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PMB-248 GPS Receiver indoors — Parallax Forums

PMB-248 GPS Receiver indoors

stonechildstonechild Posts: 4
edited 2010-02-10 16:05 in Accessories
Does the PMB-248 GPS Receiver work at all indoors? I know GPS receivers are designed to work outdoors, but I've seen people report that other brands of GPS receivers work for them indoors.

Comments

  • WBA ConsultingWBA Consulting Posts: 2,934
    edited 2010-01-08 08:02
    GPS receivers will work anywhere they can obtain the minimum required number of satellite signals. (obviously for triangulation reasons, 3 is the normal minimum, but some units may require more).

    While working on my Reverse Geo-Cache project that uses the Parallax GPS module, I was getting GPS lock while sitting in the living room with the GPS module mounted inside the briefcase. It was ~4 feet away from my front window which is 5'x8'. My Garmin GPS can get satellite lock about 8 feet away from the same window.

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    Andrew Williams
    WBA Consulting
    WBA-TH1M Sensirion SHT11 Module
    Special Olympics Polar Bear Plunge, Mar 20, 2010
    Propeller-Based Reverse Geo-Cache Birthday Present Project
  • stonechildstonechild Posts: 4
    edited 2010-01-09 21:01
    Thanks for the reply, Andrew. I was looking at the 32 Channel LS20033 GPS 5Hz Receiver and the EM-406A GPS Module both of which go for about $60.00. They don't seem to perform any better than the PMB-248 GPS for my purposes and at half the price. The receiver is for a robot.

    I thought this would do away with buying a compass module to determine direction as well as position, but I didn't think about the case of a motionless robot until it was pointed out in another forum. Now I wonder how far it would have to move to determine its direction.

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    Dennis
  • WBA ConsultingWBA Consulting Posts: 2,934
    edited 2010-01-12 07:00
    Glad I could help. In regards to the direction, only consistent movement will give you a valid direction (heading) with a GPS. While my GPS was sitting stationary in the living room, the lat/long values changed slightly as the GPS module recalculated based upon signals. You would have to adjust your code to ignore these slight changes and only provide a heading while the GPS' position moved more than X distance so that you know the direction was accurate. You should also average readings over a few seconds to prevent your direction from jumping around. If you need "always available" accurate heading info, you must use a compass module of some sort.

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    Andrew Williams
    WBA Consulting
    WBA-TH1M Sensirion SHT11 Module
    Special Olympics Polar Bear Plunge, Mar 20, 2010
    Propeller-Based Reverse Geo-Cache Birthday Present Project
  • stonechildstonechild Posts: 4
    edited 2010-01-12 20:39
    I guess I will just have to do some experimenting as far as direction goes. I wonder if the location data could be fed to a neural net to get the information I want. That's assuming I get far enough in understanding neural nets to do this. I'm not so much interested in direction as I am in writing software that can learn about the layout of a room.

    I think I'm going for the LS20031 GPS Receiver. Pololu has just started selling them for $49.95.

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    Dennis
  • DufferDuffer Posts: 374
    edited 2010-01-13 05:08
    Even if you can get a good fix indoors, the accuracy of nearly all civilian GPS receivers is +-3 meters at best. That means your position data could be off as much as 18-20 feet. Even in a 30'x30' room, you couldn't be sure that you position would even be inside the room, let alone indicate the position of the receiver in the room. Mapping a room is just not a good job for a GPS system.

    Duffer
  • stonechildstonechild Posts: 4
    edited 2010-01-13 06:06
    I understand that GPS would not allow me to map a room based on the coordinates received, but the question that has yet to be answered for me is: Are the coordinates accurate relative to one another.

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    Dennis
  • max72max72 Posts: 1,155
    edited 2010-01-13 06:47
    Stonechild said...
    I understand that GPS would not allow me to map a room based on the coordinates received, but the question that has yet to be answered for me is: Are the coordinates accurate relative to one another.


    If you have a GPS to play with you can test if it works, but I think it is not possible to use a GPS this way.

    In open space you can assume relative coordinates are quite accurate. In a room not at all. You have a geometric issue and a multipath one. You see a small amount of satellites, and they are not evenly distributed. So error source is higher, and if you move you might loose o gain a satellite, and it would change the errors, so relative position would not be reliable, and change a lot between two readings. Moreover you probably get the position accepting signals not arriving directly to the sensors, but bouncing in the room.

    The error in a room would be higher than 3-5 meters, both absolute and relative.

    Massimo
  • kf4ixmkf4ixm Posts: 529
    edited 2010-01-13 12:34
    stonechild,
    in a word, no, they will not be accurate reletive to each other. you may get lucky every once in a while and get a few consistent coordinates, but not enough for positioning with any accuracy.

    Post Edited (kf4ixm) : 1/13/2010 1:41:08 PM GMT
  • kf4ixmkf4ixm Posts: 529
    edited 2010-01-13 14:54
    here is an example of the inaccuracies in indoor gps signals. these are taken using the 28146 parallax gps module one foot from the window in a second story apartment with no cloud cover. if you look at the deviation, it could only achieve about 42 feet of accuracy. also if you have a wireless network around your location, it will detune the gps reciever some, further throwing off accuracy,·depending on what channel your network is using, proximity to wifi tranciever, etc.

    Post Edited (kf4ixm) : 1/13/2010 3:43:07 PM GMT
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  • try388try388 Posts: 8
    edited 2010-02-09 07:17
    hi.. i have a parallax gps receiver module also. However, i cant seem to make it work. Is there anyway i could actually get this stuff work.
    Also, is there any programming code available for it.
    Thank
  • hover1hover1 Posts: 1,929
    edited 2010-02-09 18:32
    Which model number is it? What processor are you using to read the NMEA strings. What are you using for output device?

    There are usually sample code for Parallax products on the individual product pages.

    Jim
    try388 said...
    hi.. i have a parallax gps receiver module also. However, i cant seem to make it work. Is there anyway i could actually get this stuff work.
    Also, is there any programming code available for it.
    Thank
  • try388try388 Posts: 8
    edited 2010-02-10 00:12
    i using parallax gps receiver module code number 28146. i uses the basic stamp 2 for programming.
  • hover1hover1 Posts: 1,929
    edited 2010-02-10 00:55
    I have 2 of these units and have not had a problem with interfacing them with a BS2 or the Propeller chip.
    The demo program in the attachment works well with the BS2 chip. I have found you need to have the GPS module outside in order to get a good signal. I sometimes takes 1-2 minutes to aquire the minimum of three satellites to get a good fix.
    Could you specify if you are using the GPS in Smart or Raw mode?
    Jim
    try388 said...
    i using parallax gps receiver module code number 28146. i uses the basic stamp 2 for programming.
    zip
    zip
    GPSDemoV1.1.zip
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  • try388try388 Posts: 8
    edited 2010-02-10 00:58
    i using the raw. i follow the instruction from the step give by parallax. i plug the gps into the basic stamp education board. i plug the sio to pin 15. gnd to vss and vcc to vdd. i could not accuquire any satellite.
  • hover1hover1 Posts: 1,929
    edited 2010-02-10 02:29
    Does the LED on the GPS stay blinking or does it come on solid?
  • try388try388 Posts: 8
    edited 2010-02-10 14:54
    it blinking. i try it indoor and outdoor, however, it cant seem to detect any satellite.
  • try388try388 Posts: 8
    edited 2010-02-10 15:04
    [img][http://forums.parallax.com/forums/ppop.aspx?upg={aada9658-1f49-4f28-b071-41e58f0d555e}][/img][noparse][[/noparse]http://forums.parallax.com/forums/ppop.aspx?upg={aada9658-1f49-4f28-b071-41e58f0d555e}]

    above are pictures of my connection
  • try388try388 Posts: 8
    edited 2010-02-10 15:12
    [img][http://forums.parallax.com/forums/ppop.aspx?upg={e05786b8-5948-4d89-99f5-e1f0acf1d2ad}][/img]
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  • hover1hover1 Posts: 1,929
    edited 2010-02-10 15:23
    In order to set the GPS in RAW mode, the /RAW pin must go to VSS.

    But nothing will work unless you have a satellite lock.·Bring the unit outside and let it try and aquire a signal for at least 10 minutes. The LED should stay on steady when you have a lock.

    Jim
    try388 said...
    [img][http://forums.parallax.com/forums/ppop.aspx?upg={e05786b8-5948-4d89-99f5-e1f0acf1d2ad}][/img]
  • try388try388 Posts: 8
    edited 2010-02-10 15:25
    u mean i also connect the raw to vss?
  • hover1hover1 Posts: 1,929
    edited 2010-02-10 15:30
    You might want to re-read page 2 and 3 of the manual.
    try388 said...
    u mean i also connect the raw to vss?
    pdf
    pdf
    GPSManualV1.1.pdf
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  • try388try388 Posts: 8
    edited 2010-02-10 15:34
    ok. i will take a look at it.
    do u know anything about max232 or fastrack supreme wavecom?
  • hover1hover1 Posts: 1,929
    edited 2010-02-10 16:05
    I'm not familiar with the Fastrack Supreme, but that's a topic for another thread.
    try388 said...
    do u know anything about max232 or fastrack supreme wavecom?
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