11ME SECONDS (3100'S) 
Figure 7.—-Sample profile plotted and annotated from Great Salt Lake Desert Profiling Project data 
  
After all of the flight data were processed and terrain profiles computed, 
a check of the repeatability of the profiles was performed at the inter- 
gections of diagonal profiles. For example, the intersection of the pro- 
file between retroreflectors Al and B2 and the profile between A2 and B1 
provided one opportunity for comparing the elevations at the intersection 
point. There were 31 intersections at retroreflector sites. However, 
these intersections were not used because the tracker is locked to the 
retroreflector, forcing the profile elevations to agree. Since the loca- 
tion of each intersection is unknown, and the likelihood of two points 
being compared having the same latitude and longitude is remote, the inter- 
sections were compared manually. Fortunately, there were only 31 to com- 
pare. The results of this comparison are shown in Table 2. 
The largest difference is 4.60 feet and the smallest 0.10 feet. For those 
profiles bounded by retroreflector locks, the rms difference is 1.28 feet 
and the average difference is 0.91 feet. Table 2 also lists reasons for 
the discrepancies. It should be kept in mind when reviewing these results 
that, first, the rms difference between profiles is 1.4 times as large as 
the rms error of individual profiles and, secondly, that the diagonal 
flight time was 4.5 minutes, which is considerably longer than the pre- 
ferred 3 minutes between updates. 
APTS FUTURE 
The concept of the APTS began with an engineering analysis study performed 
by the Draper Laboratory for the USGS in 1975. The study addressed the 
application of an airborne laser profiler augmented by an inertial naviga- 
tion system to measure ground profiles across stream valleys as needed for 
flood-plain mapping. The optimistic conclusion of the study led to a 
series of contracts for system design, component fabrication, system inte- 
gration, aircraft installation, flight testing, performance evaluation, 
and application testing. The program has reached a milestone, a point 
where the original goals have been reached. The APTS is operational and 
performs profile surveys to the accuracy specified. Its reliability, 
however, is somewhat less than anticipated, and possible causes are being 
investigated. 
Plans call for the APTS to be applied to profile surveys in support of 
earth science projects of the U.S. Geological Survey and other Government 
agencies. Most of the requests received call for APTS profile surveys 
dense enough to define the topography for the analysis and modeling of 
surface-water runoff. Other earth science applications point toward a 
similar need, i.e., to define the topography or develop a high-accuracy 
digital terrain model. A scanning mode for the APTS laser profiler would 
make the APTS much more suitable for these applications. The other re- 
quests for APTS capabilities take advantage of the high accuracy position 
and velocity information provided for the aircraft. These include measu- 
ring the accuracy of positions determined by an airborne GPS receiver and 
measuring atmospheric pressure and winds through convective storms. 
The development and fabrication of an APTS II is under consideration at 
this time. A refined version would require less power and be lighter and 
less costly than the present prototype model. It would consist of a com- 
mercially available inertial measuring unit (IMU), laser altimeter, and 
A GPS receiver would be incorporated into the system to 
microprocessors. 
A scanning 
improve the operating capabilities and final position accuracy. 
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