From b4be6e0dd3bf361d3b32208174ec985bf5895e38 Mon Sep 17 00:00:00 2001
From: Oleg Alexandrov <oleg.alexandrov@gmail.com>
Date: Tue, 26 Apr 2022 08:56:23 -0700
Subject: [PATCH] SAR model: Fix an infinite loop issue (#382)
* SAR model: Fix an infinite loop issue
* Add note that convergence may not be in one step for a nonspherical datum
---
src/UsgsAstroSarSensorModel.cpp | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)
diff --git a/src/UsgsAstroSarSensorModel.cpp b/src/UsgsAstroSarSensorModel.cpp
index 95f1822..291c45a 100644
--- a/src/UsgsAstroSarSensorModel.cpp
+++ b/src/UsgsAstroSarSensorModel.cpp
@@ -524,12 +524,16 @@ csm::EcefCoord UsgsAstroSarSensorModel::imageToGround(
// The cross-track unit vector is orthogonal to the position so we ignore it
double nadirComp = dot(spacecraftPosition, tHat);
- // Iterate to find proper radius value
+ // Iterate to find proper radius value. Limit the number of iterations
+ // because the desired precision may not drop beyond 1e-10 no matter
+ // how many iterations one does. As it was observed, this in fact
+ // converges at the first iteration; this is expected for a spherical
+ // datum and not for an ellipsoidal one.
double pointRadius = m_majorAxis + height;
double radiusSqr;
double pointHeight;
csm::EcefVector groundVec;
- do {
+ for (int it = 0; it < 10; it++) {
radiusSqr = pointRadius * pointRadius;
double alpha =
(radiusSqr - slantRange * slantRange - positionMag * positionMag) /
@@ -542,7 +546,10 @@ csm::EcefCoord UsgsAstroSarSensorModel::imageToGround(
pointHeight = computeEllipsoidElevation(
groundVec.x, groundVec.y, groundVec.z, m_majorAxis, m_minorAxis);
pointRadius -= (pointHeight - height);
- } while (fabs(pointHeight - height) > desiredPrecision);
+
+ if (fabs(pointHeight - height) <= desiredPrecision)
+ break;
+ }
if (achievedPrecision) {
*achievedPrecision = fabs(pointHeight - height);
--
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