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Understanding GPS, continued

Understanding GPS Signal Acquisition and Signal Strength

Overview:

The Global Positioning System, GPS, originally developed for military purposes, was deployed over the span of years

beginning in the 1980s, completing deployment in 1995. Today the GPS system is primarily used for civilian

applications. Within the next 5 to 6 years there will be 3 fully independent Global Navigational Satellite Systems in

service - the United States will continue to provide and improve GPS, the European Union will complete deployment

of their system, GALILEO and Russia will complete deployment of their system GLONASS.

The GPS is a constellation of 32 fully operational satellites orbiting the earth in 6 different orbital planes, with 5 to 6

satellites per orbit. At any one time 24 of the satellites are considered “in service.” The others are available in case

one of the 24 needs to come ofﬂine for maintenance issues. The satellites circle the earth at an attitude of 20,180

kilometers (about 12 miles). Each orbit takes about 12 hours, but due to the earth’s rotation, each satellite returns to

it’s starting point above earth in just under 24 hours. The 6 orbital planes, including the number and spacing of

satellites in each plane are designed to ensure that there are a minimum of 4 satellites in view of any location on

earth at any time. Typically there are more than 4. The more satellites a receiver locks onto, the more accurate the

position data. Receivers positioned in higher latitudes will typically “see” less satellites. The master control station

for the entire GPS network is located in Colorado. There are 5 additional monitoring stations around the world.

GPS Signal Strength:

GPS signal strength can be affected by many conditions. These include weather, the environment, receiver

movement, and the orbital position of the satellites. This is especially noticeable at the point in time when the

receiver is attempting to get a full GPS lock (typically means the reciever is locked onto a minimum of 4 satellites for

accurate location data).

• When considering the weather, the signal strength is affected by the density of any clouds. Heavy rain or snow

clouds can be a negative when it comes to GPS signal locking. This type of atmosphere tends to slow the locking

process. The best conditions are cloudless sunny days or partial high, thin cloud cover.

• Environmental considerations include whether the receiver is in a mountainous or ﬂat area, urban or rural area,

under a tree canopy, or some combination of the above. Mountainous topography can include deep valleys or

canyons. It can be hard to receive a satellite signal when not much sky is visible. Similarly, in urban areas, tall

buildings, sometimes referred to as urban canyons, can cut down on the amount of viewable sky. Also, in rural

areas, where the receiver may be under tree-lined roads or trails, the density of the tree canopy could have an

effect on the GPS signal. The best environmental conditions for a GPS signal would be ﬂatter terrain, rural areas,

with lots of open sky, and light to nonexistent tree cover.

• The global satellite network itself can also affect the GPS signal. As mentioned earlier, the GPS network consists

of 24 satellites in 6 different orbits around the earth. There are brief times during the day where a “coverage

hole” may temporarily exist, over a particular geographical area, due to the location of each satellite in its orbit,

at that particular point in time. In this case, the user of the receiver may need to wait a few minutes for satellite

movement to “close the coverage hole” in that geographical area.

• Receiver movement can have an affect on the initial GPS signal lock. Typically, it is better to keep the receiver

unit stationary while the unit is trying to complete the lock of the GPS signal. If movement is necessary during

the locking process, the process may take more time to complete. This occurs because, as part of the locking

process, distances to each satellite are being calculated. Any signiﬁcant movement would cause the distance to

change and thus a recalculation.

• The GPS signal is a radio frequency (RF) signal. Anything that affects RF can have an impact on GPS signal

reception. Indoors, this can mean the thickness of the walls around the reciever, the roof over the reciever, or the

type of material the roof or walls are constructed from. Outdoors, this can mean paying attention to events that

can affect electro-magnetic radiation (EMR), like solar ﬂares from the Sun. Solar activity, like the weather, is

tracked and predicted, by various organizations. On days when a major solar ﬂare produces a huge EMR blast

toward the earth, your GPS signal will be affected.

Technical Details, Cell Phones:

As mentioned above, the GPS network was designed prior to the 1980s. The transmission rate from satellite to a

receiver on the ground is only 50 bits per second. In perfect weather conditions, once a communication is established

with the ﬁrst satellite, it takes a minimum of 30 seconds to receive the initial information, a 1500 bit message block.

A receiver will need to lock on a minimum of 4 satellites to get an initial good position location. In order to ﬁnd other

satellites the receiver will reference a copy of a data ﬁle, containing position data of all other satellites in the system. This

ﬁle is commonly referred to as the almanac ﬁle. Each satellite is in constant communication with the other satellites in

the system. Each satellite continually gathers up-to-date position data of the other satellites. This information becomes

the bulk of the almanac ﬁle. Receivers typically keep their copy of the almanac ﬁle updated without notice to the user.

However, receiver almanac ﬁles can become out of date by moving great distances between powering up the device or

not powering it up for a long time. If this is the case, downloading a completely new copy of the almanac will take

approximately 12.5 minutes under good sky conditions (the almanac ﬁle is approximately 25 message blocks in size,

downloading at 50 bits/sec). Once the ﬁrst satellite is locked, the almanac ﬁle is determined to either already be

up-to-date or a new one is downloaded, the receiver begins to look for and lock other satellites. The complete lock

process is accelerated at this point because ﬁnding additional satellites is much quicker once the receiver knows where

to look for them via position info from the now up-to-date almanac data.

Cell phones acquire a GPS lock faster then other receivers. Most of the newer phones use what is called A-GPS,or

Assisted GPS technology. This is in essence a hybrid communication technology that allows the receiving cell phone to

acquire a GPS signal lock quickly using assistance from other technologies. Assisting technologies include cell tower

triangulation, the high speed data transmission link between the phone and the tower, and the fact that the tower, in a

ﬁxed position, is always locked onto and monitoring the GPS satellite constellation, 24 hours per day, 7 days per week.

Leveraging these technologies, the cell phone can acquire a GPS signal lock quickly compared with other receivers

communicating directly with the satellites at 50 bits per second.

Joule GPS User Guide page 65

Appendix D: GPS OVERVIEW

Powertap Joule GPS Handleiding

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