Why buy a 12 Channel GPS Receiver?

Many people wonder which GPS to buy.  Some very basic units have a single or dual channel scanning receiver that can scan and 'lock' on 8 to 12 satellites.  A frequently asked question is "Why buy a 12 channel GPS receiver?

This article  answers the questions regarding 12 channel receivers and why you might want one. Note that sometimes I will refer to satellites as SVs (Space Vehicles) and when I use the term gps I will usually mean the receiving unit and not the entire system.

What is a 12 Channel receiver?

This is the first question you need to ask since there are two different interpretations of the answer. One is, any receiver capable of tracking 12 satellites. The second answer is a receiver having dedicated hardware to receive 12 channels simultaneously, also known as a twelve channel parallel receiver.

What kind of receivers are there?

Today there are basically three kinds of receivers. Multiplexing receivers, sequential receivers, and parallel receivers. There are also mixed types where some channels are dedicated to a single satellite and some channels are multiplexing or sequential.

What is a parallel receiver?

The receiver has dedicated separate hardware to receive each satellite that it needs for a solution.

What is a multiplexing receiver?

This kind of receiver uses a single, dual receiver, or even 3 receiver hardware design to time division multiplex among the satellites that it is viewing. This mean it gathers some data from one SV for a slice of time and then switches to another SV to gather some more data. If it is able to perform this switch fast enough it seems to be tracking all of the satellites simultaneously.

What is a sequential receiver?

This kind of receiver also switches limited receiver hardware among all of the satellites. It, however, gathers all of the data from one SV before moving on to the next as opposed to the multiplexing unit that switches using a time slice algorithm. Sequential units tend to offer sluggish performance, particularly with the first fix, since, if it has some trouble getting information from one SV, it can get bogged down before giving up and switching to the next SV. This unit can give good performance if it has a minimum of 3 channels. You are unlikely to find a new sequential receiver.

Is a parallel receiver better than one that time division multiplexes?

Parallel receivers are faster and will generally provide a more reliable fix. However, multiplexing receivers are fine under many conditions where there is a clear view of the sky. Under marginal conditions, such as tree cover or in city canyons, having a dedicated hardware channel remaining continuously synced to a particular satellite can be advantageous. (For an in depth article on what this means check Tom Clark's article on How a GPS gets a lock on Joe and Jack's GPS Information Website  under HOW GPS WORKS.

If I want a parallel receiver, how many channels do I need?

The minimum number of parallel channels you need is 4 to obtain a fast fix since a 3D solution needs 4 satellites. But that doesn't leave any spares to look for other SV's that may be just coming into view to replace the ones you are using that are about to drift out of view so you really need a minimum of 5. There have been 5 channel parallel receivers made and they work pretty well. Hardware costs have changed the picture and it is now easier to just dedicate one channel to one bird than to try and multiplex receivers around among the available birds.

Why do I want more than 5 parallel channels

Since 5 would only allow for a margin of 1 SV having more channels would permit you to maintain a solution when you moved behind a building where you suddenly lost several SV's. Ideally you would like to track all available satellites simultaneously to maintain the likelihood of a fix under the worse possible conditions.  Thus,  if you turn a corner and your automobile obscures several SVs there is a good likelihood that the extra channels will instantly bring several others "on line" thus maintaining continuous lock.

Are there any other reasons to track more than 4 at once?

Given that you want all parallel receivers then you get an added benefit or being able to calculate an over determined solution if you wish (not all will do this). An over determined solution uses extra satellite measurement data to help improve the accuracy of the solution.

Why aren't all units parallel?

Cost. Back when hardware was more expensive, multiplexing and sequential receivers were widely used and could track 6, or 8, or even 12 SV's by switching among them using gps units equipped with from 1 to 5 receivers. The performance was OK but not nearly as stable and reliable  as the  12 channel dedicated receivers.

So why 12 channels?

The government maintains a minimum of 21 working satellites in orbit around the world. To ensure 21 working satellites there are usually 3 working spares. Sometimes there are more, currently there are 27 working SV's. With 24 in the sky there is some likelihood that 12 might be in your hemisphere at once. In practice many people have reported that, under some conditions, they have tracked 12 simultaneously for short periods. Often you cannot track nearly that many, sometimes as few as 6 SV's may be present in the sky.

In addition some satellites may be in view but are so low in the sky that their signal is not reliable and couldn't be used in a solution anyway. Therefore 12 is a good maximum number but isn't a gating item for performance. The fact that most 12 channel units are also parallel receivers is what makes them great choices.

Is 12 channels good for multiplexing receivers too?

Since multiplexing receiver do not compute an over determined solution the tracking of 12 channels is less important for these units. Generally a multiplexing receiver picks the best 4 SV's and computes its solution. Tracking 4 more means that if all of the originals were lost for some reason there would be a full replacement available. Tracking additional SV's can actually eat up some of the time slices and make the unit less responsive. Each manufacture designs their unit to maximize the performance or marketing potential they can deliver. Most folks find no difference in a unit that tracks 8 versus a unit that tracks 12.

How do I know what I have?

Check the documentation for you unit. Sometimes however the manufacturer doesn't tell you. The second method of determining whether you have a parallel unit or a multiplexing unit is to determine the time for the first fix of the day. To get the first fix the gps must download ephemeris data from 3 SV's to compute a 2D fix (4 for a 3D fix). The needed ephemeris data is transmitted every 30 seconds from each satellite in orbit. A multiplexing receiver will need 90 seconds (3 times 30) to obtain this data and then some time to compute the fix. A parallel receiver can receive the data in parallel and will thus have all the data for a 2D or 3D fix in the same 30 seconds. The specifications for a multiplexing receiver is usually about 2 minutes for the first fix of the day while a parallel unit can get a fix in about 45 seconds. Thus it is pretty easy to figure out what kind of unit you have.

Aren't 12 channel parallel units more accurate?

Not necessarily. The accuracy specifications for both parallel and multiplexing units are often the same. In practice a unit that can calculate an over determined solution is likely to produce a slightly more accurate position. However, Selective Availability reduces the accuracy below the threshold of any differences between the units in this regard.

Comments? 


Dale DePriest

Last modified: Fri Sep 18 16:32:15 PDT 1998