Satellite Signal Delay

Bump, I am bumping this so that it appears in the unread post area.

Miss Poi

The satellite propogation delay for signals to geostationary satellites is about 270 milliseconds round trip.

I read somewhere that the satellites only update once per second. Then there's the processing and screen refresh lag...

I don't think they work without the delay! That's what helps the GPS figure out where it is.. the delay with the Sat..... and I would think a lot depends on the local chipset being able to handle the math and time properly without a doubt!

The satellites transmit a constant signal. It is the individual GPS receiver that updates it's screen once per second.

Yep! and that's why it's soooooo important to have the least (comfortable, of course) graphic details on the screen: no fancy vehicle icons (the triangle would do just fine) and the map details set to "normal" (want more details? use the zoom in/out).
These and, from my own experience, disabling WAAS and using the "track-up" mode are giving the fastest, real-time refresh rate for the displayed map, thus better accuracy.
The reason is very simple: our gizmos are little not-so-powerful computers and the rule is: "less data for processing, better processing time".
Don't forget the screen has only 1 second to refresh, before the next satellite signal arrives!

my nuvi has trouble acquiring satellites every time i turn it on. should my car be stationary until the gps locates the satellites? does placing it near the window help? (i normally place it near the cup holder.) not sure what the best method is since i find it a bit annoying for it sometimes takes 3-5 minutes before it finds the satellite while my car is moving. but once it locates the satellites its perfectly fine.

Yeap that is what I do...

Track Up (at the "More" detail)
Blue Triangle
WAAS off.

I haven't done a comparison myself, but can't help but wonder why track up would be have a faster refresh rate than north up.

Is would seem that north up only requires only requires the vehicle to rotate when truning, whereas track up requires the entire map to rotate. Both require the entire map to move with forward progress.

Can anyone speculate as to why track up is turning out to have the faster refresh rate?

I haven't noticed much, if any difference, but I always have mine in track up.

Regarding several posts towards this issue and pertaining to the LG LN740 that I've had for several months now, aquiring satellite signal is amazingly fast ~ 20 seconds, even when the car is in motion, referesh rate is 1 second, regardless of which map view is being displayed.

On another note, I have used both GARMIN 350 and TOMTOM ONE GPS systems and I still find the LG LN740 to come out on top. (Again this is based on what I need it to do). Although the NAVTEQ map is Q2 2006, an updated 2008 map is due this January, might I add I have yet to find any major issues regarding navigation. The sound is fine, multistop routes are great to have and one I use often, traffic updates along with re-routing are quick.... although the screen of the Garmin was great to look at, I honestly hardly have to look at the screen anymore because of TTS. It's a must have in any GPS system.

The GPS will find the satellites when the vehicle is moving, as well as when it is stationary. From what I've read, placing the GPS unit near the windshield is the best place to get a strong signal with the built-in antenna. An outside antenna will produce an even stronger signal.
I've also read that if the battery needs to be recharged or if it's on its last legs, the GPS will take much longer to acquire a signal.
In any case, it should take only a few seconds to acquire a signal if there are no overhead obstructions. That would include trees, tall buildings and even the car's sheet metal roof.
I have mine suction-cup mounted at the extreme left, and low on the windshield. It doesn't obstruct the view of a person of average height or taller when mounted in this position (in most cars and trucks).

Mine takes much longer to acquire when the car is moving for the first fix of the day. After that, it's darned near instantaneous for the rest of the day.

Each of the satellites is in constant motion, they are not desynchronized. The signal they send out includes a time stamp and when your unit receives this signal, it calculates the time lag. This time lag is used to determine where you are located. That is why it is harder for the unit to find your location when you are in motion - the calculations are not consistent.

Here is a partial excerpt from Wikipedia that helps describe how it works:

A typical GPS receiver calculates its position using the signals from four or more GPS satellites. Four satellites are needed since the process needs a very accurate local time, more accurate than a clock can provide, so the receiver internally solves for time as well as position.

Each GPS satellite has an atomic clock, and continually transmits messages, each containing the current time at the start of the message, parameters to calculate the location of the satellite (the ephemeris), and the general system health (the almanac). The signals travel at a known speed - the speed of light through outer space, and slightly slower through the atmosphere. The receiver uses the arrival time to compute the distance to each satellite, from which it determines the position of the receiver using geometry and trigonometry. If the local time is known very precisely, this process (known as trilateration[4]) can determine the receiver's position using three satellites. However, most receivers do not contain clocks of this accuracy (an atomic clock would be required), and so require tracking four or more satellites so that the receiver can compute both the accurate time and its location.

Agreed. Without the delay, no triangulation is possible! Thank God for the delay, even at the speed of light! As far as using a triangle or a vehicle icon, I doubt if using the triangle saves much, if any processing power. Better to turn off the extra detail (normal) mode.

I have also noticed the apparent delay when in motion. As you pass a side street the map still shows it a little ahead of you. When you come to rest at an intersection the vehicle icon creeps a little after you stop. Once it's finished creeping (half a second or so) it shows your vehicle's position very accurately relative to the intersection. Not a big problem when you consider the amount of data these things are crunching, but you would think that Garmin would advance the position of the vehicle icon as the vehicle speed increases.

A couple of questions:

- How do you disable WAAS (Nuvi 200W)?
- Why will this improve the screen refresh (less data processing)?

The Nüvi 200 doesn't have WAAS, so you don't need to worry about it. WAAS is mostly wasted on an automotive unit anyway, as the display location is "snapped" to the road, so the added accuracy provided by WAAS isn't necessary. It's more of a marketing tool than a practical feature for road navigation.

For hiking and marine handhelds, it does have value.

As far as positional lag is concerned, it's always going to be there to a certain degree. At 60 miles an hour you are moving at 88 feet per second. If your unit refreshes once a second, plus takes a finite time to communicate with the satellite, then your car will always be at least 88 feet ahead of the display. At 30 mph the lag error is 44 feet. Remember too that the error looks about the same at slower speeds on the display for units like mine that have autozoom... as I slow down the display zooms in, so the visual error seems constant, when in reality the lag is less. Those users who say that they don't suffer any lag at all either have units with very quick refresh rates (very costly) or they are deluding themselves. (Or your GPSr is capable of predicting the future... a feature I haven't heard of yet)

I assume you're talking about digital satellite transmissions. Keep in mind that GPS satellites orbit at about half the altitude of geostationary satellites, and the delay is only a single path from satellite to GPSr, since the message is generated on-satellite. In the case of satellite TV, the program is being up- and down-linked at least once, passing through at least one MPEG encoder, and being buffered at multiple stages to absorb jitter.