Experimental architecture the GPS trackers allows you to reduce power consumption by three orders of magnitude
Determination of coordinates with GPS is highly energy-intensive process. Unlike smartphones, which is still every night I put on charging, or car navigators, GPS trackers, that are used by biologists to track migratory animals, have very strict requirements for power consumption. Quite often they provide not only a powerful battery, but with solar panels to provide work for many months.
In Microsoft Research have proposed to divide the process of collecting geo-location information in two stages — recording the "raw" signal from GPS satellites and to calculate coordinates on the basis of this information. It is possible to reduce power consumption for recording one track point three orders of magnitude. To understand how it managed to achieve this, you have to first understand how GPS works.
The determination of the coordinates in the GPS occurs in a rather complicated and intricate scheme. Every GPS satellite equipped with high-precision atomic clock, which also synchronized with ground stations. These stations are used to Refine the orbital parameters of satellites. The orbit is selected in such a way that at any time over any point of the Earth were simultaneously visible to at least six satellites. The orbital altitude of approximately 20200 km, orbital period — half earth days.
Each satellite transmits radio signals in the range of 1227,60 and 1575,42 MHz. Civil receivers are typically used first. All the satellites operate on the same frequency, and to distinguish their signals uses a multiple access code division (CDMA) each satellite has a unique pseudo-random 1023-bit code (code gold), which is 1000 times per second. This code is used to identify the satellites and determine the delay of arrival of signals from each of the satellites.
On top of this signal with the help of additional modulation of the carrier frequency with a speed of 50 bits/sec are transmitted in the so-called navigation messages that contain the exact coordinates of the satellite orbit ("ephemeris"), the exact system time and almanac data — ephemeris of other satellites of the system. These data are divided into 25 fragments of 1500 bits, so the complete navigation message is transmitted 12 minutes 30 seconds.
Time delays, the receiver calculates (via the speed of light) the exact distance to satellites and navigation messages gets their coordinates, and then can calculate their. The deciphering and interpretation of data the receiver is a challenging task because of the need to consider many factors that introduce errors — the Doppler effect caused by motion of satellite and receiver, the speed change signal when passing the ionosphere, and reflection from surrounding objects, and others.
Typically, the receiver no need to receive the complete navigation message, often missing the first fragment contains the minimal information. If the receiver has been off for quite a long time, we have to take more fragments, and obtaining coordinates of the first point can take up to several minutes (cold start). In continuous tracking of the coordinates of the receiver based on previously received navigation data and determines the position almost immediately.
If the device is able to receive information from other sources — GSM or Wi-Fi network, often used Assisted GPS (A-GPS). In this case, generated from a relatively high-speed network current ephemeris and almanac data and coarse location information based on the position of the towers and access points provide a much faster to consider all the necessary amendments and to calculate coordinates.
So, back to the tracker of Microsoft Research. The solution proposed by scientists, is called Cloud-Offloaded GPS (CO-GPS). It simply and effectively. Is ready of coordinates, for which you need to do all of these manipulations, it is proposed to record on the built-in flash memory only short fragments of "raw" GPS signal, which will then be uploaded to the cloud server and all the complex calculations will be made there. Instead of a navigation message, the exact coordinates of the satellites orbits can be obtained from publicly available databases GNSS.
In order to determine coordinates with an accuracy of approximately 30 meters is required only five fragments of the GPS signal with a duration of 2 milliseconds with intervals between them not greater than 50 msec. A thousand points so recorded, it takes about one megabyte is more than willing to coordinate, but flash memory is worth a penny.
Experimental tracker, developed by engineers at Microsoft Research to test the concept and experiments, found that a record of one 2-millisecond fragment requires only 0.407 MJ of electricity. To determine the exact coordinates need only a few fragments, so the total consumption will be about one millijoule, whereas conventional modern smartphone with A-GPS spends about Joule point. If recording points every second, two AA batteries will last for a year and a half of continuous work.
When recording raw data, another problem arises — sync accurate time, which is impossible without decrypting the GPS signal. If the clock skew of the tracker system is more than a minute, from the raw signal it is difficult to obtain accurate data. To resolve this issue in the tracker built in the small receiver of signals of exact time WWVB — these signals are broadcast by the National Institute of standards and technology USA and confidently accept virtually any condition throughout the United States (the transmitter Power is 70 kW, the time synchronized by atomic clocks). To achieve acceptable accuracy it is sufficient to synchronize the internal clock with WWVB signals only a few times a month.
Thus, technology CO-GPS allows you to get rid of the most part difficult filling the tracker, producing calculations, replacing it with flash memory larger volume and a simple and economical receiver of signals of exact time. Most of the time of operation of the tracker will spend in sleep mode, the GPS receiver signal is sufficient to include only 10 milliseconds to record a single point. Naturally, such a receiver will not be able to get the coordinates in real time, which suits biologists who are willing to wait a few months until the tracker will not be removed from the animal.
However, given the rapid proliferation of broadband wireless in cities, CO-GPS can find application in the consumer, "urban" devices. Enough every few seconds to send to the server for a cheap high-speed network a few kilobytes of raw data, receiving the coordinates almost as fast as in the case of conventional receiver.
Download a PDF with a detailed description of CO-GPS can here.
Article based on information from habrahabr.ru
In Microsoft Research have proposed to divide the process of collecting geo-location information in two stages — recording the "raw" signal from GPS satellites and to calculate coordinates on the basis of this information. It is possible to reduce power consumption for recording one track point three orders of magnitude. To understand how it managed to achieve this, you have to first understand how GPS works.
The determination of the coordinates in the GPS occurs in a rather complicated and intricate scheme. Every GPS satellite equipped with high-precision atomic clock, which also synchronized with ground stations. These stations are used to Refine the orbital parameters of satellites. The orbit is selected in such a way that at any time over any point of the Earth were simultaneously visible to at least six satellites. The orbital altitude of approximately 20200 km, orbital period — half earth days.
Each satellite transmits radio signals in the range of 1227,60 and 1575,42 MHz. Civil receivers are typically used first. All the satellites operate on the same frequency, and to distinguish their signals uses a multiple access code division (CDMA) each satellite has a unique pseudo-random 1023-bit code (code gold), which is 1000 times per second. This code is used to identify the satellites and determine the delay of arrival of signals from each of the satellites.
On top of this signal with the help of additional modulation of the carrier frequency with a speed of 50 bits/sec are transmitted in the so-called navigation messages that contain the exact coordinates of the satellite orbit ("ephemeris"), the exact system time and almanac data — ephemeris of other satellites of the system. These data are divided into 25 fragments of 1500 bits, so the complete navigation message is transmitted 12 minutes 30 seconds.
Time delays, the receiver calculates (via the speed of light) the exact distance to satellites and navigation messages gets their coordinates, and then can calculate their. The deciphering and interpretation of data the receiver is a challenging task because of the need to consider many factors that introduce errors — the Doppler effect caused by motion of satellite and receiver, the speed change signal when passing the ionosphere, and reflection from surrounding objects, and others.
Typically, the receiver no need to receive the complete navigation message, often missing the first fragment contains the minimal information. If the receiver has been off for quite a long time, we have to take more fragments, and obtaining coordinates of the first point can take up to several minutes (cold start). In continuous tracking of the coordinates of the receiver based on previously received navigation data and determines the position almost immediately.
If the device is able to receive information from other sources — GSM or Wi-Fi network, often used Assisted GPS (A-GPS). In this case, generated from a relatively high-speed network current ephemeris and almanac data and coarse location information based on the position of the towers and access points provide a much faster to consider all the necessary amendments and to calculate coordinates.
So, back to the tracker of Microsoft Research. The solution proposed by scientists, is called Cloud-Offloaded GPS (CO-GPS). It simply and effectively. Is ready of coordinates, for which you need to do all of these manipulations, it is proposed to record on the built-in flash memory only short fragments of "raw" GPS signal, which will then be uploaded to the cloud server and all the complex calculations will be made there. Instead of a navigation message, the exact coordinates of the satellites orbits can be obtained from publicly available databases GNSS.
In order to determine coordinates with an accuracy of approximately 30 meters is required only five fragments of the GPS signal with a duration of 2 milliseconds with intervals between them not greater than 50 msec. A thousand points so recorded, it takes about one megabyte is more than willing to coordinate, but flash memory is worth a penny.
Experimental tracker, developed by engineers at Microsoft Research to test the concept and experiments, found that a record of one 2-millisecond fragment requires only 0.407 MJ of electricity. To determine the exact coordinates need only a few fragments, so the total consumption will be about one millijoule, whereas conventional modern smartphone with A-GPS spends about Joule point. If recording points every second, two AA batteries will last for a year and a half of continuous work.When recording raw data, another problem arises — sync accurate time, which is impossible without decrypting the GPS signal. If the clock skew of the tracker system is more than a minute, from the raw signal it is difficult to obtain accurate data. To resolve this issue in the tracker built in the small receiver of signals of exact time WWVB — these signals are broadcast by the National Institute of standards and technology USA and confidently accept virtually any condition throughout the United States (the transmitter Power is 70 kW, the time synchronized by atomic clocks). To achieve acceptable accuracy it is sufficient to synchronize the internal clock with WWVB signals only a few times a month.
Thus, technology CO-GPS allows you to get rid of the most part difficult filling the tracker, producing calculations, replacing it with flash memory larger volume and a simple and economical receiver of signals of exact time. Most of the time of operation of the tracker will spend in sleep mode, the GPS receiver signal is sufficient to include only 10 milliseconds to record a single point. Naturally, such a receiver will not be able to get the coordinates in real time, which suits biologists who are willing to wait a few months until the tracker will not be removed from the animal.
However, given the rapid proliferation of broadband wireless in cities, CO-GPS can find application in the consumer, "urban" devices. Enough every few seconds to send to the server for a cheap high-speed network a few kilobytes of raw data, receiving the coordinates almost as fast as in the case of conventional receiver.
Download a PDF with a detailed description of CO-GPS can here.
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