GPS Q&A
By Frank Houzvicka

Q.If you are planning to use differential (post-processing) mode with two GPS receivers, should the two devices be the same? C.K. Arlington, Va.

A.For most post-processing GPS applications, it is not necessary to use GPS receivers of the same type or even from the same manufacturer, but there are certain advantages to using similar GPS receivers as the base (or reference) station and remote unit. I will give you some examples of these conditions later, but first let me address the first part of my response - units can be different and/or from different manufacturers.
     When performing a post-processing differential session, at least one GPS receiver must occupy a point whose coordinates are known with some certainty. The higher the accuracy for a given reference point, the greater the accuracy of the result (all other things being equal).
     The GPS receiver that occupies the reference point does not have to be of the same type or even from the same manufacturer as the GPS receiver being used at the unknown or remote location. However, both units should (in some cases must) be able to log and process the same kind of raw satellite information.
     It will do you no good to have a remote unit logging carrier phase data while the base station receiver only logs pseudorange information. Such a condition will still allow you to compute a differential correction for the remote location, but you will be limited to the lower accuracy of the pseudorange solution rather than getting the higher precision of the carrier phase computation. The GPS unit located at the base station can vary as long as it is capable of logging the raw satellite data at an appropriate update rate and the format of the raw data is either compatible with the format of the data being recorded in the remote receiver or is capable of being reformatted into the RINEX (Receiver Independent Exchange) format.
     Most GPS receivers that can be used for base station operations have the capability to vary the rate at which they store the raw satellite data, from once per second to every 30 seconds or more. Logging data at a slower rate means that there is less data to store, which is important for some community base station operations. But such data rates also mean reduced accuracy if the remote receiver can not synchronize its data collection or interpolate between data epochs. Most survey-grade GPS receivers can handle data rates of every 30 seconds for pseudorange operations, but carrier phase (and certainly centimeter) operations require that data be recorded at a higher rate, such as once per second.
     Virtually every GPS receiver manufacturer records its raw satellite data in its own internal storage format. The data can easily be shared and processed using that manufacturer's post-processing software and is generally fully compatible between the various models of that manufacturer's equipment. However, when the base station is from one manufacturer and the remote unit is from another, it is generally necessary to convert one of the logged data files (usually from the base station) to the RINEX format before post-processing can begin.
     RINEX is an industry standard adopted by most GPS manufacturers in order to ensure that base station coverage can be maximized and made available to the broadest number of GPS users. In virtually all cases, the manufacturer of the base station unit provides software that will permit conversion of their logged raw satellite data to the RINEX format. This would allow raw data files to be used by operators of other manufacturer's equipment.
     So now you can see that the GPS units at the base station and remote location do not have to be of the same make or manufacturer for you to successfully obtain differential corrections. The main advantages of having both units be the same are:
• You do not have the additional steps of converting the files to RINEX.
• Your post-processing software will generally run faster with the manufacturer's data format.
• You will frequently obtain higher accuracy since you can take advantage of different data format and truncation rules. Why does GPS tracking under trees in the same location seem to differ from day to day? S.U. Ontario, Canada Successfully tracking GPS satellites under trees depends upon several factors:
• Units with more dedicated receiver channels and quality front-end signal processing will operate the best under a tree canopy.
• The higher the satellites are above the horizon, the more likely it is that your receiver will find enough satellites to track despite masking and signal attenuation.
• Moisture retained in leaves will increase signal attenuation, so the dryer the forest the better.
     Your receiver may track better one day to the next. Because some days are wetter than others, there may be fewer satellites visible overhead because the network is constantly on the move and some satellites may be obstructed, or your batteries may be running out.
     For better reception in the trees, try raising the antenna anywhere from a few inches to a few feet. Operators have been known to successfully operate in a triple canopy jungle by raising their antenna on measured cables with helium balloons. A little extreme perhaps, but you do what you have to do.

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