GPS Operation: Is there a service charge?

Question

Hello!

Some questions on GPS receiver module:

1. If we purchase a USB GPS receiver module, do we have to pay monthly/time based charges for using the facility?

2. Is it possible to use the data from a receiver to detect acceleration? Approximately, how much acceleration can we measure? Would the data transmission rate between the receiver and the laptop be a problem?

Answer 1

1. No. But the GPS network might go offline whenever, it's just provided as a niceness, there's no "service guarantee".
2. Probably yes, but expect a rather coarse measurement.

Answer 2

Q1:

After Korean Air Lines Flight 007 was shot down in 1983 after straying into the USSR's prohibited airspace, President Ronald Reagan issued a directive making GPS freely available for civilian use, once it was sufficiently developed, as a common good. The first satellite was launched in 1989, and the 24th and last satellite was launched in 1994.

Initially, the highest quality signal was reserved for military use, and the signal available for civilian use intentionally degraded ("Selective Availability", SA). Selective Availability was ended in 2000, improving the precision of civilian GPS from about 100m to about 20m.

Source: Wikipedia - History of Global Positioning System

---

Q2:

You can calculate acceleration from GPS data by finding the change in velocity (Δv) divided by the change in time (Δt). The accuracy of this will depend on the GPS receiver's ability to acquire very frequent position fixes. Most consumer-grade GPS receivers can only acquire a position every second, which will not be enough for accurate acceleration readings.

Answer 3

Simple (autonomous) GPS is free, and depending on the reciever will be accurate to about 10 meters. You can purchase various types of corrections to improve on this ranging from WAAS, DGPS to NRTK depending on the capability of your receiver and how much you want to pay. Sub-meter broadcast corrections via WAAS are available on many mid-range receievers, whereas you will require a survey grade reciever and RTK or NRTK corrections for centimeter accuracy. The accuracy of an acceleration signal is based on positional accuracy and how often your position is updated, see NMEA 0183 for data formats. Some corrections are better suited to moving observers, e.g. RTCA is designed for aircraft whereas RTCM is designed for marine use. On survey grade receivers you can typically configure the dynamics of the unit to be static, slowly moving or quickly moving to suit your application.

Edit: With regards to other comments about the Americans turning the signal off, this is highly unlikely, as the russians also have actively maintained sattelites out there (Glonass), and europe is putting more into space (Gallileo). Newer receievers, often dubbed GNSS and G3, combine corrections from all these constellations to improve accuracy and reliability.

Edit 2 Using autonomous GPS without any other sensors, time tends to be very accurate and position much less so, so a good way of getting velocity is to model the probable dynamics of your movement, and use that model for smoothing purposes. e.g. If you join the dots on raw GPS position you get a zig zag line, whereas if you are driving in a car you know that you are generally travelling in a reasonably straight line, and changes in acceleration are similarly smooth. Thus if you take your recent positions, and do a least squares line fit, arc fit, or similar curve fit through them, you get the most probable line travelled. You can use this line to get a better running average determination of velocity. If you have background mapping of a road, you can also use that for regression analysis and velocity prediction. If the average direction you are travelling is varying wildly in this scenario, and the position isn't changing much, chances are that your are static. In a ship or plane the dynamics are different, you might have much larger radii that you can possibly turn through, and can factor these into your analysis. If you are walking, you can zig zag all over the place, although you usually don't. For this reason, many hillwalking GPS units include a magnetic compass and correct for bearing.

Answer 4

1: No, but you can't assume it will be available, GPS is not totally reliable. Apparently US policy is not to turn it off, but of course that can change.

2: You can, in so much as acceleration is the derivative of velocity, and GPS receivers are pretty good velocity measurement devices. But, even an avionics-grade GPS only gives you a sample at 5 Hz, most cheap ones only 1 Hz, so that's not very good acceleration measurement; it will tell you about very large accelerations sustained for a period of time.

Basically, if you want acceleration, get an accelerometer, it's the right thing.

Answer 5

No charge for use of the multi-billion dollar satellite network! Free information, well, timing signals!

You could use GPS data to measure acceleration if you are expecting large changes and measuring over an extended interval. The GPSr will compute speed, and you could compute acceleration between readings. But there is an inherent inaccuracy. First, the average accuracy for a GPSr might be 50 feet . At 65 miles per hour, you are doing 95 feet per second. See the problem? Second, the accuracy for a particular device changes with the terrain and weather (local and outer space). So you couldn't compute a stable adjustment factor.

No, the speed of your data transmission would not have a measurable effect on the computation, although you could compensate for that. The most interesting compensation, which is done inside your receiver, is made for the dilation of time caused by the high speed of the satellite. Einstein was right!