Design Challenge: Assisted GPS to Improve Spatial Resolution Through Cell Triangulation

Editor's note: The following is Planet Analog's first in a series of Design Challenges. This one was sent to me by a reader/design team interested in using GPS in a particular application. We are looking for design ideas to solve this challenging GPS design. We ask you, our readers, to comment on this challenge and suggest possible solutions.

This effort is meant to stimulate the creative juices of the Planet Analog Community as well as solicit any responses from IC companies or related software or even module suppliers that may have solutions or close solutions that can be modified to fit the application.

This series is also meant as a tutorial to Planet Analog readers by fellow readers who can act as mentors to stimulate and guide your design ideas.

GPS minimum specifications:

Communications: GSM/GPRS/GPS/APRS
Frequency: 850/900/1800/1900Mhz
GPS chip: Best available — SiRFstarIV looks like good possibility
GPS sensitivity: -163dBm
Tracking: Indoor & Outdoor
Accuracy: 3 meters
GPS Start (Hot): 1 second
Battery: 3.7V lithium Ion or lithium polymer or other light weight rechargeable
Battery Charging: port for charging and setting up GPS
SIM Card: Nano
Networks: GSM networks
Operating Temp: -40°C to +85°C
Certification: FCC & CE Certified
Size: 25mm x 25mm x 15mm or 25mm diameter x 15mm
Antennas: On board GPS antenna and communications antenna

The unit will likely be dormant for up to one year. The reader's thoughts are possibly a reed switch on the unit. The unit will be placed on a magnetic field so there is no power consumption until the unit is removed from the magnetic field. Once removed from the magnetic field it needs to wake up on motion of two minutes and be able to track three to four days or more.

GPS should be capable of setting up Geo-Fence parameters manually, by dragging the fence to specific needs. GPS should be capable of triggering an email, ringtone, or SMS when the device wakes up, leaves, or enters the designated area. It should then send position to a server or cellphone every five minutes or less.

Additional batteries, if required, should be no more than 6 mm or 7 mm square or round, to increase time of operation.

New technology indicates that batteries can now be made on 3D printers for more capacity and minimal size.

It must be possible to charge the battery while unit is in motion.

Software package should monitor GPS units online, then live track and record location of GPS unit when activated. The GPS and/or the software should be able to notify monitoring computers and cellphones by SMS or email when activated.

Apps for iPhone and Android are needed for monitoring GPS movement.

The least expensive monthly charge is desired for providing cellphone or data service since the unit will likely be dormant for extended periods of time.

The following is a flow chart for operation of the device in the particular application:

Flow chart

Flow chart

Please feel free to ask any questions on Planet Analog in the comments section so all readers will benefit from complete information and answers by the design team. The team will answer any questions and add any insights in the comments section of Planet Analog, as well.

19 comments on “Design Challenge: Assisted GPS to Improve Spatial Resolution Through Cell Triangulation

  1. eafpres
    February 20, 2014

    @Steve–most of the requirements are very standard.  Let's consider the ones that are not:

    1) Dormancy plus rapid start and push notification.

    2) Sensing a magnetic field.

    3) On-board geofencing.

    4) Volume < 10 cm^3

    5) Indoor AGPS < 3m accuracy

    I didn't see a specification requirement for the magnetic sensing.  Am I missing something there?

    Some other questions about the application.  It sounds like this little module has a magnet holding it onto something, then something/someone comes along and removes it and transports it elsewhere, potentially outside a geofence.  What do we know about the thing doing the transporting?  As small as the unit is, a person could put it in their pocket, or into a purse, backpack, briefcase, satchel, etc.  At that point it might be really hard to ensure good GPS.  Any more info on that environmental question?


  2. Steve Taranovich
    February 20, 2014

    That's why I love our Planet Analog audience of designers! Blaine, you break a design down into sections that are routine, then examine the challenging areas and begin to gather more data to work toward solutions. A great lesson to all designers right here in your thought process.

    We begin our first Design Challenge: The author of this Design Challenge will answer your questions shortly.

  3. burkettbob
    February 20, 2014

    Item1 and 2   Dormant state and rapid start.

    The thought was to use a reed switch to initially power the unit. The item or items will be lying on a magnetic filed such as rubber magnet sign material. The magnet will be concealed. The unit will not be held in place by the magnetic field. It is purely for allowing initial power up when the unit is removed. We are not looking to have a hot start at that time, only after the unit is intialized. 

    Item 3  Geofencing

    Geofencing will need to be accomplished remotely or direct connection to the unit when it is initially setup by use of software program from server or computer.

    Item 4 Volume

    Might be able to use a little bit larger, but not much.

    Item 5  Indoor AGPS accuracy is not as critical. Identifying the location is most critical (House or address)

  4. geek
    February 20, 2014

    “As small as the unit is, a person could put it in their pocket, or into a purse, backpack, briefcase, satchel, etc.  At that point it might be really hard to ensure good GPS”

    @eafpres1: As far as the size of the GPS receiver is concerned, I think there's a trade-off between accuracy and convenience. The big sized units are of course much more accurate but they're bulky and inconvenient to carry around. For most applications, the small units that can fit into pockets can easily do the job.

  5. geek
    February 20, 2014

    @burkettbob: What do you think about the part where the switching takes place from GPS to A-GPS and vice-versa? How does the module handles that and how effective that is in terms of time and robustness?

  6. eafpres
    February 20, 2014

    @burkettbob–thanks for the addition information.  Is there a particular reason you want to detect removal of the device using the hidden magnet?  The technology for MEMS accelerometers is very good, low cost, and the integration into portable devices is well proven.  Why not just use such a sensor to detect motion then use the GPS to see if it is more than incidental jostling?  Can whatever this tracking unit is attached to move, and you don't care about that, only the removal of the tracker?

    -40 to +85 is pretty standard for things like automotive etc. and is easily met by the electronics.  Are there other environmental standards, and related, any requirements for the housing?  If the unit will be exposed to the elements then more expensive plastic resins will be necessary.  However, more critical is the level of sealing required.  IP-67?

    Regarding (3), Geofencing, my note as “special” comes from my interpretation of the requirement that the geofence boundary data will have to be stored in the device itself, and compared to current position.  That may be pretty easy if the boundary isn't too complex.  But it does require some memory and code.  You mention nano-SIM; do you have any particular processor and memory in mind?

  7. eafpres
    February 20, 2014

    @tzubair–my comment on the issue of getting decent GPS lock was not in relation to the unit being so small as to affect performance, although that is indeed a consideration.  My point is that, guessing at what the application is, it seems like it could easily be foiled by just tossing the little unit into a child's lunch pail.  

    Consider trackers for location of pets or children. For pets, the units are typically on the collar, and if that stays on the animal, there is some chance of finidng it.  For children, a popular approach is to conceal it in a knapsack.  If the ankle biters are wearing their pack, again you have a shot.  But if someone takes the pack, and puts it into the trunk of a (metal) car, that's the end of tracking.  

    So if this application has something to do with security, safety, monitoring, or tracking and there so some value to being able to do so, then there is exposure to the risk the basic functions of the device can easily be blocked.  Being small and removable increases that risk in my view, at least with my limited understanding of the application.

  8. chirshadblog
    February 20, 2014

    @tzubair: In terms of time I think it will be really good and will save lots of time but the conversion cost will be the issue. Im not sure how it will vary but I have a feeling that the cost will be a concern for sure

  9. chirshadblog
    February 20, 2014

    @Steve: Yes breaking down into smaller sections would be the ideal way forward. That way it will be easy to evaluate and figure out and address the issues in a much faster and an effective manner.  

  10. geek
    February 21, 2014

    “Being small and removable increases that risk in my view, at least with my limited understanding of the application.”

    @eafpres1: I agree. It does increase the risk of the device being misplaced but I guess being small has a lot of merits itself.  The pet tracking application is only possible because your pet won't feel there's something attached to it. It'd be impossible to achieve this if it's a bulky hardware. Same goes for children.

  11. burkettbob
    February 21, 2014

    for this particular use we have in mind, the time to switch off is not critical. Receiving a ping every 2 to 3 minutes is acceptable. I'm no expert on GPS, so I'm looking for ways to accomplish a job. I can't say what we intend to do with the GPS, other than that I'm in the  law enforcement community

  12. burkettbob
    February 21, 2014

    The main reason I came up with the reed switch and magnet is to conserve power. This unit may sit a year or more with no activity. My understanding of accelerometers is that there will be some power consumption using that method. I have used microprocessors and acceleromters in the past but the units I used drew about 15 microamps. Over a period of a year, that adds up.

    There are no environmental issues that I see for this application, but it would be nice to have it waterproofed for other applications.

    Geofencing can be as simple as a 200 to 300 foot diameter circle or square with relation to it's current position

    I don't have any particular processor or modules in mind. I'm open for suggestions.

    Getting a GPS lock is the chance we will have to take if the unit is placed in any shielded container, we've just lost it. I've had some success with GPS in trunks, because a lot of vehicles use a lot of plastic for body parts. This is particularly true when using AGPS.

  13. burkettbob
    February 21, 2014

    I did a look at MEMS Accelerometer and was surprize at it characteristics and size. It has some definite possibilities. The issue I see, is if motion has stopped, can the GPS remain powered once it left the geofence.

  14. Davidled
    February 22, 2014

    When looking at the flow chart, GPS need to notify server and cellphone for Geofence notification. GPS need to keep up the power on.  I wonder what type server is connected to GPS unit. I guess that GPS unit is on register with cell carrier network or some type of server. For Geofence notification, map data might be necessary with GPS location. 

  15. Netcrawl
    February 22, 2014

    @burkettbob, GPS is a all-weather, satellite-based round the clock positioning system, its an extremely highly reliable systems when it comes to navigation. Its the only operational global navigation system and have been used all over the globe to access location-based services, it provides the US military the advantages it need in the battle field, the ability to strike targets anywhere in this planet. GPS is  a big help in law enforcement, with GPS asset can be monitored round the clock using fewer resources instead of an entire police team.  

  16. samicksha
    February 25, 2014

    I guess they are simplest MEMS devices possible, by the theory i guess yes, not sure what pratical results will be.

  17. PCR
    February 27, 2014

    Yes Goo suggestion chirshadblog , it will be definitely speedup the process and will increase the accuracy. 

  18. RedDerek
    March 4, 2014

    I see another challenge here. It is the power source and the duration of operation. The OP says that the unit needs to last 3 to 4 days once powered and transmitting. When looking at the volume, consideration for how often the transmit is made will be very dependent on overall operation duration. Furthermore, without looking at the GPS specs, it may be wise to shut down the GPS in between transmit periods and have the GPS come back up to acquire position before an update transmission. I suspect the GPS antenna is active in order to keep the size small, thus power is constantly consumed for the GPS operation that could limit overall operation time.

    My Garmin lasts about 20 hours on two AA batteries for example (granted, there is the LCD display being updated in this case).

  19. Davidled
    March 4, 2014

    ->My Garmin lasts about 20 hours on two AA batteries

    I wonder if Garmin is still active for navigation or just idle. Power consumption time is depending on what this device is doing.  To tracking location on road, GPS should be “On” continuously. If GPS shuts down during transmission period, it would degrade the navigation performance due to too many interrupt of GPS On and Off.

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