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GPS reception in trees

People seem to have a variety of opinions about how various GPS receivers perform when used under tree cover, as can be seen in the sci.geo.satellite-nav newsgroup thread titled Tree Canopy.

While there are reports available about how GPS receivers perform under tree canopy, such as the US Forest Service GPS Receiver Testing Reports, most people just want a GPS receiver that will "work for them". A GPS receiver's technical specifications don't necessarily determine whether or not that GPS receiver will work well for you in your situation.

Knowing something about how the Global Positioning System works, and how your GPS receiver works, can help you make the best use of your GPS.

• The GPS satellites broadcast the GPS signals towards the earth at quite a low power. The frequencies used for the signals minimize the effects of clouds, rain, snow, or fog. GPS signals are easily diminished or blocked by liquid water such as wet foliage or wet fabric(e.g. a case used to carry a GPS, or clothing).
• The GPS satellites are orbitting the earth, in various orbits, so the number of satellite signals that can theoretically be received at any one place and time varies. The number of satellites that are "overhead" may be an issue when using a GPS in tree cover, where it's unlikely that GPS signals closer to the horizon can be received. WAAS satellites are located over the equator, and therefor closer to the horizon the further you are from the equator.
• Your GPS receiver needs to receive the GPS signals "directly"(line of sight) from the GPS satellites for it to be able to properly determine your position. GPS signals that are received "indirectly", such as after bouncing off nearby mountains, will reduce the accuracy of the position reported by your GPS.
• Anything that blocks the GPS satellite signals will reduce the chances that your GPS can acquire, and maintain, a position fix. This includes your body, terrain such as hills and mountains, vegetation such as trees, and manmade obstacles such as buildings. Vegetation that is wet will block more signals, such as during, or after, it has rained.
• If your GPS has a "battery save" mode it should be turned of for any difficult reception conditions, such as tree cover.
• Some GPS receivers may obtain, or re-obtain, a position fix more easily if they remain stationary. If you hike with your GPS turned off, it may be a good idea to turn it on at each rest stop, let it get a fix, and mark a waypoint.
• Some GPS receivers may function better if they are allowed to obtain sufficient signals for a position fix before moving into an area of reduced signals. If you plan to have your GPS receiver on while hiking in the trees, it may help to allow it to get a fix before you start your hike. One approach is to turn the GPS on as soon as you are parked at the trailhead, and leave it on top of the car while getting ready for the hike. In difficult reception conditions, it may be worthwhile to stop at clearings to make sure you still have a position fix, and to mark a waypoint.
• Knowing how your GPS receiver was designed to be used is important, particularly in regards to it's antenna. Some GPS receivers are designed to be operated horizontally, others vertically. Some GPS receivers have an external antenna, or can have one attached, or can make use of an external re-radiating antenna, in which case the GPS orientation doesn't matter, but the external antenna needs to be oriented properly.

On March 31, 2004, I went on a hike on the Deeks Creek Bluffs trail, which is located north of Vancouver, off highway 99. Other than at the highway and a few clearings and viewpoints, the trail is in the forest, with what would be considered moderate to heavy tree cover. Near the start of the trail there are some steep cliffs next to the trail, and there are mountains in the area. I turned on my GPS receiver, a Garmin etrex Venture, at the car at the start of the hike. After it had obtained a position fix, I marked the "CAR" waypoint, and then carried the GPS on the right-hand shoulder strap of my backpack, in a cloth holder designed for an FRS radio. The holder was positioned below my shoulder, which meant that the GPS was in a position closer to vertical than horizontal. At a number of points along the trail, such as at viewpoints and clearings, I checked the GPS. If it had lost satellite reception, I regained reception before continuing. At one stop, the GPS had turned off, because the batteries had run down, so they were replaced with fresh batteries.

After lunch, I followed the same trail back to the car, again with the GPS on the right-hand shoulder strap of my backpack. As can be seen from the map image, unlike the first tracklog, the tracklog recorded after lunch didn't have any breaks, although the route was along the exact same trail. This was primarily because after lunch, I had moved the holder for the GPS further up on the backpack strap, almost on top of my shoulder, so the GPS was being held in a position that was close to horizontal. Another factor is that the signal reception possibilities may have been improved after lunch because the GPS was generally facing 'downhill', towards the ocean to the West.

One factor in GPS signal reception is the number of satellite signals that are available to be received, and where in the sky the signals are originating. In tree cover, it will usually be easier to receive signals from satellites that are "overhead", because the GPS receiver may be able to 'see' vertically up through the trees, but not horizontally through the forest. This is why you'll often get better reception in clearings, and especially large clearings, or ones on a slope that expose more of the sky. The hike took place on March 31, 2004, between 08:51AM and 14:27PM, PDT. The WGS84 location of the "CAR" waypoint is 49°30'15.26"N 123°15'02.52"W. Using a current almanac, the Mission Planning software shows which GPS satellite signals were theoretically available during the hike. The vertical grid lines on the map image are the North-South grid lines of a UTM grid, and the nearby mountains are on the Eastern side of the hike. Particularly in clearings, the GPS signal reception would have benefitted the most from the GPS satellites that were in the Western part of the sky during the hike.

The etrex Venture is designed to be used when held horizontally, in the left hand. It uses a patch antenna, which can be seen as the light-colored square under the 'globe logo' on the top of the Venture. While the Garmin etrex series of GPS receivers do not have connectors for an external antenna, they can be used with an external re-radiating antenna, such as the one from Pc-Mobile.

The above shows why it's important to understand, and consider, all the relevant factors before coming to conclusions, especially anecdotal ones, about whether or not particular antennas, or GPS receivers, perform well under tree cover. In this example, the main factor was how the GPS was used. There have been reports in the GPS newsgroups of people complaining about poor signal reception, but it often turns out they didn't know how to make the best use of their GPS receiver.