will cause a 3dB signal degradation on the gps. When the interfering signal strength gets into the -20 to -15 dBm range, you'll lose lock entirely. Even with a wimpy little 50mW card, you can probably swamp the GPS.

0 dBm = 1 mW
3 dBm = 2 mW
6 dBm = 4 mW
8 dBm = 6.3 mW
15 dBm = 32 mW
17 dBm = 50 mW
20 dBm = 100 mW

I'm a unix guy without an hPaq, so you can take my advice with a big grain of salt.[/quote]
_________________
http://www.ualberta.ca/~ckuethe/gps/

[ckuethe]

Something else - if you've got a Pharos iGPS, there's a pretty good chance you're running the "ES" enhanced sensitivity version of firmware. The software version string reported by the receiver will probably be "231.000.000ES". As I understand it, the ES firmware works by sampling a lot more data before feeding it to the FFT and integrators something like 3 to 5 times as much, meaning that the firmware seems to slow down by the same amount. Still... 10 minutes to first fix seems like an awful lot. You're sure you're not trying to do direct P-code acquisition? ;)

I suppose you've already tried going someplace with lots of skyview, and tested without the wireless card to rule out the possibility of local interference?
_________________
http://www.ualberta.ca/~ckuethe/gps/

[matt_field]

Thanks for the replies.

Today, TTFF took 20 minutes, however once locked in, I had between 8 and 10 permanent locks.

Cosmetic mods - having looked at the antenna casing, I removed less than 1mm from the bottom left hand corner, which has not gone through the plastic.

(Talking of which, what is the brass connector under the rubber foot?)

Wireless is only run when im in the office, so thats that one ruled out.

According to Pharos, cold TTFF should be no more than 1 minute.

I've got a re-radiating antenna that I will fit back in this weekend. Took it out of the car as the Tomtom GPS would lock within seconds of starting.

I guess that its loosing both almanac and as well as ephemeris data as I have to remove the card each time I start the iPAQ else it comes back as un recognised card.

PS - Im a Unix guy too ;-)

[matt_field]

Meant to ask,

What do you mean by "direct P-code acquisition" and how do I check the firmware?

Cheers

Matt

[ckuethe]

http://www.pharosgps.com/products/hp/iPAQ5450/PF080.htm
http://www.pharosgps.com/products/accessories/GPS_Receivers/REC12.htm

Pharos is right: your TTFF should be less than a minute if you have half-decent skyview. The SiRF chipset is pretty quick about finding signal if there is signal to be found. When I got mine, I stuck it vertically in my living room window, where it had a pretty obstructed view with lots of multipath, and I still had satellites within a minute. I take it you've seen the TTFF comparisons? http://www.pocketgpsworld.com/ttffcomparisons.php

I'd be really interested in running your GPS in full debug mode to see WTF is going on that it takes 10 minutes to get a fix. We've ruled out a messed-up radome if you only shaved a bit off a corner on the underside - it was more the plastic over the patch antenna I was worried about. We've ruled out wavelan interference as you said you only use the wavelan in the office (you did eject the card while running the gps, right?) The almanac, ephemeris and RTC are battery backed, and should be stored for at least two weeks, along with any settings like serial port speed, SBAS and NMEA message rate. You could have some bad hardware. I do seem to recall reading something about ipaq processors interfering with gps receivers. Sorry, no reference.

As best I can tell, Pharos makes one main receiver, the iGPS360 and a couple of derivatives. If you can plug your receiver into a laptop with a CF slot you can probably watch the NMEA output in something like hyperterm. You can also just point SiRFdemo at it. If you're quock enough about connecting, when the receiver boots up, you should see output like this:

$PharNav SW Version 07S203
$Build date: Sep 23 2003 10:53:18
$SW Version 231.000.000ES

As for direct P-code aquisition, that was gps geek humor: that your poor TTFF times were the result of you trying to get a lock on the annoyingly ... ephemeral P-code.

A more complete explanation: the gps satellites currently transmit two codes C/A-code on 1575.42MHz and P-code on 1575.42MHz and 1227.60MHz. (M-code somewhere in L5 is planned.) The C/A (coarse/acquisition) code is what we poor civilians have access to, and those with acceptable political credentials get access to the P (precise) code. C/A-code is short, low-speed, easy to acquire, high-power and openly documented; P-code is not. Without too much trickery, code lock can be achieved on C/A in a few seconds - under 8 seconds for a warm start on the SiRFstarII - then you wait at least 30 seconds for the ephemeris and almanac. Once you have enough almanacs and ephemerides, a navigation solution can be computed. This process is much more difficult with the P-code; The code is more complex, and is transmitted at much lower power. This can make for delays of several to many minutes to get a direct P-code aquisition. Luckily there is a shortcut: use some of the information you get from the C/A-code: that's why there's the A-for-Acquisition in the name. What ephemerides and almanacs are to C/A-code fixes, time to within a millisecond and position to within a few thousand meters are to P-code fixes. So why not just use C/A to bootstrap P-code? Because there are times when The Management may disable the C/A code For Security Reasons, or because the enemy is jamming the C/A code. Trying for a direct P-code fix while under fire, dodging between shelters doesn't sound particularly pleasant. This some nifty work on hardware assisted direct P-code acquisition going on, mostly done with FPGAs. see the following google scholar link.

http://scholar.google.com/scholar?q=direct+P-code+acquisition
_________________
http://www.ualberta.ca/~ckuethe/gps/

[matt_field]

Blimey!!!

Thankyou for a really comprehensive answer :D

I will try later to link into my laptop and see what happens.

You mention that there is battery backup, is this a rechargeable battery stored in the Pharos GPS itself? I had wondered about this, especially as I have to eject the unit every time i start the iPAQ up.

Will let you know how I get on

Cheers

Matt

[ckuethe]

Yes, there's a little "energy storage cell" onboard. It's either a little lithium cell or a supercap of some sort. The exact technology used varies by vendor and board revision. My pharos receiver has everything hidden under RF shields that I'm not about to pry open, and my tn-200 receivers all have a little lithium cell soldered down.
_________________
http://www.ualberta.ca/~ckuethe/gps/

[matt_field]

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