During some holiday planning round the H kitchen table recently, H junior is playing with the Garmin 1490T sat nav and finds a screen I hadn't seen before. It's the maximum speed that the unit has ever travelled which says 121mph. Although the sat nav has been on both our cars, my motorcycle and my bicycle, in theory we don't possess a vehicle that can do 121mph. The C8 is supposed to be able to do 116mph, the C3 114mph and the motorcycle 110mph.
H junior thinks I'm some mad outlaw, Mrs H thinks I can't be trusted with her car again and I'm bewildered. The sat nav was bought new from Halfords in Sept 2009.
Where did this figure come from? I'm not conscious of trying to go flat out at any time. I would have remembered, wouldn't I? Anyone else with sat nav speed anomalies ?
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My old BMW RT had a Garmin made unit. I topped it out at a 120 mph. which closely matched what it said on the speedo a few hours earlier. All perfectly legal etc etc...
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Position error at some point. It might only have measured that over a very short interval. Walking back from the paper shop yesterday at a brisk 4mph, my speed according to Runkeeper on the iPhone was fluctuating up to 8mph.
Is that going to be enough to get you off the hook?
Last edited by: Manatee on Sun 17 Jun 12 at 10:53
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Was it in your pocket on an Intercity train journey? ;-)
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I know of at least one fatal accident (biker) where sat-nav data was downloaded as part of the investigation.
My own friend survived his major fall - to back track his speeds and was confident enough to be able to identify his exact speed when his tyre blew out.
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I once inadvertantly left my TomTom on in my briefcase while on a flight to Italy. Must check it !
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I left mine on on a flight to scotland once. At 560mph it made apoplectic shouting noises about speed cameras! Not sure the driver was bothered about them tho.
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>> Position error at some point. It might only have measured that over a very short
>> interval. Walking back from the paper shop yesterday at a brisk 4mph, my speed according
>> to Runkeeper on the iPhone was fluctuating up to 8mph.
>>
>> Is that going to be enough to get you off the hook?
Wife " are you seriously trying to tell me it thinks you walked at 120 mph?"
Err no, you are seriously going to have to do better than that.
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My walkers Garmin Etrex has just once thrown a major wobbly. Showed me has having done 500+ miles in under an hour around Thornthwaite Forest - max speed 570!!
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Back to the motorised transport; I can imagine the bike with a different screen and a less porky rider maybe achieving 120mph down hill but that would be road speed along the hypotenuse of the triangle wouldn't it? The sat nav would see a different, lesser speed from above because the calculation algorithms assume the earth's surface is smooth. I think.
Where's number muncher when you need him?
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The car sat nav usually calculates speed over the ground distance using its mapping, not triangulation speed which would be distorted by rise and fall of terrain.
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"triangulation speed which would be distorted by rise and fall of terrain."
I wonder how much effect this would actually have. I can understand that if you climb, say Snowdon, on foot it might make a difference to distance walked but what about on a trip from E Midlands to Glasgow on M1, A50, M6, A 74, M74? Would it make much difference?
Just asking!
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I think its the speed of change of height that would upset triangulation speed over ground.
Plus of course you have the issue of "cutting corners" if you don't use mapping for speed calcs.
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>> "triangulation speed which would be distorted by rise and fall of terrain."
>> I wonder how much effect this would actually have.
Surprisingly little effect. Imagine a 6% gradient on a motorway - a typical maximum gradient.
(100*100) + (6*6) = 10036
Square root of the above is 100.18
So a difference of less than 0.2 MPH at a nominal 100 MPH
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Without bothering to Google I would think Satnavs would measure instantaneous speed by calculating doppler shift on the satellites' RF signals rather than triangulation. Shaking/dropping it voilently might well reigster a high speed - dunno about a ton plus though.
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a shake or a drop measures nothing. insufficient distance covered.
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You don't need to cover distance if you're measuring instantaneous speed using Doppler shift.
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The car sat nav registers nothing when shaken or dropped no matter what way it measures it.
Doppler shift however is all about changes in distance to the signal source., it has to be. thats what doppler shift is.
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>> Doppler shift however is all about changes in distance to the signal source., it has
>> to be. thats what doppler shift is.
>>
And depending on what you are processing the shift for it helps to know the base frequency.
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>> Without bothering to Google I would think Satnavs would measure instantaneous speed by calculating doppler
>> shift on the satellites' RF signals rather than triangulation. Shaking/dropping it voilently might well reigster
>> a high speed - dunno about a ton plus though.
>>
Satnav is a time based system not frequency shift.
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>> The car sat nav usually calculates speed over the ground distance using its mapping, not
triangulation speed
>> which would be distorted by rise and fall of terrain.
The satnav/ datahead thingies in our car transporters give an altitude readout as well as a dozen other parameters. I would think they're smart enough to compensate for gain/loss in altitude when calculating roadspeed.
The GPS on my phone has occasional blips in both speed and altitude - it can record a 5-mile hike with a CN Tower-like 1000ft spike in the middle, or a sedate stroll punctuated by a short blast of 90mph in the middle of a field. Given that, I wouldn't rely on the OP's unit's peak speed figure.
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My old old Garmin bike kit and less old Garmin did this, mildly entertain to while away an hour or so. Penypass (Snowdon) 335 meters.
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>> >> The car sat nav usually calculates speed over the ground distance using its mapping,
>> not
>> triangulation speed
>> >> which would be distorted by rise and fall of terrain.
>>
>> The satnav/ datahead thingies in our car transporters give an altitude readout as well as
>> a dozen other parameters. I would think they're smart enough to compensate for gain/loss in
>> altitude when calculating roadspeed.
The satnavs in trucks are cars give misleading height, they are simply not set up or engineered to use it properly. No way would you go to the programming aggro of setting up algorithms to compensate speed by height, its not required when its far easier to measure time over distance on the map.
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Why is my netbook double posting?
EDIT - sods law it didn't that time!
Last edited by: Old Navy on Sun 17 Jun 12 at 21:33
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Done a restart, that usually fixes minor doppler gremlins. :-)
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Got a new Sat Nav the Tom Tom start.Very nice posh English voice and also gives the street names.Pleased with it up to now.
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It's straightforward really :-)
Ref: Misra & Enge "GPS: Signals, Measurements, and Performance" (2001)
Sec. 5.2.1 (pgs 196-197) Velocity Estimation
"The relative motion of a satellite and the user results in changes in
the observed frequency of the satellite signal. This Doppler shift is
measured routinely in the carrier tracking loop of a GPS receiver
[Section 9.6]. Given the satellite velocity, the Doppler shift can be
used to estimate the user velocity. The Doppler shift, or equivalently,
the range rate [Section 1.3.3], can be written as a projection of the
relative velocity vector on the satellite line-of-sight vector. The
measurement, however, is biased by the receiver clock bias rate (i.e.,
frequency offset), and what's actually measured is the pseudorange
rate.
"The delta pseudoranges obtained from carrier phase measurements are
proportional to the average pseudorange rates or the line-of-sight
velocity of the user relative to the satellite over the time interval.
The model for pseudorange rates can be obtained by differentiating
(5.1). It is left as an exercise to show that
[equation 5.28 is true]
where v_sup(k) [a vector quantity] is the satellite velocity vector,
known from the navigational message broadcast by the satellite; v is
the user velocity vector, to be estimated. Both v_sup(k) and v are
expressed in the ECEF coordinate frame. The user-to-satellite unit
vector 1_sup(k) is determined from an estimate of the user position;
b_dot is the bias of the receiver clock (m/s), and the
epsilon_sub_phi_sup(k) denotes the combined error doe to changes during
the measurement interval in the satellite clock, ionosphere and
troposphere. Note that the velocity of an object attached to the earth
is zero in the ECEF coordinate frame.
"The principal source of error in (5.28) throughout the 1990s was the
satellite clock frequency dithering due to SA. Now with SA gone, the
remaining errors arise from changes in the ionospheric and tropospheric
delays and in multipath, and are generally small. Problems, however,
can arise if the user dynamics are excessive. The delta ranges give
only average velocity over a time interval. High accelerations and
jerks would clearly be problematic. The PPS performance specifications
for velocity estimation (0.1 m/s rms in any direction; 0.2 m/s 2drms)
are based on a constant-velocity scenario [JPO(1991)].
"Equation (5.28) is linear in user velocity components, and can be
rewritten...
the combined set of measurements from K satellites can be written as a
set of equations compactly in matrix notation as
[equation 5.29]
where matrix G characterizes the user-satellite geometry, as defined
previously (5.10). It is interesting that the problem of estimation of
user velocity based on pseudorange rates is identical in structure to
that of estimation of user position from pseudoranges (5.9). A
least-squares solution and the DOP parameters can be defined, as
before, and related to the rms error in these estimates".
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Tunnel perhaps?
I noticed the sat-nav assumes you maintain a constant speed when it loses a signal.
Say you entered a tunnel in a jam at very slow speed, traffic cleared and you were able to, legally, accelerate to NSL. You then exit tunnel and sat-nav calculates your speed from the time you re-appeared and your expected time from your speed when you 'disappeared'.
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First time tested my Nokia phone's sat nav. I found it recalculating routes much faster than traditional sat navs. Also unlike my Navman sat nav, it finds satellites quickly.
I momentarily went up to 40 MPH in a 30 MPH road and it told me "observe speed limit"!
It must be downloading data on the fly as I didn't download the offline map data to the phone.
I roughly calculated that it ate 2 MB of data for 30 minute/10 miles trip.
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I'm not a big satnav user - I have a very good natural sense of direction and location - but the Ovi Maps software on my Nokia is brilliant. As movilogo says, it picks up satellites very quickly and updates itself with speed limits and new roads. It also voices every street name (a neat party trick) and can overlay satellite photo images on the map. The phone can also use the GPS receiver to track country walks etc and generate calorie consumption figures for exercise.
I mostly use it to "bookmark" where I've left the car in unfamiliar towns or at the Alton Towers car park :)
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I used to run TomTom 5 on a HP PDA with an external GPS.
The highest speed it recorded in the car was somewhere around 750mph (I don't recall exactly) on the A38 near Burton on Trent.
The car was an Accord Diesel estate.
The software had crashed in some way and the speed rose by 20odd mph every couple of seconds, until I reset it.
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IIRC someone (Zeddo maybe) explained better my tunnel theory above quite some while ago.
I've done a search, but can't find it.
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Well... The tom tom has tunnels noted on its map, so as you say it assumes the speed at loss of signal in the tunnel, is carried on and plots itself on the map accordingly through the tunnel.. When you get to the end and get a new fix, it catches up (or is spot on - it actually works quite well)
So I suppose its possible that if you enter at 20mph, and speed up inside, when it gets a refix it will have been plotted lots of yards short, and will suddenly catch up. Could be interpreted as a massive burst of speed I suppose.
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Sat Nav perjures itself - R.P.
