I bought my Magellan Blazer 12 back in '99 for $100. At the time, it advertised an accuracy of 100 feet (33m). Today, we had a quick refresher on how to use 'em. I got to take a look at some newer models, with better displays and all (mine is LCD–yeah, I said L. C. D.)
The Question:
I understand the DoD has allowed the signals from the GPS satellites to be ‘less noisy’, to allow people to get down to 30 feet of accuracy. The GPS surveying equipment we have at work can get down to a 1/2" horizontally. But, given my old unit, will I be able to get that sort of accuracy from a better signal, or has the hardware changed too?
Tripler
I’m curious if my old dinosaur performs as well as a new Ferrari.
Well I have a MY04 Maxima, and when I am approching an intersection it tells me that I am am approaching 300 feet, it then warns me that I am approaching 50 feet and when it says 0 feet I had better turn. I have had to tell myself to look at the damn road instead of the GPS to make sure, but damn, it’s been correct almost (OK almost) everytime. Sometimes it gets confused on very complicated intersections or on VERY back country roads. But most of the times it is dead on .Scary. :eek:
Just to add from my experience. Some counties give better GPS map info than others. I believe that some poorer parts of the countries get there maps “mapped out” by contracters that don’t do a full job. I have seen areas where the maps are accurate down to the paths in the grave yards! :eek: And others where where the road ends and you are driving thru an open field :smack: Depends on who gave the info to the GPS map people.
Although the government has relaxed its accuracy restrictions on GPS signals, probably the biggest difference between your old GPS and the newer ones is the development of Wide Area Augmentation System (WAAS). If I remember correctly, WAAS is a system of error correcting radio signals that, combined with the regular GPS signal, can achieve accuracies down to the single digits in feet. I regularly achieve 9ft accuracy with my WAAS enabled Garmin e-trex Legend GPS.
Another difference might be in the number of GPS satellites the receiver can track at one time. The maximum is 12, but some older GPS receivers were limited to tracking 6 or 8.
I think most units are still LCD, power savings being a big reason for that.
Most units have 12 channels, meaning it can lock on it 12 sat’s at a time, but will select 4 to calculate postion. If yours has less then 12 then yur unit has less options to get a position and might be less accurate depending on the one’s it locks on to.
As mentioned above newer units can use the WAAS sat to compensate for atmospheric distortion (WAAS sat sends correction data to the handheld gps). It also used the WAAS sat as one of the 4 which I suspect would lessen accuracy due to it’s position but it is more then made up for by the correction data it sends.
Patch (internal) antennas have also become much better, but if you have an external antenna (or a antenna that is internal but ‘bluges’ out of the gps) you won’t knowicew any difference.
The removal of selective availability (the intentional distortion of the signal) means that your old GPS will be significantly more accurate. But it probably won’t do much better than ±30 feet. Good enough for geocaching, not for surveying.
The WAAS system apparently is very good when it works, but you have to receive a separate signal which I guess is often difficult to pick up. It may not be much good in the woods.
I think later GPS models have somewhat more sensitive receivers but I don’t think they’re inherently more accurate.
The timing difference is measured using the receiver’s internal clock. Think “stopwatch” rather than “clock on the wall” Short term stabilitity (lack of jitter) is critical.
With light travelling at some 300,000,000 meters per second (30 meters every 1,000,000th of a second) clock accuracy does more to seprate the good from the bad.
So GPS receivers depend on internal oscillators to convert timing difference into distance? Hmm, never thought about that…
So have there been any recent improvements in oscillator chips that resulted in improved GPS performance? Are newer commercial GPS receivers noticeably and/or measurably more accurate than older ones, even in cases where WAAS is not used and the number of channels is not an issue (e.g. receiver locked onto the same 3 or 4 satellites)?
An important source of errors in GPS measurements is the Earth’s atmosphere. Light travels at C in a vacuum, but at slower speeds in the atmosphere. This delay is variable and frequency dependent. It can be affected by weather, dust, ionospheric activity, etc. The GPS receiver has an atmospheric model that tries to compensate for these factors, but it’s an approximation based on incomplete information. Military receivers have the advantage of receiving signals on two frequencies, which allows them to measure the frequency dependent delay.
Damn, shouldn’t have hit reply so quick. I should ask in IMHO but since this is already started who has suggestions for a GPS that can download to a PC preferably by USB or by RS-232? I’m looking to get track data to apply location information to photos.
What do you want to spend? Trimble makes the best GPS units and data can be easily downloaded via USB.
FYI- For the highest accuracy, you want real time differential (it picks up a local radio beacon to fine-tuned your location). It’ll get you down to sub-meter accuracy. Try the Trimble GEoExplorer XT. (no I’m not a Trimble rep)
Pretty much any GPS with a data port will do it. I have an old yellow eTrex that does that via serial. Just look for a GPS with NMEA 0183 support (that’s the most common protocol). Garmin has a proprietary protocol, but most of their older recievers have several different communication options (NMEA is one of the options). Their new ones that work via USB use Garmin only (although I have heard of programs that convert Garmin singnals to NMEA in realtime)
RM Mentock, I should have stated I knew it had different equipment. I work around the stuff every day (or at least my coworkers do) :smack:
So, what I’m gathering from more of the start of the thread is that the handhelds basically won’t change in hardware, but the accuracy comes from the ‘give-and-take’ of the DoD. Back when I first bought it in ‘99, the signals were pre-set to give a 100’ circle of error. But with this WAAS signal, it can get much, much closer than that.
Now, my Blazer 12 had 12-channel capability and at one point had 11 sats tracking. But I don’t know if it has the WAAS capability. Is there a specific point this came into use?
Side note: For all intents and purposes, when I’m walking around in the woods, I don’t need less than 100’ capability. If I can’t navigate back to 100’ of my truck, I deserve to die in the woods. It’s just the engineer in me that demands perfection!
Tripler
. . . as perfect as I can get in a handheld unit.
Never heard this, the GPS unit gets it’s time from the atomic clocks in the GPS sats, it may only have to go a second on 2 on it’s own till it gets it’s next clock reading.
From what I’ve read, the receiver synthesizes a local clock from a combination of the local oscillator and GPS time tags in the received signals. The GPS data is used to synchronize the local oscillator with GPS time. So the short-term stability of the local oscillator is an issue. How far can it wander before it is corrected by incoming GPS data.
It appears similar to the Network Time Protocol (NTP), which is used to synchronize clocks in computer networks. The stability of the local clock is a major factor in the accuracy of the time produced by NTP.
