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GPS Consumer Series: Number of Channels - How Many Are Enough?
By Chuck Gilbert Marketing people have a little acronym; "FAB." FAB represents "Features, Advantages, and Benefits." The idea is that users care not a whit about the features in a particular product, nor do they really care about the advantage that a specific feature gives to them. Users care about the benefit they derive from that specific advantage/feature.
GPS product data sheets are typically an endless list of features, with nary a mention of benefits to the user. This month I will address one of the many items that are mentioned on a typical GPS data collection system data sheet. Number of channels
When a GPS sales person proudly proclaims that his receiver '...has 29 channels!', the honest response of most customers is a slightly bewildered and lethargic 'So?'. This is the classic example of a datasheet feature that provides no clue regarding the associated advantages or benefits.
The "channels" in a receiver are what the receiver uses to track satellites. The number of channels in a GPS receiver will have a direct impact on how well that receiver will perform under difficult satellite reception circumstances; here's a little background. What is a channel anyway?
A single channel in a GPS receiver is analogous to a radio in an automobile. The channel can be used to receive information from a satellite; similar to the car radio being used to receive information from a radio station. The similarity continues in that one channel in your GPS receiver can only listen to one satellite at a time, just as you can only receive one radio station at a time in your car.
But what if, for some reason, you wanted to simultaneously listen to more than one radio station? You have two choices. You can either get more radios (e.g. more channels) or you can use your single radio and turn the dial every few seconds to sequence between multiple stations (e.g. satellites).
One very common classification scheme for GPS receivers is based upon how the receiver uses its channels to track satellites. Of the receivers on the GIS data capture market today, most fall into one of two categories; sequential tracking or continuous tracking.
1) Sequential tracking receivers will usually have one, two, or three channels. Note that for all GPS users, your receiver must collect data from at least four satellites to obtain an accurate GPS position. In order to obtain an accurate position, your receiver will have to listen to at least four satellites. Therefore if a receiver has less than four channels, some channels must sequence between multiple satellites (in order to track four satellites with less than four channels.) Because of this sequencing a sequential receiver has to reacquire the satellite signal every time it sequences to the next satellite. Similar principles are employed by receivers described as multi-tracking or multiplexing.
Constantly having to reacquire satellites with a sequential receiver has two problems; time, and signal strength. First, it takes time. Second, the typical signal strength required to locate and obtain lock on a GPS signal is several dB higher than the signal strength required to simply maintain signal tracking. An undesirable result of this technique is that sequential receivers may sometimes have difficulty reacquiring a satellite. This problem is most severe in environments where the GPS signals are already intermittent due to blockage by trees, mountains, or buildings.
2) Continuous tracking receivers typically have four or more channels. Each channel is dedicated to the continuous tracking of a single satellite. Since each channel is dedicated to a satellite, a continuous receiver will tend to have very good performance in a dynamic environment and will have good receiver sensitivity. Improved sensitivity allows the receiver to track satellites more effectively under trees or in other difficult circumstances. All other things being equal, continuous tracking receivers will have superior performance to sequencing receivers. Similar principles are employed by receivers described as parallel receivers.
Note that there are receivers that do both continuous and sequential tracking. A receiver that has, for example, six channels but can track eight satellites may track five satellites continuously with the first five channels; and sequence through several other less necessary satellites with channel number six. This allows the receiver to have the good characteristics of a continuous receiver while still keeping extra satellites relatively available in case they are needed. This type of receiver will tend to perform better than a sequential receiver but not quite as well as a pure continuous receiver. Why the industry variation?
Why is there such a wide variation in the number of channels in GPS receivers today? The wide variation in number of channels is simply a reflection of the variety of different GPS receiver chip designs by a multitude of manufacturers. For example, a GPS manufacturer who has designed a six-channel chip is likely to be selling receivers with six-to-12 channels. (A 12 channel receiver is constructed by putting two GPS chips in the receiver.) Likewise, a GPS manufacturer who has designed a five channel chip is likely to be selling receivers with five-to-10 channels. This does not explain all the variation on the market, for example, it is possible to make a receiver with lots of channels, but to electronically activate only the channels that are necessary for the intended application. How many are enough?
What are the maximum number of channels that are worthwhile? Since each channel is used to track a satellite, there is no benefit in having more channels than there are satellites. The total GPS constellation is designed to have 24 satellites in orbit. However, from any particular location on the surface of Earth, there are typically five to 12 satellites visible at any given time. Therefore, I would suggest that as a very general rule, users should not consider GPS receivers with less than five channels.
More specifically, how many satellites you need to be able to track depends very much upon whether your GPS receiver is being used as a base station or as a roving receiver. It is vital that a GPS base (or reference) station is able to track every satellite in the sky. Since it is unlikely that a base station will be able to ever see more than 12 useable satellites, I would suggest a minimum of 12 channels for a GPS base station.
A rover receiver on the other hand needs only to be able to track enough satellites to provide a reliable position of the roving user. The superior performance characteristics of a continuous tracking receiver dictate that roving receivers should have at least four channels. However, there are very definite benefits in having a few additional channels above the minimum. Benefits of even more channels
Additional channels, up to a maximum of eight or nine, can yield tremendous benefits in difficult GPS reception circumstances. Although only four channels are required to continuously track four satellites, often there are situations where 'backup' channels can be valuable for tracking additional, 'backup' satellites.
For example: Imagine a situation where a GPS user is collecting attribute data about telephone poles in a residential neighborhood. The user may be either walking or driving down the street, recording attribute data along the way. There are a moderate number of things nearby that occasionally occlude parts of the sky; usually trees, and buildings. However, if the user were able to 'see' the entire sky, there would be eight satellites visible to the GPS antenna.
The situation outlined above is quite typical. The result of the sky obstructing trees and buildings etc. is that although eight satellites are above the horizon, there would probably only be five-to-seven visible at any given moment. It is very important to realize, however, that which five-to-seven are visible is constantly changing. (Don't try to keep track of the following example too carefully, it merely illustrates how the available satellite set can change from moment to moment. Just read fast and have fun with it. I did.) For example, at one moment, perhaps satellite numbers 1, 2, 3, 4, 5, and 6 are visible. However, whenever the car turns onto an east-bound road satellites 7 and 8 are no longer occluded by building on the side of the road and instead are visible directly in front of the car. This is good. Unfortunately, satellites 1, 3, and 4 are now lost behind other buildings due to the new road orientation. Meanwhile, every time a tree is passed on the left side of the car, satellites 3 and 6 are blocked for a few seconds, then are visible again after the tree has passed. At the same time, satellite 5 is blinking in and out each time a tall truck passes nearby. Of course as the road curves back to the north you lose satellites.... (I think you get the idea.)
Why does the mess illustrated above matter to the GPS receiver? It matters because it is not always 'easy' to regain lock on a satellite signal that you need. GPS receiver channels that are each dedicated to a single satellite will regain lock more quickly and successfully than a channel that must sequence through multiple satellites one at a time. In a severe and unlucky situation, a sequencing channel may end up spending a lot of time repeatedly trying very hard to regain lock on a satellite that is no longer available (simply because it has been momentarily blocked by a tree). On the other hand, an eight channel continuous receiver will always have instant access to eight satellites so that a position can be computed instantly utilizing whatever satellites are available at that moment. This is why additional channels beyond the minimum four can be very useful for roving receivers. Summary
I'll tell you a secret. The summary part of this column is almost always the easiest part to write; simply because it's usually the same every month. It always amounts to this; "Test it yourself, in your work environment." A high quality sequential receiver may be more than sufficient for your application. Especially if you work in the desert or in locations where dynamics and sky occlusion are not problems. A continuous receiver will probably cost a little more than a sequential receiver and only you can decide if the extra performance is worth the extra money. The sales people will happily provide the price differences. However, you can only obtain the performance difference by trying out all of the candidates in the environment where you will be doing your work. Happy testing. About the Author:
Chuck Gilbert has over a decade of experience as a GPS user. He has been employed as an applications engineer for Trimble Navigation since 1989.
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