Delorme InReach

[alexmtn]

InReach and Spot both use short burst data messages. Thus a sender only needs a link to be functional for a short period (eg through a gap in the forest cover) and the sender can keep trying until an acknowledgement is received. (This works for short messages, but not for a long continuous service such as a telephone call.)

IMO, the most powerful single reason to pay the extra money for InReach over SPOT (I'm currently a 2nd-gen SPOT user) is that acknowledgement. In an emergency (as opposed to periodic "I'm OK" messages) with just SPOT, I'd be pretty persistent with 'hitting the button' from a variety of instrument positions.

Alex

 

[Remix]

A real time iridium satellite tracker is here:

http://www.satflare.com/track.asp?q=iridium#MAP

Click on the grey satellite to see coverage.

 

[Remix]

FWIW, I put my Inreach next to my computer monitor. I am inside my office. There is a window 6 feet away.

At 2011 I sent a test message to [email protected]. Within 4 minutes, the wheel on the display stopped spinning, and I got the "up-chirp"

At 2028 I received a ring tone and a return email from [email protected].

Also, had a 3d gps fix indoors.

Usually the whole process takes 2-3 minutes max...clearly there were repeated attempts.


Note- If the inreach does not get an acknowledgement receipt in 15 minutes, a message will appear on the screen advising you to move to clear sky.

 

[DougPaul]

IMO, the most powerful single reason to pay the extra money for InReach over SPOT (I'm currently a 2nd-gen SPOT user) is that acknowledgement. In an emergency (as opposed to periodic "I'm OK" messages) with just SPOT, I'd be pretty persistent with 'hitting the button' from a variety of instrument positions.

I was referring to an acknowledgement to the hardware so that it could stop transmitting to save the batteries, but I agree--passing that acknowledgement back to the human is also very desirable. Plus the ability to compose and send messages is very valuable too.

A PLB keeps transmitting until the rescuers arrive or the batteries run out. This allows the rescuers to track the user and home in on the beacon, but there is no mechanism for returning an acknowledgement or custom messages.

Doug

 

[iAmKrzys]

I was referring to an acknowledgement to the hardware so that it could stop transmitting to save the batteries, but I agree--passing that acknowledgement back to the human is also very desirable.

Acknowledgement can be very important in maintaining hope and will of survival.

I remember watching a movie (some 30+ years ago) about an experiment with two rats that were placed in two aquaria filled with water side-by-side. One rat was left alone and it sank after about 15 minutes. The other one was allowed to climb a piece of wood for a couple of minutes and then was placed back in water. It kept swimming for almost 15 hours afterwards before it sank repeatedly returning back to the place where earlier it climbed out on the piece of wood.

This is not an experiment on people, so one could question whether there is link to human behavior, but I really believe that people will persist in their will of survival if they have some reason to believe that there is still hope they will get rescued.

Apparently, this movie is now on youtube ( https://www.youtube.com/watch?v=ypPDSuE0Q2w ), so you can watch it for yourself. The commentary is in Polish only, but a rough transcript is available below the movie so you can use google translate if you wish.

I don't mean to hijack this thread, so I will add on a separate note, then when I compared locations transmitted by my Spot to traces from my Garmin eTrex 30, most of the time they were quite a bit closer then 100 meters discussed earlier. Also, I often can get gps lock in my house if I stay close to a window, however, I had first 3 cases of lost satellite reception on my eTrex two weeks ago when we were hiking in deep canyons of Zion National Park, so I cannot even be sure exact distance we hiked!

 

[DougPaul]

I don't mean to hijack this thread, so I will add on a separate note, then when I compared locations transmitted by my Spot to traces from my Garmin eTrex 30, most of the time they were quite a bit closer then 100 meters discussed earlier. Also, I often can get gps lock in my house if I stay close to a window, however, I had first 3 cases of lost satellite reception on my eTrex two weeks ago when we were hiking in deep canyons of Zion National Park, so I cannot even be sure exact distance we hiked!

Typical accuracy of a consumer GPS is within 10m 95% of the time (with a good skyview). The 100 meter accuracy of a PLB is due to rounding the data to the nearest 4 sec (up to 61m rounding error) of lat and lon for transmission. The accuracy of the GPS within the PLB is presumably similar to that of consumer GPSes.

Spot presumably does not use the PLB message formats so it may not suffer the large rounding errors. This could account for the better location match to the eTrex.


Inside a house is generally a pretty difficult environment for a GPS. Performance depends on construction of the house and location within the house. I have read that GLONASS works better than GPS indoors.

I'm not surprised that you lost lock within Zion's canyons--your view can be limited to a thin strip of sky. Urban canyons can also be difficult environments.

Doug

 

[skiguy]

Both the 121.5 MHz and 406 MHz signals will be affected by the local vegetation and topography but less so than the ~1.6 GHz signals used by SPOT and InReach. The 100 meter accuracy of the GPS position should get rescuers within range of the 121.5 MHz and the (direct) 406 MHz signals in almost all cases (except maybe under salt water or in a cave...).

Doug

Is this due to a lower take off angle of the VHF vs. UHF wave?

 

[DougPaul]

Is this due to a lower take off angle of the VHF vs. UHF wave?

No--it is primarily due to the longer wavelengths at 121.5 MHz (2.5 m) and 406 MHz (.7 m) than at 1.6 GHz (.19 m). Obstacles become smaller (in terms of wavelengths) at lower frequencies and anything that is significantly smaller than a wavelength is usually unnoticed by the wave. (This is why GPS is not affected by the weather--raindrops and snow flakes are much smaller than a wavelength.)

FWIW, VHF is 30-300 MHz and UHF is 300-3000MHz so 121.5 MHz is VHF and the other two are UHF.

Signals at the above frequencies generally travel line-of-sight, although (like light) they can be refracted by temperature and humidity gradients in air. (Like UHF TV: 470-806MHz)


Takeoff angle applies to horizontally polarized antennas* and is determined by the antenna height above the ground. The ground acts as a reflector so there is effectively a phase reversed mirror image of the antenna below ground. The signals from the two antennas cancel at a zero elevation angle and the take-off angle are the elevation angle at which the signals add constructively. These angle depend on the antenna height in wavelengths--lower antennas produce higher take-off angles.
* A highly directional antenna which is aimed upward will not illuminate the ground and will not be subject to this issue.

Vertically polarized antenna and their image antenna signals add constructively at zero elevation, but may have nulls at higher elevation angles.

Doug

 

[skiguy]

No--it is primarily due to the longer wavelengths at 121.5 MHz (2.5 m) and 406 MHz (.7 m) than at 1.6 GHz (.19 m). Obstacles become smaller (in terms of wavelengths) at lower frequencies and anything that is significantly smaller than a wavelength is usually unnoticed by the wave. (This is why GPS is not affected by the weather--raindrops and snow flakes are much smaller than a wavelength.)

FWIW, VHF is 30-300 MHz and UHF is 300-3000MHz so 121.5 MHz is VHF and the other two are UHF.

Signals at the above frequencies generally travel line-of-sight, although (like light) they can be refracted by temperature and humidity gradients in air. (Like UHF TV: 470-806MHz)


Takeoff angle applies to horizontally polarized antennas* and is determined by the antenna height above the ground. The ground acts as a reflector so there is effectively a phase reversed mirror image of the antenna below ground. The signals from the two antennas cancel at a zero elevation angle and the take-off angle are the elevation angle at which the signals add constructively. These angle depend on the antenna height in wavelengths--lower antennas produce higher take-off angles.
* A highly directional antenna which is aimed upward will not illuminate the ground and will not be subject to this issue.

Vertically polarized antenna and their image antenna signals add constructively at zero elevation, but may have nulls at higher elevation angles.

Doug

Thanks for your response and explanation. I understand what you are saying about wavelength and it's theoretical application in the disscusion of tracking devices. Although I disagree with your statement that take off angle only applies to horizontally polarized antennas.
http://www.dx-antennas.com/Height versus take off angle.htm

 

[DougPaul]

Thanks for your response and explanation. I understand what you are saying about wavelength and it's theoretical application in the disscusion of tracking devices. Although I disagree with your statement that take off angle only applies to horizontally polarized antennas.
http://www.dx-antennas.com/Height versus take off angle.htm

With a horizontally polarized antenna, the image antenna is 180 degrees out of phase with the real antenna so there is complete cancellation at 0 degrees elevation. At very low elevations above ground, it will have a single vertically oriented lobe. As the antenna is raised above ground, the lobe will split into multiple lobes.

With a vertically polarized antenna, the image signal is phased so there is constructive addition at 0 degrees elevation. A quarter wave vertical with its base at ground level will combine with its image to form a virtual half-wavelength long vertical dipole which will have a single broad single lobe with its peak at 0 deg elevation. (This is how a 1/4 wave whip with ground-plane antenna operates, except an it uses its own ground plane (3 or 4 radials) rather than an earth ground plane so it can be mounted well above the physical ground.)

As you raise a vertically polarized (actually any) antenna above ground, its vertical radiation pattern will break into multiple lobes because the antenna and its image will be separated by long distances (eg multiple wavelenths). For a real antenna, the 0 degree elevation lobe can be absorbed by a lossy ground leaving the next lobe (as seen at a distance) as the main lobe at a takeoff angle of >0 degrees.

The above radiation patterns assume a perfectly conducting infinite level ground. Real world antennas exist above imperfectly-conducting, uneven, and tilted grounds. This imperfect ground can alter the phase (and amplitude) of the signal "radiated" by the image antenna which can alter the takeoff angles of all of the lobes. Thus the phase shift can cause the take off angle of the main lobe of a vertical antenna to be greater than 0 deg, but to a first approximation, all vertical antennas will have a main lobe at 0 deg.

The radiation patterns of (non-active) antennas are the same for receive and transmit although they may be easier to understand in one or the other.

Bottom line: the radiation patterns of antennas can be altered in weird and wondrous ways by objects in their near-field.

When analyzing the vertical radiation pattern of any antenna, one must consider both the real antenna and its image. If they are close together, there will be only a few lobes and only one if very close together. If they are far apart, then there will be many lobes will nulls in between them and the directions of these lobes will depend on the antenna locations, signal phasing, and signal amplitudes. (This is related to a 2-slit interference pattern https://en.wikipedia.org/wiki/Double-slit_experiment https://en.wikipedia.org/wiki/Young's_interference_experiment)

<meta comments>
This is quite a tangent--hope those who are not interested in this level of detail have had the good sense to skip it...

It is actually relevant for getting the best signals between a satellite and a portable device, but sticking to the rules-of-thumb with regard to antenna positioning and orientation is probably the best strategy in the field in most cases. However, the above can explain why a small change in the location of an antenna can sometimes make a significant change in the signal strength.
</meta comments>

Doug