High Altitude Balloon
RADIO COMMUNICATIONS COVERAGE
Second Approximation based on Topographic Terrain
and Radio Mobile Deluxe
Ralph Wallio, WØRPK W0RPK at netINS.net
discussion of high altitude balloon communications range was based on the
seemingly simple model
DISTANCE [statute miles] = 1.23 x SQRT(ALTITUDE [feet])
This model has a heritage in spherical trigonometry and is a reasonable first approximation of radio coverage for we flatlanders here in corn and soybean country. See http://showcase.netins.net/web/wallio/ALTvDIST.htm for a more thorough discussion.
Model distances can be plotted using mapping software (Delorme Street Atlas in this case) for increasing balloon and payload altitude. During September 2002 Don Pfister, KA0JLF, flew a HABITAT SkyLab mission from Herington, Kansas (see http://habitat.netlab.org/index.shtml). Radio coverage was predicted at various altitudes and plotted with locations of three stations than completed QSOs via Don's simplex repeater payload.
This simple model considers our Earth to be a smooth and perfect sphere that is acceptable here in our Midwest flatlands. However, the model does not take topographical terrain into account so it does not know when mountains are blocking a desired path. Therefore predictions for flights in mountainous areas need to come from a higher level, a second much more accurate approximation, by using radio propagation prediction software that applies digital terrain elevation data.
We are fortunate that Roger Coudé, VE2DBE, of Quebec, Canada developed and actively supports Radio Mobile Deluxe that answers our needs. Roger has written a comprehensive interface between users and the Longley-Rice Model that calculates and predicts radio propagation over terrain described by digital elevation data. See http://www.cplus.org/rmw/english1.html for further discussion and to download Roger's software. See http://elbert.its.bldrdoc.gov/itm.html for further information about the Longley-Rice Model including descriptions and source code in FORTRAN (we Old Timers remember those days) and C++.
Roger's Radio Mobile Deluxe system uses digital topographical data that is free and in the public domain. Large data sets can be downloaded or ordered on inexpensive CD-ROMs. Data sets are available of 1000m, 100m and 30m resolution. I use 30-arcsecond 1000m resolution data for the large coverage areas created by high altitude balloon payloads. (The 2000x2000 software elevation matrix limits paths to 2000km for 1000m data (30-arc second), 200km for 100m data (3-arc second) and 130km for 30m data (1-arc second).
Roger provides instructions for downloading and installing Radio Mobile Deluxe and free topographic data sets as well as first examples of use. The next step is to read through Roger's extensive Help facility to start understanding the power and possibilities in his system. An email discussion group is available, Radio_Mobile_Deluxe@yahoogroups.com (subscribe using Radio_Mobile_Deluxeemail@example.com).
Radio Mobile Deluxe and Amateur Radio High Altitude Ballooning
[I don't intend for this discussion to be a tutorial on using Radio Mobile Deluxe
and will freely use a lot of terms that will be learned by new users during early experiments.]
Longley-Rice is a comprehensive model of 20Mhz to 20GHz radio propagation for paths that are close to the earth's surface and entirely in the lower refractive atmosphere. This is entirely reasonable for typical terrestrial applications but radio paths from high altitude balloon payloads start well outside of this model envelope. A long series of experiments comparing radio propagation model predictions with observations during flights found the Longley-Rice model (and therefore Radio Mobile Deluxe) to slightly overstate radio coverage and signal strength.
The substantial fraction of Earth-Balloon (EB) and Earth-Balloon-Balloon-Earth (EBBE) radio paths are above the refractive atmosphere. EB paths start well above the lower denser more humid atmosphere then penetrate the lower refractive atmosphere at various angles (depending on distance to a given earth station) all the way to the ground. Path geometry is doubly complicated for EBBE paths that start well above the refractive atmosphere, approach earth to some degree at the mid-point and then leave the refractive atmosphere before arriving at the distant balloon and payload. The Longley-Rice model overstates bending for the substantial fraction of these paths that are above the refractive atmosphere.
Roger may not have had our unusual high altitude ballooning application of his software in mind but he is way ahead of us by including VISUAL coverage in his Radio Mobile Deluxe tool bag. We can start with visual coverage (with no bending), then experiment with statistical and sensitivity variables in the Longley-Rice Model to match observations and then use these variables to predict coverage for future missions.
Our first example starts with visual coverage to targets only 2m above terrain from NSTAR02F (see http://members.cox.net/mconner1/nstar.html). The map is centered on NSTAR02F so that magenta coverage is from an altitude of 104,567ft ASL while red coverage is from 60,000ft ASL. Digital topographic data is GTOPO30 (30-arcsecond) with 1000m resolution. Map overlay is from MapBlast (automatically downloaded and registered with topographic data by Radio Mobile Deluxe). Broken visual coverage at the edge is due to shadowing from intervening terrain. Note that visual coverage is boringly close to circular here in our Midwestern flatlands.
During the flight of NSTAR02F while it was at 104,567ft ASL and the balloon was almost ready to burst, Jerry Havill, K5OL, and Rick Vidmar, K9KK, both copied a couple of packet radio telemetry frames from the payload and set our #1 and #2 long distance telemetry reception records (http://showcase.netins.net/web/wallio/ARHABrecords.htm). Both stations near Norman, Oklahoma were using gain antennas 15-20m above the ground but were well outside of predicted visual coverage (locations denoted with antenna icons on above map.
study this further we zoom in on the path from NSTAR02F and K5OL-K9KK by recentering
the map and coverage to the midpoint. Near solid red visual coverage is from
104,567ft ASL and stops short of Oklahoma
City as shown on the previous map. We
then experiment with radio coverage by varying statistical and sensitivity
values. Note that use of SPOT 50% and receiver sensitivity set to 1.0uV yield
broken radio coverage near Norman, Oklahoma and our two stations of interest (yellow coverage
with distant red edging). This map uses another Radio Mobile Deluxe feature of limiting coverage arc as
now complete the process by creating a coverage map that predicts both visual
coverage and extended radio coverage from near maximum altitude. This
prediction of radio coverage should be reasonably close for distant stations
that are similarly equipped to K5OL and K9KK. If these stations had been near Indianapolis,
Indiana and Pierre, South
Dakota we might have an almost
unbeatable QSO distance record (unbeatable for EBE). Now lets move our
discussion to the Rockies and EOSS-52 (see http://www.eoss.org/).
few mountains and it gets much more interesting (at least that is what the
tourism folks want us to think). It all looks familiar to the east and flat
land but to the west line-of-sight visual paths are seriously blocked by the Rockies. Note
broken coverage at both 60,000ft (red) and 92,000ft (magenta).
Back to the east, Craig McManus, KCØIUW and Rex Easton, KCØIUY, copied a few EOSS-52 telemetry packets in east central Kansas that put them in the records list. Similar to the NSTAR-02F example, they were slightly off the edge of visual coverage.
predict Craig and Rex's reception of a few packets we show visual coverage when
EOSS-52 was at 92,148ft ASL and add radio coverage based on SPOT 50% and 1.0uV
sensitivity. Note increase in coverage from visual to radio to the east looks
similar to the NSTAR02F example but the increase to the west (over mountainous
terrain) is not so dramatic. Interesting. Now on to different mountains in the land of Tony Hillerman.
coverage for ANSR-2 at 60,000ft (red) and 86,000ft (magenta) is limited by
mountainous terrain in all directions (see http://www.ansr.org and http://www.kd7lmo.net). This
flight was launched near Flagstaff and had a very short track. Bill Cook, K5LPS, and
Rick Sohl, K5RIC, were set up on a mountaintop east of White Sands, New Mexico
at 9600ft ASL. Note that visual coverage skipped almost completely over White
Sands (the green rectangular feature) but narrowly hit mountaintops to the
we add radio coverage to visual coverage from 86,000ft. Note that there is very
little difference. The vantagepoint for K5LPS and K5RIC is again very narrowly
when we zoom in with a slice of coverage, mountains to the east of White Sands
are just barely illuminated. We should note that Bill and Rick did not have the
advantage of this map and prediction before they picked their listening post
for ANSR-2. Now we consider an Earth-Balloon-Balloon-Earth (EBBE) prediction.
Visual line-of-sight coverage to targets 2m above terrain for ANSR, EOSS and NSTAR are shown with all three balloons and payloads at 50,000ft ASL. ANSR is near Flagstaff. EOSS is near Colorado Springs. NSTAR is near Omaha. Slices of visible coverage are shown for line-of-sight paths between adjacent payloads when they are all three at 50,000ft. Assuming the typical 1000ft/min ascent rate and near simultaneous burst of all three balloons at 100,000ft, there would be 50-minutes for EBBE, even EBBBE, QSOs.