The Effects of Large Ionospheric Gradients on Single Frequency Airborne Smoothing Filters for WAAS and LAAS
The Effects of Large Ionospheric Gradients on Single Frequency Airborne Smoothing Filters for WAAS and LAAS
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发表时间：
2004-01
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通讯作者：
T. Walter;S. Datta‐Barua;J. Blanch;P. Enge
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作者：
T. Walter;S. Datta‐Barua;J. Blanch;P. Enge
WAAS 和 LAAS 接收器都使用载波平滑滤波器来减少多径和热噪声对飞机的影响。对于大多数用户来说，这可以减少错误的程度并提高准确性。然而，显着电离层梯度的存在可能会给该滤波器的输出带来偏差。如果不加以缓解，这种偏差可能会明显大于滤波器用来减少的噪声和多径效应。这种梯度在中纬度地区很少见。然而，它们在极地地区更为常见，有时在赤道地区每天都会发生。因此，这个在美国或欧洲可能很少见的问题在世界其他地区将更加严重。产生偏差的原因是 GPS 代码和载波测量受到电离层的影响不同。代码的正延迟会为承运人带来相等但相反的提前。因此，当电离层总电子含量 (TEC) 沿着用户到卫星的视线发生变化时，代码和载波测量值彼此之间的偏差是电离层变化率的两倍。名义上，几百秒内的变化率非常小（每秒不到几毫米），导致分米大小的偏差或更小。然而，如果用户的视线穿过显着的电离层梯度，TEC 可能会在短短几分钟内发生数十米的变化。这反过来又会产生大于 20 米的有偏差滤波器估计值。与小于 0.4 m 1-sigma 的总体误差预算相比，此误差较大。本文研究了 TEC 变化和滤波器偏差对 WAAS/LAAS 用户差分距离误差的综合影响，并提出了减轻影响的补救措施。该解决方案包括对机载算法的更改，以检测代码和运营商之间的巨大差异。机载接收器检测这种偏差的能力取决于它所经历的热噪声和多径的量。本文将回顾当前预期的噪声和多径水平，并指出应可实现的性能水平。
Both WAAS and LAAS receivers use carrier-smoothing filters to reduce the effects of multipath and thermal noise at the aircraft. For the majority of users this reduces the magnitude of the errors and leads to improved accuracy. However, the presence of a significant ionospheric gradient can introduce a bias into the output of this filter. If unmitigated, this bias can grow to be significantly larger than the noise and multipath effects the filter is employed to reduce. Such gradients are rare at midlatitudes; however, they are much more common in polarregions and sometimes a daily occurrence in equatorial regions. Therefore, this problem which may be rare in the United States or Europe will be much more significant in other parts of the world. The bias arises because the GPS code and carrier measurements are affected differently by the ionosphere. A positive delay for the code creates an equal, but opposite advance for the carrier. Therefore, as the ionospheric Total Electron Content (TEC) changes along a user’s line of sight to the satellite, the code and carrier measurements diverge from each other at twice the rate of ionospheric change. Nominally, the rate of change over a few hundred seconds is very small (less than a few millimeters per second) leading to decimeter-sized biases or less. However, if the user’s line of sight traverses a significant ionospheric gradient, the TEC can change by tens of meters in just a few minutes. This in turn can create a biased filter estimate of greater than 20 meters. This error is large compared to the overall error budget less than 0.4 m one-sigma. This paper investigates the combined effect of the TEC change and the filter bias on the WAAS/LAAS user’s differential range error and suggests a remedy to lessen the impact. This solution includes changes to the airborne algorithms to detect large differences between the code and carrier. The ability of the airborne receiver to detect this bias depends on the amount of thermal noise and multipath it experiences. This paper will review the current expected levels of noise and multipath and indicate level of performance that should be achievable.