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Sunday, March 18, 2012

What to do about GPS reception problems

GPS satellite reception problemsIs your GPS not locking onto satellites? There’s usually a simple solution that works for both auto and handheld GPS units, but first, let’s look at why this is happening.

The science of GPS

GPS satellites are always on the move – they are not geostationary. If your GPS receiver knows where to look for them, it can speed reception. The satellites transmit several signals, one of which is almanac data, which provides the orbital information for the satellites in the system. Most modern GPS receivers store several days worth of almanac data, which can help speed acquisition, by predicting where to search for the satellites.

The causes

So if you haven’t used your GPS for several days, it may be slow to lock onto satellites. And since the GPS constellation is always shifting, there are good reception days and bad reception days, which is why I say it MAY be slow to lock.

This can be exacerbated by other things as well, such as:

  • Being in an environment where signals are blocked (urban canyons are notorious for this)
  • In my own experience, GPS receivers are slower to lock when moving, so it can help to wait for a good signal before putting your car in drive
  • Moving your GPS a great distance is also problematic (see “The airport problem” below)

The solution

What to do if your receiver won’t lock? The answer is really quite simple. Power your GPS on, and let it sit under open sky, stationary, for half an hour. If you don’t have a good spot outdoors, put it on your dashboard, giving it as much of a view of the sky as possible. DO NOT turn it off as soon as it locks onto satellites; it needs time to download the almanac.

Special case: The airport problem

Flying cross-country or around the globe? Sorry to anthropomorphize, but your GPS ends up confused about where it is — it’s looking for satellites in relation to where you last turned it on. So here’s my suggestion. Use your GPS the morning of or the day before your trip; let it guide you to the airport. As soon as possible, after your flight and you are outside of the airport, turn it on. I often do this while riding a shuttle bus to pick up a rental car. Another option is to set it in a hotel window, but remember, don’t turn it off as soon as it locks. Give it a full half hour to download the satellite almanac data.

Cell phone pros and cons

Mobile phones use assisted GPS (A-GPS), meaning they are able to get GPS satellite almanac data from cell towers, which happens much faster than downloading it from satellites (which transmit data at a paltry 50 bits per second). Unfortunately many cell phones struggle with GPS reception in the absence of a cellular signal, so I wouldn’t exactly call them superior in this regard.

About Rich Owings

Rich is the owner, editor and chief bottle-washer for GPS Tracklog. Connect with him on Twitter, Facebook or Google Plus.

Comments

  1. You say receivers are slower to lock when moving, and this sounds plausible to me. But you recently interviewed a TeleNav GPS expert who said

    ”The impact to GPS signal acquisition is negligible.  A car speeding at 118 miles per hour is moving at 50 meters per second.  The satellites are moving at 3,873 meters per second.  The relative speed along the line-of-sight between the satellite and the receiver determines how much the GPS downlink signal is Doppler shifted in frequency.  This line-of-sight speed is dominated by the satellite speed, and the receiver’s speed is inconsequential.’

    So who’s right?

    • Which is exactly why I said “in my own experience!”

    • Rich, feel free to correct me if you feel I am wrong, but I’ve owned numerous GPS receivers over the last 10 years or so and even have experience with several models of military receivers. Satellite acquisition in a moving vehicle has always been a problem.
      Every single device I’ve ever used acquires quicker if it is stationary. The faster the receiver is moving, the longer it takes to acquire. Many GPS units warn that they may not acquire at all, or track reliably over a certain velocity, typically around 300 or 400 mph. The reason is not so much the logistics of satelites and the relatively slow movement of the GPS receiver, but rather the software algorithms used by the specific receivers. The software determines when the GPS receiver has successfully acquired stable satellite tracking. If the receiver sees large changes in position each time it samples, it continues to crunch through it’s initialization routines, repeatedly checking for stable readings. After it receives additional satellites that can be integrated into the position calculations that correlate with the existing readings, it will eventually decide it has enough confidence in the readings to switch from acquisition to tracking mode even though the readings are still drifting. My older GPS units had substancially less capable software and would occasionally refuse to lock, even in a fast moving car, let alone an aircraft. The newer units are substancially faster and more sensitive making their acquisitions considerably faster and more reliable. Especially units designed specifically for vehicular use. These modern units are so fast and reliable it is quite rare for you to notice the effect velocity has on satellite acqisition time.
      I believe the simple answer to GMAN’s original queary is Yes, expect longer acquisition times when a GPS receiver is moving, especially if you have an older device.

  2. Pythagoras says:

    Excellent article!

  3. Hi,

    I have one Question! Is there any other way for using the GPS in indoors.. by which we can know the exact location or approx location of the device located on maps.

    What i mean is, The way we determine a cell phone location from the nearest tower it is accessing for communication, the same way!, any other means available ??

    • Most GPS units don’t have any other built-in positioning capability. Many smartphones already do a pretty good job of it though, and they may be using WiFi to do so.

  4. Springfield Harrison says:

    Hello:
    Just bought a Garmin Nuvi 2495LMT for use in our truck camper. My Garmin GPSmap76 requires an external antenna to work with the camper overhead. It covers the cab, half the hood and extends a foot to the sides.

    A Garmin tech assured me that reception would be OK with the 2495 although there is no provision for an external antenna with any of the Nuvis. I haven’t had a chance to test it yet.

    Question: the Quick Start guide refers to an external antenna which plugs into the power cable (?!). The 2495 power cable does have a box-like bulge near the receiver with a jack hole in it. Is this for an external antenna? If so where are they obtained?

    Thanks very much, cheers . . . . Springfield Harrison

  5. Dean Sonneborn says:

    I asked garmin tech support if the etrec20 would download the almanac data and they said the it would not. That only the fancy, expensive scientific type gps units would do that. What I wanted to find out from them is how long would the unit maintain the almanac data and would it survive turning off the unit and then on again at a later point and/or a battery change. So I was quite surprised when they responded that it would not. Rick could you clarify this downloading of the almanac data on an etrex 20 unit?
    thanks, Dean

  6. Rich thanks for the explanation. My Garmin 2597 is experiencing the airport problem, I guess. I bought it in AU, and worked no problem. Then I bought a new map and took it to the US, which also worked beautifully. Then the problem comes after I brought it back to AU again. It is now always took 30 min to find the satellite with extremely poor signal.

    I will take your advise to put it on my balcony overnight to down load the full almanac data to see how it goes from there

    Thanks
    William

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