Page 1 of 2 1 2 LastLast
Results 1 to 20 of 33

Thread: Speed?

  1. #1
    Join Date
    Aug 2010
    Location
    Kempton park
    Age
    36
    Posts
    1,597
    Thanked: 15

    Default Speed?

    Hi Guys,

    Just would like to know, if i am using my GPS as a speedometer, is that the correct speed? or must I stay to my Toyota's Speedometer?There is about a 10km/h difference, will i be ok according to my GPS?dont want to get any fines....
    Alex
    2019 Mitsibushi Triton 2.4DiD

  2. #2
    Join Date
    Dec 2008
    Location
    Suffolk, UK
    Age
    63
    Posts
    5,617
    Thanked: 47

    Default

    Your GPS is much more accurate than your car speedo. Stick to the lower speed of the two, then you never have to worry!!

    Mike
    "A poxy, feral, Brit architect who drinks bad beer and supports the wrong rugby team." Tony Weaver

    "Mike for President" Freeflyd

  3. #3
    Join Date
    May 2009
    Location
    Jo'burg
    Age
    49
    Posts
    2,753
    Thanked: 8

    Default

    Your GPS speed will be fine, if it has good reception.
    You car will over-read compared to the GPS.

    Just be careful...
    If it's a Nuvi, it also records maximum speed achieved. You can check it in the trip log, by clicking on the arrival-time, when you have a destination selected.
    I only know this because the Garmin techy commented on the 201km/h I had my unit at a time.
    SWAMBO
    2009 Suzuki Grand Vitara 3.2i V6 Auto
    Me
    2003 Ford Focus 2.0

    Eurard: For T=2; T=1 but, 1/2T=

  4. #4
    Join Date
    May 2009
    Location
    Ruimsig
    Age
    60
    Posts
    8,790
    Thanked: 45268

    Default

    definately GPS. If you know the tolerences that Toyota use in their manufacturing process the GPS will be the safer bet

    PS: They use GPS Telemetry in some land speed recordings
    Estee = S T = Sean Towlson, A Schrodingers Douche Bag GOF

    ''In Western society, it is not the facts or the truth that are important, but the correct presentation of information, even if it is a lie.''

    Clank, a 1979 Ex-SADF Series III 109 SW powered by a Toyota 2B Oil Burner

  5. #5
    Join Date
    Mar 2010
    Location
    Klein Karoo (CBR)
    Age
    51
    Posts
    2,683
    Thanked: 984

    Default

    Quote Originally Posted by MikeAG View Post
    Your GPS is much more accurate than your car speedo.
    Mike
    Really? What about Satelite time delays, plus it is only accurate to max of 2m.... Wouldn't a direct couple like a speedo cable be more accurate?
    Everybody needs to believe in something.... I believe I will have another Beer!

    Travel Blog: https://since1652.wordpress.com/

    https://www.instagram.com/since1652travel/

  6. #6
    Join Date
    May 2009
    Location
    Ruimsig
    Age
    60
    Posts
    8,790
    Thanked: 45268

    Default

    What satelite time delays?
    Estee = S T = Sean Towlson, A Schrodingers Douche Bag GOF

    ''In Western society, it is not the facts or the truth that are important, but the correct presentation of information, even if it is a lie.''

    Clank, a 1979 Ex-SADF Series III 109 SW powered by a Toyota 2B Oil Burner

  7. #7
    Join Date
    Sep 2006
    Location
    Undisclosed
    Posts
    3,569
    Thanked: 2248

    Default

    GPS is spot on. I travelled for 2 years at 80 000 km per year and my cruise control was always set on "no admission of guilt" speed minus 3 km/h and I trusted it enough to go that way.

    That said, I was caught only once. At 135 km/h in a 120 km/h zone by camera. Have no clue where they put it and I also have no clue where I went that slow that day.
    The price of stability is unpreparedness.

  8. #8
    Join Date
    Mar 2010
    Location
    Klein Karoo (CBR)
    Age
    51
    Posts
    2,683
    Thanked: 984

    Default

    Quote Originally Posted by Estee View Post
    What satelite time delays?
    The time it takes for data to travel from earth to satelite & then back again.... it is not as fast as you think..... (in data terms in any case)
    Everybody needs to believe in something.... I believe I will have another Beer!

    Travel Blog: https://since1652.wordpress.com/

    https://www.instagram.com/since1652travel/

  9. #9
    Join Date
    Aug 2010
    Location
    Kempton park
    Age
    36
    Posts
    1,597
    Thanked: 15

    Default

    Cruise control?
    Alex
    2019 Mitsibushi Triton 2.4DiD

  10. #10
    Join Date
    May 2009
    Location
    Jo'burg
    Age
    49
    Posts
    2,753
    Thanked: 8

    Default

    Quote Originally Posted by Papsak View Post
    The time it takes for data to travel from earth to satelite & then back again.... it is not as fast as you think..... (in data terms in any case)
    As long as the delay is constant, it makes no difference on the measurement.

    The only time you pick up problems, is when you turn lots. Then you're changing satelites all the time.
    It's made worse by high buildings around you, the location of your GPS etc.
    All that boils down to signal quality/strength.

    But on a highway, where you're pretty much using the same bunch of satelites, the delay doesn't interfere at all wrt speed measurement.
    SWAMBO
    2009 Suzuki Grand Vitara 3.2i V6 Auto
    Me
    2003 Ford Focus 2.0

    Eurard: For T=2; T=1 but, 1/2T=

  11. #11
    Join Date
    Aug 2009
    Location
    Squamish, BC
    Age
    41
    Posts
    3,478
    Thanked: 504

    Default

    Transmission delay in the signal, GPS is clever enough to compensate, go and google it!
    Office Johnny
    2008 Triton CC 3.5V6 4x2 - Sold
    2007 Triton DC 2.5DiD 4x4 - Sold
    2007 Pajero GLS 3.2DiD - Sold
    2011 Ford Escape 2.5 4x2

  12. #12
    Join Date
    Jul 2010
    Location
    Johannesburg
    Age
    45
    Posts
    2,043
    Thanked: 16

    Default

    Satellite time delays for GPS are very cleverly dealt with through differencing.
    You are quite correct that CA code GPS can be out by metres, but here we are talking about accuracy. To measure speed, all one requires is precision, which GPS will provide with two measurements taken with a short time between them. (Accurately measured time.)

  13. #13
    Join Date
    Sep 2006
    Location
    Undisclosed
    Posts
    3,569
    Thanked: 2248

    Default

    Quote Originally Posted by Goofaloop View Post
    Cruise control?
    Well uhm yeah!!!

    If you drive that much, it is soon a very rustige, comfortable speed where things happens well within your honed reaction times. I did not sweat pulling over at that speed in the gravel shoulder is the need was there.

    Sjoe, at some days it went so hard that I was driving 160 km/h AFTER some hard braking to stay ready for something that looked remotely if it would get in my way.

    That said, as I sit here today, I remind myself on a daily basis that I never never ever in my life, want to drive that much again. The toll it took on me and my family life was just not worth it.
    The price of stability is unpreparedness.

  14. #14
    Join Date
    Aug 2009
    Location
    Squamish, BC
    Age
    41
    Posts
    3,478
    Thanked: 504

    Default

    Quote Originally Posted by Wikipedia
    Correcting a GPS receiver's clock

    One of the most significant error sources is the GPS receiver's clock. Because of the very large value of the speed of light, c, the estimated distances from the GPS receiver to the satellites, the pseudoranges, are very sensitive to errors in the GPS receiver clock; for example an error of one microsecond (0.000 001 second) corresponds to an error of 300 metres (980 ft). This suggests that an extremely accurate and expensive clock is required for the GPS receiver to work. Because manufacturers prefer to build inexpensive GPS receivers for mass markets, the solution for this dilemma is based on the way sphere surfaces intersect in the GPS problem.


    It is likely that the surfaces of the three spheres intersect, because the circle of intersection of the first two spheres is normally quite large, and thus the third sphere surface is likely to intersect this large circle. It is very unlikely that the surface of the sphere corresponding to the fourth satellite will intersect either of the two points of intersection of the first three, because any clock error could cause it to miss intersecting a point. However, the distance from the valid estimate of GPS receiver position to the surface of the sphere corresponding to the fourth satellite can be used to compute a clock correction. Let denote the distance from the valid estimate of GPS receiver position to the fourth satellite and let denote the pseudorange of the fourth satellite. Let . is the distance from the computed GPS receiver position to the surface of the sphere corresponding to the fourth satellite. Thus the quotient, , provides an estimate of
    (time indicated by the receiver's on-board clock) - (correct time),
    and the GPS receiver clock can be advanced if is positive or delayed if is negative. However, it should be kept in mind that a less simple function of may be needed to estimate the time error in an iterative algorithm as discussed in the Navigation equations section.
    Pretty cool..
    Office Johnny
    2008 Triton CC 3.5V6 4x2 - Sold
    2007 Triton DC 2.5DiD 4x4 - Sold
    2007 Pajero GLS 3.2DiD - Sold
    2011 Ford Escape 2.5 4x2

  15. #15
    Join Date
    Jul 2008
    Location
    Stellenbosch
    Age
    64
    Posts
    1,258
    Thanked: 160

    Default

    Garmin publishes the speed accuracy of their GPSr's as 0.1 mph or 0.16 km/h absolute, obviously dependent on satellite signal reception strength. Some studies have shown that practical speed accuraries of 0.1 m/s (that's 0.1 meters per second or 0.36 km/h) have been demonstrated for 50% of the valid measurement period, and 0.4 m/s or ~ 1.4 km/h for a further 20% of the measurement time.

    Rather trust your 0.1% accurate GPS speed over the ~10% accuracy of your car's speedo.
    - Nothing comes at a higher price than that which is free.

  16. #16
    Join Date
    May 2009
    Location
    Jo'burg
    Age
    49
    Posts
    2,753
    Thanked: 8

    Default

    Quote Originally Posted by petermoffat View Post
    Pretty cool..
    Interesting!

    I made my observation through a mind experiment only...

    Looking at a car from above, like from a helicopter or such...

    I spot the car at t=0
    And I see where the car is a t=1
    So I know the distance traveled in 1s.

    Now if there is a 1sec delay between what I'm seeing, and what's actually on the ground, it doesn't make a difference.

    At t=0, I see the car. But the car is actually at t=1 position, but I don't know it.
    At t=1, I note the car's position again. But the car is actually at t=2 position.
    I will still accurately measure it's speed, though my data is 1 second old.
    Last edited by RedLineR; 2011/03/24 at 04:21 PM.
    SWAMBO
    2009 Suzuki Grand Vitara 3.2i V6 Auto
    Me
    2003 Ford Focus 2.0

    Eurard: For T=2; T=1 but, 1/2T=

  17. #17
    Join Date
    Aug 2009
    Location
    Squamish, BC
    Age
    41
    Posts
    3,478
    Thanked: 504

    Default

    You assume that light from t0 to t1 takes the same amount of time to reach your eyes. But if the car is driving up a hill, and you are following directly above, then there would be a difference. It's all got to do with relativity. I think?
    Office Johnny
    2008 Triton CC 3.5V6 4x2 - Sold
    2007 Triton DC 2.5DiD 4x4 - Sold
    2007 Pajero GLS 3.2DiD - Sold
    2011 Ford Escape 2.5 4x2

  18. #18
    Join Date
    May 2009
    Location
    Jo'burg
    Age
    49
    Posts
    2,753
    Thanked: 8

    Default

    Yes, the major assumptions are...
    1) The delay is constant.
    2) With 4 or more satellites, you'll actually have 3D position, which will allow compensation for altitude as well. Though the earth is relatively flat when viewed from such great distances, that the error is massively reduced anyways.
    SWAMBO
    2009 Suzuki Grand Vitara 3.2i V6 Auto
    Me
    2003 Ford Focus 2.0

    Eurard: For T=2; T=1 but, 1/2T=

  19. #19
    Join Date
    Aug 2009
    Location
    Squamish, BC
    Age
    41
    Posts
    3,478
    Thanked: 504

    Default

    Science. F*** YEAH!!

    Quote Originally Posted by http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html

    Real-World Relativity: The GPS Navigation System
    People often ask me "What good is Relativity?" It is a commonplace to think of Relativity as an abstract and highly arcane mathematical theory that has no consequences for everyday life. This is in fact far from the truth.
    Consider for a moment that when you are riding in a commercial airliner, the pilot and crew are navigating to your destination with the aid of the Global Positioning System (GPS). Further, many luxury cars now come with built-in navigation systems that include GPS receivers with digital maps, and you can purchase hand-held GPS navigation units that will give you your position on the Earth (latitude, longitude, and altitude) to an accuracy of 5 to 10 meters that weigh only a few ounces and cost around $100.
    GPS was developed by the United States Department of Defense to provide a satellite-based navigation system for the U.S. military. It was later put under joint DoD and Department of Transportation control to provide for both military and civilian navigation uses.
    The current GPS configuration consists of a network of 24 satellites in high orbits around the Earth. Each satellite in the GPS constellation orbits at an altitude of about 20,000 km from the ground, and has an orbital speed of about 14,000 km/hour (the orbital period is roughly 12 hours - contrary to popular belief, GPS satellites are not in geosynchronous or geostationary orbits). The satellite orbits are distributed so that at least 4 satellites are always visible from any point on the Earth at any given instant (with up to 12 visible at one time). Each satellite carries with it an atomic clock that "ticks" with an accuracy of 1 nanosecond (1 billionth of a second). A GPS receiver in an airplane determines its current position and heading by comparing the time signals it receives from a number of the GPS satellites (usually 6 to 12) and triangulating on the known positions of each satellite. The precision is phenomenal: even a simple hand-held GPS receiver can determine your absolute position on the surface of the Earth to within 5 to 10 meters in only a few seconds (with differential techiques that compare two nearby receivers, precisions of order centimeters or millimeters in relative position are often obtained in under an hour or so). A GPS receiver in a car can give accurate readings of position, speed, and heading in real-time!
    To achieve this level of precision, the clock ticks from the GPS satellites must be known to an accuracy of 20-30 nanoseconds. However, because the satellites are constantly moving relative to observers on the Earth, effects predicted by the Special and General theories of Relativity must be taken into account to achieve the desired 20-30 nanosecond accuracy.
    Because an observer on the ground sees the satellites in motion relative to them, Special Relativity predicts that we should see their clocks ticking more slowly (see the Special Relativity lecture). Special Relativity predicts that the on-board atomic clocks on the satellites should fall behind clocks on the ground by about 7 microseconds per day because of the slower ticking rate due to the time dilation effect of their relative motion.
    Further, the satellites are in orbits high above the Earth, where the curvature of spacetime due to the Earth's mass is less than it is at the Earth's surface. A prediction of General Relativity is that clocks closer to a massive object will seem to tick more slowly than those located further away (see the Black Holes lecture). As such, when viewed from the surface of the Earth, the clocks on the satellites appear to be ticking faster than identical clocks on the ground. A calculation using General Relativity predicts that the clocks in each GPS satellite should get ahead of ground-based clocks by 45 microseconds per day.
    The combination of these two relativitic effects means that the clocks on-board each satellite should tick faster than identical clocks on the ground by about 38 microseconds per day (45-7=38 )! This sounds small, but the high-precision required of the GPS system requires nanosecond accuracy, and 38 microseconds is 38,000 nanoseconds. If these effects were not properly taken into account, a navigational fix based on the GPS constellation would be false after only 2 minutes, and errors in global positions would continue to accumulate at a rate of about 10 kilometers each day! The whole system would be utterly worthless for navigation in a very short time. This kind of accumulated error is akin to measuring my location while standing on my front porch in Columbus, Ohio one day, and then making the same measurement a week later and having my GPS receiver tell me that my porch and I are currently about 5000 meters in the air somewhere over Detroit.
    The engineers who designed the GPS system included these relativistic effects when they designed and deployed the system. For example, to counteract the General Relativistic effect once on orbit, they slowed down the ticking frequency of the atomic clocks before they were launched so that once they were in their proper orbit stations their clocks would appear to tick at the correct rate as compared to the reference atomic clocks at the GPS ground stations. Further, each GPS receiver has built into it a microcomputer that (among other things) performs the necessary relativistic calculations when determining the user's location.
    Relativity is not just some abstract mathematical theory: understanding it is absolutely essential for our global navigation system to work properly!
    Office Johnny
    2008 Triton CC 3.5V6 4x2 - Sold
    2007 Triton DC 2.5DiD 4x4 - Sold
    2007 Pajero GLS 3.2DiD - Sold
    2011 Ford Escape 2.5 4x2

  20. #20
    Join Date
    Jul 2010
    Location
    Johannesburg
    Age
    45
    Posts
    2,043
    Thanked: 16

    Default

    The only occasion that relativity is taken into account for GPS is the need for correcting the atomic clocks on the satellites.

Page 1 of 2 1 2 LastLast

Similar Threads

  1. Part catalogs & other useful info
    By charlatan in forum Jeep
    Replies: 93
    Last Post: 2020/06/01, 01:02 PM
  2. AT Tyre Tests
    By kalahari in forum Tyre chat
    Replies: 67
    Last Post: 2016/04/02, 08:58 PM
  3. Installing chips to increase power
    By stevan in forum Vehicle & Technical Chat - General
    Replies: 171
    Last Post: 2012/09/10, 10:02 AM
  4. Grand Cherokee Error code
    By Nick47 in forum Jeep
    Replies: 4
    Last Post: 2010/08/30, 01:28 PM
  5. Using Diff-lock on steep descent
    By piers in forum General 4x4 Discussion
    Replies: 82
    Last Post: 2008/12/09, 01:19 PM

Tags for this Thread

Bookmarks

Posting Permissions

  • You may post new threads
  • You may post replies
  • You may post attachments
  • You may edit your posts
  •