Join Early Retirement Today
Reply
 
Thread Tools Search this Thread Display Modes
Old 10-23-2012, 04:07 PM   #41
Give me a museum and I'll fill it. (Picasso)
Give me a forum ...
RonBoyd's Avatar
 
Join Date: Dec 2007
Location: Denver, Colorado
Posts: 5,280
Quote:
Originally Posted by Midpack View Post
Yes there are lots of them, some come with the phones, and they provide turn-by-turn maps with voice guidance. Garmin even sells a smartphone GPS app.

And evidently some carmakers are going one better, displaying your smartphone GPS screen on the car's center stack screen (image below)...http://www.nytimes.com/2012/10/12/au...ions.html?_r=0.

One day your smartphone will do everything but breathe for you, I'm looking forward to it...

This graphic doesn't tell one which lane to be in nor the number of lanes from which to choose -- note the images I posted.

I am unsure here. Maybe I need to familiar myself with what turn-by-turn maps are. (Not saying this isn't an apt description... just that I need to make sure we are speaking of the same thing as live tracking.)

I don't know what a Car's "center stack screen" is.

Quote:
"A better, safer solution would be to feed a phone’s maps and instructions through the car’s larger, built-in display and sound system."
How is this safer than being placed in your field of vision? I never have to take my eyes off the road (well. you know what I mean -- not look away). And yes, I can pipe the sound through the vehicle's speaker system -- I elect not to but it is an option.

Nevertheless, I don't really don't have a dog in this fight. My praise is for GPS per se. Whether it is on a laptop with MS S&T, a Smart Phone, A Tablet, or a dedicated GPS unit (in-dash or not) is not important to me (all, BTW, devices I have and use extensively). The Garmin device I use is the result of many years of personal experimentation to arrive at what is most comfortable for me and YMMV.

Truth be told, I enthusiastically support anyone's navigational choice... well, paper maps are down the list, I guess.
__________________

__________________
"It's tough to make predictions, especially when it involves the future." ~Attributed to many
"In theory, there is no difference between theory and practice. But, in practice, there is." ~(perhaps by) Yogi Berra
"Those who have knowledge, don't predict. Those who predict, don't have knowledge."~ Lau tzu
RonBoyd is offline   Reply With Quote
Join the #1 Early Retirement and Financial Independence Forum Today - It's Totally Free!

Are you planning to be financially independent as early as possible so you can live life on your own terms? Discuss successful investing strategies, asset allocation models, tax strategies and other related topics in our online forum community. Our members range from young folks just starting their journey to financial independence, military retirees and even multimillionaires. No matter where you fit in you'll find that Early-Retirement.org is a great community to join. Best of all it's totally FREE!

You are currently viewing our boards as a guest so you have limited access to our community. Please take the time to register and you will gain a lot of great new features including; the ability to participate in discussions, network with our members, see fewer ads, upload photographs, create a retirement blog, send private messages and so much, much more!

Old 10-23-2012, 04:12 PM   #42
Moderator Emeritus
 
Join Date: May 2007
Posts: 11,034
Quote:
Originally Posted by rodi View Post
Cell phone wasn't practical because we were going to some places with no reception.
With most GPS apps, cellular reception is not required for the GPS to work (just like for a dedicated GPS device).

For the app I use for example:

Quote:
NAVIGON MobileNavigator carries the complete map material within the phone. The app relies on the GPS receiver, independent from a cell or wireless signal, providing directions and recalculations even when no cell phone signal is available.
No cell phone reception, no problem.
__________________

__________________
FIREd is online now   Reply With Quote
Old 10-23-2012, 05:43 PM   #43
Give me a museum and I'll fill it. (Picasso)
Give me a forum ...
NW-Bound's Avatar
 
Join Date: Jul 2008
Posts: 19,400
Quote:
Originally Posted by Alan View Post
That's why Europe decided to put up their own independent system. The 4th satellite was launched about 10 days ago.

BBC News - Galileo: Europe's version of GPS reaches key phase
I first heard of the Galileo proposal many years ago, but have not been following up on it (well, that happens when you drop out of the business).

So, I looked at the link and learned that the first 2 Galileo sats were launched last year, and with the two newly launched satellites, they now have the minimum of 4 satellites for a ground receiver to be able to compute its position. However, the window in the day when all 4 satellites are in view for test is fairly short. And that only happens at a small preplanned area on earth.

This situation was similar to the US GPS system, back in the early 80s. There were only 5 or 6 NAVSTAR satellites up then, with their orbits arranged such that Yuma Proving Ground got a coverage of an hour or two each day for testing. And as that window slowly slipped from day to day, people had to work according to that schedule. And that meant working night or early morning.
__________________
"Old age is the most unexpected of all things that can happen to a man" -- Leon Trotsky
NW-Bound is online now   Reply With Quote
Old 10-23-2012, 05:56 PM   #44
Thinks s/he gets paid by the post
 
Join Date: May 2008
Posts: 3,416
Smart phones have GPS chipsets which can tune into GLOSNAS, the Russian system, and maybe Galileo as well.

Plus with A-GPS from cell towers, it's more accurate.

But now, you can ask for directions through Siri. Google also has their equivalent, called Google Now.
__________________
explanade is offline   Reply With Quote
Old 10-23-2012, 05:59 PM   #45
Give me a museum and I'll fill it. (Picasso)
Give me a forum ...
NW-Bound's Avatar
 
Join Date: Jul 2008
Posts: 19,400
Quote:
Originally Posted by BigNick View Post
Are you sure this is up to date? I thought President Clinton ordered the "selective availability" feature (i.e., degraded signal for civilians) to be turned off in 2000, partly because the Cold War was more or less over, partly because (I've heard) military units were hard to come by and US troops were buying their own civilian-grade units. Maybe a forthcoming version of GPS will once again introduce a military and civilian distinction. Or maybe I missed something, as usual.
It was true that during the first Gulf War, many US troops had to buy their own Trimble GPS (one of the 1st civilian GPS makers).

However, the C/A and P(Y) signals as described in the Wikipedia article are still in use as we speak. Let me explain further.

S/A or Selective Availability was the intentional degradation of the civilian C/A code, and it was indeed turned off in 2000. Back when S/A was on, the position output was god-awful, but GPS receivers were expensive and not in wide use, so not too many people remember this. They dithered the signal, so that the GPS receiver position output would jump around a few hundred feet.

Now, with S/A gone, using just the civilian GPS signals, one can get down to a few meters (subject to all the caveats I explained earlier), which is quite good for most purposes. By using the military signal with the special receivers (which must be loaded with crypto key), one can squeeze out a few more meters of errors.

However, the real advantage of the military GPS receivers is this: 1) they are more tolerant of signal jamming by the enemy due to the use of the P(Y) code, and 2) when GPS signals are denied to the enemy (who are using civilian GPS receivers), the military GPS receivers will keep on working. How and why?

The old and obsolete S/A was a global degradation "feature". There are ways to deny GPS signals to a smaller localized area of conflict (talk of conflict, I can think of guys running around and hiding in the mountain ). In fact, here's a statement from the US Government made in 2003 (Reference: GPS.gov: Special Notices Regarding GPS Selective Availability)
The United States Government recognizes that GPS plays a key role around the world as part of the global information infrastructure and takes seriously the responsibility to provide the best possible service to civil and commercial users worldwide. This is as true in times of conflict as it is in times of peace.

The U.S. Government also maintains the capability to prevent hostile use of GPS and its augmentations while retaining a military advantage in a theater of operations without disrupting or degrading civilian uses outside the theater of operations.
__________________
"Old age is the most unexpected of all things that can happen to a man" -- Leon Trotsky
NW-Bound is online now   Reply With Quote
Old 10-23-2012, 06:31 PM   #46
Give me a museum and I'll fill it. (Picasso)
Give me a forum ...
Midpack's Avatar
 
Join Date: Jan 2008
Location: Chicagoland
Posts: 11,970
Quote:
Originally Posted by RonBoyd View Post
This graphic doesn't tell one which lane to be in nor the number of lanes from which to choose -- note the images I posted.

I am unsure here. Maybe I need to familiar myself with what turn-by-turn maps are. (Not saying this isn't an apt description... just that I need to make sure we are speaking of the same thing as live tracking.)

I don't know what a Car's "center stack screen" is.

How is this safer than being placed in your field of vision? I never have to take my eyes off the road (well. you know what I mean -- not look away). And yes, I can pipe the sound through the vehicle's speaker system -- I elect not to but it is an option.

Nevertheless, I don't really don't have a dog in this fight. My praise is for GPS per se. Whether it is on a laptop with MS S&T, a Smart Phone, A Tablet, or a dedicated GPS unit (in-dash or not) is not important to me (all, BTW, devices I have and use extensively). The Garmin device I use is the result of many years of personal experimentation to arrive at what is most comfortable for me and YMMV.

Truth be told, I enthusiastically support anyone's navigational choice... well, paper maps are down the list, I guess.
I wasn't trying to convince you of anything, I was simply answering your question. If you want to buy a dash mount for a smartphone that's fine (not everyone does), but many have such small screens, you end up relying mostly on the voice guidance (at least I do, I don't look at the display as often). If you prefer Garmin, great, I think they're great products...
__________________
No one agrees with other people's opinions; they merely agree with their own opinions -- expressed by somebody else. Sydney Tremayne
Retired Jun 2011 at age 57

Target AA: 60% equity funds / 35% bond funds / 5% cash
Target WR: Approx 2.5% Approx 20% SI (secure income, SS only)
Midpack is offline   Reply With Quote
Old 10-23-2012, 06:51 PM   #47
Moderator
Ronstar's Avatar
 
Join Date: Aug 2007
Location: A little ways southwest of Chicago
Posts: 9,343
DW has a garmin nuvi and loves it. I've worked 20 some years in the gps industry but I don't use the nuvi - just my phone. It gets me where i need to go. The gps navigation world needs an upgrade. The problem isnt the accuracy of the gps - its the makeup of the address geodatabase that couples with gps in providing directions. Many of us have seen that a navigational gps gives us "wrong" directions. The error is due to approximations in the algorithm that translates the user's desired address or poi to latitude and longitude coordinates that the gps understands.

I use survey grade gps at work that in conjunction with data link to continuous operating reference stations yield accuracies of approx 1/4 inch horizontal. Makes planning/design/survey of long highways/pipelines, etc easy compared to the old days. But now we're dealing with a periodic shift of positions (put out by the national geodetic survey) that will "move" points in our area by almost 3/4".
__________________
Ronstar is offline   Reply With Quote
Old 10-24-2012, 05:22 AM   #48
Recycles dryer sheets
 
Join Date: Aug 2009
Posts: 141
I have not gone through all posts but you will see a ton of recommendations. By now, you must have gone dizzy. I did too when I bought rather expensive Garmin GPS at that time. Down the line 6 months of use, I found what a dumb GPS I bought, so set your feature list and then go get a Garmin GPS. I don't think I will buy a GPS anymore (because I have HTC One X and it has in-built GPS, just downloaded offline google map so I am good to go with it) but if I buy one, my requirements are -

- Free lifetime map updates (they cost fortune)
- Free live traffic update
- Ability to export route from PC
- Ability to record route I am driving on
- Ability to set my default driving directions on certain route
- Better navigation (this is subjective)
- Better lane explanation.
- Yari yari yara. I know you are bored by now.

All of us are most likely just the users but the folks at GPS Review Forums are experts. Post your question with your requirements and a specific model will show up

Good Luck
__________________
noelm is offline   Reply With Quote
Old 10-24-2012, 10:08 PM   #49
Give me a museum and I'll fill it. (Picasso)
Give me a forum ...
NW-Bound's Avatar
 
Join Date: Jul 2008
Posts: 19,400
I felt that I should make another post to hopefully dispel some confusions over GPS accuracy.

If you search the Web, you may stumble across some US Government official documents that describe the accuracy of even the military signal (called P/Y code, or PPS for Precise Positioning Service) as something like 6 m horizontal error, 95% probable. What that means is that over all conditions, all places on earth, and all time of day (remember that the satellites move and do not stand still), you can be sure that the horizontal error will be less than 20 ft, 19 out of 20 times.

That may appear crummy, and contradicts with the centimeter accuracy that is claimed by people who use GPS for surveying. So, what is the truth?

The answer is that both claims are correct. The difference is like trying to compare apples and oranges.

The surveyors use a method called differential GPS. And by using a technique called carrier phase tracking, they do get errors to less than 1". However, this accuracy is with respect to a reference GPS receiver set at a known reference location, and is achieved by logging data over a period, then doing some post-processing. It is not to be compared to the normal use of a stand-alone GPS like the normal use in a mobile application.

So, for surveying one can see that the absolute accuracy depends on the precision of location of the reference station. That reference location itself may not be known that well within the WGS-84 reference system, and may have to be adjusted. I guess that's what Ronstar said about the survey point coordinates being moved every so often.

I hope that clears up some questions readers may have about some apparent conflicting info.

But, can I raise another question, if anybody is really curious about all this GPS stuff?

Now, we know that the basic principle of operation of a GPS receiver is that of measuring the ranges to the satellites. The range to a satellite is measured by the propagation delay of the satellite signal to the receiver. Knowing the satellite positions and the ranges to them, we should be able to compute the receiver 3-D position using 3 satellites. However, because we do not have an atomic clock that is synchronized to the GPS system time, we need a 4th satellite to solve for that 4th unknown, meaning time. And how do we know the satellite positions? They tell us that by broadcasting their orbital parameters.

Here's the question. How do the satellites know their position?
__________________
"Old age is the most unexpected of all things that can happen to a man" -- Leon Trotsky
NW-Bound is online now   Reply With Quote
Old 10-24-2012, 10:39 PM   #50
Give me a museum and I'll fill it. (Picasso)
Give me a forum ...
Lsbcal's Avatar
 
Join Date: May 2006
Location: west coast, hi there!
Posts: 5,675
Quote:
Originally Posted by NW-Bound View Post
...(snip)...
Here's the question. How do the satellites know their position?
One of the great benefits of not working is only solving the problems I want to solve.

Am waiting breathlessly for the answer.
__________________
Lsbcal is offline   Reply With Quote
Old 10-24-2012, 10:44 PM   #51
Thinks s/he gets paid by the post
 
Join Date: Mar 2010
Location: Kerrville,Tx
Posts: 2,708
Quote:
Originally Posted by NW-Bound View Post
I felt that I should make another post to hopefully dispel some confusions over GPS accuracy.

If you search the Web, you may stumble across some US Government official documents that describe the accuracy of even the military signal (called P/Y code, or PPS for Precise Positioning Service) as something like 6 m horizontal error, 95% probable. What that means is that over all conditions, all places on earth, and all time of day (remember that the satellites move and do not stand still), you can be sure that the horizontal error will be less than 20 ft, 19 out of 20 times.

That may appear crummy, and contradicts with the centimeter accuracy that is claimed by people who use GPS for surveying. So, what is the truth?

The answer is that both claims are correct. The difference is like trying to compare apples and oranges.

The surveyors use a method called differential GPS. And by using a technique called carrier phase tracking, they do get errors to less than 1". However, this accuracy is with respect to a reference GPS receiver set at a known reference location, and is achieved by logging data over a period, then doing some post-processing. It is not to be compared to the normal use of a stand-alone GPS like the normal use in a mobile application.

So, for surveying one can see that the absolute accuracy depends on the precision of location of the reference station. That reference location itself may not be known that well within the WGS-84 reference system, and may have to be adjusted. I guess that's what Ronstar said about the survey point coordinates being moved every so often.

I hope that clears up some questions readers may have about some apparent conflicting info.

But, can I raise another question, if anybody is really curious about all this GPS stuff?

Now, we know that the basic principle of operation of a GPS receiver is that of measuring the ranges to the satellites. The range to a satellite is measured by the propagation delay of the satellite signal to the receiver. Knowing the satellite positions and the ranges to them, we should be able to compute the receiver 3-D position using 3 satellites. However, because we do not have an atomic clock that is synchronized to the GPS system time, we need a 4th satellite to solve for that 4th unknown, meaning time. And how do we know the satellite positions? They tell us that by broadcasting their orbital parameters.

Here's the question. How do the satellites know their position?
As stated in this Wikipedia article there is a contingent of the airforce that tracks the GPS satellites and updates their clocks and onboard models of the orbit of the satellite. Global Positioning System - Wikipedia, the free encyclopedia

The European and Russian systems must have a similar tracking and update service as well.
__________________
meierlde is offline   Reply With Quote
Old 10-24-2012, 10:50 PM   #52
Give me a museum and I'll fill it. (Picasso)
Give me a forum ...
NW-Bound's Avatar
 
Join Date: Jul 2008
Posts: 19,400
Quote:
Originally Posted by Lsbcal View Post
One of the great benefits of not working is only solving the problems I want to solve.
Well, it is not really that hard like actually solving the problems. I never did work on the so-called Space Segment of the GPS system.

It's more like asking about how things work, generally speaking of course, without using any math or equation. It's really something we are usually curious about, but often overlook or do not think of asking ourselves.

Here's a personal example. Though not a mechanic, I have worked on gasoline engines all my life, from small 2-cycle engines to big V-8s. But I have not been exposed to diesel engines. So it was only recently that it occurred to me that diesel engines do not have a throttle like gasoline engines. Yes, I knew how diesel fuel was injected, blah blah blah, but I did not make the connection to the missing butterfly throttle, and that diesel engines draw full air all the time.
__________________
"Old age is the most unexpected of all things that can happen to a man" -- Leon Trotsky
NW-Bound is online now   Reply With Quote
Old 10-24-2012, 10:54 PM   #53
Give me a museum and I'll fill it. (Picasso)
Give me a forum ...
NW-Bound's Avatar
 
Join Date: Jul 2008
Posts: 19,400
Quote:
Originally Posted by meierlde View Post
As stated in this Wikipedia article there is a contingent of the airforce that tracks the GPS satellites and updates their clocks and onboard models of the orbit of the satellite. Global Positioning System - Wikipedia, the free encyclopedia

The European and Russian systems must have a similar tracking and update service as well.
Ah yes! The satellites do not know their position and have to be told. Ground stations are used to track the satellites and upload their orbital parameters, which are then broadcast back to the GPS user receivers.

The ground stations compute the satellite positions by the inverse ranging, by knowing their own position and measuring the range to the satellites.

OK! Now, how do the ground tracking stations know their own positions?
__________________
"Old age is the most unexpected of all things that can happen to a man" -- Leon Trotsky
NW-Bound is online now   Reply With Quote
Old 10-24-2012, 11:23 PM   #54
Thinks s/he gets paid by the post
 
Join Date: Mar 2010
Location: Kerrville,Tx
Posts: 2,708
Quote:
Originally Posted by NW-Bound View Post
Ah yes! The satellites do not know their position and have to be told. Ground stations are used to track the satellites and upload their orbital parameters, which are then broadcast back to the GPS user receivers.

The ground stations compute the satellite positions by the inverse ranging, by knowing their own position and measuring the range to the satellites.

OK! Now, how do the ground tracking stations know their own positions?
As with all surveying you need to define a point as the origin. Then using todays technology of very long base baseline interferometry you can find out to inches how far it is to another point from the point of origin (this is how they track plate movements) do enough of these and assume a figure for the earth such as wgs 84 or newer and you have the location for the tracking stations. Here is Wikipedia on vlbi Very-long-baseline interferometry - Wikipedia, the free encyclopedia.

It was vlbi that discovered that surveying over an ocean had large errors in the distance between continents. VLBI then corrected it.
__________________
meierlde is offline   Reply With Quote
Old 10-25-2012, 04:54 AM   #55
Moderator
Ronstar's Avatar
 
Join Date: Aug 2007
Location: A little ways southwest of Chicago
Posts: 9,343
Quote:
Originally Posted by NW-Bound View Post
I felt that I should make another post to hopefully dispel some confusions over GPS accuracy.

If you search the Web, you may stumble across some US Government official documents that describe the accuracy of even the military signal (called P/Y code, or PPS for Precise Positioning Service) as something like 6 m horizontal error, 95% probable. What that means is that over all conditions, all places on earth, and all time of day (remember that the satellites move and do not stand still), you can be sure that the horizontal error will be less than 20 ft, 19 out of 20 times.

That may appear crummy, and contradicts with the centimeter accuracy that is claimed by people who use GPS for surveying. So, what is the truth?

The answer is that both claims are correct. The difference is like trying to compare apples and oranges.

The surveyors use a method called differential GPS. And by using a technique called carrier phase tracking, they do get errors to less than 1". However, this accuracy is with respect to a reference GPS receiver set at a known reference location, and is achieved by logging data over a period, then doing some post-processing. It is not to be compared to the normal use of a stand-alone GPS like the normal use in a mobile application.

So, for surveying one can see that the absolute accuracy depends on the precision of location of the reference station. That reference location itself may not be known that well within the WGS-84 reference system, and may have to be adjusted. I guess that's what Ronstar said about the survey point coordinates being moved every so often.

I hope that clears up some questions readers may have about some apparent conflicting info.

But, can I raise another question, if anybody is really curious about all this GPS stuff?

Now, we know that the basic principle of operation of a GPS receiver is that of measuring the ranges to the satellites. The range to a satellite is measured by the propagation delay of the satellite signal to the receiver. Knowing the satellite positions and the ranges to them, we should be able to compute the receiver 3-D position using 3 satellites. However, because we do not have an atomic clock that is synchronized to the GPS system time, we need a 4th satellite to solve for that 4th unknown, meaning time. And how do we know the satellite positions? They tell us that by broadcasting their orbital parameters.

Here's the question. How do the satellites know their position?
Well said NW-Bound. Part of the reason for the coordinate change is improved accuracy made possible by an increased number of GPS baselines, and part is due to movement (1-2 mm per year) of the North American tectonic plate.

As to post processing - we no longer need it in the Chicago area. We subscribe to the regional Trimble VRS system that consists of several base stations whose data is transmitted to our receivers instantaneously. That coupled with observations on US and Glonass satellites (maybe 10-20 at a time) provide us with sub centimeter accuracy in a 1 or 2 second observation.
Trimble - Positioning Services - Trimble VRS Now

As to satellites knowing their position - I do not know the specifics, but after the launch, the government performs several weeks if not months of analysis on the orbit before it's ephemeris is derived.

In the old days of post processing data from few satellites, it was important to know the health of satellites. Toward the end of their life cycles, satellites would produce bad data for a while until the gov't turned off the satellite. During that time, the data had to be analyzed and faullty satellite data manually removed from the solution.
__________________
Ronstar is offline   Reply With Quote
Old 10-25-2012, 12:21 PM   #56
Give me a museum and I'll fill it. (Picasso)
Give me a forum ...
 
Join Date: Sep 2005
Location: Northern IL
Posts: 18,272
Quote:
Originally Posted by NW-Bound View Post
But, can I raise another question, if anybody is really curious about all this GPS stuff?

...

Here's the question. How do the satellites know their position?
Yes, very interesting to some of us. I attended an in-house seminar at MegaCorp about 12 years ago that spent about half a day getting us familiar with GPS basics. I was really impressed with the technology, and the thought that went behind it.

Don't know the answer to your question, but I did remember why 4 satellites were the minimum required, and I recall that they picked up a repeating signal and just kept accumulating that signal until the sum (correlated) popped out of the noise (uncorrelated). And something about an 'almanac', that I think told the GPS unit where (or which?) to look for the satellites at that time.

So the summing explains how they can pick up such low level signals. What I don't understand is how this is resistant to being jammed by the enemy. Seems it would easy to mess with such a low level signal.

-ERD50
__________________
ERD50 is online now   Reply With Quote
Old 10-25-2012, 04:31 PM   #57
Give me a museum and I'll fill it. (Picasso)
Give me a forum ...
NW-Bound's Avatar
 
Join Date: Jul 2008
Posts: 19,400
Quote:
Originally Posted by meierlde View Post
As with all surveying you need to define a point as the origin. Then using todays technology of very long base baseline interferometry you can find out to inches how far it is to another point from the point of origin (this is how they track plate movements) do enough of these and assume a figure for the earth such as wgs 84 or newer and you have the location for the tracking stations. Here is Wikipedia on vlbi Very-long-baseline interferometry - Wikipedia.

It was vlbi that discovered that surveying over an ocean had large errors in the distance between continents. VLBI then corrected it.
VLBI indeed plays a part in modern geodesy, but there are other techniques that allow researchers to collect a lot more data and over more places on earth a lot quicker. See: Satellite geodesy - Wikipedia.

The reason for collecting a lot of data is to establish a global reference system, which is more involved than defining some origin points like done in the past. But what kind of data? Please read further.

Surveying used to be done with reference to some known landmarks, called geodetic datums (not data!). See: http://en.wikipedia.org/wiki/Datum_(geodesy).

Starting from the 1950s, the drive was towards a common and global reference system, hence the procession of World Geodetic System 60 (WGS60), WGS66, WGS72, and the current WGS84. See: World Geodetic System - Wikipedia.

WGS defines a reference ellipsoid that best matches the earth's mean sea level (MSL) surface. The problem is that the earth core is not homogenous, and its actual equipotential surface at MSL has significant deviation from the ideal ellipsoid. This is caused by gravity anomalies and deflections of the vertical. See: Gravity anomaly - Wikipedia and Vertical deflection - Wikipedia.

Hence, latitude and longitude of a point as defined in WGS84 may differ significantly from latitude and longitude of the same point, but obtained the old way by star sighting. See: Latitude - Wikipedia, the free encyclopedia and Longitude - Wikipedia.

The altitude as defined in WGS84 (or GPS altitude) also differs from the MSL altitude by a significant amount. The difference can be as high as a few hundred feet. Many GPS receivers now incorporate an altitude correction table, so that they can display MSL altitude. Many early GPS users were perplexed when they saw that their GPS receivers showed a big number (may be negative too) for altitude when they were right at the beach. They might not know what the old latitude and longitude was, but they surely expect an altitude of 0 right at the beach. No, GPS was not wrong; it was just different!

The actual MSL surface of the earth is called the geoid which undulates above and under the reference ellipsoid. As the geoid surface is defined by the earth gravity with all its local variations from the ideal values, the two are tightly related. Spherical harmonics is the most convenient way to express the earth gravity model, using field measurements that were collected. As more and more data is incorporated, the degree and order of the spherical harmonics keep increasing. See: Geoid - Wikipedia.

I was surprised to learn that they now have up to degree 2160 and order 2160. Moreover, the entire model with over 4 million coefficients is now available to the public. Fifteen years ago, it used to be that even much smaller models were classified data, as they would be useful for ICBM trajectory calculations among other things. Amazing!
__________________
"Old age is the most unexpected of all things that can happen to a man" -- Leon Trotsky
NW-Bound is online now   Reply With Quote
Old 10-25-2012, 04:53 PM   #58
Give me a museum and I'll fill it. (Picasso)
Give me a forum ...
NW-Bound's Avatar
 
Join Date: Jul 2008
Posts: 19,400
Quote:
Originally Posted by Ronstar View Post
As to post processing - we no longer need it in the Chicago area. We subscribe to the regional Trimble VRS system that consists of several base stations whose data is transmitted to our receivers instantaneously. That coupled with observations on US and Glonass satellites (maybe 10-20 at a time) provide us with sub centimeter accuracy in a 1 or 2 second observation.
Trimble - Positioning Services - Trimble VRS Now
Very cool! As I know, Trimble has been into higher-end products for specialized applications, and this service they provide would be much appreciated by people like yourself.

Back in the early 90s, I was involved in a civilian GPS application. As S/A degradation was on during that time, the GPS position would have jitters as high as a few hundred feet, which made it not usable for what we wanted to do. We did not need precision to the inch, only a couple of feet. The way to do it then was to establish our own reference station, and use a dedicated radio link to transmit the correction data. That was the way everybody was doing it then. The problem is the hassle of getting FCC frequency clearance.

And then, in the late 90s, there was a company that sold subscription to their differential correction, which was broadcast on the SCA subcarrier of some FM radio stations around the country. We bought their data receiver and incorporated it in our product. Then, the S/A was turned off and the business model of this company vanished overnight! I do not even remember the name of this company, but just realized I may still have one or two of their differential data receivers in my attic or garage. Gosh, I still keep a lot of junk!

Further more, there was WAAS that the FAA started working on in the mid 90s, and what everybody could use. See: Wide Area Augmentation System - Wikipedia

Quote:
As to satellites knowing their position - I do not know the specifics, but after the launch, the government performs several weeks if not months of analysis on the orbit before its ephemeris is derived.
Even once in operation, each GPS satellite ephemeris must be uploaded by the ground tracking station every few hours. Yes, the satellite actual orbit deviates from the ideal orbital calculation that quickly. In fact, one of the drivers of the error budget of GPS is called AOD (age of data).

This frequent update of the satellite ephemeris causes another problem. The discontinuity in satellite position when the satellite starts broadcasting a new ephemeris means a corresponding jump in the GPS ground user position! That may cause problems in some critical applications.
__________________
"Old age is the most unexpected of all things that can happen to a man" -- Leon Trotsky
NW-Bound is online now   Reply With Quote
Old 10-25-2012, 05:05 PM   #59
Give me a museum and I'll fill it. (Picasso)
Give me a forum ...
NW-Bound's Avatar
 
Join Date: Jul 2008
Posts: 19,400
Quote:
Originally Posted by ERD50 View Post
I recall that they picked up a repeating signal and just kept accumulating that signal until the sum (correlated) popped out of the noise (uncorrelated). And something about an 'almanac', that I think told the GPS unit where (or which?) to look for the satellites at that time.

So the summing explains how they can pick up such low level signals. What I don't understand is how this is resistant to being jammed by the enemy. Seems it would easy to mess with such a low level signal.

-ERD50
The "payload" of the GPS C/A signal is a very low 50 bits/sec data rate, mainly to broadcast the satellite ephemeris and various status bits. As any radio amateur would know, this low data rate could be transmitted in an extremely narrow bandwidth signal. However, GPS uses "spread spectrum", whereas the signal is further modulated by a repeating pseudo-random code of a higher bit rate. That spreads out the bandwidth of the signal to a few megahertz. Each GPS satellite uses a different code, although they all transmit on the same frequency.

To tune into a particular satellite, the receiver uses correlation to match the intended signal. The correlator then rejects the unmatched or undesired codes.

This spread spectrum technique is quite common, and used by some US cell phone carriers (IS-95 standard by Qualcomm).

About jamming, one can see that if the code is known for the public to receive, anybody can transmit the same for jamming. On the other hand, since the military code is secret, one needs a much higher level signal to overcome the correlator for a P(Y) code, which is designed to tune in to only the wanted signal (and known only to the GPS satellites and intended receivers).

PS. Worse than jamming is "spoofing", where one can pretend to be a GPS satellite and transmit false data to drive the receivers to the wrong position. Very, very bad! Some receivers do have logic to protect themselves against this. See Receiver autonomous integrity monitoring - Wikipedia.
__________________
"Old age is the most unexpected of all things that can happen to a man" -- Leon Trotsky
NW-Bound is online now   Reply With Quote
Old 10-25-2012, 05:21 PM   #60
Give me a museum and I'll fill it. (Picasso)
Give me a forum ...
NW-Bound's Avatar
 
Join Date: Jul 2008
Posts: 19,400
You asked about the "almanac". This is an abbreviated version of the orbital parameters. It does not allow the receiver to compute a satellite position to the level of accuracy good enough for navigation, but only so that the receiver would know what satellites are above the horizon. Else, the receiver software would keep looking (with its correlators) for signals that are not there.

Because the almanac is only used for acquiring the signals, it does not have to be accurate or fresh. Six months or a year old would still be good. Once the receiver has locked onto a signal with its correlator, it then decodes the 50 bps data stream to get the ephemeris, then the ranging computation to the satellite can begin.

Note that if the receiver does not know roughly where it is, nor a rough time, then it is the same as not having the almanac. It would have to do a cold search, and it would take longer to obtain a position fix.
__________________

__________________
"Old age is the most unexpected of all things that can happen to a man" -- Leon Trotsky
NW-Bound is online now   Reply With Quote
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


 

 
All times are GMT -6. The time now is 12:57 PM.
 
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.