[gps-talkusers] GPS Factors

Since the topic of GPS accuracy and the comparison between receivers comes 
up frequently, I thought I'd make a list of all the factors that may have 
to do with GPS accuracy and acquisition time. These factors certainly apply 
to the BrailleNote GPS, Trekker and all other receivers. I don't go into 
detail about these factors but here they are.

GPS Factors
Accuracy and Acquisition Time

The following factors may impact both time-to-GPS-Acquisition and GPS 
accuracy. These factors will vary among receivers.

Time since last successful acquisition. The shorter the better.

Lengthy time period being unable to acquire. Some receivers remember your 
last position and a lengthy acquisition time will erase this position.

Distance from last acquisition. The shorter the better for fast acquisition.

Tinted windows or windows with defrost elements. Metal interferes with GPS 
signals even if it is within windows.

Material comprising the roof of a building or vehicle. Signals can pass 
right through a fiberglass roof of a bus or the wooden roof of a house.

Surrounding obstacles like buildings or trees. Interfere with acquisition 
and accuracy.

Position of the receiver on one's body. The clearer view the antenna has of 
the horizon, the better.

Orientation of the antenna relative to the satellites. Patch or flat GPS 
antennas can be tilted one way or the other to increase sensitivity to 
satellite signals.

Blockage by guide. A person walking right next to you may be between your 
GPS antenna and the satellite signals.

Stationary while acquiring. This may improve acquisition time.

WAAS. Increases accuracy when available. It usually takes 5 minutes at 
least to acquire.

Patch or quadrafiler antenna. The latter is more omni directional.

Type of chip and receiver. The SIRF chip in the Earthmate is supposed to 
reduce the impact of reflected signals.

Quality and position of satellite signals. The satellites are constantly in 
different positions relative to you and your receiver.

The organization, which maintains the satellites constantly, is taking one 
or another satellite temporarily offline for maintenance.  They publicize 
each outage ahead of time. This may mean that in a spot that's always been 
fine for you in the past, you'll suddenly have a day when you don't get 
adequate signal. This may be the reason.

Interference from strong electronic forces, cell phones and other radio 
frequencies. These other forces may interfere with accuracy.

Multi-path signal reflections. Signals may bounce among tall buildings 
reducing accuracy.

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