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I'm working with Estimote beacon from some weeks but I have a doubt. What is the unit of measurement of the Estimote's distance?

NSString *distance=[NSString stringWithFormat:@"%@",beacon.distance];

If I place the Estimote at 10 cm from the iOS device the distance is 0.055.

Julian
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Carioni
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5 Answers5

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You can not calculate the exact distance - prove. It is rather some indication (but not too strict) whether you are getting closer or not to the beacon.

Morover, according to Apple's documentation:

"The array of beacons is ordered by approximate distance from the device, with the closest beacons at the beginning of the array. You can use the information in these objects to determine the proximity of each beacon and its identifying information (proximity UUID, major, and minor values). For instance, the value of the proximity property of the CLBeacon gives a general sense of the relative distance to a beacon."

And to be honest I haven't found any information about the units, it is rather a value to distinguish somehow the relative distance to that devices.

ANSWER: it is not any specific unit but only a relative value (still worth to read above text)

Julian
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The distance should be in meters.

Bear in mind the BTLE is very susceptible to interference. We played with our Estimote beacons and found that at 1m the device's distance read would very from 0.7m to 2.1m.

rnystrom
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    where have you found that this unit is in meters? I haven't found anything like that – Julian Feb 04 '14 at 22:46
  • @viperking maybe you can find the clue from Apple sample code AirLocate File: APLRangingViewController.m: NSString *formatString = NSLocalizedString(@"Major: %@, Minor: %@, Acc: %.2fm", @"Format string for ranging table cells."); cell.detailTextLabel.text = [NSString stringWithFormat:formatString, beacon.major, beacon.minor, beacon.accuracy]; Check "%.2fm" where the unit is meter. – tedyyu May 07 '14 at 02:32
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The iBeacon protocol provides in one signal essentially two information:

  1. the unique code of the device( the so called proximity UDID)
  2. the power of the signal at the emission point

Using this second information is possible to elaborate an approx distance. Knowing the emitting power and the power at the receiver, is possible to evaluate a distance knowing the drop of the power each meters. High freq signals are really susceptible to interference. The Estimote advertising signal I'm pretty sure is the same of iBeacon. The distance measure is really a heavy approximation.

Andrea
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The distance measured is in meters, it is derived from Apple's Class CLBeacon, and the Accuracy property: "The accuracy of the proximity value, measured in meters from the beacon. (read-only)". "https://stackoverflow.com/questions/19007306/what-are-the-nominal-distances-for-ibeacon-far-near-and-immediate" Describes a bit about what this value means.

Estimote's API simply takes this value, and renames it to Distance.

The value is not very accurate in certain circumstances, this is because environmental variables influence the result. I've recently investigates some of these effects for Estimotes, you can read about them in this presentation: "Factors effecting positional accuracy of iBeacons". The presentation also has references to some academic articles that will help you understand distance estimation from RSSI better, Apple's API is doing exactly this.

Community
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Chris Thomson
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This is Wojtek Borowicz, I'm a community evangelist at Estimote.

Beacons transmit data packets including their unique ID and information about signal power (value called TxPower). With that info, the receiving device measures RSSI (received signal strenght indicator), which serves the purposes of estimating proximity. Both TxPower and RSSI use dBm as their unit of measurement, which is a dimensionless unit (http://en.wikipedia.org/wiki/Dimensionless_unit). It is NOT in meters.

Cheers.

Wojtek Borowicz
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    While providing a good reminder of the limitations of the technology, this is not actually an answer to the question asked. The numbers cited in the question are pretty clearly not dBm, but rather something derived from that using a mathematical model. Also, you seem to be confusing **dB** which **is** dimensionless, with **dBm**, which is in fact an absolute power level (specifically, a ratio relative to 1 mW) – Chris Stratton Feb 27 '14 at 15:45