December 25, 2012 at 22:57The experimental architecture of GPS trackers reduces power consumption by three orders of magnitude
Determining coordinates using GPS is a very energy-intensive process. Unlike smartphones, which are still charged every night, or car navigators, stand-alone GPS trackers, which biologists use to track animal migrations, have very stringent energy requirements. Quite often they are supplied not only with powerful batteries, but also with solar panels to ensure work for many months.
Microsoft Research proposed to divide the process of collecting geolocation information into two stages - recording the "raw" signal from GPS satellites and calculating coordinates based on this information. This allowed us to reduce the energy consumption for recording one track point by three orders of magnitude. To understand how it was possible to achieve this, you first need to figure out how GPS works.
The determination of coordinates in the GPS system takes place according to a rather complicated and confusing scheme. Each GPS satellite is equipped with ultra-precise atomic clocks, which are also periodically synchronized with ground stations. The same stations are also used to refine the parameters of the satellite’s orbit. The orbits are selected so that at any moment at least six satellites are visible above any point on the Earth. The height of the orbit is about 20,200 kilometers, the period of revolution is half of the earth's day.
Each satellite emits radio signals in the bands 1575.42 and 1227.60 MHz. In civilian receivers, the first one is usually used. All satellites operate at the same frequency, and code division multiple access ( CDMA) is used to distinguish their signals.) Each satellite has its own unique pseudo-random 1023-bit code ( Gold code ), which is broadcast 1000 times per second. This code is used to identify satellites and determine the arrival delay 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 only 50 bits / s, so-called navigation messages are transmitted that contain the exact coordinates of the satellite’s orbit (“ephemeris”), the exact system time and almanac data — the ephemeris of the remaining satellites of the system. This data is divided into 25 fragments of 1,500 bits, so a complete navigation message is transmitted 12 minutes 30 seconds.
Based on the delay time, the receiver calculates (through the speed of light) the exact distances to the satellites, and from the navigation messages receives their coordinates, after which it can calculate its own. Deciphering and interpreting receiver data is a rather difficult task, since it is necessary to take into account many factors that introduce errors - the Doppler effect caused by the movement of satellites and the receiver, the change in the speed of the radio signal during the passage of the ionosphere, reflection from surrounding objects, and others.
As a rule, the receiver does not need to receive a full navigation message, most often the first fragment containing the necessary minimum information is enough. If the receiver has been turned off for a rather long time, you have to take more fragments, and obtaining the coordinates of the first point can take up to several minutes (“cold start”). In the constant tracking mode, the receiver relies on already received navigation data and determines the position almost immediately.
If the device is capable of receiving information from other sources - GSM or WiFi networks, Assisted GPS ( A-GPS) technology is often used.) In this case, the actual ephemeris and almanac data obtained from a relatively high-speed network, as well as rough geolocation data based on the position of towers and access points, allow you to take into account all the necessary corrections much faster and calculate the coordinates.
So, back to the Microsoft Research tracker. The solution proposed by scientists is called Cloud-Offloaded GPS (CO-GPS). It is simple and effective. Instead of the ready-made coordinates, for obtaining which you need to do all of the above manipulations, it is proposed to record on the built-in flash memory only short fragments of the “raw” GPS signal, which will then be uploaded to the cloud server and all complex calculations will be made there. Instead of navigational messages, the exact coordinates of the satellite’s orbit can be obtained from publicly available databases.Of GNSS .
In order to determine the coordinates with an accuracy of about 30 meters, you need only five fragments of a GPS signal with a duration of 2 milliseconds with intervals between them no more than 50 ms. A thousand points recorded this way takes about one megabyte - this is much more than the finished coordinates, but flash memory costs a penny.
An experimental tracker created by Microsoft Research engineers to test the concept and experiments showed that it takes only 0.407 mJ of electricity to record one 2-millisecond fragment. To determine the exact coordinates, only a few of these fragments are needed, so that the total consumption will be about one millijoule, while an ordinary modern smartphone with A-GPS spends about a joule per point. If you record points every second, two AA batteries will last for a year and a half of continuous operation.
When recording raw data, another problem arises - the exact time synchronization, which is impossible without decoding the GPS signal. If the tracker’s clock out of sync with the system’s time is more than a minute, it’s difficult to get accurate data from the raw signal. To solve this problem, a small receiver of accurate time signals WWVB was built into the tracker - these signals are broadcast by the U.S. National Institute of Standards and Technology and are confidently received in almost any environment throughout the United States (transmitter power is 70 kW, time is synchronized by atomic clock). To achieve acceptable accuracy, it is enough to synchronize the internal clock with WWVB signals only several times a month.
Thus, CO-GPS technology allows you to get rid of most of the complex stuffing of a GPS tracker that performs calculations, replacing it with a larger flash memory and a simple and economical receiver of accurate time signals. The tracker will spend most of the operating time in sleep mode, it is enough for the GPS-signal receiver to turn on for only 10 ms to record one point. Naturally, such a receiver will not be able to obtain coordinates in real time, which is quite suitable for biologists who are ready to wait a few months until the tracker is removed from the animal.
However, given the rapid spread of broadband wireless in cities, CO-GPS can also be used in consumer, “urban” devices. It is enough to send several kilobytes of raw data to the server via a cheap high-speed network once every few seconds, receiving the coordinates almost as quickly as in the case of a conventional receiver.
Download a PDF with a detailed description of CO-GPS here .
Microsoft Research proposed to divide the process of collecting geolocation information into two stages - recording the "raw" signal from GPS satellites and calculating coordinates based on this information. This allowed us to reduce the energy consumption for recording one track point by three orders of magnitude. To understand how it was possible to achieve this, you first need to figure out how GPS works.
The determination of coordinates in the GPS system takes place according to a rather complicated and confusing scheme. Each GPS satellite is equipped with ultra-precise atomic clocks, which are also periodically synchronized with ground stations. The same stations are also used to refine the parameters of the satellite’s orbit. The orbits are selected so that at any moment at least six satellites are visible above any point on the Earth. The height of the orbit is about 20,200 kilometers, the period of revolution is half of the earth's day.
Each satellite emits radio signals in the bands 1575.42 and 1227.60 MHz. In civilian receivers, the first one is usually used. All satellites operate at the same frequency, and code division multiple access ( CDMA) is used to distinguish their signals.) Each satellite has its own unique pseudo-random 1023-bit code ( Gold code ), which is broadcast 1000 times per second. This code is used to identify satellites and determine the arrival delay 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 only 50 bits / s, so-called navigation messages are transmitted that contain the exact coordinates of the satellite’s orbit (“ephemeris”), the exact system time and almanac data — the ephemeris of the remaining satellites of the system. This data is divided into 25 fragments of 1,500 bits, so a complete navigation message is transmitted 12 minutes 30 seconds.
Based on the delay time, the receiver calculates (through the speed of light) the exact distances to the satellites, and from the navigation messages receives their coordinates, after which it can calculate its own. Deciphering and interpreting receiver data is a rather difficult task, since it is necessary to take into account many factors that introduce errors - the Doppler effect caused by the movement of satellites and the receiver, the change in the speed of the radio signal during the passage of the ionosphere, reflection from surrounding objects, and others.
As a rule, the receiver does not need to receive a full navigation message, most often the first fragment containing the necessary minimum information is enough. If the receiver has been turned off for a rather long time, you have to take more fragments, and obtaining the coordinates of the first point can take up to several minutes (“cold start”). In the constant tracking mode, the receiver relies on already received navigation data and determines the position almost immediately.
If the device is capable of receiving information from other sources - GSM or WiFi networks, Assisted GPS ( A-GPS) technology is often used.) In this case, the actual ephemeris and almanac data obtained from a relatively high-speed network, as well as rough geolocation data based on the position of towers and access points, allow you to take into account all the necessary corrections much faster and calculate the coordinates.
So, back to the Microsoft Research tracker. The solution proposed by scientists is called Cloud-Offloaded GPS (CO-GPS). It is simple and effective. Instead of the ready-made coordinates, for obtaining which you need to do all of the above manipulations, it is proposed to record on the built-in flash memory only short fragments of the “raw” GPS signal, which will then be uploaded to the cloud server and all complex calculations will be made there. Instead of navigational messages, the exact coordinates of the satellite’s orbit can be obtained from publicly available databases.Of GNSS .
In order to determine the coordinates with an accuracy of about 30 meters, you need only five fragments of a GPS signal with a duration of 2 milliseconds with intervals between them no more than 50 ms. A thousand points recorded this way takes about one megabyte - this is much more than the finished coordinates, but flash memory costs a penny.
An experimental tracker created by Microsoft Research engineers to test the concept and experiments showed that it takes only 0.407 mJ of electricity to record one 2-millisecond fragment. To determine the exact coordinates, only a few of these fragments are needed, so that the total consumption will be about one millijoule, while an ordinary modern smartphone with A-GPS spends about a joule per point. If you record points every second, two AA batteries will last for a year and a half of continuous operation.When recording raw data, another problem arises - the exact time synchronization, which is impossible without decoding the GPS signal. If the tracker’s clock out of sync with the system’s time is more than a minute, it’s difficult to get accurate data from the raw signal. To solve this problem, a small receiver of accurate time signals WWVB was built into the tracker - these signals are broadcast by the U.S. National Institute of Standards and Technology and are confidently received in almost any environment throughout the United States (transmitter power is 70 kW, time is synchronized by atomic clock). To achieve acceptable accuracy, it is enough to synchronize the internal clock with WWVB signals only several times a month.
Thus, CO-GPS technology allows you to get rid of most of the complex stuffing of a GPS tracker that performs calculations, replacing it with a larger flash memory and a simple and economical receiver of accurate time signals. The tracker will spend most of the operating time in sleep mode, it is enough for the GPS-signal receiver to turn on for only 10 ms to record one point. Naturally, such a receiver will not be able to obtain coordinates in real time, which is quite suitable for biologists who are ready to wait a few months until the tracker is removed from the animal.
However, given the rapid spread of broadband wireless in cities, CO-GPS can also be used in consumer, “urban” devices. It is enough to send several kilobytes of raw data to the server via a cheap high-speed network once every few seconds, receiving the coordinates almost as quickly as in the case of a conventional receiver.
Download a PDF with a detailed description of CO-GPS here .