The news, opinions and events of VE3OIJ / VE3EEE

Archive for the APRS tag


Searching for the balloon

Astrophysical and Geophysical, Technology

As you’ve read in a previous post, or perhaps on VE3XGD’s blog, I assisted in a balloon chase recently. VE3XGD and I were the chase team that recovered the first balloon. We are experienced geocachers, and that experience really helped when we were looking for the balloon on the ground.

What I learned on this chase is that there is a bit of wisdom and experience that can be passed on to other people who may wish to chase balloons in the future.

GPS isn’t as accurate as you might think

First off, it’s important to know what GPS accuracy actually means. There are a number of factors that can affect how close the position calculated by the GPS receiver actually is to the true location on the Earth’s surface. Among these factors are:

  • Radio propagation effects between the receiver and the satellite – refraction delays and reflections in particular… every three nanosecond (3×10-9 seconds, 0.000000003s) of delay means 1 m of error
  • The number of satellites seen
  • Signal quality
  • The relative positions of the satellites – you might see 12 satellites, but if they’re all bunched up, they don’t give as good a position.
  • Whether or not you have Wide Area Augmentation System (WAAS) turned on and can hear the WAAS “satellites” plus all the factors above as they apply to WAAS.
  • The weather, tree cover, terrain, etc. This is usually the biggest factor. In trees or urban terrain, GPS accuracy is typically 10-15m

Indeed, if you have a high-end GPS receiver, with a clear sky, a view to the horizon, and good coverage of the WAAS system, you can get 2m accuracy out of your GPSr. The number displayed on your GPSr for accuracy is an estimate of the “Circular Error Probable” or CEP. That number means that there is a 50% chance you are within that distance of the position displayed on the GPSr. If you multiply that number by 2.5, you come up with an approximation of the second diamater Root Mean Square error. Basically that means there’s a 95% chance that you are within that distance of the position displayed on the GPSr.

Thus, if you are showing a position of N 45° 0.00″ and W 75° 0.00″ with an estimated error of 5m, there is a 50% chance you are, in fact, within 5m of that position, and a 95% chance that you are within 12.5m of that position.

Now consider the display itself. At best, you can expect +/- 1 in the last digit for accuracy (that’s probably being kind). If you are using dd mm.mmm (degrees and decimal minutes to 3 places), that’s accuracy of 0.001 minutes added to the CEP error. That works out to 1.8m of latitude, and at my latitude, it’s 1.3m of longitude. If you are using only 2 digits of decimal minutes (like APRS from this balloon was), the accuracy is at least 10x worse (18m and 13m respectively).

So what?

Well, if you’re looking for coordinates set by someone else, when they measured the position, they have all those accumulated errors. Then you go look for their point, and you get all YOUR accumulated errors too. Your errors add to the position errors from the person who set the waypoint you’re looking for.

What does that mean for the balloon?

The balloon we were chasing was sending position updates to 2 decimal places on the minutes. That’s 18m of accuracy, plus whatever other errors. It was a rainy, miserable day when we went out, and the balloon had landed in a field, but in a generally forested area near a big reflective building. My own GPS was reporting estimated errors of 15m or more, depending on whether I was under the trees or not. It’s fair to estimate, then, that the balloon on the ground would be in a similar situation.

Therefore the balloon position is a CEP of 15m (for the GPSr) plus 18m (accuracy of the numbers transmitted), for 33m. The 2dRMS circle is 2.5x that or 82m. And that’s just the balloon position.

When I show up, I have a 15m accuracy from the GPSr, and I can work in 3 decimal places, so there’s an extra 1.8m I have to take into account. That all gets added to the error for the balloon. So I am searching with an error of 33m (balloon) + 17m (my GPSr) or 50m. There is a 50% chance that the balloon is within 50m of what I see as “Ground Zero” on my GPSr, and a 95% chance the balloon is within 125m.

You thought geocaching was easy because you can look up the coordinates, eh?

As you can see, it’s not necessarily so. In the geocaching game, you get the coordinates to 3 decimal places, but that can still leave you a huge search circle. For this balloon, we only had 2 decimal places and an effective search circle that was bigger than two football fields and mostly tree-covered. Suddenly you wish the balloon could signal with a flare or an air horn…

After searching the 50m circle quite extensively for nearly an hour, we did not find the balloon – even though there were at least three teams of searchers. In this case, our 50% chance failed… the balloon was likely in the wider, 95% circle.

I went back and talked with one of the other searchers (VE3JGL) who had seen the balloon come down. He had a direction for the balloon, so I used a geocaching technique: walk the line. I knew roughly how far, and approximately what direction. There’s a note here about estimating the distance of a falling object: If you see something fall from the sky, unless you see it actually hit the ground, it’s probably a lot farther away than you think. With no reference for size of a falling object in the sky, it is REALLY hard for a human to estimate how far away an object in the air actually is. The important thing is to accurately guage the DIRECTION in which the object fell.

VE3XGD and I determined there was a good chance that the balloon had landed on the roof of a nearby building based on this. In the process of looking for a good vantage point to see on the roof, we discovered the balloon near the building in a little field.

How far off was the balloon?

Here is an aerial view (click to embiggen):

[image lost to the ravages of time and database failure]

As you can see, ground zero was 107m from the APRS transmitted location… inside the estimated 95% circle, but well outside the CEP circle. I guess it wasn’t a good day to buy lottery tickets since we failed our 50% chance. Oddly enough, there was sufficient tall grass around that little spot of field that even though I personally had walked around the building once already, I did not see the balloon package.

I marked the wall and glass from VE3XGD’s blog post. The pile of broken glass was about 3 m tall. This goes to illustrate another important point:

When someone has a sign up that says “Danger, do not tresspass here” they probably mean it. The company that owns this land did have such warnings up. That’s something else to think about, especially if you’re doing this sort of thing with kids.

You can read about the Lanark Space Agency’s balloon chases on their site.

You can track the travels of this balloon here.


Yaesu VX-8R, the good, the bad, the ugly

Digital Modes, Equipment, Operating, Voice Modes

As noted in the previous post, I have a Yaesu VX-8R now and I thought I’d post some of my impressions in my standard review format…

The Good

This radio is top-notch in a number of areas:

  • The audio quality on send and receive seems to be excellent.  Although nobody buys an HT for its awesome sound quality on receive, it’s nice to have a radio that puts out clear audio right up to max volume.
  • The radio only has 1 knob and setting the volume involves pushing a function button and turning the knob.  My initial reaction to this was not good, but as I played with the radio it occurred to me that something was different… unlike when handling my Kenwood TH-F6A, handling the VX-8R never resulted in the volume being bumped to a weird setting.  I have to give props to this, because it really annoys me when the volume knob on my Kenwood gets bumped off the preferred setting.
  • The radio is light, even compared to the tiny Kenwood that I have.  This means that it can clip on clothing a little easier.  The radio is also quite thin, thinner than my wallet – you could carry it in a pocket if you had to.  Perhaps that says more about the junk I carry in my wallet than the radio, but you get the idea.
  • The radio is submersible.  The primary purpose of an HT is outdoor use for me, and that means exposure to rain and a non-zero chance of being dropped in a puddle or pond.  Submersible is a major bonus in my opinion.
  • APRS is built in.  This is also a great feature for outdoor use.  I hike and geocache, and it’s nice to know that I can carry a beacon with me in the event that I get into trouble.  I use my Kenwood D-700 as a repeater when I am in the woods, and now I can use it to digipeat my location when I’m on a trail or seeking Tupperware by GPS 🙂
  • The radio seems to have good performance on 50 and 220 MHz.  I haven’t really played with it on the other two bands.  There is no problem getting into the 6m repeater that is in my grid square (VE3RVI: 53.030, minus, 1 MHz), and no problem making the 220 MHz repeater even on the reduced power that this radio puts out in that band (VE2REH: 224.760, minus, 110.9 Hz tone).

The GPS unit works well, even from inside my house.  It has a nice little display that gives all your position info.

The Bad

These are really snivels.  There’s only one real issue with this radio as noted in the next section.

  • The lithium-ion battery that comes with the radio is, at 1100 mAH, a bit light for a radio of this power.  I wish the radio was delivered with a pack that accepted AA cells as well, but that is a separately purchased option.  There is, however, a higher capacity battery available separately.  I’ll have to look into that I guess.
  • I don’t understand why this radio is so low powered on the 1.25 m band.  It’s definitely better than nothing, but it would be nice if it had full power like the Kenwood TH-F6A.  I’m sure there’s some design reason, but my gut tells me it should have been easy enough to overcome.
  • The radio only has AM, Narrow FM and Wide FM modes.  This really reduces the functionality of receive in the other amateur bands.  The radio has wide frequency coverage, but within the amateur bands, you can’t listen to the SSB traffic.  Considering that competing radios (like the Kenwood TH-F6A) have SSB and CW reception, it surprises me that this radio does not.  That said, I don’t spend a lot of time listening to that stuff with the Kenwood, so it’s not a major issue with this radio for me.
  • Sending and receiving APRS messages is a bit of a PITA.  If you’re used to the Kenwood D-700 system, you’ll be disappointed.  Per the previous paragraph, the APRS messaging is buried a couple levels down in menus, and my first impression is that the whole interface is not especially intuitive.

The Ugly

I have only one major complaint about this radio, and that it uses a complicated menu system.  The main menu has something like 100 items.  So many of the features of this radio are accessed from the menu, that there is a very steep learning curve.  Sure, the basic functionality is straightforward, but if all you wanted was a couple of VFOs for talking, you would buy a much less expensive radio.  I am certain that another row of front-panel keys could have reduced the menu complexity a bit and not added significantly to the size and weight of the radio.  Even simple features like squelch are in the menu system, making them hard to use.

Whatever you do… DO NOT LOSE THE MANUAL!  I guarantee you’ll need the manual often.


I am very pleased so far.  I have some accessories on order (speaker-mic, GPS antenna).  I may explore the Bluetooth board, although I am not convinced I want to use a bluetooth headset – mostly because I usually have such a headset for my telephone and don’t think I need two headsets on at the same time.  I will be seeking that AA cell battery pack.  That’s a must-have in my book.

The stock battery seems to have a lifetime of about 3 hours while using high power to talk on a repeater AND transmit an APRS beacon every two minutes.  I haven’t decided if that is good, bad, or ugly.  For most of my use, I don’t expect to use high power, so I would expect to get more battery life.