- Dual-Frequency GNSS – An important location feature your phone is probably missing
- What is the Xiaomi Mi 8’s dual frequency GPS?
- Dual-Frequency GNSS – An important location feature your phone is probably missing
- Dual-Frequency GPS vs Single-Frequency GPS: What’s the Difference?
- What is the Xiaomi Mi 8’s dual frequency GPS?
Dual-Frequency GNSS – An important location feature your phone is probably missingThat makes Mi 8 a very special device. That said, exactly what is the new dual-frequency GPS technology and how is it better than the regular single GPS frequency technology which is being used in all the other smartphones, tablets, and wearables? That is the question that we are going to try to answer in this article. For the GPS to work flawlessly, the receiver for example, your smartphone should have an unobstructed line of sight to four or more GPS satellites. So, basically in optimal conditions, four or more satellites are sending navigational signals to your device which your device receives and interprets to tell you your exact location. Now that happens when the conditions are optimal, however, this is not always the case. There are a lot of factors in play which escalate the potential error. The most significant ones are the interference in the ionosphere, signal blocking due to large physical structures such as mountains, and highly urbanized area with tall buildings. There are many other factors that affect GPS accuracy, but the above three are the main culprit. The dual-frequency GPS technology was developed to pacify the signal problems caused by the above-mentioned factors so that we can have a more reliable and accurate location recognition. Again, not getting into the technicalities, for civilian usage purposes, there are three main different bands that are available to use. What are bands you ask? Simply put, a satellite sends signals in radio frequencies. The Global Positioning System utilizes frequencies between 1. The main frequency range that is being used by satellites today is the L1 band which uses frequencies roughly between 1. Just like the L1 band, there are two other major frequency bands that can be used in for GPS which are the L2 and L5 bands shown in the picture below. Now, as it can be interpreted from the name dual-frequency GPS, it will be using two different bands. The use of dual-frequency GPS helps in alleviating the problems that we discussed above. Again, not getting in the mathematics and the physics of it all, the dual-bands basically give more information to the receiver so there are fewer chances of an error occurring. Another benefit of using dual-GPS frequency is that even if one of them fails, the other one is present as a backup. Even the signal make-up is different which results in better readings in different environments. For example, the L5 band makes it easier to distinguish real signals from the ones reflected by buildings thereby reducing the MultiPath effect caused by tall buildings in urban environments. In conclusion, the dual-GPS frequency reduces the GPS error rate by bringing more information thereby reducing the error rate and giving out a more accurate positioning reading.
What is the Xiaomi Mi 8’s dual frequency GPS?
Of course, this goes beyond just casual app usage. In essence, what this means is that your smartphone tracks a single radio signal from each satellite. However, single-frequency GNSS is prone to multipath errors. This can result in inaccuracies of around 5 meters. So how does dual-frequency GNSS rectify this issue? The short answer is two is better than one. Instead of relying on just one signal to determine your location, devices track more than one signal from each satellite, each on a different radio frequency. For Europeans with the Galileo satellites, the frequencies are called E1 and E5a. They can be used to refine position accuracy to as low as 30cm versus the 5m mentioned earlier. Dual-frequency GNSS is not exactly new technology. Support for it was added in Android 8. Broadcom followed this by launching a dual-frequency chipthe BCM, in September of Broadcom said that its BCM would be featured in smartphones, but never elaborated on which ones exactly. Other vendors have been slow to jump on the wagon, however. At the moment, Huawei and Xiaomi are the only two vendors of note whose smartphones officially claim to support dual-frequency. But, Xiaomi is the only vendor where the use of dual-frequency GNSS has been observed in third-party apps. So why is dual-frequency GNSS such a niche feature? The most likely answer is cost. Dual-frequency chips are not exactly commonplace at the moment, so it may be hard for manufacturers to get their hands on them. By and large, accuracy to within 5m is enough for day-to-day usage. That being said, there are still numerous groups of people who would benefit from increased accuracy. The first groups that jump to mind, for example, are joggers and hikers. Very few phones at the moment support dual frequency, so the most likely answer is no. There are a multitude of apps out there that can help you determine if your device supports dual-frequency GNSS. All you have to do is load up the app preferably while outsidelet it lock your location, and then check the CF column to see if there are any L5 or E5a readings. There are detailed analyses of many devices available there. Currently, the Xiaomi Mi 8 and the Xiaomi Mi 9 are the only devices that both officially claim to support dual-frequency. The use of dual-frequency has been confirmed using 3rd party apps. The Hong Kong Snapdragon variant of the Samsung Galaxy S10 also appears to support it, based on 3rd party apps, but this is as yet unofficial as Samsung does not list support in the official specifications for the device. We reached out to Samsung for clarification but did not receive a response by publication time. Below is a table of popular phones detailing whether or not they support dual-frequency GNSS, and what global systems they support too. Google maintains a list of devices that support raw GNSS measurements, which is the basis of the following table. Xiaomi certainly seems to view dual-frequency GNSS as a valuable investment, however. Dual-frequency GNSS is readily available for every vendor, but they are being very slow to adopt it.
Dual-Frequency GNSS – An important location feature your phone is probably missing
We expect that for professional applications that need precision positions, a dedicated system that employs a custom GNSS chipset and purpose-built applications will continue to be the right solution. A range of new use models and applications will be enabled by consumer mobile phones with technology that improves positioning performance. Then we show the position performance achievable using precision engine with measurements from a dual-frequency GNSS chipset targeted for the cellular handset market. This class of device is expected to be integrated into consumer cellular devices on the market within the next 1 to 2 years. We tested various devices including the Nexus 9 which provides phase data and various other Android devices that implement the new API. Most devices tested do not support phase data; of the few devices tested that do provide phase data, all except the Nexus 9 implement GNSS power duty cycling. This is a mode where the GNSS chipset is only active for a fraction of each second to reduce power consumption. This results in cycle slips each epoch, which makes carrier-phase processing for real-time kinematic RTK unusable. During the testing a wide range of performance across devices was observed. The units were located in a clear environment less than a meter apart. Deep fades are present, most likely caused by deconstructive multipath. Figure 1. Before attempting to position with observables from Android devices the measurement quality was analyzed. One of the devices tested was a Samsung S7 device. However, the phone implements power duty cycling so after a short period of operation the duty cycling mode was enabled which resulted in a cycle slip on the phase every epoch. To derive an improved position from this class of device pseudorange and Doppler can be fed into a code-phase positioning engine. Fortunately, the Doppler provided by the device is of reasonable quality as can be seen from Figure 2. Figure 2. In this simple analysis measurements from a single high elevation satellite were analyzed. The Doppler is plotted along with the differenced pseudorange converted into L1 cycles. It can be seen that as expected the Doppler has much lower noise and so can be used in a pseudorange smoother. A simple way to view the pseudorange noise is to subtract the carrier phase from the pseudorange. If there are no cycle slips this should show ionospheric divergence with the noise dominated by the pseudorange noise. The absolute level is arbitrary as it includes integer carrier cycles. Figure 3 shows an example from an Android device. Figure 3. Android GNSS observables: pseudorange — carrier phase. The data was captured on a building roof in an open environment.
Dual-Frequency GPS vs Single-Frequency GPS: What’s the Difference?
The introduction of a new generation of mass-market chips based on multi GNSS dual frequency measurements, already being commercialized and integrated in smartphones by major manufacturers, is contributing to a new level of positioning accuracy in the mass-market location-based services. Today, we are assisting to a proliferation of high accuracy applications on smartphones, thanks to the availability of dual frequency measurements along with the capability to process GNSS raw measurements on Android devices. Here the authors address a new level of sub-meter positioning accuracy, before unimaginable without professional grade equipment, and now accessible to everyone on smartphones, to people on all budgets. While a new generation of mass-market chips using multi GNSS dual frequency measurements provides great potential, there are still some hardware limitations to overcome, most notably related to the poor quality of the GNSS antenna integrated in smartphones. The selection of this dual frequency combination is particularly appealing for two main reasons:. This is very important for multipath rejection as it will be shown further in this article. It is important to stress that the final positioning accuracy in mass-market devices is not only driven by GNSS measurements, either single or dual frequency. All these ingredients contribute to the fused location and its ultimate accuracy. The tests results shown in this article have been conducted in a variety of configurations and scenarios, including static, pedestrian and vehicular setups. The quality of the raw measurements has been evaluated through code multipath analysis and cycle slips occurrence probability. Similarly to other authors Riley et aliathe development kit enabled the comparison of the standalone performance of the chip against its integrated smartphone version. The main differences in the test setup between this evaluation kit and the smartphone are the following:. It can be fed with raw measurements coming either from the chipset evaluation kit connected to the professional grade antenna or from smartphones, accepting as input either broadcast or final orbital and clock products MGEX GBM. The evaluation kit was used as a simple source of dual-frequency measurements for static and kinematic users. Live static data were collected in open sky conditions. One test was performed near the ESTEC football pitch, with the evaluation kit connected to a geodetic-grade antenna installed on the roof of the van, while the smartphone was placed on the van dashboard. The satellite signal power levels are set to values corresponding to open sky conditions in AWGN channel model and then decreased by steps of 2 dB per sec. The football pitch is mainly an open sky area, with only a slight building shadowing on its west side. The average walking speed was 3. Lever arms to the reference anten. The campus presents some obstructions in a few points of the route due to trees and surrounding buildings and the environment can generally be considered mild. The main contributor to the accuracy of the pseudorange measurements is definitely the multipath error. In the case of Galileo E5a and GPS L5 as well the error instead results much smaller and confined between few meters amplitude max 3 meters in the case shown as example. Such combination is a code minus carrier observation where the ionospheric error contribution is removed by the combination of carrier phase measurements from L1 and L5, as described in the following equation:. N represents the unknown ambiguity. During a continuous period of tracking satellite i, N is a constant as long as no cycle slips have occurred. Therefore, the multipath plus noise estimate. To compute instead the multipath error on L5, the first frequency is L5 and the second frequency is L1. The same methodology has been applied to all the satellites visible during the static tests T. In these cases, the variance of the measurements is higher and there is a degradation of performance with respect to the same chipset working with a professional grade antenna, demonstrating the impact of the planar inverted F antenna PIFA assumed to be mounted on the smartphone and of a noisier environment probably due to internal interference in the phone. In the case of carrier phase measurements, the multipath combination cannot be used to determine the accuracy. A double difference combination approach has been used, where the difference of carrier phase measurements from two satellites and two different receivers are combined. Carrier phase measurements from the GNSS RF simulator and the evaluation kit were double differenced for cycle slips detection. In the case of a zero-baseline test, double differencing allows to cancel errors from the ionosphere, troposphere and satellite clock biases. Since a static test was simulated, the position component of the double differences is constant and can be ignored. Estimating the receiver clock bias and the cancelling it from the double differences, the dominant remaining component will be the phase bias.